JP7189170B2 - Planar lighting device - Google Patents

Description

The present invention relates to a planar illumination device used for a backlight unit of a liquid crystal display device or the like.

2. Description of the Related Art LCDs (Liquid Crystal Displays) are widely used year by year as space-saving image display devices with low power consumption.
In recent years, liquid crystal display devices are required to have a higher dynamic range, lower power consumption, and improved color reproducibility as performance improvements. In particular, from the viewpoint of achieving both a high dynamic range and low power consumption, a so-called direct type backlight is preferably used, which is capable of in-plane partial driving and so-called local dimming.

A direct type backlight is a type of backlight that emits planar light by emitting light from light sources such as LEDs (Light Emitting Diodes) arranged two-dimensionally.
Therefore, in the direct type backlight, a light diffusion member such as a diffuser plate and a prism sheet is used to make the light uniform in order to make the in-plane (plane direction) uniform brightness.

In recent years, there has been a demand for thinner LCDs, and along with this demand, thinner backlight units have also been demanded. In order to reduce the thickness of the direct type backlight unit, it is necessary to place the light source and the light diffusing member close to each other.
However, in the direct type backlight unit, when the light source and the light diffusion member are brought close to each other, a so-called hot spot occurs, in which only the area directly above the light source becomes extremely bright. If a hot spot occurs in the backlight unit, the displayed image on the LCD will become brighter only at the hot spot portion, resulting in deterioration of the display image quality.

On the other hand, Patent Document 1 describes that in a direct type backlight unit, a shielding layer (reflection pattern) that reflects light from the light source is provided at a position facing a light source such as an LED. .
As described in Japanese Unexamined Patent Application Publication No. 2002-200000, hot spots can be prevented by providing a shielding layer facing the light source.

JP 2006-012818 A

However, according to studies by the present inventors, hot spots can be prevented by providing a shielding layer corresponding to the light source, but the brightness is partially increased at the edges of the shielding layer. For example, when a circular shielding layer is provided corresponding to the light source, a ring-shaped portion with high brightness is generated at the edge of the shielding layer.
Similarly, when such a high-brightness portion occurs at the edge of the shielding layer, the image displayed by the LCD becomes bright only at the edge of the shielding layer, resulting in deterioration of the display image quality.

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems of the prior art. To provide a planar lighting device capable of preventing high luminance and irradiating planar lighting with high luminance uniformity.

The present invention solves this problem with the following configuration.
[1] a substrate;
a plurality of light sources discretely mounted on one main surface of the substrate;
a first reflective layer provided on the main surface of the substrate on which the light source is mounted;
a shielding layer provided on the light emitting side of the light source and spaced apart from the light source, corresponding to the position of the light source in the plane direction of the main surface of the substrate;
a second reflective layer provided on the light emitting side of the light source and spaced further from the light source than the shielding layer;
The second reflective layer has a cholesteric liquid crystal layer in which the cholesteric liquid crystal phase is fixed, and the difference between the emission center wavelength of the light source and the reflection center wavelength of the integrated reflectance spectrum of the second reflective layer is 50 nm or less. A planar lighting device characterized by the following.
[2] The planar lighting device according to [1], wherein the second reflective layer has diffuse reflectivity.
[3] The planar lighting device according to [1] or [2], wherein the second reflective layer has an integrated transmittance of 40% or less when light of the emission center wavelength of the light source is incident at an incident angle of 0°. .
[4] The second reflective layer has an integrated transmittance of T(0) when the light of the emission center wavelength of the light source is incident at an incident angle of 0°, and the light of the emission center wavelength of the light source is incident at an incident angle of 40°. When the integrated transmittance at the time is T (40),
The planar illumination device according to any one of [1] to [3], wherein the integrated transmittance T(40) is 1.2 times or more the integrated transmittance T(0).
[5] The planar illumination device according to any one of [1] to [4], wherein the difference between the reflection center wavelength of the integrated reflectance spectrum of the second reflective layer and the half-value wavelength on the longer wavelength side is 70 nm or less. Device.
[6] The planar illumination device according to any one of [1] to [5], wherein the distance between adjacent light sources is 10 mm or less.
[7] The planar illumination device according to any one of [1] to [6], wherein the light source has an emission central wavelength in the wavelength region of blue light.
[8] The planar lighting device according to any one of [1] to [7], wherein the light source and the shielding layer are provided in the same pattern in the plane direction of the main surface of the substrate.
[9] The planar illumination device according to any one of [1] to [8], wherein the second reflective layer has a combination of two cholesteric liquid crystal layers that selectively reflect circularly polarized light with opposite rotation directions. Device.

According to the planar illumination device of the present invention, for example, when it is used in a direct type backlight unit used in an LCD or the like, it is possible to prevent the emitted light from becoming partially high in brightness and to improve the brightness uniformity. A high backlight (planar illumination) can be emitted.

1 is a diagram conceptually showing an example of a backlight unit using the planar lighting device of the present invention; FIG. It is a figure which shows notionally an example of a 2nd reflection layer. FIG. 4 is a conceptual diagram for explaining the action of an example of a cholesteric liquid crystal layer; FIG. 4 is a conceptual diagram for explaining the action of another example of the cholesteric liquid crystal layer; FIG. 3 is a conceptual diagram for explaining a conventional backlight unit;

DESCRIPTION OF THE PREFERRED EMBODIMENTS The planar illumination device of the present invention will be described in detail below with reference to preferred embodiments shown in the accompanying drawings.

The description of the constituent elements described below may be made based on representative embodiments of the present invention, but the present invention is not limited to such embodiments.
In the present invention, a numerical range represented by "-" means a range including the numerical values described before and after "-" as lower and upper limits.
In the present invention, "(meth)acrylate" is used to mean "one or both of acrylate and methacrylate".
In addition, although not limited thereto, in the present invention, light in a wavelength range of 400 nm or more and less than 495 nm is blue light, light in a wavelength range of 495 nm or more and less than 595 nm is green light, and 595 nm or more and less than 750 nm. is red light.

In the present invention, the integrated transmittance T(θ, λ) at the incident angle θ and the wavelength λ is determined by adjusting the position of the light source so that the light from the surface of the object to be measured has a predetermined angle of incidence with respect to the object to be measured. A commercially available light source, a light receiving element, an integrating sphere device, and the like may be combined for measurement so that the light is incident.
In the present invention, the integrated reflectance R (λ) at the wavelength λ is measured by a commercially available spectrophotometer (for example, V-550 manufactured by JASCO Corporation) so that the light is incident from the surface of the measurement object. A large integrating sphere device (eg, ILV-471 manufactured by JASCO Corporation) may be used for measurement.

In the present invention, the reflection center wavelength of the integrated reflectance spectrum is obtained by the following method.
When the integrated reflectance at each wavelength is measured by the above-described method, a spectrum waveform (integrated reflectance spectrum) of the integrated reflectance is obtained which has a mountain shape (upper convex shape) with the wavelength as the horizontal axis. An average reflectance (arithmetic mean) of the maximum value and the minimum value of the integrated reflectance of this integrated reflectance spectrum is obtained. Furthermore, of the two wavelengths at the two intersections of the waveform and the average reflectance, the wavelength on the short wavelength side (half-value wavelength on the short-wavelength side) is λα [nm], and the wavelength on the long-wavelength side (half-value wavelength on the long wavelength side) is λβ [nm], the reflection center wavelength is calculated by the following formula.
Reflection center wavelength = (λα + λβ)/2
As another method, there is a method of measuring the reflection center wavelength by measuring the transmission spectrum of the object to be measured with Axoscan manufactured by Axometrix. When the transmission spectrum is measured, a valley-shaped (downwardly convex) transmission spectrum waveform is obtained with the wavelength as the horizontal axis. At this time, the average transmittance (arithmetic average) of the maximum value and minimum value of the transmittance is obtained. By setting the wavelength on the side to λβ (nm), the reflection center wavelength is calculated by the above formula.

FIG. 1 conceptually shows an example in which the planar illumination device of the present invention is used as a backlight unit (direct type backlight unit) of an LCD. In the following description, the backlight unit using the planar lighting device of the present invention is also simply referred to as "backlight unit of the present invention".
A backlight unit (planar lighting device) 10 shown in FIG.
Note that the planar lighting device of the present invention is not limited to those used in backlight units such as LCDs. It can be used for various uses such as outdoor lighting and outdoor lighting. Among others, the planar illumination device of the present invention can be suitably used for a backlight unit such as an LCD in that planar illumination with high luminance uniformity can be obtained.

In addition to the illustrated members, the backlight unit 10 of the present invention may have various members that a backlight unit used as a light source for emitting a direct backlight (planar illumination) in an LCD or the like has. good. Examples of the members include diffusion plates, diffusion sheets, light guide plates, prism sheets, wavelength conversion members, polarizers, brightness enhancement films, reflective linear polarizers, and color filters.
As an example, the backlight unit of the present invention has a diffusion plate above the second reflective layer 20 of the backlight unit 10 shown in FIG. and a wavelength conversion member that emits fluorescence of red light, and two prism sheets may be provided on the upper side in the drawing so that the ridge lines are perpendicular to each other. Examples of the wavelength conversion member include a film containing a KSF phosphor, a quantum dot film, and the like.

The substrate 12 has a light source 14 mounted on one principal surface thereof, and a first reflective layer 16 provided on the surface on which the light source 14 is mounted. The main surface is the maximum surface of the sheet-like material (film, plate-like material, layer).
Various known substrates can be used for the substrate 12 as long as the substrate 12 can stably mount the light source 14 thereon. Examples include paper phenol substrates, paper epoxy substrates, glass epoxy substrates, composite epoxy substrates, glass composite substrates, glass polyimide substrates, BT (bismaleimide triazine resin) substrates, Teflon (registered trademark) substrates, and metal base substrates. mentioned. Moreover, when flexibility is required for the base material, a flexible substrate made of a resin such as polyester resin, polyimide resin, or liquid crystal polymer can be used.
Also, as the substrate 12, a printed circuit board such as a printed wiring board and a printed circuit board provided with wiring and circuits for driving the light source 14 can be used. For example, when the light source 14 is an LED, various known so-called LED substrates can be used as the substrate 12 .

In the present invention, the substrate 12 may have light reflectivity by a method of kneading a white pigment or a method of containing fine air bubbles. That is, in the backlight unit of the present invention, the substrate 12 may act as a first reflective layer, which will be described later.

The thickness of the substrate 12 is also not limited, and may be appropriately set according to the size of the substrate 12, the type of the substrate 12, the material forming the substrate 12, the type of the light source 14, the number of the light sources 14 mounted, and the like. .
In order to make the backlight unit 10 thinner, it is preferable that the substrate 12 is thinner. The thickness of the substrate 12 is preferably 10-1000 μm, more preferably 30-400 μm.

In the backlight unit 10 of the present invention, the light sources 14 are two-dimensionally arranged (mounted) on one main surface (upper surface in the figure) of the substrate 12 .
A light source 14 is the light source of the backlight unit 10 . In the illustrated backlight unit 10, the light source 14 is, as a preferred example, a blue light source having an emission center wavelength in the wavelength range of blue light. More preferably, light source 14 has an emission center wavelength within the range of 450 nm±25 nm.
The emission center wavelength is the wavelength (peak wavelength) at which the emission intensity is the highest. That is, the illustrated light source 14 is a light source that emits light with a peak intensity of 420 to 490 nm.

In the backlight unit 10 of the present invention, the light source 14 is not limited to a blue light source. That is, the light source 14 may be a green light source having an emission central wavelength in the green light wavelength region or a red light source having an emission central wavelength in the red light wavelength region, and the emission central wavelength is in the ultraviolet wavelength region (280 nm or more and less than 400 nm). may be an ultraviolet light source having
Moreover, the backlight unit 10 of the present invention may have a plurality of types of light sources 14 having different emission center wavelengths. For example, the backlight unit 10 of the present invention may have a blue light source and a red light source, may have a blue light source and a green light source, or may have a blue light source, a green light source, and a red light source.
When the backlight unit 10 of the present invention has a plurality of types of light sources 14 with different emission center wavelengths, the second reflective layer 20 described later is arranged so that each light source 14 It is preferable to have a plurality of types of cholesteric liquid crystal layers that selectively reflect emitted light. That is, when the backlight unit 10 has a plurality of types of light sources 14 with different emission center wavelengths, the second reflective layer 20 includes a plurality of types with different selective reflection center wavelengths according to the emission center wavelengths of the respective light sources 14. of cholesteric liquid crystal layers.
For example, when the backlight unit 10 has a blue light source and a red light source, the second reflective layer 20 also includes a cholesteric liquid crystal layer that selectively reflects blue light and a cholesteric liquid crystal layer that selectively reflects red light. It is preferable to have two types of cholesteric liquid crystal layers having different selective reflection wavelength ranges (selective reflection central wavelengths) from the layers.

As the light source 14, various known light sources (light sources) used in a direct backlight unit or the like can be used. Examples of the light source 14 include an LED, a laser diode, an organic EL (Electro Luminescence), an inorganic EL, and the like. Among them, LEDs are preferably used.

In the backlight unit 10 of the present invention, the arrangement (arrangement) of the light sources 14 may be similar to that of a general direct type backlight unit used in LCDs.
Therefore, the arrangement of the light sources 14 may be regular or irregular, but is usually regular. Examples of the regular arrangement include a square lattice, a houndstooth lattice, an orthorhombic lattice, a hexagonal lattice, and the like. Also, the arrangement density of the light sources 14 may be uniform or may vary in the direction of the main surface of the substrate 12 .

The interval between the light sources 14 is not limited, and may be appropriately set according to the size of the backlight unit, the required brightness of the backlight, and the like.
The interval between the light sources 14 is preferably 10 mm or less, more preferably 8 mm or less, and even more preferably 5 mm or less, which is the distance between adjacent light sources 14 . By setting the distance between the adjacent light sources 14 to 10 mm or less, uniformity is improved and finer luminance control (local dimming) becomes possible.
The distance between light sources is the distance between centers (usually optical axes).

A first reflective layer 16 is provided on the main surface of the substrate 12 on which the light source 14 is arranged.
The first reflective layer 16 is for reflecting the emitted light of the light source 14 reflected by the shielding layer 18 and the second reflective layer 20, which will be described later, toward the shielding layer 18 and the second reflective layer 20 again. be.
The first reflective layer 16 may have an opening corresponding to the light source 14 as shown, or the light source may be placed above the first reflective layer 16 .

The first reflective layer 16 is not limited, and various known substrate-side reflective layers used in LCD backlights and the like can be used. Also, the first reflective layer 16 may be one that reflects light as a whole, such as a diffuse reflector, or may reflect light only on the surface, such as a structure in which a reflective film is provided on the surface of a metal plate and a resin layer. It may be reflective.
As the first reflective layer 16, a white reflective layer formed by using a white solder resist, white paint, etc., or a diffusion reflector in which white particles such as titanium oxide are dispersed in a resin matrix such as a silicone resin or an epoxy resin. , metal plates such as aluminum plates and silver plates, dielectric multilayer films of inorganic compounds such as _SiO2 and _TiO2 , and polymer multilayers in which high refractive index polymer layers and low refractive index polymer layers are alternately laminated Examples include membranes, as well as combinations of at least two of these.
For the first reflective layer 16, commercially available light reflecting materials such as Furukawa Electric's MCPET and MCPOLYCA and 3M's ESR (Enhanced Specular Reflector) can also be suitably used.

The thickness of the first reflective layer 16 is not limited, and the thickness that can reflect the light emitted by the light source 14 with sufficient reflectance can be appropriately set according to the material forming the first reflective layer 16. Just do it.
The thickness of the first reflective layer 16 is preferably 0.01-1000 μm, more preferably 0.02-100 μm.

The backlight unit 10 of the present invention is provided with a shielding layer 18 on the side from which light is emitted from the light source 14 , spaced apart from the light source 14 .
The shielding layer 18 is for reflecting the light emitted from the light source 14 directly above the light emitting direction of the light source 14 . In the following description, "directly above the light emitting direction of the light source 14" is also simply referred to as "directly above the light source 14".
Therefore, the shielding layer 18 is arranged at a position corresponding to the light source 14 in the planar direction of the main surface of the substrate 12 . That is, the shielding layer 18 is provided in a pattern having the same arrangement pattern as the arrangement pattern of the light sources 14 according to the arrangement of the light sources 14 . In the following description, unless otherwise specified, the surface direction indicates the surface direction of the main surface of the sheet-like material.
The light source 14 and the shielding layer 18 are preferably arranged so that the optical axis of the light source 14 coincides with the center of the shielding layer 18 in the planar direction. For example, if the shielding layer 18 is circular, the shielding layer 18 is arranged so that the optical axis of the light source 14 passes through the center of the circle. If the shielding layer 18 is square or rectangular, the shielding layer 18 is arranged so that the optical axis of the light source 14 passes through the intersection of the diagonals.

As described above, in the direct type backlight unit, the luminance of emitted light is high immediately above the light sources 14 arranged two-dimensionally, and the emitted backlight (planar illumination) emits light directly above the light sources 14. high-brightness hot spots.
On the other hand, as described in Japanese Unexamined Patent Application Publication No. 2002-100000, hot spots are prevented by providing a shielding layer 18 that reflects the light emitted from the light source 14 directly above the light source 14 of the direct type backlight unit. You can prevent it from happening.

The transmittance of the shielding layer 18 for the light emitted from the light source 14 is not limited, and depends on the output of the light source 14, the distance between the light source 14 and the shielding layer 18, the distance between the light sources 14, the arrangement of the light sources 14, and the like. can be set as appropriate.
The transmittance of the shielding layer 18 is preferably 1 to 80%, more preferably 2 to 70%, and even more preferably 5 to 60%, as an integrated transmittance for the emission center wavelength of the light source 14 .
By setting the transmittance of the shielding layer 18 to 1 to 80%, the generation of hot spots is prevented, and the decrease in brightness directly above the light source 14 is also prevented, so that the brightness of the illuminated backlight is more preferably uniformed. It is preferable in that it is possible to

If the light emitted from the light source 14 is absorbed by the shielding layer 18, that amount of light is lost.
Therefore, the shielding layer 18 preferably reflects light other than transmitted light with the highest possible reflectance.

The area of the shielding layer 18 is also not limited, and is appropriately adjusted according to the diffusibility of the light emitted from the light source 14, the distance between the light source 14 and the shielding layer 18, the arrangement of the light sources 14, the distance between the light sources 14, and the like. You can set it.
The area of the shielding layer 18 is preferably an area where 100 to 2500% of the beam spot of the light emitted from the light source 14 is irradiated to the shielding layer 18 at the arrangement position of the shielding layer 18, and 120 to 1600% is irradiated to the shielding layer 18. The irradiated area is more preferable, and the area where 140 to 900% of the shielding layer 18 is irradiated is even more preferable.

The distance between the shielding layer 18 and the light sources 14 is also not limited, and may be appropriately set according to the intended thickness of the backlight unit 10, the output of the light sources 14, the distance between the light sources 14, and the like.
A shorter distance between the shielding layer 18 and the light source 14 is advantageous in that the backlight unit 10 can be made thinner. Moreover, since the backlight unit 10 of the present invention has the shielding layer 18, even if the shielding layer 18 and the light source 14 are brought close to each other, hot spots can be prevented from occurring. In addition, since the backlight unit 10 of the present invention has the second reflective layer 20, even if the shielding layer 18 and the light source 14 are brought close to each other, high luminance portions (fringes) occur at the edge of the shielding layer 18, which will be described later. can also be prevented.
Considering this point, the distance between the shielding layer 18 and the light source 14 is preferably 5 mm or less, more preferably 3 mm or less, and even more preferably 2 mm or less.

The shape of the shielding layer 18 (the shape of the main surface) is also not limited, and various shapes such as circular, square, rectangular, elliptical, and triangular can be used.
Since the beam spot shape of the light emitted from the light source 14 such as an LED is usually circular, the shielding layer 18 is also preferably circular.

When the shielding layer 18 is elliptical and rectangular, light from a plurality of light sources 14 may be incident on one shielding layer 18 .
In addition, in the backlight unit 10 of the present invention, shielding layers 18 having different shapes, such as a circular shielding layer 18 and a square shielding layer 18, may be mixed.

The material for forming the shielding layer 18 is not limited, and various materials used for various known light reflecting members can be used according to the emission center wavelength of the light source 14 . Further, the shielding layer 18 may diffusely reflect incident light, or may reflect incident light in a specular manner.
Examples of the material for forming the shielding layer 18 include white ink, a diffuse reflection material obtained by dispersing light diffusion particles such as titanium oxide in a matrix binder, an interference reflection film such as a laminated film, and a metal thin film. exemplified. Commercially available products of these light reflecting materials are also suitable for use.

The shielding layer 18 may be formed on a transparent support, or may be formed on a second reflective layer 20 which will be described later. Alternatively, the sheet-like shielding layer 18 may be held directly above the light source 14 in a known manner using holding means.
Transparent supports forming the shielding layer 18 include films made of resin materials such as polyethylene terephthalate (PET), triacetyl cellulose (TAC), cycloolefin polymers, polyacrylic resins, and polycarbonate resins, and glass plates. etc. are exemplified.

The method for forming the shielding layer 18 is not limited, and depending on the material to be formed, it may be formed by a coating method. It may be worn and formed.
As the coating method, various known methods such as screen printing, ink jet method, and coating with a dispenser can be used. As the adhesive, various known adhesives such as an optically transparent adhesive (OCA (Optical Clear Adhesive)), an optically transparent double-sided tape, and an ultraviolet curable resin can be used.

In the backlight unit 10 of the present invention, the second reflective layer 20 is provided at a position spaced apart from the shielding layer 18 on the light emitting side from the light source 14 . That is, the shielding layer 18 is arranged between the light source 14 and the second reflective layer 20 on the light exit side from the light source 14 .
The second reflective layer 20 has a cholesteric liquid crystal layer with a fixed cholesteric liquid crystal phase.
Further, in the backlight unit 10 of the present invention, the difference between the emission center wavelength of the light source 14 and the reflection center wavelength (selective reflection center wavelength) of the integrated reflectance spectrum of the second reflective layer 20 is 50 nm or less. For example, when the emission center wavelength of the light source 14 is 450 nm, the reflection center wavelength of the integrated reflectance spectrum of the second reflective layer 20 is in the range of 400-500 nm.
As will be described in detail later, the backlight unit 10 of the present invention has such a second reflective layer 20 having a cholesteric liquid crystal layer in addition to the shielding layer 18 provided corresponding to the light source 14 described above. It is possible to irradiate a backlight with high luminance uniformity by preventing the backlight to be irradiated (planar lighting) from being partially high in luminance.
In the following description, "reflection central wavelength of the integrated reflectance spectrum of the second reflective layer 20" is also simply referred to as "reflection central wavelength of the second reflective layer 20".

FIG. 2 conceptually shows an example of the second reflective layer 20 .
The second reflective layer 20 shown in FIG. 2 has a support 28, an underlayer 30, a first cholesteric liquid crystal layer 32R and a second cholesteric liquid crystal layer 32L.
In addition, in the backlight unit 10 of the present invention, the second reflective layer 20 may be in the form of one sheet or may be divided into a plurality of pieces. However, the second reflective layer 20 is provided so as to include the shielding layer 18 at least in the planar direction. Preferably, the second reflective layer 20 is provided so as to cover the beam spot of the light emitted from the light source 14 in the planar direction.

<Support>
The support 28 of the second reflective layer 20 supports the underlayer 30, the first cholesteric liquid crystal layer 32R and the second cholesteric liquid crystal layer 32L.

Support 28 may be a single layer or multiple layers. Supports 28 in the case of a single layer include supports made of glass, TAC, PET, polycarbonate, polyvinyl chloride, acrylic, polyolefin, and the like. Examples of the support 28 in the case of a multi-layer include those including any of the above-described single-layer supports as a substrate and providing another layer on the surface of this substrate.

Further, the film thickness of the support 28 is not limited, and the thickness that has sufficient transparency and can support the cholesteric liquid crystal layer may be appropriately set according to the forming material and the like. The thickness of the support 28 is preferably 10-300 μm, more preferably 12-100 μm, and even more preferably 12-65 μm.
Also, the support 28 is preferably transparent. Specifically, the support 28 preferably has a total light transmittance of 85% or more as measured according to JIS K 7361.

<Underlayer>
The foundation layer 30 is a layer that serves as a foundation for forming the first cholesteric liquid crystal layer 32R and the second cholesteric liquid crystal layer 32L.
The underlying layer 30 is preferably a resin layer, more preferably a transparent resin layer. Examples of underlayers include an alignment film for adjusting the alignment of the liquid crystal compound when forming the first cholesteric liquid crystal layer 32R and the second cholesteric liquid crystal layer 32L, and the support 28 and the first cholesteric liquid crystal layer 32R and A layer for improving adhesive properties with the second cholesteric liquid crystal layer 32L, and the like.
The thickness of the underlying layer 30 is not limited, and the thickness that allows the underlying layer 30 to exhibit its intended function may be appropriately set according to the material forming the underlying layer 30 .

In the second reflective layer 20, the support 28 and the underlying layer 30 are provided as necessary and are not essential constituents.
Therefore, the second reflective layer 20 may be composed of the support 28 and the cholesteric liquid crystal layer, or may be composed of the underlayer 30 and the cholesteric liquid crystal layer, or may be composed of the cholesteric liquid crystal layer only. may

<Cholesteric liquid crystal layer>
A first cholesteric liquid crystal layer 32R is formed on the surface of the base layer 30, and a second cholesteric liquid crystal layer 32L is formed on the surface of the first cholesteric liquid crystal layer 32R.
Both the first cholesteric liquid crystal layer 32R and the second cholesteric liquid crystal layer 32L have a fixed cholesteric liquid crystal phase.
The cholesteric liquid crystal layer has wavelength selectivity in reflection and selectively reflects circularly polarized light in a specific sense of rotation. The first cholesteric liquid crystal layer 32R selectively reflects right-handed circularly polarized light, and the second cholesteric liquid crystal layer 32L selectively reflects left-handed circularly polarized light.

In the backlight unit 10 of the present invention, the second reflective layer 20 is not limited to having two cholesteric liquid crystal layers in which the circularly polarized light to be reflected rotates in opposite directions. That is, the second reflective layer 20 may have only one of the first cholesteric liquid crystal layer 32R and the second cholesteric liquid crystal layer 32L.
However, the second reflective layer 20 has two layers of cholesteric liquid crystal layers in which the circularly polarized light to be reflected is rotated in the opposite direction according to the emission center wavelength of the light source 14 in order to more preferably prevent hot spots. is preferred.
Further, as described above, when the backlight unit has a plurality of types of light sources 14 with different emission center wavelengths, the second reflective layer 20 also includes a plurality of light sources 14 with different selective reflection wavelength ranges according to each light source 14 . It is preferred to have a seed cholesteric liquid crystal layer. In this case, the second reflective layer 20 may have a plurality of configurations including the support 28, the base layer 30 and the cholesteric liquid crystal layer. In this case as well, it is preferable to have a plurality of combinations of two cholesteric liquid crystal layers in which the rotating directions of the reflected circularly polarized light are opposite to each other.

In the following description, if there is no need to distinguish between the first cholesteric liquid crystal layer 32R that selectively reflects right-handed circularly polarized light and the second cholesteric liquid crystal layer 32L that selectively reflects left-handed circularly polarized light, both are used. It is collectively called a cholesteric liquid crystal layer.

Cholesteric liquid crystal phases are known to exhibit selective reflectivity at specific wavelengths.
In a general cholesteric liquid crystal phase, the selective reflection center wavelength (selective reflection center wavelength) λ depends on the helical pitch P in the cholesteric liquid crystal phase, and follows the relationship between the average refractive index n of the cholesteric liquid crystal phase and λ = n × P. . Therefore, by adjusting the helical pitch P, the selective reflection central wavelength can be adjusted. The selective reflection center wavelength of the cholesteric liquid crystal phase becomes longer as the helical pitch P becomes longer.
Note that the helical pitch P is the length in the thickness direction of one pitch of the helical structure of the cholesteric liquid crystal phase (the period of the helix), in other words, it is the number of turns of the cholesteric liquid crystal. It is the length in the helical axis direction in which the director of the liquid crystal compound constituting the phase (long axis direction in the case of rod-like liquid crystal) rotates 360°.

When the cross section of the cholesteric liquid crystal layer is observed using a scanning electron microscope (SEM), a striped pattern of bright portions B and dark portions D is usually observed as conceptually shown in FIG. . That is, in the cross section of the cholesteric liquid crystal layer, a layered structure in which bright portions B and dark portions D are alternately laminated can be observed.
In the cholesteric liquid crystal layer, two bright portions B and two dark portions D correspond to one spiral pitch of the cholesteric liquid crystal phase.

The helical pitch P of the cholesteric liquid crystal phase depends on the type of chiral agent used together with the liquid crystal compound and the addition concentration of the chiral agent when forming the cholesteric liquid crystal layer. Therefore, the desired helical pitch P can be obtained by adjusting these.
Regarding the adjustment of the helical pitch P, refer to Fuji Film Research Report No. 50 (2005) p. 60-63 for a detailed description. Methods for measuring the helical sense and the helical pitch P are described in "Introduction to Liquid Crystal Chemistry Experiments", edited by the Japan Liquid Crystal Society, published by Sigma Publishing, 2007, page 46, and "Liquid Crystal Handbook", Liquid Crystal Handbook Editing Committee, Maruzen, page 196. can be used.

In the backlight unit 10 of the present invention, the difference between the emission central wavelength of the light source 14 and the reflection central wavelength of the second reflective layer 20 is 50 nm or less.
Therefore, the first cholesteric liquid crystal layer 32R and the second cholesteric liquid crystal layer 32L set the selective reflection center wavelength λ, that is, the spiral pitch P so that the reflection center wavelength of the second reflective layer 20 satisfies the above conditions. For example, the helical pitch P and the like are set so that the selective reflection center wavelength λ of the first cholesteric liquid crystal layer 32R and the second cholesteric liquid crystal layer 32L is within ±50 nm of the light emission center wavelength of the light source .

If the difference between the emission central wavelength of the light source 14 and the reflection central wavelength of the second reflective layer 20 is 50 nm or less, the selective reflection central wavelength λ of the first cholesteric liquid crystal layer 32R and the second cholesteric liquid crystal layer 32L There is no limit to the difference between the selective reflection center wavelength λ of .
The difference between the selective reflection central wavelength λ of the first cholesteric liquid crystal layer 32R and the selective reflection central wavelength λ of the second cholesteric liquid crystal layer 32L is preferably 40 nm or less, more preferably 20 nm or less. .

A cholesteric liquid crystal phase exhibits selective reflectivity for either left or right circularly polarized light at a specific wavelength. Whether the reflected light is right-handed circularly polarized light or left-handed circularly polarized light depends on the twist direction (sense) of the spiral of the cholesteric liquid crystal phase. The selective reflection of circularly polarized light by the cholesteric liquid crystal phase reflects right circularly polarized light when the spiral of the cholesteric liquid crystal phase is twisted to the right, and reflects left circularly polarized light when the spiral is twisted to the left.
The direction of rotation of the cholesteric liquid crystal phase can be adjusted by the type of liquid crystal compound forming the cholesteric liquid crystal layer and/or the type of chiral agent added.

In the second reflective layer 20, the first cholesteric liquid crystal layer 32R selectively reflects right-handed circularly polarized light, and the second cholesteric liquid crystal layer 32L selectively reflects left-handed circularly polarized light.
Therefore, the liquid crystal compound and/or the chiral agent are selected so that the first cholesteric liquid crystal layer 32R selectively reflects right-handed circularly polarized light, and the second cholesteric liquid crystal layer 32L is selected so as to selectively reflect left-handed circularly polarized light. , liquid crystal compounds and/or chiral agents are selected.

<<Method for Forming Cholesteric Liquid Crystal Layer>>
A cholesteric liquid crystal layer can be obtained by fixing a cholesteric liquid crystal phase.
The structure in which the cholesteric liquid crystal phase is fixed may be any structure as long as the alignment of the liquid crystal compound in the cholesteric liquid crystal phase is maintained. A structure in which the polymer is polymerized and cured by ultraviolet irradiation, heating, etc. to form a layer having no fluidity and at the same time changes to a state in which the orientation is not changed by an external field or external force may be used.
In the structure in which the cholesteric liquid crystal phase is fixed, it is sufficient that the optical properties of the cholesteric liquid crystal phase are maintained, and the liquid crystal compound does not have to exhibit liquid crystallinity. For example, the polymerizable liquid crystal compound may be polymerized by a curing reaction and lose liquid crystallinity.

A liquid crystal composition containing a liquid crystal compound and a chiral agent can be given as an example of a material used for forming a cholesteric liquid crystal layer in which a cholesteric liquid crystal phase is fixed. The liquid crystal compound is preferably a polymerizable liquid crystal compound.
A liquid crystal composition containing a liquid crystal compound used for forming a cholesteric liquid crystal layer preferably further contains a surfactant. Moreover, the liquid crystal composition used for forming the cholesteric liquid crystal layer may further contain a polymerization initiator.

(liquid crystal compound)
The liquid crystal compound used in the present invention may be a rod-like liquid crystal compound or a discotic liquid crystal compound, but is preferably a rod-like liquid crystal compound.
Examples of rod-like liquid crystal compounds forming a cholesteric liquid crystal structure include rod-like nematic liquid crystal compounds. Rod-shaped nematic liquid crystal compounds include azomethines, azoxys, cyanobiphenyls, cyanophenyl esters, benzoic acid esters, cyclohexanecarboxylic acid phenyl esters, cyanophenylcyclohexanes, cyano-substituted phenylpyrimidines, and alkoxy-substituted phenylpyrimidines. , phenyldioxanes, tolanes, and alkenylcyclohexylbenzonitriles are preferably used. Not only low-molecular-weight liquid crystal compounds but also high-molecular liquid-crystal compounds can be used.

The liquid crystal compound used in the present invention is preferably a polyfunctional liquid crystal compound.
A polyfunctional liquid crystal compound is obtained by introducing a polymerizable group into a liquid crystal compound. Examples of polymerizable groups include unsaturated polymerizable groups, epoxy groups, and aziridinyl groups, with unsaturated polymerizable groups being preferred, and ethylenically unsaturated polymerizable groups being particularly preferred. Polymerizable groups can be introduced into molecules of liquid crystal compounds by various methods. The number of polymerizable groups possessed by the polyfunctional liquid crystal compound is preferably 1 to 6, more preferably 1 to 3, per molecule. Examples of polyfunctional liquid crystal compounds are described in Makromol. Chem. , 190, 2255 (1989), Advanced Materials 5, 107 (1993), U.S. Pat. , WO97/00600, WO98/23580, WO98/52905, JP-A-1-272551, JP-A-6-16616, JP-A-7-110469, JP-A-11-80081, JP-A-2001-328973, Compounds described in WO2016/194327, WO2016/052367 and the like are included. You may use together 2 or more types of polyfunctional liquid crystal compounds. When two or more kinds of polyfunctional liquid crystal compounds are used together, the alignment temperature can be lowered in some cases.

In a liquid crystal composition prepared using a liquid crystal compound, a chiral agent, etc., for forming a cholesteric liquid crystal layer, the amount of the polyfunctional liquid crystal compound added in the liquid crystal composition is determined by the solid content mass of the liquid crystal composition (excluding the solvent). It is preferably from 80 to 99.9% by mass, more preferably from 85 to 99.5% by mass, and particularly preferably from 90 to 99% by mass, based on the total mass).

(Chiral agent (optically active compound))
A chiral agent (chiral agent) has a function of inducing a helical structure of a cholesteric liquid crystal phase. The chiral agent may be selected depending on the purpose, since the helical twist direction or helical pitch induced by the compound differs.
The chiral agent is not particularly limited, and known compounds (for example, Liquid Crystal Device Handbook, Chapter 3, Section 4-3, chiral agent for TN (twisted nematic), STN (Super Twisted Nematic), page 199, Japan Society for the Promotion of Science 142nd Committee, 1989), isosorbide and isomannide derivatives can be used.
A chiral agent generally contains an asymmetric carbon atom, but an axially chiral compound or planar chiral compound that does not contain an asymmetric carbon atom can also be used as the chiral agent. Examples of axially or planarly chiral compounds include binaphthyl, helicene, paracyclophane, and derivatives thereof. The chiral agent may have a polymerizable group. When both the chiral agent and the liquid crystal compound have a polymerizable group, the polymerization reaction of the polymerizable chiral agent and the polymerizable liquid crystal compound causes a repeating unit derived from the polymerizable liquid crystal compound and a repeating unit derived from the chiral agent. A polymer having repeating units can be formed. In this aspect, the polymerizable group possessed by the polymerizable chiral agent is preferably the same type of group as the polymerizable group possessed by the polymerizable liquid crystal compound. Therefore, the polymerizable group of the chiral agent is also preferably an unsaturated polymerizable group, an epoxy group or an aziridinyl group, more preferably an unsaturated polymerizable group, and an ethylenically unsaturated polymerizable group. More preferred.
Also, the chiral agent may be a liquid crystal compound.

When the chiral agent has a photoisomerizable group, it is possible to form a desired reflection wavelength pattern corresponding to the emission wavelength by irradiating with a photomask such as actinic rays after coating and orientation. The photoisomerizable group is preferably an isomerization site of a compound exhibiting photochromic properties, an azo group, an azoxy group, or a cinnamoyl group.
Specific compounds include JP-A-2002-80478, JP-A-2002-80851, JP-A-2002-179668, JP-A-2002-179669, JP-A-2002-179670, JP-A-2002- 179681, JP-A-2002-179682, JP-A-2002-338575, JP-A-2002-338668, JP-A-2003-313189, and the compounds described in JP-A-2003-313292 can be used.

The content of the chiral agent in the liquid crystal composition is preferably 0.01 to 200 mol %, more preferably 1 to 30 mol % of the amount of the liquid crystal compound.

(Polymerization initiator)
The liquid crystal composition preferably contains a polymerization initiator. In the embodiment in which the polymerization reaction is advanced by ultraviolet irradiation, the polymerization initiator to be used is preferably a photopolymerization initiator capable of initiating the polymerization reaction by ultraviolet irradiation. Examples of photopolymerization initiators include α-carbonyl compounds (described in US Pat. Nos. 2,367,661 and 2,367,670), acyloin ethers (described in US Pat. No. 2,448,828), and α-hydrocarbon-substituted aromatic acyloins. compounds (described in US Pat. No. 2,722,512), polynuclear quinone compounds (described in US Pat. Nos. 3,046,127 and 2,951,758), combinations of triarylimidazole dimers and p-aminophenyl ketones (US Pat. No. 3,549,367 No.), acridine and phenazine compounds (described in JP-A-60-105667, US Pat. No. 4,239,850), acylphosphine oxide compounds (JP-B-63-40799, JP-B-5-29234). Publications, JP-A-10-95788, JP-A-10-29997, JP-A-2001-233842, JP-A-2000-80068, JP-A-2006-342166, JP-A-2013-114249, JP 2014-137466, JP 4223071, JP 2010-262028, JP 2014-500852), oxime compound (JP 2000-66385, JP 4454067) ), and oxadiazole compounds (described in US Pat. No. 4,212,970).

Acylphosphine oxide compounds, oxime compounds, or thioxanthone compounds can also be used as polymerization initiators.
As the acylphosphine oxide compound, for example, a commercially available IRGACURE810 (compound name: bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide) manufactured by BASF Japan Ltd. can be used. Examples of oxime compounds include IRGACURE OXE01 (manufactured by BASF), IRGACURE OXE02 (manufactured by BASF), TR-PBG-304 (manufactured by Changzhou Tenryu Electric New Materials Co., Ltd.), Adeka Arkles NCI-831, and Adeka Arkles NCI-930. (manufactured by ADEKA), and ADEKA Arkles NCI-831 (manufactured by ADEKA). As the thioxanthone compound, commercially available products such as Kayacure DETX (manufactured by Nippon Kayaku Co., Ltd.) can be used.
Only one polymerization initiator may be used, or two or more polymerization initiators may be used in combination.
The content of the photopolymerization initiator in the liquid crystal composition is preferably 0.01% by mass to 4.00% by mass, more preferably 0.1% by mass to 2.00% by mass, relative to the content of the liquid crystal compound. % is more preferred.

(radical scavenger)
In the present invention, the liquid crystal composition that forms the cholesteric liquid crystal layer may have a radical scavenger as one having a functional group capable of scavenging radicals.
As the radical scavenger, known compounds can be used, for example, phenol-based compounds (preferably hindered phenol-based compounds), hindered amine-based compounds, diphenylamine-based compounds, phosphorus atom-containing compounds compounds), and sulfur atom-containing compounds.
The content of the radical scavenger is preferably 0.1 to 10% by mass, more preferably 1.1 to 3.3% by mass, based on the content of the liquid crystal compound.

(crosslinking agent)
The liquid crystal composition may optionally contain a cross-linking agent in order to improve film strength and durability after curing. As the cross-linking agent, one that is cured by ultraviolet rays, heat, humidity, or the like can be preferably used.
The cross-linking agent is not particularly limited and can be appropriately selected depending on the intended purpose. For example, polyfunctional acrylate compounds such as trimethylolpropane tri(meth)acrylate and pentaerythritol tri(meth)acrylate; , epoxy compounds such as ethylene glycol diglycidyl ether; 2,2-bishydroxymethylbutanol-tris[3-(1-aziridinyl)propionate], aziridine compounds such as 4,4-bis(ethyleneiminocarbonylamino)diphenylmethane; hexa isocyanate compounds such as methylene diisocyanate and biuret isocyanate; polyoxazoline compounds having an oxazoline group in the side chain; alkoxysilane compounds such as vinyltrimethoxysilane and N-(2-aminoethyl)3-aminopropyltrimethoxysilane; be done. Also, a known catalyst can be used depending on the reactivity of the cross-linking agent, and productivity can be improved in addition to the enhancement of membrane strength and durability. These may be used individually by 1 type, and may use 2 or more types together.
The content of the cross-linking agent is preferably 3% by mass to 20% by mass, more preferably 5% by mass to 15% by mass, relative to the content of the liquid crystal compound. By setting the content of the cross-linking agent to 3% by mass or more, the effect of improving the cross-linking density can be obtained, and by setting the content of the cross-linking agent to 20% by mass or less, the decrease in the stability of the cholesteric liquid crystal structure can be prevented. can be prevented.

(Orientation control agent)
The liquid crystal composition may contain an alignment control agent that contributes to stably or quickly form a cholesteric liquid crystal structure of planar alignment. Examples of alignment control agents include fluorine (meth)acrylate polymers described in paragraphs [0018] to [0043] of JP-A-2007-272185, paragraphs [0031] to [0034] of JP-A-2012-203237. ] and the like, and compounds represented by formulas (I) to (IV) described above.
As the alignment control agent, one type may be used alone, or two or more types may be used in combination.

The amount of the alignment control agent added in the liquid crystal composition is preferably 0.01% by mass to 10% by mass, more preferably 0.01% by mass to 5% by mass, and more preferably 0.02% by mass with respect to the total mass of the liquid crystal compound. % to 1% by weight is particularly preferred.

(Surfactant)
The liquid crystal composition preferably contains a surfactant. The surfactant is preferably a compound capable of functioning as an alignment control agent that contributes to stably or quickly form a planar alignment cholesteric structure. Examples of surfactants include silicone-based surfactants and fluorine-based surfactants, with fluorine-based surfactants being preferred.

Specific examples of the surfactant include compounds described in [0082] to [0090] of JP-A-2014-119605, compounds described in paragraphs [0031] to [0034] of JP-A-2012-203237, Compounds exemplified in [0092] and [0093] of JP-A-2005-99248, [0076] to [0078] and [0082] to [0085] of JP-A-2002-129162 and fluorine (meth)acrylate polymers described in paragraphs [0018] to [0043] of JP-A-2007-272185.

The amount of the surfactant added in the liquid crystal composition is preferably 0.01 to 10% by mass, more preferably 0.01 to 5% by mass, and 0.02 to 1% by mass with respect to the total mass of the liquid crystal compound. is particularly preferred.

(Other additives)
In addition, the liquid crystal composition may contain at least one selected from various additives such as polymerizable monomers. In addition, antioxidants, ultraviolet absorbers, light stabilizers, coloring materials, metal oxide fine particles, etc., can be added to the liquid crystal composition as necessary, as long as the optical performance is not lowered. .

The cholesteric liquid crystal layer is formed by applying a liquid crystal composition in which a liquid crystal compound and a chiral agent, etc. are dissolved in a solvent, to a base layer, drying it to obtain a coating film, and irradiating the coating film with an actinic ray to polymerize the liquid crystal compound. By doing so, a cholesteric liquid crystal structure in which the cholesteric regularity is fixed can be formed.
When the second reflective layer 20 has a plurality of cholesteric liquid crystal layers, the above manufacturing steps for the cholesteric liquid crystal layer may be repeated.

(solvent)
The solvent used for preparing the liquid crystal composition is not particularly limited and can be appropriately selected according to the purpose, but organic solvents are preferably used.
The organic solvent is not particularly limited and can be appropriately selected depending on the purpose. Examples include ketones, alkyl halides, amides, sulfoxides, heterocyclic compounds, hydrocarbons, esters and ethers. , and so on. These may be used individually by 1 type, and may use 2 or more types together. Among these, ketones are particularly preferable in consideration of the load on the environment.

(coating, orientation, polymerization)
The method of applying the liquid crystal composition is not particularly limited and can be appropriately selected depending on the purpose. A coating method, a spin coating method, a dip coating method, a spray coating method, a slide coating method, and the like are included. It can also be carried out by transferring a liquid crystal composition separately coated on a support. It is also possible to eject droplets of a liquid crystal composition. An ink jet method can be used as the dot printing method.
By heating the applied liquid crystal composition, the liquid crystal compound is aligned. The heating temperature is preferably 200° C. or lower, more preferably 130° C. or lower. By this alignment treatment, a structure is obtained in which the polymerizable liquid crystal compound is twisted so as to have a helical axis in a direction substantially perpendicular to the film surface.

(Curing of liquid crystal composition)
By further polymerizing the oriented liquid crystal compound, the liquid crystal composition can be cured. Polymerization may be either thermal polymerization or photopolymerization using light irradiation, but photopolymerization is preferred. It is preferable to use ultraviolet rays for light irradiation.
The irradiation dose is preferably 100 to 1,500 mW/cm ² , more preferably 100 to 600 mW/cm ² . The irradiation energy is preferably 20 mJ/cm ² to 50 J/cm ² , more preferably 100 to 1,500 mJ/cm ² . It is preferable to irradiate with a light source that emits light in the wavelength range of 200 to 430 nm, and more preferably with a light source that emits light in the range of 300 to 430 nm. In addition, from the viewpoint of preventing decomposition and side reactions of materials used, a wavelength cut filter or the like can be used in order to suppress the transmittance of light with a wavelength of 300 nm or less to 20% or less.

In the backlight unit 10 of the present invention, the distance between the shielding layer 18 and the cholesteric liquid crystal layer of the second reflective layer 20 is not limited.
However, from the viewpoint of reducing the thickness of the backlight unit 10, it is preferable that the distance between the shielding layer 18 and the cholesteric liquid crystal layer of the second reflective layer 20 is short.
The distance between the shielding layer 18 and the cholesteric liquid crystal layer of the second reflective layer 20 is preferably 2 mm or less, more preferably 0.5 mm or less, and even more preferably 0.2 mm or less.

Hereinafter, the backlight unit (planar illumination device of the present invention) of the present invention will be described in detail by describing the operation of the backlight unit 10. FIG.

A shielding layer 18 is provided directly above the light source 14 in the backlight unit 10 . Therefore, of the light emitted from the light source 14 , the brightest light near the optical axis partially passes through the shielding layer 18 , but most of it is reflected by the shielding layer 18 .
As described in Patent Document 1, this can prevent a so-called hot spot in which only the position of the light source 14 of the backlight to be illuminated has high brightness in the direct type backlight unit.

However, if the shielding layer 18 is provided just above the light source 14, the luminance of the backlight becomes high at the edge of the shielding layer 18. FIG. In the following description, the phenomenon in which the brightness of the backlight increases at the edge of the shielding layer 18 is referred to as "fringe" for convenience.
As an example, when the shielding layer 18 is circular, when the light emitted from the light source 14 is observed through a light diffusion member 50 such as a diffuser plate and a prism sheet, as conceptually shown in FIG. Although the hot spot directly above is eliminated, a fringe occurs in which the brightness of the backlight increases in a ring shape at the edge of the circular shielding layer 18 . Similar to hotspots, fringes also cause deterioration in the quality of images displayed by LCDs and the like.

In contrast, the backlight unit 10 of the present invention has the second reflective layer 20 on the opposite side of the shielding layer 18 from the light source 14 .
The second reflective layer 20 has a cholesteric liquid crystal layer, and the difference between the emission center wavelength of the light source 14 and the reflection center wavelength of the second reflective layer 20 is 50 nm or less. For example, the cholesteric liquid crystal layers (the first cholesteric liquid crystal layer 32R and the second cholesteric liquid crystal layer 32L) have a selective reflection center wavelength λ within ±50 nm of the light emission center wavelength of the light source 14 .
Since the backlight unit 10 of the present invention has such a second reflective layer 20 in addition to the shielding layer 18 arranged directly above the light source 14, not only hot spots but also fringes can be prevented.

The cholesteric liquid crystal layer selectively changes the reflection wavelength range depending on the incident angle of light. That is, the cholesteric liquid crystal layer selectively reflects light in a set wavelength range for light incident from the normal direction, but selectively reflects light incident from a direction having an angle with respect to the normal. A so-called blue shift occurs in which the reflection wavelength range shifts to the short wavelength side.
The change in selective reflection wavelength due to blue shift increases as the angle of incidence on the cholesteric liquid crystal layer increases.
The normal direction is a direction perpendicular to the main surface of the sheet-like material, that is, a direction perpendicular to the main surface of the cholesteric liquid crystal layer. Also, the incident angle is the angle between the normal line and the incident light incident on the sheet-like object.
Therefore, in the cholesteric liquid crystal layer, the reflectance of light in the set selective reflection wavelength range is highest at an incident angle of 0° where light is incident from the normal direction, and decreases as the incident angle increases. That is, the light emitted from the light source 14 is reflected with a high reflectance when the light is incident on the cholesteric liquid crystal layer at an incident angle of 0° close to the optical axis. Increases transmittance.

On the other hand, the light emitted from the light source 14 has the highest luminance in the direction of the optical axis, and the luminance decreases as the angle with respect to the optical axis increases.

A shielding layer 18 is positioned directly above the light source 14 . Preferably, the optical axis of the light source 14 and the center of the shielding layer 18 are aligned in the planar direction of the substrate 12 .
High-intensity light near the optical axis of the light source 14 incident on the shielding layer 18, such as the light indicated by arrow a, is generated directly above the light source 14 because most of it is reflected by the shielding layer 18, as described above. It can prevent hotspots.

Of the light emitted from the light source 14 , the light that has not entered the shielding layer 18 enters the second reflective layer 20 .

The light incident near the end portion of the shielding layer 18 of the second reflective layer 20 indicated by the arrow b travels at an angle close to the optical axis of the light source 14, and therefore has high brightness. However, the light incident here has a small incident angle to the second reflective layer 20, that is, the cholesteric liquid crystal layer, so the blue shift of the cholesteric liquid crystal layer is small. That is, the reflectance of the light emitted by the light source 14 is high.
Therefore, most of the light incident near the edge of the shielding layer 18 indicated by the arrow b is reflected by the second reflective layer 20 . As a result, it is possible to prevent the occurrence of fringes at the edge of the shielding layer 18 where the brightness of the backlight increases.

On the other hand, the light incident on the second reflective layer 20 at a position away from the shielding layer 18, indicated by the arrow c, travels at a large angle with respect to the optical axis of the light source 14, and therefore has low luminance. However, the light indicated by arrow c also has a large angle of incidence on the second reflective layer 20, that is, the cholesteric liquid crystal layer. Therefore, the selective reflection wavelength range of the cholesteric liquid crystal layer is greatly shifted toward the short wavelength side due to the blue shift. That is, the reflectance of the light emitted by the light source 14 is low.
Therefore, most of the light incident on the position away from the shielding layer 18 indicated by the arrow c passes through the second reflective layer 20 although the brightness is low.

As described above, blue shift increases as the incident angle of light increases. That is, in the backlight unit 10 of the present invention, the reflectance of light by the second reflective layer 20 increases and the transmittance increases as the distance from the shielding layer 18 increases.
On the other hand, the light emitted from the light source 14 has the highest optical axis, and the brightness decreases as the angle from the optical axis increases.
Therefore, according to the backlight unit 10 of the present invention having such a second reflective layer 20 in addition to the shielding layer 18, the occurrence of hot spots and fringes can be prevented, and the luminance uniformity without high luminance portions can be achieved. It is possible to irradiate a backlight (planar lighting) with high brightness.

In the backlight unit 10 of the present invention, the difference between the emission center wavelength of the light source 14 and the reflection center wavelength (selective reflection center wavelength) of the integrated reflectance spectrum of the second reflective layer 20 is 50 nm or less.
If the difference between the emission center wavelength of the light source 14 and the reflection center wavelength of the second reflective layer 20 exceeds 50 nm, the function of the second reflective layer 20 cannot be sufficiently exhibited, and the effect of suppressing fringes becomes insufficient.
The difference between the emission center wavelength of the light source 14 and the reflection center wavelength of the second reflective layer 20 is preferably 30 nm or less, more preferably 20 nm or less, and more preferably matched (substantially matched).

In the backlight unit 10 of the present invention, the transmittance of the light emitted from the light source 14 through the second reflective layer 20 is not limited. The transmittance of the light emitted from the light source 14 by the second reflective layer 20 may be appropriately set according to the output of the light source 14, the reflectance of the shielding layer 18, and the like.
The second reflective layer 20 preferably has an integrated transmittance of 40% or less, more preferably 30% or less, more preferably 20% when the light of the emission center wavelength of the light source 14 is incident at an incident angle of 0°. More preferably:
By setting the integrated transmittance of the second reflective layer 20 to 40% or less when the light of the emission center wavelength of the light source 14 is incident at an incident angle of 0°, the generation of fringes can be more preferably prevented.

The second reflective layer 20 has an integrated transmittance of T(0) when the light of the emission central wavelength of the light source 14 is incident at an incident angle of 0°, and the light of the emission central wavelength of the light source 14 is incident at an incident angle of 40°. It is preferable that the integrated transmittance T(40) is 1.2 times or more as large as the integrated transmittance T(0), where T(40) is the actual integrated transmittance. That is, the second reflective layer 20 preferably has T(40)/T(0) of 1.2 or more. T(40)/T(0) is more preferably 1.25 or more, even more preferably 1.3 or more.
By setting the integral transmittance T(40) to 1.2 times or more of the integral transmittance T(0), the reflection of the second reflective layer 20 is adjusted to an appropriate range, and the brightness of the surroundings does not become too dark. This is preferable in that the degree of spots can be reduced.

Further, in the backlight unit 10 of the present invention, the difference between the reflection center wavelength of the integrated reflectance spectrum of the second reflection layer 20 and the half-value wavelength on the long wavelength side (the above-mentioned λβ [nm]) is 70 nm or less. It is preferably 65 nm or less, more preferably 65 nm or less.
By setting the difference between the reflection center wavelength of the integrated reflectance spectrum of the second reflective layer 20 and the half-value wavelength on the long wavelength side to 70 nm or less, the reflection of the second reflective layer 20 is adjusted to an appropriate range, and the ambient brightness It is preferable in that the degree of hot spots can be reduced without making the image too dark.

As an example, the ratio of the integrated transmittance T(40) to the integrated transmittance T(0) and the half-value wavelength on the longer wavelength side of the integrated reflectance spectrum of the second reflective layer 20 are can be controlled by adjusting the width of the reflection wavelength range of the cholesteric liquid crystal layer constituting the .

A cholesteric liquid crystal layer typically specularly reflects incident light. However, in the backlight unit 10 of the present invention, the second reflective layer 20, ie, the cholesteric liquid crystal layer, also preferably has diffuse reflectivity.
Specifically, the second reflective layer 20 preferably has an integrated reflectance of 50% or more at a peak wavelength in the integrated reflectance spectrum and a specular reflectance of 20% or less at a peak wavelength in the integrated reflectance spectrum.
The diffuse reflectivity of the second reflective layer 20 is preferable in that the luminance uniformity of the backlight can be improved more effectively.

A cholesteric liquid crystal layer having diffuse reflectivity can be formed, for example, by the following method.

When a cross section of the cholesteric liquid crystal layer is observed with an SEM, a striped pattern of bright portions B and dark portions D is observed as conceptually shown in FIG. That is, in the cross section of the cholesteric liquid crystal layer 36 having a fixed cholesteric liquid crystal phase, a layered structure in which the bright portions B and the dark portions D are alternately laminated can be observed.
In a general cholesteric liquid crystal layer, the striped patterns of the bright portions B and the dark portions D are formed so as to be parallel to the surface of the film forming substrate 38, that is, the surface on which the cholesteric liquid crystal layer 36 is formed, as shown in FIG. . In this case, the helical axes of the liquid crystal compounds in the cholesteric liquid crystal phase are arranged in a state perpendicular to the surface of the film-forming substrate 38 . Therefore, as indicated by arrows in FIG. 3, the cholesteric liquid crystal layer 36 exhibits specular reflectivity.

On the other hand, as conceptually shown in cross section in FIG. On the other hand, when it has unevenness, it has a region where the helical axis of the liquid crystal compound in the cholesteric liquid crystal phase is tilted.
Therefore, when light is incident from the normal direction on the cholesteric liquid crystal layer 36 having unevenness in the bright portion B and the dark portion D, there is a region where the helical axis of the liquid crystal compound is tilted, as indicated by the arrow in FIG. Therefore, part of the incident light is obliquely reflected.
That is, in the layer in which the cholesteric liquid crystal phase is fixed, the bright portions B and the dark portions D have unevenness, so that the cholesteric liquid crystal layer exhibits diffuse reflectivity.

Such a cholesteric liquid crystal layer having unevenness in the bright part B and the dark part D of the cross section has a formation surface (film formation substrate) without an alignment film, a formation surface without alignment control force, and a small alignment control force. It can be formed by forming a cholesteric liquid crystal layer on any of the formation surfaces by the coating method as described above.

Although the backlight unit of the present invention has been described in detail above, the present invention is not limited to the above examples, and various improvements and modifications may be made without departing from the gist of the present invention. Of course.

EXAMPLES The present invention will be specifically described below based on examples. The materials, reagents, amounts and ratios of substances, operations, etc. shown in the following examples can be changed as appropriate without departing from the gist of the present invention. Accordingly, the invention is not limited to the following examples.

<Preparation of support having underlayer>
As a support, a single-sided easy-adhesive single-sided highly smooth PET film (manufactured by Toyobo Co., Ltd., Cosmoshine A4100) having a thickness of 50 μm was prepared.
An underlayer coating solution having the following composition was applied to the highly smooth layer side surface of the support using a #4.4 wire bar coater. Next, the resulting coating film was dried at 45°C for 60 seconds, and then irradiated with ultraviolet rays of 500 mJ/cm ² at 25°C using an ultraviolet irradiation device to obtain a film thickness of 1.5 µm. A support with strata was prepared.

[Base layer coating solution]
PET30 50 parts by mass DCP 50 parts by mass IRGACURE 907 (manufactured by Ciba-Geigy) 3.0 parts by mass Kayacure DETX (manufactured by Nippon Kayaku Co., Ltd.) 1.0 parts by mass The following surfactant F1 0.01 parts by mass Methyl ethyl ketone 156 parts by mass Cyclohexanone 156 parts by mass

PET30: Nippon Kayaku KAYARAD PET-30 (mixture of the following two compounds)

DCP: NK ester DCP manufactured by Shin-Nakamura Chemical Co., Ltd.

Surfactant F1

<Preparation of chiral agent>
The following four chiral agents A to D were prepared. Using this chiral agent, various cholesteric liquid crystal layers described later were formed.

Chiral agent A

Chiral agent B

Chiral agent C

Chiral agent D

Chiral agent A and chiral agent C are chiral agents that form a right-handed helix.
Chiral agent B and chiral agent D are chiral agents that form a left-handed helix.

<Preparation of Second Reflective Layer Ch-1>
(Formation of first cholesteric liquid crystal layer)
A composition shown below was stirred in a container kept at 25° C. to prepare a coating liquid Ch-A1.

[Coating liquid Ch-A1]
Methyl ethyl ketone 152.2 parts by mass Mixture of the following rod-like liquid crystal compound 100.0 parts by mass Photoinitiator A 0.056 parts by mass Photoinitiator B 0.019 parts by mass Chiral agent A 6.06 parts by mass Above-mentioned surface activity Agent F1 0.027 parts by mass Surfactant F2 below 0.067 parts by mass

mixture of rod-like liquid crystal compounds

Photopolymerization initiator A; IRGACURE 907 (manufactured by Ciba-Geigy)
Photopolymerization initiator B; Kayacure DETX (manufactured by Nippon Kayaku Co., Ltd.)

Surfactant F2

The prepared coating solution Ch-A1 was applied to the surface of the underlayer of the prepared support using a #12 wire bar coater and dried at 105° C. for 60 seconds to obtain a coating film.
After that, in a low oxygen atmosphere (100 volume ppm or less), the coating film was irradiated with light from a metal halide lamp at 100° C. with an irradiation dose of 500 mJ/cm ² to form a first cholesteric liquid crystal layer.

(Formation of second cholesteric liquid crystal layer)
A composition shown below was stirred in a container kept at 25° C. to prepare a coating liquid Ch-B1.

[Coating liquid Ch-B1]
Methyl ethyl ketone 145.0 parts by mass Mixture of rod-like liquid crystal compound 100.0 parts by mass Photopolymerization initiator B 0.50 parts by mass Chiral agent B 12.69 parts by mass Surfactant F1 0.027 parts by mass Above interface Activator F2 0.067 parts by mass

The prepared coating liquid Ch-B1 was applied to the surface of the prepared first cholesteric liquid crystal layer using a #4.2 wire bar coater and dried at 105° C. for 60 seconds to obtain a coating film.
Thereafter, the coating film was irradiated with light from a metal halide lamp at 100° C. in a low oxygen atmosphere (100 volume ppm or less) with an irradiation dose of 500 mJ/cm ² to form a second cholesteric liquid crystal layer.
Further, the produced laminate was irradiated with light from a metal halide lamp at 60° C. in a low oxygen atmosphere (100 volume ppm or less) with an irradiation dose of 1500 mJ/cm ² to produce a second reflective layer Ch-1. .
Therefore, the second reflective layer Ch-1 has two cholesteric liquid crystal layers, one for selectively reflecting right-handed circularly polarized light and the other for selectively reflecting left-handed circularly polarized light.

<Preparation of Second Reflective Layer Ch-2>
(Formation of first cholesteric liquid crystal layer)
A composition shown below was stirred in a container kept at 25° C. to prepare a coating liquid Ch-A2.

[Coating liquid Ch-A2]
Methyl ethyl ketone 152.2 parts by mass Mixture of rod-like liquid crystal compound 100.0 parts by mass Photoinitiator A 0.038 parts by mass Photoinitiator B 0.012 parts by mass Chiral agent A 6.06 parts by mass Above surfactant Agent F1 0.027 parts by mass Surfactant F2 0.067 parts by mass

The prepared coating solution Ch-A2 was applied to the surface of the underlayer of the prepared support using a #12 wire bar coater and dried at 105° C. for 60 seconds to obtain a coating film.
Thereafter, the coating film was irradiated with light from a metal halide lamp at 100° C. in a low oxygen atmosphere (100 volume ppm or less) with an irradiation dose of 500 mJ/cm ² to form a first cholesteric liquid crystal layer.

(Formation of second cholesteric liquid crystal layer)
On the surface of the formed first cholesteric liquid crystal layer, the same coating liquid Ch-B1 as that for the second reflective layer Ch-1 was applied with a #5 wire bar coater and dried at 105° C. for 60 seconds to form a coating film. Obtained.
Thereafter, the coating film was irradiated with light from a metal halide lamp at 100° C. in a low oxygen atmosphere (100 volume ppm or less) with an irradiation dose of 500 mJ/cm ² to form a second cholesteric liquid crystal layer.
Furthermore, the produced laminate was irradiated with light from a metal halide lamp at 60° C. in a low oxygen atmosphere (100 volume ppm or less) with an irradiation dose of 1500 mJ/cm ² to produce a second reflective layer Ch-2. .
Therefore, the second reflective layer Ch-2 has two cholesteric liquid crystal layers, a cholesteric liquid crystal layer selectively reflecting right-handed circularly polarized light and a cholesteric liquid crystal layer selectively reflecting left-handed circularly polarized light.

<Production of second reflective layer Ch-3>

(Formation of cholesteric liquid crystal layer)
A composition shown below was stirred in a container kept at 25° C. to prepare a coating liquid Ch-C1.

[Coating liquid Ch-C1]
Methyl ethyl ketone 152.2 parts by mass Mixture of rod-like liquid crystal compounds 100.0 parts by mass Photopolymerization initiator A 1.0 parts by mass Chiral agent C 4.68 parts by mass Surfactant F1 0.027 parts by mass Above interface Activator F2 0.067 parts by mass

The prepared coating solution Ch-C1 was applied to the surface of the underlayer of the prepared support using a #6 wire bar coater and dried at 95° C. for 60 seconds to obtain a coating film.
Thereafter, in a low-oxygen atmosphere (100 volume ppm or less), the coating film was irradiated with light from a metal halide lamp at 80° C. with an irradiation amount of 500 mJ/cm ² to prepare a cholesteric liquid crystal layer. 3 was produced.
Therefore, the second reflective layer Ch-3 has a single cholesteric liquid crystal layer that selectively reflects right-handed circularly polarized light.

<Preparation of second reflective layer Ch-4>
(Formation of first cholesteric liquid crystal layer)
In the same manner as in the second reflective layer Ch-3, a cholesteric liquid crystal layer was formed on the surface of the base layer of the prepared support to form a first cholesteric liquid crystal layer.

(Formation of second cholesteric liquid crystal layer)
A composition shown below was stirred in a container kept at 25° C. to prepare a coating liquid Ch-D1.

[Coating liquid Ch-D1]
Methyl ethyl ketone 145.0 parts by mass Mixture of rod-like liquid crystal compounds 100.0 parts by mass Photopolymerization initiator A 1.0 parts by mass Chiral agent D 4.68 parts by mass Surfactant F1 0.027 parts by mass Above interface Activator F2 0.067 parts by mass

The prepared coating liquid Ch-D1 was applied to the surface of the formed first cholesteric liquid crystal layer using a #6 wire bar coater and dried at 95° C. for 60 seconds to obtain a coating film.
After that, in a low oxygen atmosphere (100 ppm by volume or less), the coating film is irradiated with light from a metal halide lamp at 100° C. with an irradiation amount of 500 mJ/cm ² to form a second cholesteric liquid crystal layer. A reflective layer Ch-4 was prepared.
Therefore, the second reflective layer Ch-4 has two cholesteric liquid crystal layers, one for selectively reflecting right-handed circularly polarized light and the other for selectively reflecting left-handed circularly polarized light.

<Preparation of second reflective layer Ch-5>

(Formation of first cholesteric liquid crystal layer)
A composition shown below was stirred in a container kept at 25° C. to prepare a coating liquid Ch-C2.

[Coating liquid Ch-C2]
Methyl ethyl ketone 152.2 parts by mass Mixture of rod-like liquid crystal compound 100.0 parts by mass Photopolymerization initiator A 1.0 parts by mass Chiral agent C 3.83 parts by mass Surfactant F1 0.027 parts by mass Above interface Activator F2 0.067 parts by mass

The prepared coating solution Ch-C2 was applied to the surface of the underlayer of the prepared support using a #6 wire bar coater and dried at 95° C. for 60 seconds to obtain a coating film.
Thereafter, the coating film was irradiated with light from a metal halide lamp at 80° C. in a low oxygen atmosphere (100 volume ppm or less) with an irradiation dose of 500 mJ/cm 2 to form a first cholesteric liquid crystal layer.

(Formation of second cholesteric liquid crystal layer)
A composition shown below was stirred in a container kept at 25° C. to prepare a coating liquid Ch-D2.

[Coating liquid Ch-D2]
Methyl ethyl ketone 145.0 parts by mass Mixture of rod-like liquid crystal compound 100.0 parts by mass Photopolymerization initiator A 1.0 parts by mass Chiral agent D 3.83 parts by mass Surfactant F1 0.027 parts by mass Above interface Activator F2 0.067 parts by mass

The prepared coating liquid Ch-D2 was applied to the surface of the formed first cholesteric liquid crystal layer using a #6 wire bar coater and dried at 95° C. for 60 seconds to obtain a coating film.
After that, in a low oxygen atmosphere (100 volume ppm or less), the coating film is irradiated with light from a metal halide lamp at 100 ° C. with an irradiation amount of 500 mJ / cm to form a second cholesteric liquid crystal layer and a second reflection. Layer Ch-5 was made.
Therefore, the second reflective layer Ch-5 has two cholesteric liquid crystal layers, one for selectively reflecting right-handed circularly polarized light and the other for selectively reflecting left-handed circularly polarized light.

<Measurement of properties of the second reflective layer>
For the fabricated second reflective layers Ch-1 to Ch-5, the integrated transmittance of light incident at an incident angle of 0° (integrated transmittance of incident light at 0°) and the integrated transmittance of light incident at an incident angle of 40° (integrated transmittance of 40° Incident light integrated transmittance), and the reflection center wavelength and the half-value wavelength on the long wavelength side of the integrated reflectance spectrum were measured.
The integrated transmittance was measured using a blue LED with a central emission wavelength of 450 nm as a light source and a handheld spectrophotometer (Illumia lite, manufactured by Labphere) as a light receiving element with an integrating sphere. The measurement was performed while adjusting the position of the light source so as to obtain a predetermined incident angle.
Measurement of the integrated reflectance is performed by attaching a large integrating sphere device (ILV-471, manufactured by JASCO Corporation) to a spectrophotometer (V-550, manufactured by JASCO Corporation) such that light is incident from the surface of the second reflective layer. It was carried out using the attached one, and the value at a wavelength of 450 nm was adopted.
Results are shown in the table below.
<Preparation of pattern shielding film>

As a support, a single-sided easy-adhesive single-sided highly smooth PET film (Cosmoshine A4100 manufactured by Toyobo Co., Ltd.) having a thickness of 50 μm was prepared.
Circular dots with a diameter of 0.5 mm were screen-printed on the easy-adhesion layer side surface of the support in a square grid pattern at intervals of 3 mm using a commercially available evaporative drying white ink (manufactured by Jujo Chemical Co., Ltd.). Spacing is the spacing between the centers of the circular dots.
After drying the printed coating film at 100° C. for 60 seconds, it was allowed to stand for 1 day to prepare a patterned shielding film having a circular dot-like shielding layer in a square grid pattern at intervals of 3 mm.

<Production of a substrate with a first reflective layer mounted with a light source>
On a substrate made of glass epoxy of 100 mm×100 mm in size, 30 rows of blue LEDs having an emission center wavelength of 450 nm were arranged in a square lattice pattern at intervals of 3 mm and mounted as light sources. The spacing of the blue LEDs is the spacing between the optical axes.
Also, a white solder resist was used to make the surface of the substrate after mounting the blue LED white, thereby forming the first reflective layer.
A silicone resin was applied onto the mounted substrate to form a transparent protective layer having a thickness of 0.3 mm.

[Examples 1 to 4 and Comparative Examples 1 to 4]
Laminate of pattern shielding film and second reflective layer Ch-1 (Example 1),
Laminate of pattern shielding film and second reflective layer Ch-2 (Example 2),
Laminate of pattern shielding film and second reflective layer Ch-3 (Example 3),
Laminate of pattern shielding film and second reflective layer Ch-4 (Example 4), and
Laminate of pattern shielding film and second reflective layer Ch-5 (Comparative Example 4),
was made.
Lamination was performed using OCA with the support of the patterned shielding film facing the cholesteric liquid crystal layer of the second reflective layer.

The produced laminate was placed on the light emitting side of the substrate on which the light source was mounted with a gap of 0.2 mm, with the shielding layer facing the light source side. At this time, the optical axis of the light source (blue LED) was aligned with the center of the shielding layer (circular dot) of the laminate in the planar direction of the substrate.
Furthermore, on the support side of the second reflective layer of the laminate, a single-sided smooth single-sided matte diffusion plate (manufactured by Japan Polyester Co., Ltd., Aroma Bright KT-1070B), a wavelength conversion member (manufactured by 3M, QDEF-360), and two prisms A sheet (BEF2-T-155n, manufactured by 3M) and a reflective polarizer layer (DBEF-D2-400, manufactured by 3M) were arranged to prepare a backlight unit. The two prism sheets were arranged such that the linear prism arrays were orthogonal to each other.

In addition, except for not using a laminate (Comparative Example 1), except that the laminate does not have a second reflective layer (Comparative Example 2), except that the laminate does not have a pattern shielding film (shielding layer) (Comparative Example In 3), a backlight unit was produced in the same manner as in Example 1.

[Evaluation of uniformity]
The produced backlight unit was visually observed from the front with the light source turned on to evaluate the uniformity of luminance. Evaluation of luminance uniformity was performed according to the following criteria.
A: Brightness unevenness is not visually recognized.
B: Brightness unevenness is faintly felt, but acceptable.
C (NG): Brightness unevenness is clearly visible and unacceptable.
Results are shown in the table below.

As shown in the table above, the backlight unit of the present invention has a shielding layer and a second reflective layer, and the difference between the emission center wavelength (450 nm) of the light source and the reflection center wavelength of the second reflective layer is 50 nm or less. , the luminance uniformity of the illuminated backlight is good.
In contrast, strong hot spots were observed in Comparative Example 1, which did not have the patterned shielding film and the second reflective layer. In Comparative Example 2, which does not have the second reflective layer, fringes were observed at the edge of the shielding layer. Hot spots were observed in Comparative Example 3 without the pattern shielding film. Furthermore, fringes were also observed at the edges of the shielding layer in Comparative Example 4, in which the difference between the reflection center wavelength of the integrated reflectance spectrum of the second reflective layer and the emission center wavelength of the light source exceeded 50 nm.

Some hot spots were observed in Example 2, which was rated B. In Example 3, which is also rated B, some fringes were observed. However, in both cases, there is no problem at all in practical use, and they have sufficient brightness uniformity.
From the comparison between Example 1 and Example 2, the integrated transmittance when the light of the emission center wavelength of the light source is incident at an incident angle of 40° is 1.2 times or more of the integrated transmittance at the time, and the difference between the emission central wavelength of the light source and the reflection central wavelength of the integrated reflectance spectrum of the second reflective layer and the half-value wavelength on the long wavelength side is 70 nm or less. By doing so, it is possible to more preferably prevent the occurrence of hot spots.
From the comparison between Example 3 and Example 4, the occurrence of fringes can be more preferably prevented by setting the integrated transmittance to 40% or less when the light of the emission center wavelength of the light source is incident at an incident angle of 0°. .
From the above results, the effect of the present invention is clear.

It can be suitably used for LCD backlights and the like.

REFERENCE SIGNS LIST 10 backlight unit 12 substrate 14 light source 16 first reflective layer 18 shielding layer 20 second reflective layer 28 support 30 base layer 32R first cholesteric liquid crystal layer 32L second cholesteric liquid crystal layer 36 cholesteric liquid crystal layer 38 film formation substrate 50 light Diffusion member B Bright area D Dark area

Claims (9)

a substrate;
a plurality of light sources discretely mounted on one main surface of the substrate;
a first reflective layer provided on a main surface of the substrate on which the light source is mounted;
a shielding layer provided on the light emitting side of the light source and spaced apart from the light source, corresponding to the position of the light source in the plane direction of the main surface of the substrate;
a second reflective layer provided on the light emitting side of the light source and spaced further from the light source than the shielding layer;
The second reflective layer has a cholesteric liquid crystal layer in which a cholesteric liquid crystal phase is fixed, and the difference between the emission center wavelength of the light source and the reflection center wavelength of the integrated reflectance spectrum of the second reflective layer is A planar lighting device characterized by having a wavelength of 50 nm or less. 2. The planar illumination device according to claim 1, wherein said second reflective layer has diffuse reflectivity. 3. The planar illumination device according to claim 1, wherein said second reflective layer has an integrated transmittance of 40% or less when light of the emission center wavelength of said light source is incident at an incident angle of 0[deg.]. The second reflective layer has an integrated transmittance of T(0) when the light of the emission center wavelength of the light source is incident at an incident angle of 0°, and the light of the emission center wavelength of the light source is incident at an incident angle of 40°. When the integrated transmittance at the time is T (40),
4. The planar illumination device according to claim 1, wherein said integrated transmittance T(40) is 1.2 times or more as large as said integrated transmittance T(0). 5. The planar lighting device according to claim 1, wherein a difference between a reflection center wavelength of the integrated reflectance spectrum of said second reflection layer and a half-value wavelength on the longer wavelength side is 70 nm or less. 6. The planar illumination device according to claim 1, wherein the distance between said adjacent light sources is 10 mm or less. 7. The planar lighting device according to claim 1, wherein the light source has an emission central wavelength in the wavelength region of blue light. 8. The planar illumination device according to claim 1, wherein said light source and said shielding layer are provided in the same pattern in the plane direction of said main surface of said substrate. 9. The planar lighting device according to claim 1, wherein the second reflective layer has a combination of two cholesteric liquid crystal layers in which the circularly polarized light selectively reflected has opposite rotation directions.

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