JP2006106393A - Light diffusion sheet and projection screen - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light diffusion sheet which hardly cause decrease of the luminance of picture light to be visually sensed and hardly decrease the contrast of the picture light and which has little scattering reflection (noise) of the external light. <P>SOLUTION: A light diffusion sheet of this invention is a light diffusion sheet in which a plurality of unit optical elements are formed in a one-dimensional or two-dimensional direction. Each unit optical element is constituted of translucent material whose cross-sectional shape is roughly a trapezoid, and has a total reflection part on its inner face to totally reflect a part of the incident light. The space between adjacent unit optical elements is constituted of light absorbing material or light shielding material whose cross-sectional shape is roughly a triangle, and whose refractive index is different from that of the translucent material. Each unit optical element is formed in such a manner that, when the lower base, the oblique sides and the upper base of the trapezoid are used respective as an entrance for light, total reflection parts and an exit for light, the area ratio of the light exiting parts in the plane of the light diffusion sheet seen from the side of the light exiting part is made to be equal to or lower than 20%. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to a light diffusion sheet and a projection screen using the light diffusion sheet.

In a projection display device or the like, a device using a light diffusion sheet for a screen is known to improve the visibility of an observer.
Examples of the light diffusing sheet include those obtained by subjecting the surface of a translucent film to unevenness (for example, see Patent Documents 1 to 3), and those containing light diffusing fine particles inside a resin film (for example, Patent Document 4). -8), a lens sheet having an array structure of unit lenses such as a lenticular sheet in which semi-cylindrical convex cylindrical lenses are arranged in parallel on one plane (for example, see Patent Document 9).
In addition, a combination of several of these sheets is also used.
These are based on the difference in refractive index of film, air, fine particles, etc., refracting image light in multiple directions at these boundaries, and diffusing the image light over a wide range and emitting it to the viewer side. It is intended to improve.

JP-A-3-43724 JP-A-5-61120 JP-A-7-270918 Japanese Patent Laid-Open No. 4-134440 JP-A-4-134441 JP-A-2-157735 JP-A 63-266442 JP-A-4-322240 JP 2003-50307 A

However, the surface of the sheet on which light diffusing fine particles and irregularities are formed causes image light to be irregularly reflected to generate a lot of stray light, resulting in a decrease in surface brightness and contrast of the display.
In addition, those having diffusibility due to the surface unevenness treatment have a problem that the visibility varies depending on the angle at which the display is viewed because the diffusivity and transparency have angle dependency.

On the other hand, the light diffusibility of the light diffusing sheet also leads to an increase in scattering and reflection of external light, and there is a problem in that the contrast is remarkably lowered and the image is easily blurred.
In transmissive projection TV applications, observation in a bright room is common. Reflecting external light such as sunlight or indoor illumination light on the screen surface and making the reflected light visible to the viewer is an image. This is accompanied by the vision of unnecessary light other than light, resulting in the above problem.
Accordingly, the present invention provides a light diffusing sheet with less scattered reflection (noise) of external light, and a projection screen using the light diffusing sheet, without lowering the brightness or contrast of visual image light. For the purpose.

  In order to achieve the above object, according to the present invention, a non-transmission part of image light having high external light absorption is provided, and an area ratio of the non-transmission part facing the observer is increased so as to reflect external light. Eliminates the resulting contrast degradation and image blur.

The light diffusion sheet according to the present invention is:
A light diffusion sheet in which a plurality of unit optical elements are formed in a one-dimensional or two-dimensional direction,
The unit optical element is made of a translucent material having a substantially trapezoidal cross-sectional shape, and includes a total reflection portion in which a part of incident light is totally reflected on the inner surface thereof,
Between the unit optical elements adjacent to each other, the cross-sectional shape is generally triangular, and the light-transmitting material is made of a light-absorbing material or a light-shielding material having a different refractive index,
The unit optical element has an area of the light exiting portion in the plane of the light diffusing sheet as viewed from the light exiting portion when the lower base of the trapezoid is a light entrance portion, the hypotenuse is the total reflection portion, and the upper base is a light exit portion. The ratio is 20% or less.

  In the present invention, which causes total light reflection and resulting light diffusion, total reflection is required at the interface between the light-transmitting material and the light-absorbing material (or light-shielding material). In order to further ensure total reflection in the layer, a layer having high light reflectivity may be formed on the surface where the light-absorbing material (or light-shielding material) is in contact with the light-transmitting material.

The length of the lower base of the trapezoid that is the light incident part is P, the height of the triangle of the light-absorbing material or the light-shielding material is T, and the angle between the hypotenuse forming the total reflection part and the normal line of the light emitting part is θ If
It is preferable to satisfy the relationship (T / P) tan θ ≧ 0.4.

FIG. 1 is a cross-sectional view showing a schematic configuration of a light diffusion sheet according to the present invention.
In the figure, the upper side corresponds to the projection side and the lower side corresponds to the observer side.
Since the cross-sectional shape of the translucent material in the unit optical element is substantially trapezoidal, in the figure, θ is constant, that is, the hypotenuse is basically a straight line, but the present invention is a plurality of the hypotenuses and shallow angles that form a curve. The case of a combination of straight lines is also included.

  The screen of the present invention is mainly used for a single light source projector, and by using a Fresnel lens, the incident angle on the sheet can be made substantially 0 °. It is known that the incident angle to the hypotenuse is generally in the range of 0 ° ± 10 °.

The angle θ does not necessarily have to be uniform over the entire surface of the light diffusion sheet, and may vary between adjacent hypotenuses.
The light diffusing sheet of the present invention is not limited to a projection screen designed on the assumption that the angle of light incident on the sheet is approximately 0 ° as described above. It can also be applied as an optical path control member used in a backlight unit in a display that illuminates a prescribed display element (liquid crystal panel or the like) from the back and emits image light to the viewer side.

  In the case where the backlight unit is a so-called edge light type having a light source (such as an LED) at the side end, the amount of light propagating through the light guide plate is increased at an angle close to perpendicular to the display element ( It is difficult for an existing backlight unit to emit light at an angle close to vertical from the light emitting plate's exit surface.

  The light emitted from the exit surface of the light guide plate is an angle close to the exit surface (far from 90 °), exceeding the critical angle at which the propagated light in the light guide plate is totally reflected according to the propagation direction from the light source. In order to increase the amount of light incident on the display element at an angle close to the vertical, an optical path control member such as a prism sheet is used, but there is still considerable room for the increase in the amount of light. ing.

  In consideration of the current situation where the light emitted from the exit surface of the light guide plate is far from 90 °, in the unit optical element, if only one hypotenuse of one pair of hypotenuses is responsible for total reflection. For example, the angle θ does not have to be equal between adjacent hypotenuses, and may change continuously or stepwise according to the displacement from the light source.

FIG. 2 shows an example in which the light diffusion sheet when the trapezoid (and triangle) is asymmetric according to the change in the angle θ is applied to an edge light type backlight unit optical path control member.
In the figure, the trapezoid (and triangle) has an asymmetrical shape, and the substantially vertical interface substantially contributes to the reflection of light emitted from the exit surface of the light guide plate. The display light is emitted in the direction of the person (both not shown).
In the same figure, the shape (interface angle) of the trapezoid (and triangle) forming the unit optical element is substantially uniform, but may be changed according to the displacement from the light source.

According to the light diffusing sheet of the present invention, incident light parallel to the light exit surface normal line can be totally reflected on the oblique side, and can be emitted to the viewer side without causing reflection on the light exit surface.
At this time, the lower base portion of the trapezoid (which is the upper side in FIG. 1), which is the light incident portion, is in contact with the adjacent unit optical elements without being spaced, so that the projection light (image image) that enters from the projector The light is efficiently emitted to the viewer side without being blocked, and it is possible to view high-luminance video light.
In the present invention, the area ratio of the upper base of the trapezoid in the plane of the light diffusion sheet viewed from the observer side is 20% or less (that is, the area ratio of the light-absorbing material or light-shielding material filling the space is 80% or more). Therefore, outside light other than the projected image light is effectively absorbed, and it becomes difficult for the observer to visually recognize the noise light even in a bright room environment.
Therefore, a light diffusion sheet with high brightness and contrast can be obtained.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is an explanatory view showing a horizontal section of a light diffusion sheet 10 according to an embodiment of the present invention.
In the figure, in the case of a use as a rear projection screen, an image light source is arranged on the upper side of the drawing and an observer is located on the lower side of the drawing.
The light diffusion sheet, in order from the observer side to the image light source direction, is a hard coat layer 15, a transparent substrate 14, a light shielding part (triangle) 13 made of a light absorbing material or a light shielding material / a light transmitting part made of a light transmissive material (Trapezoid) 12 and a transparent substrate 11.

  In the present invention, the light-shielding part 13 and the light-transmitting part 12 are essential requirements, and the upper and lower transparent base materials 11 and 14 and the hard coat layer 15 in FIG. Accordingly, it is preferable that either one of the transparent substrates is present.

In the present invention, when the upper side of the transparent part 12 is a light incident part, the oblique side is a total reflection part, and the lower side of the figure is a light emission part, the light emission part in the plane of the light diffusion sheet viewed from the light emission part side The area ratio is 20% or less.
That is, the area ratio of the light-shielding portion 13 facing the observer side is 80% or more, and the image light is efficiently emitted without loss from the narrow light-transmitting portion 12 with the area ratio of 20% or less, and the outside light is reflected. High-intensity images can be viewed with high contrast because they are reliably blocked over a large area.

In the figure, when the base of the light shielding portion 13 occupies a length of 80% or more of the pitch P of the unit optical elements, and the triangle height of the light shielding portion 13 is T,
Pa / P ≧ 0.8
((2Ttanθ) / P) ≧ 0.8
The relationship of ((T / P) tan θ) ≧ 0.4 is satisfied.

Because the total reflection at the hypotenuse that is the interface between the light-shielding portion 13 and the light-transmitting portion 12 expands the viewing area of the image light in the left-right direction in FIG. .
In addition, when the image light is incident on the oblique side at such an angle that total reflection does not occur, a highly reflective material may be formed on the oblique side so as to eliminate the loss that the image light is absorbed by the light shielding portion 13.

The light shielding portion 13 is colored to a predetermined concentration with a pigment such as carbon or a predetermined dye.
The transparent base materials 11 and 14 are made of a material having substantially the same refractive index as that of the light transmitting portion 12.
Not only the hard coat layer 15 but also various functional layers such as an antireflection layer, a polarizing filter layer, an antistatic layer, an antiglare treatment layer, an antifouling treatment layer, and a touch sensor layer are necessary on the outermost viewer side. Depending on the above, at least one combination is appropriately provided.

Next, an example of the method for producing the light diffusion sheet of the present invention will be described with reference to FIG.
A transparent substrate (14) is supplied from the unwinding side.
A light shielding material is supplied to a roll stamper (plate) having a concave portion which is the reverse shape of the light shielding portion 13 on the surface, nipped by a roll pair, and then irradiated with energy rays (UV, etc.) through a transparent substrate.
In order to stabilize the shape of the light-shielding portion 13 having a steep slope with respect to the transparent substrate, the light-shielding portion 13 can be supplied in a liquid state, and the characteristics that the shape can be easily maintained by curing are suitable. Materials that cure over time are preferred.

A transparent base material (11) and a roll which are unwound after supplying a light-transmitting material to be a light-transmitting portion 12 after forming a reflective film on the transparent base material peeled off from the roll stamper and having the light-shielding portion 13 formed After being sandwiched and nipped by the part, energy rays (such as UV) are irradiated through the transparent base material (11) to cure the light transmitting part 12.
Thereafter, if necessary, after cleaning, it is finally wound up.

As a modification of the above manufacturing process, the manufacturing process order of the light shielding part 13 and the light transmitting part 12 may be reversed.
In addition, the series of manufacturing methods described above are effective for continuous production in a web-like form, including modifications, but the present invention is not limited to this, and has transparent substrates above and below. In the case of a light diffusing sheet having no structure, another manufacturing method should be selected.

Sectional drawing which shows schematic structure of the light-diffusion sheet by this invention. Explanatory drawing which shows schematic structure of the other example of the light-diffusion sheet by this invention. Explanatory drawing which shows an example of the manufacturing method of a light-diffusion sheet.

Explanation of symbols

11,14 ... Transparent substrate
10 ... Light diffusion sheet
12 ... Translucent part
13 ... Shading part
15 ... Hard coat layer

Claims (5)

A light diffusion sheet in which a plurality of unit optical elements are formed in a one-dimensional or two-dimensional direction,
The unit optical element is made of a translucent material having a substantially trapezoidal cross-sectional shape, and includes a total reflection portion in which a part of incident light is totally reflected on the inner surface thereof,
Between the unit optical elements adjacent to each other, the cross-sectional shape is generally triangular, and the light-transmitting material is made of a light-absorbing material or a light-shielding material having a different refractive index,
The unit optical element has an area of the light exiting portion in the plane of the light diffusing sheet as viewed from the light exiting portion when the lower base of the trapezoid is a light entrance portion, the hypotenuse is the total reflection portion, and the upper base is a light exit portion. A light diffusing sheet having a ratio of 20% or less. A light diffusion sheet in which a plurality of unit optical elements are formed in a one-dimensional or two-dimensional direction,
The unit optical element is made of a translucent material having a substantially trapezoidal cross-sectional shape,
Between the unit optical elements adjacent to each other, the cross-sectional shape is generally triangular, and the light-transmitting material is made of a light-absorbing material or a light-shielding material having a different refractive index,
2. The light diffusing sheet according to claim 1, wherein a layer having high light reflectivity is formed on a surface where the light absorbing material or the light shielding material is in contact with the light transmitting material. When the length of the lower base of the trapezoid that is the light incident part is P, the height is T, and the angle between the hypotenuse forming the total reflection part and the normal line of the light emission part is θ,
The light diffusing sheet according to claim 1, wherein a relationship of (T / P) tan θ ≧ 0.4 is satisfied.   The light diffusion sheet according to claim 1, wherein the angle θ changes between adjacent hypotenuses.   The projection screen of the structure by which a Fresnel lens is arrange | positioned at the light-incidence part side of the light-diffusion sheet of any one of Claims 1-3.

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