JPH06202229A - Rear projection type screen - Google Patents

Abstract

PURPOSE:To prevent a decrease in the brightness of the peripheral part of the screen and to suppress the generation of ghost by forming a thin film having a thickness at which transmittance become maximum in the peripheral part of the screen on the surface of a Fresnel lens. CONSTITUTION:The thin film 5 consisting of a material having the refractive index lower than the refractive index of a base material constituting the Fresnel lens 2 is formed on at least either of the incident surface or exit surface of the Fresnel lens 2. The thickness of the thin film 5 is so determined as to maximize the transmittance in the position of 80 to 100% from the central part on the diagonal lines of the screen 4 when the brightness in the peripheral part of the screen 4 is desired to particularly improved. Generally, the above- mentioned thickness is so determined as to maximize the transmittance in the position of 90% from the center on the diagonal lines of the screen 4. The thickness is so determined as to maximize the transmittance in the region where the ghost appears if the generation of such ghost is desired to be particularly suppressed. Generally, the thickness is so determined as to maximize the transmittance in the position of 80% in the vertical direction of the screen from the center on the diagonal lines of the screen 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rear projection type screen which prevents the lowering of the brightness of the peripheral part with respect to the brightness of the central part of the screen and suppresses the generation of ghosts.

[0002]

2. Description of the Related Art As a method for displaying a large screen image, a CRT is used.
There is known a method of enlarging and projecting an optical image from a liquid crystal panel or the like on a rear projection screen with a projection lens. FIG. 5 is a general schematic configuration diagram of a display device for forming an image by such a method. In the display device shown in the figure, an optical image from the projector 1 is formed on the surface of a two-screen 4 composed of a Fresnel lens 2 and a lenticular lens sheet 3, and an observer views this projected image on the screen 4. It will be observed from the side of the lenticular lens sheet 3. Here, the Fresnel lens 2 has a function of almost directing the incident light in the direction in which the observer is positioned, and the lenticular lens sheet 3 appropriately distributes the light emitted from the Fresnel lens 2 into predetermined horizontal and vertical angles. It has the function of spreading the viewing angle within a predetermined range by dispersing it at a ratio.

However, in such a rear projection type screen, since the projection light L from the projector 1 is reflected at each interface of the Fresnel lens 2 and the lenticular lens sheet 3, the transmittance of the entire screen is lowered. The problem that a bright screen cannot be obtained as a screen, and as shown in FIG. 4, in addition to the light L incident on the Fresnel lens 2 being emitted as a light ray La, the light L is reflected at the emission side interface and then at the incident side interface. Re-reflected ghost light L
There was a problem of emitting as b.

Further, since external light is also reflected at each interface between the Fresnel lens 2 and the lenticular lens sheet 3, there is a problem that a dark portion of an image becomes bright and the contrast is lowered.

To solve such a problem, a thin film made of a fluororesin compound is formed on at least one of the Fresnel lens surface and the lenticular lens surface, thereby reducing the reflectance at each interface and transmitting the amount of light transmitted through the screen. It has been proposed to increase the number (Japanese Patent Laid-Open No. 3-220542).
Issue). As a result, it was confirmed that the brightness at the central portion of the screen was increased and the screen was brightened, and the reflection of external light was reduced to improve the contrast.

[0006]

However, even if a thin film is formed on the interface of the lenses constituting the screen according to the above-mentioned Japanese Patent Laid-Open No. 3-220542, the brightness of the peripheral portion of the screen is smaller than that of the central portion. However, it was not possible to sufficiently suppress the occurrence of ghosts.

The present invention is intended to solve the problems of the prior art as described above, and in the rear projection type screen, prevents the lowering of the luminance of the peripheral portion with respect to the central portion and suppresses the occurrence of ghost. Is intended.

[0008]

SUMMARY OF THE INVENTION The inventors of the present invention have heretofore found that when a thin film of fluororesin or the like is formed on the interface of a lens forming a screen to increase the amount of light transmitted through the screen, the brightness of the central portion of the screen is conventionally reduced. In order to improve the brightness of the entire screen by improving the film thickness, the thickness of the thin film was set to the thickness that minimizes the reflectance at the center of the screen. The reflectance of the peripheral part cannot be efficiently reduced,
That is, the thickness of the thin film that reduces the reflectance is different between the central portion and the peripheral portion, and in order to reduce the difference between the peripheral luminance and the central luminance of the screen, the thickness that reduces the reflectance of the peripheral portion is the maximum. They found that forming a thin film was effective, and completed the present invention.

That is, according to the present invention, in a rear projection type screen having a Fresnel lens, a thin film made of a material having a refractive index lower than that of the constituent base material of the Fresnel lens is formed on at least one of the entrance surface and the exit surface of the Fresnel lens. And a thin film thereof having a thickness that maximizes the transmittance in the peripheral portion of the rear projection screen.

Generally, in the rear projection type screen, the peripheral brightness monotonously decreases as the distance from the center of the screen increases, so that the brightness decreases largely at the four corners farthest from the center of the screen, and the screen of that part increases. The uneven brightness of the image becomes more noticeable. Therefore, in the rear projection type screen of the present invention described above,
In particular, in order to suppress the decrease in the brightness of the peripheral part with respect to the central part of the screen, 80-100 from the four corner parts of the screen, that is, the center part on the diagonal line of the rear projection type screen.
It is preferable to set the thickness of the thin film to a thickness that maximizes the transmittance at the position of%.

Further, in general, the ghost light is generated on the rear projection type screen as shown in FIG. 4 described above. Further, since the incident angle θ ₃ on the exit surface of the Fresnel lens is small and the reflectance is also small, the amount of light thereof becomes small and becomes inconspicuous. Further, since the rear projection type screen is usually used together with a lenticular lens, the ghost light Lb whose horizontal emission angle is different from the chief ray La is shielded by a light absorbing portion usually provided on the emission side of the lenticular lens. It becomes invisible and is invisible. Therefore, the ghost varies depending on the characteristics of the projection optical system, Fresnel lens, lenticular lens, etc. used for the screen, but is generally within the fan-shaped range of about 120 ° from the center of the Fresnel lens above and below the screen, respectively. And, it is observed outside about 250 mm from the center. Therefore, in the rear projection type screen of the present invention, particularly in the case of suppressing the generation of a ghost, the thickness of the thin film is set to a thickness that maximizes the transmittance in the ghost generation region of such a rear projection type screen. It is preferable.

Hereinafter, the rear projection type screen of the present invention will be described in more detail with reference to the drawings. In the drawings, the same reference numerals represent the same or equivalent constituent elements.

As described above, according to the present invention, when a thin film is formed on the Fresnel lens, the film thickness is 80 to 100% from the center on the diagonal line of the screen or the peripheral portion of the screen such as the ghost generation area. In, the thickness is set to maximize the transmittance,
Such a film thickness is determined as follows. That is, in general, when light enters from a medium having a refractive index n _{1 to} a medium having a refractive index n ₂ , the following Snell's equation (1)

[0014]

N ₁ sinφ ₁ = n ₂ sinφ ₂ Equation (1) (wherein φ ₁ represents the incident angle and φ ₂ represents the outgoing angle), so that the amplitude reflectances r _S and P of the S-polarized light are obtained. The amplitude reflectance r _P of the polarized light is expressed by the following equations (2) and (3), respectively.

[0015]

## EQU00002 ## r _S = (n ₁ cosφ ₁ −n ₂ cosφ ₂ ) / (n ₁ cosφ ₁ + n ₂ cosφ ₂ ) Equation (2)

[0016]

## EQU00003 ## r _P = (n ₂ cosφ ₁ −n ₁ cosφ ₂ ) / (n ₂ cosφ ₁ + n ₁ cosφ ₂ ) Expression (3) is obtained.

On the other hand, as shown in FIG. 6, light is generally incident from a medium having a refractive index n ₀ to a thin film 5 having a refractive index n ₁ and a thickness d ₁ , and further emitted to a medium having a refractive index n _2. Then, the reflectance r of the thin film of the light is calculated by the following equation (4).

[0018]

Equation 4] _{r = (r 0 + r 1} e -2iδ) / (1 + r 0 r 1 e -2iδ) (4) (wherein, r ₀ is a thin film having a refractive index n ₁ and medium refractive index n ₀ Where r ₁ is the reflectance of the thin film with the refractive index n ₁ and the refractive index n ₂
Represents the reflectance at the interface with the medium. Further, δ represents a phase delay amount, and is represented by 2δ = (4π / λ) n ₁ d ₁ cos φ ₁ . Further, the energy reflectance R at this time is expressed by the following equation (5).

[0019]

## EQU00005 ## R = | r | ^{2 It} is expressed by Formula (5). The energy reflectance of the thin film represented in this way is obtained for each of S-polarized light and P-polarized light, and the total energy reflectance R is calculated by the following equation (6).

[0020]

## EQU00006 ## R = ( _R.sub.S + _R.sub.P ) / 2 can be obtained by the equation (6), and the transmittance T of this thin film can be calculated by the following equation (7).

[0021]

## EQU00007 ## T = 1-R It can be obtained from the equation (7). As described above, the transmittance T of the thin film is a function of the film thickness d ₁ of the thin film, and is also a function of the incident angle φ ₁ .

The Fresnel lens 2 of FIG.
By applying it when a thin film is formed on the projector 1,
From the above, the transmittance of the Fresnel lens 2 for the light incident on the position of the radius 1 of the Fresnel lens 2 at the incident angle θ ₁ can be obtained. That is, as shown in FIG. 4, in the case where thin films having a thickness d ₁ are formed on both the entrance surface and the exit surface of the Fresnel lens 2, the incident angle θ ₁ is the incident angle φ of the above equation (4).
_When applied to ₁ , the transmittance T _{i of the} thin film formed on the incident surface is obtained. Similarly, by applying the incident angle θ ₃ on the outgoing surface of the Fresnel lens 2 to the incident angle φ ₁ , The transmittance T _o of the formed thin film is obtained, and the total transmittance T _t is calculated from the following equation (8).

[0023]

[Expression 8] T _t = T _i × T _o It is possible to obtain from the equation (8).

FIG. 1 shows a case where a thin film 5 having a thickness d ₁ is formed on both the entrance surface and the exit surface of the Fresnel lens 2 as shown in FIG. FIG. 6 is a general relationship diagram between the thin film thickness d ₁ and the transmittance T _t when the transmittance T _t of the Fresnel lens is calculated for each of the lights incident on the light source. In the figure, the solid line represents the relationship for the light incident on the central part of the Fresnel lens, that is, the position where the incident angle θ ₁ from the projector is 0 °, and the broken line is the peripheral part of the Fresnel lens, that is, the incident angle from the projector. θ ₁
Represents the relationship for light incident on a relatively large position.

As described above, the film thickness d _{x of} the thin film that maximizes the transmittance of light incident on the central portion is different from the film thickness d _{y of} the thin film that maximizes the transmittance of light incident on the peripheral portion. In the present invention, since the thin film formed on the Fresnel lens has the thickness d _y that maximizes the transmittance of light incident on the peripheral portion, the transmittance of light incident on the central portion is maximized as in the conventional case. It is possible to reduce the difference between the central brightness and the peripheral brightness of the screen as compared with the case where the screen is formed to have a thickness d _x . Further, in such a region where the transmittance is maximized, the interface reflection that causes the ghost is most efficiently suppressed, so that according to the present invention, the generation of the ghost can also be suppressed.

In determining the thickness of the thin film in the screen of the present invention, which part of the peripheral portion of the screen is to have the maximum transmittance depends on the size of the screen, the place of contact with the ground, and the observer with respect to the screen. It is set appropriately according to the position of. For example, when it is desired to particularly improve the peripheral brightness of the screen, it is preferable to maximize the transmittance at a position of 80 to 100% from the center on the diagonal line of the screen. ), The transmittance at the position P of 90% from the center O on the diagonal line of the screen 4 may be maximized. Further, when a ghost is generated in the shaded portion of the screen 4 as shown in FIG. 2B, if it is desired to particularly suppress the generation of such a ghost, the transmittance in such a ghost generation region is maximum. In general, the transmittance may be maximized at a position Q of 80% from the center O on the diagonal line of the screen 4 in the vertical direction of the screen.

The material of the thin film formed to have such a thickness may be one having a refractive index lower than that of the constituent base material of the Fresnel lens. Generally, it is preferable to use a fluororesin from the viewpoint of easy film formation.

The method for forming such a thin film is not particularly limited, and a dip method, a spray method, a vapor deposition method or the like can be used. For example, when forming a thin film of a fluororesin to a predetermined thickness by the dipping method, by controlling the viscosity of the fluororesin composition to be dipped or by controlling the pulling rate, the incident surface and the exit surface of the Fresnel lens can be adjusted. A thin film having a desired thickness can be easily formed on both sides.

The thin film formed in the present invention is
The Fresnel lens does not necessarily have to be formed on both the entrance surface and the exit surface, and may be formed on either one of the surfaces.
Further, the present invention is not limited to the two-lens type lens composed of the Fresnel lens 2 and the lenticular lens sheet 3 as shown in FIG. 5, but may also be applied to a one-lens type lens having a lenticular lens surface formed on the back surface of the Fresnel lens. Can be applied.

[0030]

According to the present invention, when a thin film is formed on the Fresnel lens to improve the transmittance of the screen, the thin film is formed to have a thickness that maximizes the transmittance at a predetermined position in the peripheral portion of the screen. . Therefore, it is possible to suppress a decrease in peripheral brightness with respect to the central brightness of the screen. Also, by setting the position on the screen that maximizes the transmittance as the ghost generation area of the screen, it is possible to reduce the reflected light that causes the ghost, and thus it is possible to suppress the ghost of the screen. Becomes

[0031]

EXAMPLES The present invention will be specifically described below with reference to examples.

Example 1 A Fresnel lens (focal length 530 mm) made of an acrylic resin having a refractive index of 1.494 was formed on both the entrance and exit surfaces.
A fluororesin (CYTOP, manufactured by Asahi Glass Co., Ltd.) having a refractive index of 1.34 was formed by changing the thickness d (nm) as shown in Table 1, and the wavelength of 600 nm was measured from a distance of 680 mm from the center of the Fresnel lens. When irradiated with light, the transmittance T ₀ at the center of the Fresnel lens and 480 m from the center
The transmittance T ₄₈₀ at the position of m was calculated, and the increase rate of the transmittance with respect to the transmittance when the fluororesin thin film was not formed at each position was calculated. Also, this Fresnel lens and lenticular lens sheet (pitch 0.9m
m, gain 5.2), projection distance is 680
480 mm from the center and attached to the mm TV set
The brightness at the position of mm was measured. In this case, the measurement was performed using a color difference meter (CS-100, manufactured by Minolta Co., Ltd.) provided in front of the screen at a distance of 3 m from the center of the screen. The results are shown in Table 1.

[0033]

[Table 1] From Table 1, assuming that the film thickness of the fluororesin is 110 nm, 16
The transmittance in the central portion is larger than that in the case of 0 nm, but at this time, the difference in the transmittance between the central portion and the peripheral portion (position 480 mm from the center) is as large as 16.9%. On the other hand, when the film thickness of the fluororesin is 160 nm, the transmittance at the central portion is reduced, but the increase rate of the transmittance at the peripheral portion is large, so the difference in the transmittance between the central portion and the peripheral portion is 12. It is as small as 9%. Also, the screen brightness is higher in the peripheral part when the film thickness of the fluororesin is set to 160 nm than when it is set to 110 nm. Therefore, in this Fresnel lens projection system, it is understood that the peripheral brightness can be greatly improved by setting the film thickness of the fluororesin formed on the Fresnel lens to 160 nm.

Example 2 When no thin film was formed in the Fresnel lens of Example 1, a ghost was observed at a position 280 mm from the center. Therefore, in the case where the same fluororesin as in Example 1 is formed with different thicknesses d (nm) as shown in Table 2 on both the incident surface and the emission surface of the Fresnel lens, and the respective thicknesses are formed. 280m from the center of the Fresnel lens
The reflectance R ₂₈₀ at the position of m was determined. Further, these Fresnel lenses and a lenticular lens sheet (pitch 0.9 mm, gain 5.2) were combined and attached to a TV set to observe images. The results are shown in Table 2.

From Table 2, in the projection system of this Fresnel lens, by setting the film thickness of the fluororesin formed on the Fresnel lens to 140 nm, the reflection that causes ghost as much as 20% compared to the case where the film thickness is 110 nm. It can be seen that the rate decreases. In fact, when the image was observed while it was attached to the TV set, it was confirmed that the generation of ghost was well suppressed.

[0036]

According to the rear projection type screen of the present invention, it is possible to prevent the lowering of the luminance of the peripheral portion with respect to the central portion and to suppress the generation of ghost.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the thickness of a thin film formed on a Fresnel lens and the transmittance.

FIG. 2 is an explanatory diagram of a position on the screen where the transmittance is maximized when the peripheral brightness of the screen is improved (FIG. 2A) and an explanatory diagram of a ghost generation area of the screen (FIG. 2B).

FIG. 3 is an explanatory diagram of an optical path of light incident on a Fresnel lens having thin films formed on both surfaces.

FIG. 4 is an explanatory diagram of ghost light generated by a Fresnel lens.

FIG. 5 is a general schematic configuration diagram of a display device using a rear projection screen.

FIG. 6 is an explanatory diagram of an optical path of light passing through a thin film.

[Explanation of symbols]

 1 Projector 2 Fresnel lens 3 Lenticular lens sheet 4 Screen 5 Thin film

[Table 2]

[Procedure amendment]

[Submission date] March 7, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0034

[Correction method] Change

[Correction content]

Example 2 When no thin film was formed in the Fresnel lens of Example 1, a ghost was observed at a position 280 mm from the center. Therefore, in the case where the same fluororesin as in Example 1 is formed with different thicknesses d (nm) as shown in Table 2 on both the incident surface and the emission surface of the Fresnel lens, and the respective thicknesses are formed. 280m from the center of the Fresnel lens
The reflectance R ₂₈₀ at the position of m was determined. Further, these Fresnel lenses and a lenticular lens sheet (pitch 0.9 mm, gain 5.2) were combined and attached to a TV set to observe images. The results are shown in Table 2.

[Table 2]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] Explanation of code

[Correction method] Change

[Correction content]

[Explanation of symbols] 1 Projector 2 Fresnel lens 3 Lenticular lens sheet 4 Screen 5 Thin film

Claims (3)

[Claims] 1. A rear projection screen having a Fresnel lens, wherein a thin film made of a material having a refractive index lower than that of a constituent base material of the Fresnel lens is formed on at least one of an entrance surface and an exit surface of the Fresnel lens, and A rear projection type screen, wherein the thin film has a thickness that maximizes the transmittance in the peripheral portion of the rear projection type screen. 2. The rear projection screen according to claim 1, wherein the thickness of the thin film is such that the transmittance becomes maximum at a position of 80 to 100% from the center on the diagonal line of the rear projection screen. 3. The rear projection screen according to claim 1, wherein the thickness of the thin film is such that the transmittance becomes maximum in a ghost generation region of the rear projection screen.