JPH0480370B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a rear projection screen (transmissive screen) used in a video projection television or the like. [Prior Art] Rear projection screens are used as screens for video projectors, microfilm readers, and the like. In such a rear projection screen, it is important that the brightness of the entire screen is uniform. Uniformity of brightness on the rear projection screen is achieved by satisfying the following ~ over the entire screen. The optical axis of the emitted light must be directed toward the viewer. The diffusivity of the emitted light must be uniform. The brightness must be uniform. For the above, use a Fresnel lens sheet that has a light focusing effect by changing the optical axis. The above is performed by applying light diffusing means to the screen. but,
Regarding the above, with conventional Fresnel lenses,
The amount of light incident on the lens surface decreases toward the periphery of the lens, and less light contributes to the light-condensing effect, making it impossible to obtain uniform brightness over the entire screen. That is, as shown in FIG. 1, such a Fresnel lens is configured such that a large number of prisms 1 having a triangular cross section are arranged, and the prisms 1 are composed of a lens surface 11 and a non-lens surface 12. It's growing. If the Fresnel lens surface of this Fresnel lens is used as the entrance surface X, the incident light will be emitted to the exit surface Y as shown in the figure. At this time, lens surface 11
The light L that enters the lens is refracted toward the exit surface Y side as effective light and exits, but the light L' that enters the non-lens surface 12 does not contribute to the condensing effect, and the brightness seen by the observer decreases. This will lead to a decrease in the amount of water. This tendency becomes more severe as the light source approaches the screen at a location farther away from the light source or even at the same location.
This is because the amount of light incident on the non-lens surface 12 of the prism 1 increases. In addition, in such a case, since the angle of incidence of the light beam entering the prism 1 becomes large, the amount of light incident on the lens surface 11 also decreases in the amount of transmitted light due to surface reflection, making it increasingly difficult to expect an effective amount of light. This surface reflectance can be determined using Fresnel's equation, which is expressed by the following equation. Surface reflectance (R) = 1/2 {tan ² (i
-r)/tan ² (i+r)+sin ² (i-r)/sin ² (i+
r)}... Note that here, i is the incident angle and r is the refraction angle. For example, if the material of the Fresnel lens is acrylic resin (refractive index n = 1.49), the following holds true: sini/sinr = n, where n is the refractive index. The directional reflectance can be calculated from the above formula. For example, when the angle of incidence is 70°, the surface reflectance is 15%, and when the angle of incidence is 80°, it is 40%, and this amount of loss is caused by surface reflection alone. Based on this estimate, assuming that the distance from the light source is 1000 mm and the focal length of the Fresnel lens is f = 1000 mm or less, most of the incident light will be lost at a point 500 mm or more away from the center of the Fresnel lens. I know I'll get used to it. Recently, there has been a movement to make this type of screen even larger, and there is also an opportunity to reduce the depth of the device, so the above-mentioned loss of light amount has come to be seen as a problem. Furthermore, most projection-type image enlarging devices such as video projection televisions use optical systems using refractive lenses. However, in such a refractive optical system, a cosine fourth power law exists. In other words, as shown in Figure 12, the illuminance E _B at point B on the screen is the illuminance at the center of the screen.
When E _O and the angle of view are θ, it is expressed by equation (3). E _B =E _O・cos ⁴ θ...(3) This means that the brightness at point P on the screen becomes darker in proportion to the fourth power of the cosine of the angle of view, and the brightness at the outer periphery where the angle of view is larger. Get dark. Furthermore, even with the Fresnel lens shown in FIG. 11, the amount of light that does not contribute to the light-condensing effect that enters the non-lens surface 12 increases as the outer periphery of the screen increases, resulting in a darker area. Therefore, due to both of these characteristics, the outer peripheral area becomes increasingly dark, and the uniformity of the brightness of the screen is significantly impaired. In terms of uniformity of screen brightness (the above-mentioned uniformity of brightness), it is ideal that the efficiency characteristics of the Fresnel lens be the opposite of the characteristics of the optical system.
In other words, it is ideal to have a screen in which the brightness decreases less at the outer periphery than at the center. For this reason, the present inventors created a prism piece in which a part of the light beam incident on one lens surface is totally reflected on another lens surface and then exits.
As shown in the figure, a Fresnel lens provided at the periphery has already been proposed (Japanese Patent Laid-Open No. 119340/1983).
FIG. 14 shows the prism piece in this case. The prism piece 2 has two lens surfaces 21 and 22, and part of the light incident on one lens surface 21 is transmitted to the other lens surface. 22, the light is totally reflected and emitted. In this way, the amount of light is lost in the area where the angle of incidence on the Fresnel lens sheet is large (L' ray)
has the advantage of being smaller than the Fresnel lens shown in FIG. Therefore, when the projection distance to the Fresnel lens sheet is short, that is, when the angle of incidence is large, the light can be emitted to the observation side with less loss than in the conventional case. However, in the case of Fresnel lens sheets using prism pieces as shown in FIGS. 11 and 14, there was a problem in that the boundary line between the two types of prisms was conspicuous. Therefore, we decided to
In No. 52601, we proposed a Fresnel lens sheet that is uniform over the entire Fresnel lens sheet and does not have noticeable border lines, by taking advantage of the unique characteristics of the prism pieces and combining them alternately with conventional prism pieces. [Problem to be solved by the invention] However, when the above-mentioned screen is attached to an actual projector, the light utilization efficiency is high in the center and peripheral parts, so only this part becomes brighter and the middle part becomes brighter than the above-mentioned part. It was found that the low efficiency caused uneven brightness. Therefore, the present invention aims to provide a rear projection screen that can achieve relatively uniform brightness over the entire screen without uneven brightness. [Means for Solving the Problems] The purpose of the above is to provide a rear projection screen with a Fresnel lens in which a Fresnel lens is formed on the light receiving side and a light diffusion means is provided.
A plurality of annular prism pieces centered on the optical axis are formed on the light receiving side to constitute the Fresnel lens, and the surface of each prism piece on the optical axis side is the incident surface, and the other surface is the incident surface. The prism piece is formed over the entire surface of the screen, and the prism piece is formed over the entire surface of the screen. The angle formed by the screen surface increases gradually or stepwise as the distance from the optical axis increases, and the angle formed between the total reflection surface of each prism piece and the screen surface increases as the distance from the optical axis increases. This is achieved by means of a rear projection screen, which is characterized by being gradually or stepwise smaller. [Example] Hereinafter, an example of the present invention will be described based on the drawings of the example. FIG. 1 shows a cross-sectional view of the rear projection screen of the present invention. The rear projection screen of the present invention consists of a Fresnel lens 1 and an exit surface 2 of a lenticular lens surface in which a light diffusing agent is mixed and uniformly dispersed. ). In the present invention, a total reflection surface that emits at least a part of the light incident on the entrance surface of the prism to the observation side is formed on the other surface of the prism, and the prism piece is formed over the entire surface of the screen. . FIG. 14 is a schematic diagram showing the transmission characteristics of a conventional prism having a total reflection surface similar to the prism described above. That is, this prism 2 has an entrance surface 21
It is composed of a total reflection surface 22 and an entrance surface 2.
A part of the light incident on the light source 1 is totally reflected by the total reflection surface 22 and then emitted. The light that is now incident as L' ₁ will travel straight without reaching the total reflection surface 22, but in the case of such a prism 2, the part of the light that is incident on the incident surface 21 that corresponds to L' This means that the loss of light is smaller than in the case of FIG. 11, and the amount of effective light L is large. Of course, in order to cause the light to be totally reflected by the total reflection surface 22, the angle of incidence on the total reflection surface 22 must be greater than or equal to the critical angle. If a prism 2 as shown in FIG. 14 is provided on the entire surface of the screen while keeping the inclination of the incident surface 21 constant, the greater the angle of incidence of the light incident on the incident surface 21 with respect to the screen surface, the more the total reflection surface 22, the probability of total reflection increases, and the closer the light source is to the screen or the larger the screen size, the fewer rays like L' ₁ will be present, so uniform brightness can be obtained over the entire screen. Things become difficult. Therefore, in order to obtain a screen with uniform brightness, the present inventors considered gradually changing the inclination angle θ ₂ of the prism entrance surface depending on the position on the screen. In other words, if the angle of incidence of the light from the light source on the screen surface is small, the inclination of the prism entrance surface (angle with the screen surface) is made small so that the effect of the refracting surface becomes large; on the other hand, if it is large, the effect of the total reflection surface increases. The idea is to increase the inclination of the prism entrance surface so as to increase the angle of incidence. If you look at Figure 1, the optical axis
It can be seen that the angle changes between θ ₂ near ll′ and θ ₂ far away. Figure 2 is a diagram to specifically explain the above content. As shown in Figure 2, if the object distance a, the image distance b, the distance from the center (optical axis) x, and the prism tip angle θ are ₁ , then , the inclination angle θ ₂ of the prism entrance surface may be changed depending on the position on the screen as expressed by equation (4). tanθ ₂ = {sinθa+nsin(2θ ₁ +j)}/{co
sθa−ncos (2θ ₁ + j)}… (sinJ=sinθb/n tanθa=x/
a tan θb=x/b) A Fresnel lens having a shape obtained by the formula was manufactured with settings of θ ₁ =40°, a=750 mm, and b=5000 mm. The relationship between the angle θ ₂ and the distance x from the optical axis at this time was as follows.

[Table] White light is projected by a projector at a projection distance of 800 mm on the screen in the shape of Figure 1, and at each point in the horizontal and diagonal directions of the screen (each 100 mm interval) as shown in Figure 3 at an observation distance of 2 m. The luminance was measured using a luminance meter, and the illuminance on the screen projection side at that point was measured using an illuminance meter. The measured results are shown in Table 1.

〔Effect of the invention〕

Since the present invention has the configuration described in detail above, it is possible to provide a rear projection screen in which the amount of emitted light is controlled in a well-balanced manner over the entire surface of the sheet, ensuring uniform brightness over the entire surface.

[Brief explanation of drawings]

FIG. 1 is a sectional view of the rear projection screen of the present invention, FIG. 2 is a diagram for explaining how to set the inclination θ ₂ of the prism entrance surface, and FIG. 3 is a diagram showing the effect of the screen of the present invention. FIG. FIG. 4 is a sectional view of a screen showing a second embodiment of the present invention. FIGS. 5 to 10 are schematic diagrams showing embodiments of the present invention in which the lenticular lens is modified in various ways. Fig. 11 is a diagram showing the transmission characteristics of a screen having conventional prism pieces, Fig. 12 is a diagram showing the relationship between the light source and the brightness on the screen, and Fig. 13 is an example of the conventional screen. , FIG. 14 is a diagram showing the transmission characteristics of some of the prism pieces. 1... Fresnel lens, 2... Output surface, 21...
...Incidence surface, 22... Total reflection surface.

Claims (1)

[Scope of Claims] 1. A rear projection screen with a Fresnel lens, which is provided with a Fresnel lens on the light-receiving side and is provided with a light diffusing means, wherein the light-receiving side has a ring-shaped ring centered on the optical axis. A plurality of prism pieces constitute the Fresnel lens, a surface of each prism piece on the optical axis side serves as an incident surface, and the other surface totally reflects at least a portion of the light incident from the incident surface. The prism pieces are formed over the entire surface of the screen, and the angle between the incident surface of each prism piece and the screen surface increases as the distance from the optical axis increases. and the angle between the total reflection surface of each prism piece and the screen surface gradually or stepwise decreases as it moves away from the optical axis. , rear projection screen. 2. The rear projection screen according to claim 1, characterized in that a lenticular lens surface extending vertically on the viewing side is formed. 3. Claim 2, wherein the lenticular lens surface is provided with a total reflection surface.
Rear projection screen as described in section. 4. The rear projection screen according to any one of claims 1 to 3, wherein the light diffusing means is provided on a base material constituting the screen. 5. A separate sheet having a lenticular lens surface is combined on the observation side,
A rear projection screen according to any one of claims 1 to 4. 6. The rear projection screen according to claim 5, wherein the light diffusing means is provided on the separate sheet.