JPS6210637A - Rear projection screen - Google Patents

Abstract

PURPOSE:To give a uniform brightness overall by changing the angles of inclination of prism incidence faces of prism pieces gradually or stepwise in accordance with the position on a screen and forming them on all of the surface of the screen. CONSTITUTION:A prism 2 consists of an incidence face 2 and a totally reflecting face 22, and a part of the light incident on the incidence face 21 is totally reflected on the totally reflecting face 22 and is emitted. Though an incident light L' does not reach the totally reflecting face 22 and goes straight, the loss of light in a part corresponding to L' is small in case of the prism 2 and the quantity of effective light L is large. The angle of inclination of the prism incidence face is changed gradually in accordance with the position on the screen, and the angle of inclination of the prism incidence face is reduced to enhance the function of a refracting face when the angle of incidence is small, and the angle of inclination of the prism incidence face is increased to enhance the function of the totally reflecting face when the angle of incidence is large. Thus, the quantity of exit light on all of the surface of a sheet is controlled in the well-balanced state to secure a uniform brightness overall.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a transmissive screen used in video globules, televisions, and the like.

[Conventional technology]

Rear projection screens are used as screens for video projectors, microfilm readers, etc., and Fresnel lens sheets are often used to provide a light condensing effect. Incidentally, such a Fresnel lens has transmission characteristics as shown in FIG. 11, for example. That is, such a Fresnel lens is configured such that a large number of prisms 1 having a triangular cross section are arranged, and this prism 1 consists of a lens surface 11 and a non-lens surface 12. 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, the light incident on the lens surface 11 is refracted toward the exit surface Y side as effective light and exits, but the light L' incident on the non-lens surface 12 does not contribute to the light focusing effect.

This tendency becomes more severe as the light source approaches the screen at a location further away from the light source or even at the same location, but in such a case, the amount of light incident on the non-lens surface 12 of the prism 1 increases.

Furthermore, in such a case, since the angle of incidence of the light beam entering the prism 1 increases, the amount of transmitted light also decreases due to surface reflection, making it increasingly difficult to expect an effective amount of light.

This surface reflectance can be determined by Fresnel's equation, which is expressed by the following equation.

Note that here, i is the incident angle and γ is the refraction angle.

For example, the material of Fresnel lenses is acrylic resin (refractive index n
= 1.49), the following ■ holds true, 1...■ -γ where n is the refractive index.

The surface reflectance can be determined from the above equations (1) and (2). For example, when the angle of incidence is 70°, the surface reflectance is 15*, and the angle of incidence is 80°.
In the case of @, the loss is 40%, and this amount of loss occurs only due to surface reflection. Based on this estimate, assuming that the distance from the light source is 1,000 m and the focal length of the Fresnel lens is less than f 1.000 m, the amount of incident light at a point more than 500 W from the center of the Fresnel lens It can be seen that most of the amount is lost.

Recently, this type of screen has been made even larger □
Since there is movement and there is also an opportunity to reduce the depth of the device, the above-mentioned loss of light amount has come to be seen as a problem.

Also, video! Most projection-type image magnifying devices such as log-action televisions use optical systems using refractive lenses. However, in such a refractive optical system, a cosine fourth power law exists. That is, as shown in FIG. 12, the illuminance E at point B on the screen is expressed by equation (3), where Eo is the illuminance at the center of the screen and θ is the angle of view.

E=E - House 40...(3) B O This means that the brightness at point P on the screen becomes darker in proportion to the fourth power of the cosine of that angle of view, and the outer periphery where the angle of view is larger. It gets dark.

Furthermore, even with the Fresnel lens as shown in FIG. 11, the amount of light that does not contribute to the light-condensing effect that is incident on the non-lens surface 12 increases at the outer periphery of the screen, resulting in darkness. Therefore, due to both of these characteristics, the outer periphery becomes increasingly dark, and the uniformity of the brightness of the screen is significantly impaired.

In terms of uniformity of screen brightness, it is ideal for the efficiency characteristics of the Fresnel lens to be the opposite of the characteristics of the optical system. In other words, the outer edge of the screen is brighter than the center.

For this reason, the present invention, etc. has a Fresnel lens equipped with a prism piece in the peripheral part as shown in FIG. We have already proposed a lens (Japanese Patent Application Laid-open No. 5
No. 9-119340).

FIG. 14 shows the prism piece in this case. The prism piece 2 has two lens surfaces 21 and 22, and a part of the light incident on one lens surface 21 is transmitted to the other lens surface. It is totally reflected at 22 and emitted.

This has the advantage that the loss of light quantity (L/ rays) in the region where the angle of incidence on the Fresnel lens sheet is large is reduced compared to the Fresnel lens shown in FIG. 11. 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, Fresnel lens sheets using grism pieces as shown in FIGS. 11 and 14 have a problem in that the boundary line between the two types of prisms is clearly noticeable. Therefore, in Japanese Patent Application No. 59-174349, 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. did.

[Problem that the invention seeks to solve]

However, if the above screen is used as an actual projector,
It was found that when the light is attached to the center and the periphery, the light usage efficiency is high, so only these areas become brighter, while the middle area is less efficient than the above-mentioned areas, resulting in uneven brightness.

Therefore, the present invention seeks to provide a rear projection screen that can achieve relatively uniform brightness over the entire screen.

[Means for solving problems]

The purpose of the above is to provide a rear projection screen with a Fresnel lens that uses a Fresnel lens surface as a light receiving surface.
The prism piece constituting the light-receiving surface has a total reflection surface formed on the other surface of the prism to emit at least a part of the light incident on the entrance surface of the prism toward the observation side, and the prism entrance surface of the prism piece This is achieved by a rear projection screen characterized in that the tilt angle θ changes gradually or stepwise depending on the position on the screen, and is formed over the entire surface of the screen.

〔Example〕

Embodiments of the present invention will be described below based on the drawings of the embodiments.

FIG. 1 shows a sectional view of a transmission type screen of the present invention. The transmission screen of the present invention consists of a Fresnel lens 1 and an exit surface 2 such as a lenticular lens surface 9, where X is the entrance surface side and Y is the exit surface side (observation side). In the present invention, a total reflection surface that emits at least a portion of the light incident on the entrance surface of the prism toward the viewing side is formed on the other surface of the prism, and the prism pieces are formed over the entire surface of the screen.

FIG. 14 is a schematic diagram showing the transmission characteristics of a conventional prism having a lens surface and a total reflection surface similar to the prism described above. That is, this prism 2 is composed of an entrance surface 21 and a total reflection surface 22, and a part of the light incident on the entrance surface 21 is totally reflected by the total reflection surface 22 and then exits. The light that is now incident as L/1 will go 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 incidence surface 21 that corresponds to L' There is less light loss compared to the case in Figure 11,
This results in a large amount of effective original light. The incident surface 21 and the total reflection surface 22 are adjusted according to the focal length of the Fresnel lens, but considering a ray such as L'l, it is desirable that they be perpendicular to the Fresnel lens plane. Of course, in order to cause total reflection on the total reflection surface 22, the total reflection surface 2
2 must be greater than or equal to the critical angle.

If a prism 21 as shown in FIG. 14 is used over the entire surface of the screen, the larger the angle of incidence of light incident on the incident surface 21, the higher the probability that it will be totally reflected on the total reflection surface 22. The closer the light is to the screen, or the larger the size of the screen, the fewer light rays such as L/, and the more difficult it becomes to obtain uniform brightness over the entire surface of the screen.

Therefore, in order to obtain a screen with uniform brightness, the present inventors considered gradually changing the inclination angle θ of the prism entrance surface depending on the position on the screen. In other words, if the angle of incidence from the light source to the prism incidence surface is small, the prism incidence surface should be made small so that the effect of the refractive surface becomes large; This is to increase the slope of the surface. Looking at Figure 1, θ2 near the optical axis tt' and θ2 far from the optical axis tt'
It can be seen that the angle changes.

Figure 2 is a diagram for specifically explaining the above contents, and as shown in Figure 2, object distance a, image distance b, center (optical axis)
The distance X from the lens surface to the lens surface, and the prism tip angle θ, the inclination angle θ of the prism entrance surface may be changed according to the position on the screen as expressed by equation (4).

- θz= (s1110a+ndn(2θt+j))/
(cmθa −nas(2θ+j))...■(1+
nj-stn#b/n tanθa-x/a −θa=
x/b) θ1=40°, a-750w, b=5000m
A Fresnel lens with the shape obtained by Equation 0 is manufactured with the following settings, and a white light is projected by a projector at a projection distance of 800 m on a screen in the form of wJ1, and the screen horizontal direction and Table 1 shows the results of measuring the brightness at each point in the diagonal direction (at intervals of 100+ m) and the illuminance on the screen projection side at that point.

Table 1 As can be seen from Table 1, the screen of the present invention provides uniform brightness over the entire surface of the screen.

In the above embodiment, the inclination θ2 of the prism entrance surface gradually changes, but as shown in FIG. -d)
Even if a screen having a prism a % d with a limited value is manufactured, the uniformity of the brightness of the screen remains practically the same. This embodiment has the advantage of being easier to manufacture than the previous embodiments.

Next, practical usage of the screen having prism pieces as in the above two embodiments will be explained by giving an example.

5 and 6 show parts of an embodiment of the present invention, and FIG. 5 shows the most basic rear projection screen, with a reflection surface 22 having a total reflection surface on the projection side and an entrance surface. 21
A Fresnel lens as described above is formed having the following features. FIG. 6 shows an example in which a lenticular lens surface IE extending in the vertical direction is formed on the observation side in the example of FIG. Also, Figure 7 and '
Similarly, in the ig8 diagram, there is a total reflection surface I Fl s I on the projection side.
Lenticular lens surfaces IF and IG with G1 are formed, thereby providing greater horizontal light diffusivity, that is, a viewing angle. The structure and operation of Lentekeeler lens surfaces IP and IG having total reflection surfaces in FIGS. Patent application No. 56-91896, patent application No. 56-21258
No. 4, Patent Application No. 1982-29178, Patent Application No. 57-593
Since it is detailed in No. 89, the explanation here will be omitted.

9 and 10 show an example in which a separate sheet 2 is further combined on the viewing side of the rear projection screen in FIG. 5, and FIG. 9 shows a lenticular lens surface 2 extending horizontally on the projection side. A person also combines a separate sheet 2 on the viewing side with a vertical Lentekeeler lens surface 2B having a total reflection surface 2Bl similar to that shown in FIG. A rear projection screen can also be provided with light diffusing properties. Also the first
Figure 0 shows a combination of a separate sheet 2 in which a lenticular lens surface 2C extending perpendicularly to the projection side is formed, and a concave lenticular lens surface 2D and an external light absorbing layer 2E are formed on the observation side. This makes it possible to improve horizontal light diffusivity and contrast.

Acrylic resin is the most suitable material for the rear projection screen of the present invention, since acrylic resin is particularly excellent in terms of optical properties and moldability. However, in exchange for this, vinyl chloride resin,
It is also possible to incorporate polycargenate resin, olefin resin, styrene resin, etc. When using these synthetic resin materials, the rear projection screen according to the present invention can be manufactured by extrusion molding, hot press or injection molding. I can do it.

(9) The following light diffusing means for further improving light diffusing properties may be provided on the base material or separate sheet constituting the rear projection screen of the present invention. As this light diffusion means, S to
□, CaCO3, At205. Tie, ,BaSO
4゜ZnO-At(OH)s is dissolved in a liquid synthetic resin medium such as fine glass powder or an organic diffusing agent, and one or more additives of diffusive substances that do not undergo chemical changes are uniformly added to the medium. It is preferable to form a layer containing these diffusion substances. It is also effective to form a fine matte surface on the projection side surface and/or the observation side surface. If such a means for imparting light diffusivity is taken, the diffusivity of the screen in the horizontal and vertical directions is compensated for and uniformity can be improved.

〔Effect of the invention〕

Since the present invention has the above-described configuration, 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,
The fjgZ diagram is a schematic diagram for explaining how to set the inclination θ2 of the grism entrance surface, and FIG. 83 is a diagram for explaining the effect of the screen of the present invention. 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 the side of the present invention in which the lenticular lens is variously modified. Fig. 11 is a diagram showing the transmission characteristics of a conventional screen having prism pieces, Fig. 12 is a diagram showing the relationship between the light source and the brightness on the screen, and Fig. 13 is a diagram showing the transmission characteristics of a conventional screen having prism pieces. This is an example, and FIG. 14 is a diagram showing the transmission characteristics of some of the prism pieces. ]... Fresnel lens, 2... Output surface, 21...
Incident surface, 22... Total reflection surface.

Claims (6)

[Claims] (1) A rear projection screen equipped with a Fresnel lens that uses a Fresnel lens surface as a light-receiving surface, in which a prism piece constituting the light-receiving surface has a projection screen that emits at least a portion of the light incident on the incident surface of the prism toward the viewing side. A reflective surface is formed on the other surface of the prism, and the inclination angle θ_2 of the prism entrance surface of the prism piece changes gradually or stepwise depending on the position on the screen, and is formed over the entire surface of the screen. A rear projection screen. (2) The rear projection screen according to claim 1, further comprising a lenticular lens surface extending vertically on the viewing side. (3) The rear projection screen according to claim 2, further comprising a lenticular lens surface having a total reflection surface. (4) Claims 1, 2, and 3, characterized in that the base material constituting the screen is provided with a light diffusion means.
Rear projection screen as described in section. (5) Claims 1 and 2 are characterized in that they are combined with a separate sheet having a lenticular lens surface.
Rear projection screen as described in Items 1, 3 and 4. (6) The rear projection screen according to claim 5, wherein a light diffusion means is provided on a separate sheet.