JPH04226445A - Back projected screen - Google Patents

Abstract

PURPOSE:To provide the back projected screen by which the images irradiated nearly uniformly even when viewed from a front perpendicular direction and from diagonal forward are obtd. CONSTITUTION:The screen for back projection of a projector having 3 units of TV projectors which projects red, green and blue images and are adjacent to each other has a screen (A). The screen (A) has backward facing lens elements (25) which function as convex lenses for the light entering from back and extend perpendicularly in the use position behind the screen. Further, the screen (A) has masking stripes (22) which have stripe forming parts (23) facing the backward facing lens elements (25) therebetween and extend perpendicularly before the screen. The peak parts (45) of the backward facing lens elements (25) have the focal lengths (f3) shorter than the focal lengths (f1) of the side face parts (51, 52) of the backward facing lens elements (25).

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is mainly applied to a projection device having three TV projectors installed adjacent to each other for projecting red, green, and blue TV images onto the back of a screen. In particular, the rear projection screen has at the rear a rear-facing lens element formed as a convex lens and extending vertically for light incident from the rear, a stripe formation facing the rear-facing lens element and having a vertical The present invention relates to a rear projection screen comprising a screen having a masking stripe on the front side extending in the direction.

0002

BACKGROUND OF THE INVENTION Typically, rear-facing lens elements have a focal length measured perpendicular to the screen surface between the highest portion of the rear-facing lens element (as viewed in horizontal section through the screen) and the stripe formation. is formed to match the distance. This is achieved by forming the lens as an ellipse or an elliptical enlargement, also seen in plan. These known lens elements therefore have the feature of focusing light from the rear incident perpendicularly and parallel to the screen surface onto the center of the stripe formation, which can be formed as a forward-facing convex lens. .

0003

Such parallel light rays incident on the highest portion of the rear-facing lens element pass through the screen almost unchanged. Since only a small amount of energy is lost when passing through the screen, these rays predominate in the output light pattern of the stripe formation. On the other hand, light rays incident on the side portions of the rear-facing lens elements suffer considerable energy loss when passing through the screen. This is due to the fact that these rays are incident on the surface of the side part at a relatively large angle of incidence, resulting in a fairly large reflection. The same thing can be said even when such light rays are incident from the rear on the stripe forming portion, which can be formed as a convex lens, as described above, because the angle of incidence becomes large. The reflection loss in the latter case is greater than the reflection loss due to the rays incident on the screen. This is because in the latter case the light rays travel from the material with the higher index of refraction to the material with the lower index of refraction (from the screen material into the air). It is generally envisaged that said rays incident on the side portions of the rear-facing lens element near the central plane of the screen will suffer an energy loss of about 50%. As a result, the image projected on the back of the screen is more visible than the viewer viewing the image diagonally from the side of the screen.
For viewers located perpendicular to the front of the screen,
It is illuminated as something more vivid. Technically speaking, this screen has a distinct "peak gain".

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a screen that provides a screen image having substantially the same light intensity whether viewed from the front orthogonal direction or obliquely from the side.

[0005]

According to the invention, the object is to make the focal length of the top portion of the rear-facing lens element shorter than the focal length of the side portions of the rear-facing lens element; This is accomplished by making the distance less than the distance between the highest portion of the rear-facing lens element and the stripe formation, measured perpendicular to the screen plane.

[0006]

This allows light rays entering the top of the rear-facing lens element from the rear and traveling perpendicular to the screen surface to intersect each other within the screen material before exiting the stripe formation. If the stripe forming part is flat,
This diffusion will be further enhanced as the rays travel from a material with a higher refractive index to a material with a lower refractive index. If the stripe forming portion is formed as a forward-facing convex lens, diffusion can be further accelerated by setting the focus of the top portion between the forward-facing lenses and their focal points. This is because the forward-facing lens functions as a concave lens. However, the focal lengths of the top part and the forward facing convex lens can also be chosen such that the focal points of the two lenses are the same. In the latter case, therefore, the forward-facing convex lens does not influence the diffusion of parallel rays incident on the top portion of the backward-facing lens element.

[0007] A diffusion of the light beam resulting in a distinct "peak gain" is caused by the structure described above, so that this can be avoided. As a result of the improved horizontal energy dispersion of the output light, both viewers who view the screen straight from the front and viewers who view the screen from diagonally in front receive images projected with almost the same illuminance. I can do it. In other words, a screen with improved viewing conditions in the horizontal direction has been created.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to the accompanying drawings.

Reference numerals 1, 2, and 3 in FIG. 1 indicate three projectors that project TV images onto the rear surface of the projection screen 4. As shown in FIG. Each projector projects a black and white TV image, and a filter is placed in front of the projector 1 so that the image projected by this projector onto the screen 4 is red. A filter is placed in front of the projector 2 so that the image projected by the projector onto the screen 4 becomes green, and a filter is used for the projector 3 so that the image of this projector becomes blue. These three colors are represented by R, G, and B, respectively. Three projectors 1, 2, and 3 are installed in line with the optical axis 7 of the central projector, which is perpendicular to the center of the screen. Because each projector has a lateral extent, it is not possible to install the optical axes 6 and 8 so that they are aligned with the axis 7; instead, the three optical axes are placed at an angle of 7° next to each other. Form. Reducing these 7° angles any further is difficult due to the size of the projector. On the other hand, with the development of more powerful projectors and improved lateral capabilities, the above angle of 7° can be increased to 10°. The optical axis is directed toward the center of the screen 4, and the projector is configured so that the images projected onto the back of the screen 4 cover each other as much as possible. Thereby, the viewer can see from the front of the screen a higher quality and larger image, corresponding to the image produced by a color TV picture tube. The curved frame in FIG. 1 indicates the position of the viewer.

The screen shown in FIG. 1 consists of two screens, a front screen A and a rear screen B. "Front" means closest to the viewer's position, and similarly, when referring to the screen surface, "front" and "rear" refer to the seat facing the viewer and the seat facing away from the viewer, respectively. means.

[0011] The front surface of the front screen A is masked.
The screen has stripes 22 and a stripe forming portion 23 located therebetween and extending vertically in the use position of the screen. In the embodiment of the screen 4 shown in FIG.
It is formed as a forward facing convex lens adjacent to 4. Edges 24 are generally perpendicular to the screen plane and form the lateral boundaries of masking stripes 22.

The front screen A has on its back side rearwardly facing vertically extending lens elements 25, which are arranged directly behind each stripe formation 23. These lenses 25 are mainly used to bend light from the rear, so that the light exits through the stripe formation 23 on the front of the screen.

The rear screen B is formed at its front surface as a Fresnel lens 27 that collimates the light coming from the rear. Three projectors 1, 2 with optical axes 6, 7, 8
, 3 are oriented toward the center of the screen and the optical axis of the projector 2 is perpendicular to the screen. incident on the back. However, this is corrected by the action of a Fresnel lens.

Even if screen A is generally used as explained in connection with FIG. Only the direction is shown. As for the lights from projectors 1 and 3, even after they are collimated by Fresnel lens B, they still continue to form the above-mentioned 7° angle with respect to the green light incident at right angles to the back surface of screen A. However, the situation is the same.

FIG. 2 is a partial horizontal sectional view of a known embodiment of screen A. A light ray 30 incident on the highest point 31 of the rear-facing lens element 25 (upward in the projection of FIG. 2) in a direction perpendicular to the intermediate plane of the screen passes directly through the screen. The energy of the output beam is slightly reduced compared to the energy of the input beam. When the screen is made of acrylic resin, light rays suffer a light energy loss of approximately 7% as a result of reflection as they pass from the air to the acrylic resin and from the acrylic resin to the air. Light rays 33, 34 which are incident normal to the back surface of the screen and pass through the front side of the rear-facing lens element 25 close to the screen mid-plane suffer a significantly greater energy loss than light ray 30 after passing through the screen. This is due to the fact that the light rays 33, 34 are incident on the surface side at a fairly large angle of incidence, resulting in a relatively large reflection from that surface side. However, the screen A is formed so that none of the light rays incident on the side surfaces of the screen are totally reflected.

Furthermore, partial reflection also occurs when the light rays 33 and 34 impinge on the inside of the stripe forming portion 23. Here, too, since the angle of incidence is relatively large, a correspondingly large reflection occurs. Regarding the light ray 33, when it enters the deepest part of the surface side part 35 located approximately at the center of the two lens elements 25, the total energy loss is 30 to 50%.
go up to Much of this is due to the large refraction that occurs when the light rays 33 exit the stripe formation 23. The corresponding loss when the light ray 33 is incident on the lens element 25 was calculated to be 15-20%, depending on the magnitude of the refraction upon incidence and the magnitude of the angle of incidence. Also, when light diffusing particles are mixed into the screen for image reproduction, or even when the lens surface is frosted for image reproduction, the light rays are weakened as they pass through the screen.

The various losses experienced by rays 30, 33, 34 and intermediate ray 36 are illustrated in FIG. 2, where the output rays are indicated by arrows. As a result, since they represent vectors at the same time, the output vector of ray 30 is longer than the output vector of ray 36, the output vector of ray 36 is longer than the output vector of ray 34, and furthermore, the output vector of ray 33 is the shortest. I understand that.

The horizontally diffused light energy is shown in the graph of FIG. Ray 30 has the highest energy and can be read as 8 from the graph. From the vectors and graphs described above, it can be seen that a viewer viewing the screen from the front (lower part of Figure 2) at right angles receives much more light energy than when viewing the screen diagonally in front due to the reflection losses described above. I know what I can do. Technically speaking,
This means that the screen has a fairly large "peak gain."

FIG. 4 shows a cross-sectional view of the screen of FIG. 2, but in this case the stripe formations 23 are planar and furthermore the masking stripes are not shown. f1 indicates the focal length of the lens element 25, and b2 indicates the pitch of the screen.

To explain the operation of the present invention, it is necessary to recognize that it is not possible to measure the energy of a single ray of light passing through lens element 25. Any measuring instrument naturally has an object to be measured of a certain size. Here, it is assumed that the measuring device is capable of measuring a spherical angle of +/-1°. Therefore, instead of a single ray set on the optical axis of lens element 25, ray 40 is
It is appropriate to target the luminous flux defined between the ray 41 and the ray 41. The luminous flux between rays 40 and 41 is 0.40 m
+/-1 for the radius of curvature of the top of the lens element 25 of m
This corresponds to a width b1 of 0.04 mm, which corresponds to the top portion of the lens element 25 that provides a displacement of .degree., and a pitch b2 of 1.0 mm. FIG. 5 shows the light intensity curve of the lens element 25 as a function of viewing angle, which corresponds to the graph of FIG. You can see that the more you look at it from an angle, the less light energy there is. In a typical screen, the loss is about 18-20% at maximum refraction. If the screen of FIG. 4 were equipped with a forward-facing convex lens corresponding to FIG.
40% larger. As a result, the total loss through the screen at the point of maximum refraction is approximately 50%.

The present invention starts from this point,
According to the present invention, equalization of the energy spread curves shown in FIG. 5 has been achieved. A cross section of the screen of the present invention is shown in FIG. 6, which corresponds to FIG. 4 showing a conventional screen. Here, for the top portion 45 of the lens element 25, the radius of curvature is approximately 10
% has been reduced. Looking again at the bundle of rays surrounded by rays 40 and 41, it can be seen that the more convex top lens portion 45 has a focal length f3, so that the rays intersect at a focal point 44 in screen A. . Thereby, the beam defined by rays 40 and 41 is spread out more than in FIG. 4, increasing the divergence angle for the beam by delta-beta, whereas in FIG. 4 it is only beta. At the same time, "dilution" of the light intensity occurs at the right angle position on the screen surface, and due to that "dilution", Fig. 7
The graph has a saddle shape.

The difference between FIG. 4 and FIG. 6 can be explained as follows.

In FIG. 4, the exposure meter is measured from the top of the lens element 25, for example 2f1, in the shadow region between the rays 40 and 41.
Suppose that the exposure meter shows a value of E1 at a distance of . Repeating this experiment in Figure 6, the exposure meter is adjusted to rays 40 and 41.
and at the same distance from the top of the lens element 25,
The light meter shows a value of E2 smaller than E1.

In both cases, when the lens element 25 is irradiated with the same energy in the same area, that is, between the rays 40 and 41, the intensity at the point at the same distance from the top (2f1) of the lens element is as shown in FIG. 4 than in FIG. is lower. This is because the light in FIG. 4 is spread over a larger angle, Beta+DeltaBeta.

As mentioned above, the energy of the output light approaches the optical axis of the lens element 25 decreases, and conversely, as can be seen from FIG. 7, the energy increases toward both sides of the axis. The half-value angle in FIG. 5 increased from alpha to alpha + delta alpha because the peak gain decreased. (Half-maximum is defined as half the value of the peak gain, which is considered as light energy delivered substantially perpendicular to the screen.) In FIG. Shown in plan. In this case, the intermediate lens element 25 is illuminated over its entire width with parallel rays directed perpendicular to the back surface of the screen. The top lens portion 45 collects the light incident therethrough to the focal point 44 (focal length f3), so that this portion of the light is diffused again. Superimposed on this light is light entering the lens element 25 through lens portions 51 and 52 (focal length f1) located laterally of the top lens portion 45 and focused on the forward-facing lens 23. . As a result, as shown in FIG. 8, a substantially hemispherical reflection vector band 50 with a curved portion over the wide-angle area is formed, and constant light energy appears within the wide-angle area. FIG. 9 shows the light energy after the light passes through the lens portion 25, including the top lens portion 45, and it can be seen that the curve is saddle-shaped. FIG. 10 shows the light energy diffused when the light passes through the forward facing convex lens 23. It can be seen from FIG. 10 that the graph is smoothed by the loss in the forward-facing convex lens 23, and that the internal reflection is greatest in the side portion. Thus, constant light intensity is obtained at various viewing angles.

The focal point of lens 23, labeled 46, is located very close to the surface of top lens portion 45 and slightly offset inwardly. As a result, in this embodiment, the lens 23
has approximately the same focal length as defined for the lens side portions 51, 52 (FIG. 8), 54, 56 and 55, 57 (FIG. 11). In other words, if the rear-facing lens element 25 did not have a more convex top portion 45, the focal length of the forward-facing convex lens 23 would be at the highest point of the rear-facing lens element 25. This fact is made clear in FIGS. 8 and 11, where the dashed lines represent the configuration that the rear-facing lens element 25 would have if the top portion 45 were not present.

Computer calculations have shown that the lens side portions 51, 52 compensate for the focus within the screen member and that this placement does not change the color shift.

Preferred dimensions for an embodiment of the screen according to the invention are shown in FIGS. 11 and 12. Figure 11
From this, it can be seen that a homogeneous viewing condition can be obtained whether the screen is viewed from the front at right angles or laterally at an angle of 45 degrees. The top lens portion 45 has a radius R3 of 0.35 mm over an 80° square. Each side of the top lens portion 45 is provided with short side lens portions 54, 55 defined laterally from the top lens portion 45 by radii having a radius R2 of 0.40 mm and forming a 90° angle with each other. There is. Side lens part 5
4 and 55 are followed by side parts 56 and 57, respectively. Its radius of curvature R1 is 0.84 mm, and its center is located symmetrically with respect to the optical axis of each lens element 25, ie at a distance of 0.30 mm therefrom. Additionally, these centers are moved 0.24 mm toward the center from the intersection between adjacent lens elements 25. Furthermore, the screen is 0
．． A forward facing convex lens 2 with a radius of curvature R4 of 40 mm
It has 3. The pitch of the screen is 1.0 mm, and the tangent to the inner part of the lens side parts 56 and 57 is 3.
It forms a top angle of 8°. Thus, as can be seen in Figure 12, the height of lens element 25 (including top lens portion 45) is 0.48 mm. Top lens portion 45
The distance between the surface of the lens 23 and the surface of the forward convex lens 23 is 1.
18 mm, and if the screen is formed conventionally as shown by the broken line in FIG. 12, the distance between the top of the backward lens 25 and the top of the forward convex lens 23 is 1.15 m.
m.

Regarding the width of the top lens 45, its arc length is smaller than 1/3 of the arc length of the lens element 25 including the arc length of the top lens element 25, and Must be greater than 1/20 of the arc length.

Regarding the lengths of the radii R1, R2, and R3 mentioned above, in the case of a screen with a pitch of 1.0, the following example can be given. Example 1 R1=0.84mm R2=0.40mm R3=0.35mm Example 2 R1=0.90mm R2=0.43mm R3=0.40mm Example 3 R1=0.80mm R2=0.38mm R3=0. 33mm These radii must meet the condition that R2 is greater than R3 and less than R1.

[0031]

[Effects of the Invention] As explained above, according to the present invention,
A rear projection screen is provided that provides a screen image with approximately the same luminous intensity whether viewed from the front orthogonal direction or from an oblique side.

[Brief explanation of the drawing]

FIG. 1 is a perspective view schematically showing a preferred example of use of an embodiment of a rear projection screen according to the present invention.

FIG. 2 is a horizontal section through a portion of a known screen.

FIG. 3 is a graph showing the energy spread of light passing through the screen shown in FIG. 2;

FIG. 4 is a horizontal cross-sectional view of a portion of a known screen to illustrate the problem motivating the invention;

FIG. 5 is a graph showing the energy spread of light passing through the screen shown in FIG. 4;

FIG. 6 is a horizontal sectional view through a portion of an embodiment of the screen according to the invention.

FIG. 7 is a graph showing the energy spread of a portion of the light leaving the screen according to FIG. 6;

FIG. 8 is a horizontal sectional view of a portion of another embodiment of the screen according to the present invention.

FIG. 9 is a graph showing the energy spread of a portion of the light leaving the screen according to FIG. 8;

FIG. 10 is a graph showing the total energy spread of the light leaving the screen according to FIG. 8;

FIG. 11 is a diagram corresponding to FIG. 8 showing an example of each dimension of the screen.

FIG. 12 is an enlarged view showing the dimensions of the screen.

[Explanation of symbols]

1, 2, 3 Projector 4 Screen 6, 7, 8 Optical axis 22 Masking stripe 23 Stripe forming part 24 Edge 25 Rear-facing lens element 30, 33, 34, 36, 40, 41 Ray 45
Top lens part

Claims (3)

[Claims] Claim 1: Three TVs arranged adjacent to each other to project red, green, and blue TV images on the back of the screen.
Rear projection screen used primarily in projection devices with projectors (1, 2, 3), with a vertically extending rear-facing lens element formed as a convex lens for light (30, 33, 34) incident from the rear (25
) at its rear, said rearward-facing lens element (25)
consisting of a screen (A) provided at its front with a vertically extending masking stripe (22) having between it opposite stripe formations (23), the top portion ( 45), but
Side portions (54, 5) of the rear-facing lens element (25)
Focal length (f3) shorter than the focal length of 6; 55, 57)
and the focal length (f3) of the top portion (45) is
A rear projection screen, characterized in that it is shorter than the distance (f1) measured perpendicular to the screen surface between the highest part of the rearward-facing lens element (25) and the stripe-forming portion (23). 2. The focal length (f1) of the side portions (54, 56; 55, 57) of the backward-facing lens element (25) is
Rear projection screen according to claim 1, characterized in that the distance corresponds approximately to the distance between the highest part of the rear-facing lens element (25) and the stripe formation (23), measured at right angles to the screen plane. . 3. The stripe forming portion includes a forward convex lens (
23), the focal length of this lens (23) being formed as a side portion (54,
5. Rear projection screen according to claim 1, characterized in that it corresponds approximately to a focal length of 56; 55, 57).