DK167944B1 - Back-lit projection screen - Google Patents

Abstract

A screen assembly for use in back-lit projection for use with projection devices, which has here TV image projectors arranged side by side, which emit respectively a red, green, and blue image, has a screen (A). This is on its back side equipped with backwards facing lens devices (25), which work as convex lenses for light coming from behind. This screen (A) has vertically running masking bands (22) which form between themselves stripe-forming parts (23) which are located along the backwards-facing lens devices (25). In order to attain a uniformly lit picture, regardless of whether the screen (A) is viewed from straight on or from the side, the top parts (45) of the backwards-facing lens devices (25) have a focal distance (f3), which is shorter than the focal distance (f1) of the side parts (51 and 52) of the backwards-facing lens devices (25).<IMAGE>

Description

in DK 167944 B1

The present invention relates to a taillight projection screen construction, in particular for use in projection devices of the kind having three side-by-side television projectors for projecting a red, green and blue television image towards the rear of the respective screen structure, which has a screen structure a screen having (a) on its reverse side rearwardly extending vertical lens means configured as rear lens collectors and b) on its front having vertical extending masking strips forming between them striped portions disposed opposite the rearward facing lens means, each consisting of a central, convex top portion and a pair of convex side portions with a curvature deviating from the top portion.

Usually, the rear facing lens members are designed to have a focal length (seen in horizontal section through the screen) corresponding to the distance 20 between the highest portions of the rear facing lens members and the strip-shaped portions measured perpendicular to the plane of the screen. This is achieved by the fact that the lens members concerned, also in horizontal section, have the shape of the chapped end of an ellipse or ellipse-like shape. Thus, these known lens means have the feature of focusing from behind incoming light that runs parallel to the normal to the plane of the screen in the middle of the strip-shaped portions which may be formed as forward-arched lenses. Since the highest portions of such parallel rays affecting the upper portions of the rearward lens members are substantially uninterrupted through the screen, and as their energy diminution in passing the screen is only small, these rays will dominate the radiation pattern emerging from the striped portions. By contrast, those of the said 35 rays which hit the side portions of the rear facing lens members DK 167944 B1 2 will undergo a relatively greater energy decrease during their passage of the screen. This is because these rays hit the surfaces of the side parts with relatively great reflection, and the same is true when such rays from behind strike the strip-shaped portions, which, as mentioned, can be designed as convex lenses, the rays in question also having a large angle of incidence here. The reflection losses in the latter case are greater than the reflection losses that occur 10 upon the entry of the rays into the screen, in the latter case the rays must move from a stronger to a less strong refractive material (from the screen material and to the air). All in all, it can be expected that the said rays which hit the side portions of the rearward facing lens members which are closest to the center plane of the screen will be weakened approximately. 50%, in terms of their energy. The result is that the image projected on the back of the screen appears more clearly illuminated for an observer who is in the middle of the screen than for an observer who views the image obliquely from the side. This is technically expressed in that the screen in question has a pronounced "peak gain".

US Patent No. 4752116 discloses a rear light projection screen with rear facing lens means which is formed as assembly lenses and consists of a center portion having a focal length corresponding to the screen thickness and side portions having a longer focal length.

In this embodiment, the center portion occupies a large proportion of each assembly lens and the side portions occupy smaller proportions.

30 In order to achieve a deliberate light distribution, this prior art display requires additional lenses on the front of the screen.

It is the object of the present invention to provide a screen of the kind in which the screen appears at approximately uniform light intensity, whether viewed from the front or obliquely from DK 167944 B1 3, and this is achieved according to the present invention by, a ) that each top portion occupies a relatively small proportion of each of the rear lens members and has a focal length shorter than the focal length of the side portions, and b) that said top focal length is shorter than the distance between the highest portions of the rear lens members and the strip-shaped portions 10 measured perpendicular to the plane of the screen.

Hereby it is achieved that the rays which strike from behind the peaks of the rearward facing lens members, which extend perpendicular to the plane of the screen, will intersect with each other within the screen material, and thus be deflected or directed laterally outwards to obtain an increased horizontal spread and thus a uniform distribution of light before stepping out through the striped portions. If these are planar, they will further accentuate this spread as the rays in question move from a stronger to a less strongly refractive material. If the strip-shaped portions are designed as forward-arched lenses, the spreading can be further accentuated by placing the top of the focal point so as to lie between the forward-facing lenses and their focal points, in which case the forward-facing lenses will serve such as scatter lenses. However, the focal length of the top members and forward arched lenses can also be chosen in such a way that the focal points of the two lens sets are coincident. Thus, in the latter case, the forward-arched lenses will have no effect on the scattering of the parallel beams which strike the top portions of the rearward facing lens members.

Because of the explained construction, a scattering will be obtained precisely from the rays which, as previously explained, would give rise to a pronounced "peak gain", such an avoidance is obtained and an improved energy distribution in the horizontal direction with respect to the emitting light, and, consequently, an observer observing the screen in the front and an observer observing the screen in the front will perceive the projected image with substantially the same illumination. In other words, a screen is provided where the viewing conditions in the lateral direction are improved.

The invention will now be explained in more detail with reference to the drawing, in which fig. 1 schematically illustrates a preferred use of an embodiment of the taillight projection screen construction according to the invention; FIG. 2 is a horizontal section through a portion of a known screen; FIG. 2a is a graph illustrating the energy distribution of the light penetrating a screen as shown in FIG. 2, FIG. 3 is a horizontal section through a portion of a prior art screen for illustrating the problem underlying the present invention; FIG. 3a shows a graph for illuminating the energy distribution of the light passing through it in FIG. 3; FIG. 4 is a horizontal section through part of the screen embodiment of the present invention; FIG. 4a is a graph illustrating the energy distribution of a portion of the light emitting from the screen of FIG. 4, DK 167944 B1 5 fig. 5 is a horizontal section of a portion of another embodiment of the screen according to the invention; FIG. 5a is a graph illustrating the light energy distribution of a portion of the light emitting from the screen of FIG. 5, FIG. 5b is a graph illustrating the energy distribution of all light emitting from the screen of FIG. 5, 10 FIG. 6 is a view similar to FIG. 5 to illustrate a copy of the screen dimensions; and FIG. 7 is an enlarged scale of a scale, also for illuminating the dimensions of the screen 15.

In FIG. 1, 1, 2 and 3 denote three projectors for projecting television images towards the back of a projection screen assembly 4. Each projector outputs a black and white television image, but in front of projector 20 1 a filter is arranged so that the image that this projector sends the screen structure 4 turns red, in front of the projector 2 a filter is placed so that the image that this projector sends against the screen structure 4 becomes green, and for the projector 25 3 a filter is used so that the image of this projector turns blue . These three colors are marked with R, G and B. The three projectors 1, 2 and 3, respectively, are arranged side by side with the center axis optical axis 7 perpendicular to the center of the screen construction. Since the projectors extend in the lateral direction, these cannot be arranged with their optical axis 6 and 8 coinciding with the axis 7, and instead the three optical axes form angular angles of 7 °. These 7 ° are difficult to underestimate due to the size of the projectors. On the contrary, DK 167944 B1 6 is moving towards more and more bright projectors, which thereby fill more in the lateral direction, so that the said angle of 7 ° can rise very well up to 10 °. The optical axes are directed toward the center of the screen structure 4 and, moreover, the projectors are arranged so that the images projected onto the back of the screen structure 4 are as close to each other as possible. As a result, from the front of the screen structure 4, an image similar to that of a color television picture tube can be observed, but of a better quality and of a larger format. In FIG. 1, a frame with curved lines indicates a spectator room.

The FIG. 1 display structure consists of two 15 screens, namely a front screen A and a rear screen B. By "front" is meant here the closest to the viewing room, as are the terms "front" and "rear" when it comes to surfaces of the screens, denote surfaces facing and away from the spectator room respectively.

The front of the front screen 5 has masking strips 22 and interspersed strip-shaped portions 23 which extend vertically in the position of application of the screen structure. In the embodiment shown in FIG. 1, shown in Figure 25 of the screen structure 4, the strip-shaped portions 23 are formed as forwardly vaulted lenses which abut their sides to edge portions 24 which are approximately perpendicular to the plane of the screen and which constitute lateral restrictions for the masking stripes 22.

30 On its rear side, the front screen A has rearwardly extending vertical lens means 25, one of which is located directly behind each of the strip-shaped portions 23. These lenses 25 primarily serve to deflect from behind incoming light so that it exits through GB 167944 B1 7 the strip-shaped portions on the front of the screen construction.

The rear shield B, on its front face, is designed as a Fresnel lens 27 for parallel rear view light. Since the three projectors 1, 2 and 3 are directed 5 with their optical axes 6, 7 and 8 towards the center of the screen and the optical axis of projector 2 is perpendicular to the screen, the rays from the three projectors will hit the back of the screen construction 4 at increasing angles of incidence. in the direction away from the center axis 7, 10 which, however, is corrected for by the Fresnel lens.

Although screen A is generally intended for use as explained in connection with FIG. 1, in the following figures, the beam passage is shown only for the beams which strike the screen A from behind and perpendicular to its plane, ie the beams from the projector 2, but the conditions will be analogous to the beams from the projectors. 1 and 3, although these, after parallelization by means of the Fresnel lens B, will still form said angles at e.g.

7 ° to the green rays that will hit the back of screen A perpendicular to its plane.

As mentioned, FIG. 2 is a section of a horizontal section through a known embodiment of the screen A. A beam 25 30 striking the highest point 31 of a rearward facing lens member 25 (which in the projection shown in FIG. 2 faces upwards) and perpendicular to the center plane of the screen , will pass the screen continuously. The ejecting beam will, as far as its energy is concerned, be slightly diminished in relation to the energy of the incoming beam, since if the screen consists of acrylic it will lose approx. 7% of the light energy at the transition from air to acrylic and from acrylic to air as a result of reflection. Rays 33 and 34 which strike the back of the screen perpendicular to its plane and which pass 35 through the surface plane parts closest to the screen of the screen 25, after passing the screen, have undergone an energy reduction which is significantly larger than beam 30's. This is because the rays 33 and 34 strike the surface side portions 35 at rather large angles of incidence, and consequently the reflection from the surface portions in question will be relatively large. However, the screen A is constructed in such a way that none of the rays hitting the surface side portions will be totally reflected.

Furthermore, partial reflection will also occur when the rays 33 and 34 strike the inside of the strip-shaped portions 23. Here, too, the angles of incidence will be relatively large, and consequently a correspondingly large reflection will appear. As for the rays 33 which strike the innermost portions of the surface side portions 35, i.e. approximately midway between the lens means 25, the energy decrease is a total of 30-50%, the greatest loss being due to the large refraction as the rays 33 eject 20 through the strip-shaped portion 23. The corresponding loss upon the entry of the rays 33 into the lens member 25 is calculated at 15-20%, depending on the magnitude of the refraction at the onset or the angle of incidence. In addition, the rays will be attenuated during their passage of the screen if it is mixed with light-diffusing grain to make the screen imaging, or if the lens surfaces are dulled for the same purpose.

The various losses suffered by the rays 30, 33 and 34 and 30 intermediate rays 36 are shown in FIG. 2 is illustrated in that the outgoing rays are provided with arrowheads so that they simultaneously represent vectors, and it will be seen that the output vector of the beam 30 is longer than the output vector of the beams 36, which in turn is longer than the output vectors of the beams 34, DK 167944 B1 9 and that the output vectors with respect to the rays 33 are the shortest.

This light energy distribution in the horizontal direction is illustrated by the graph of FIG. 2a. For the beam 5 30, the energy is maximum and is indicated on the graph as "8". From the vectors explained and from the graph, it will be seen that a viewer observing the screen directly from the front (from below in Fig. 2) will receive considerably greater light energy than if the screen is considered obliquely from the front because of the foregoing explained reflection loss. This is technically referred to as the screen having a relatively large "peak gain".

FIG. 3 shows a section of the screen of FIG. 2, but in this case the strip-shaped portions 23 are shown planar, and furthermore the masking strips are not shown. for example, the focal length of the lens means 25 indicates, and bz indicates the pitch of that screen.

To explain the operation of the invention, it is necessary to make it clear that it is not possible to measure the energy of a single beam of light passing through the lens member 25. Any measuring instrument will, of course, have a measuring object having a given extent. It is assumed that the measuring instrument measures over an arc of + 1 °. It will therefore be natural to consider a beam of beam, as in the embodiment shown in FIG. 3 is limited between beams 40 and 41 and not a single beam located on the optical axis of lens member 25. The beam bundle between beams 40 and 41 corresponds to a width bj = 0.04 mm, which corresponds to that portion of lens member 24 peak, which gives a deflection of ± Γ for a radius of curvature at the top of the lens member 25 of 0.40 mm, and has a pitch b2 of 1.0 mm. FIG. 3 shows the light intensity curve of the lens member 25 as a function of the viewing angle. This corresponds to the graph in Figure 35 2a. It will be clearly seen that the light energy decreases at a growing side-viewing angle at DK 167944 B1 10. For a regular screen, the loss is approx. 18-22% at maximum refraction. If the screen of FIG. 3 is supplemented with a forward curved lens similar to FIG. 2, the losses in respect of the 5 most broken rays will be even greater, namely approx. 30-40% larger, so that the total loss at maximum refraction through the screen is approx. 50%.

It is at this point that the present invention is implemented, in accordance with the present invention 10, a smoothing of the energy distribution curve shown in FIG. 3a. This is shown in FIG. 4, which shows a screen section similar to that of FIG. 3, but where the radius of curvature, with respect to the top portion 25 of the lens member 25, is reduced by approx. 10%. Now, considering the 15 beams bounded by the rays 40 and 41, it will be seen that its rays will intersect at a focal point 44 located inside the screen A, the stronger vaulted top lens portion 45 having the focal length f3. Thereby, the beam of beam limited by the jets 40 and 41 is spread more than is the case in FIG. 3, which in FIG. 4 is shown by the scattering angle of the beam beam in question being increased by Δβ, where in FIG. 3 is only β. At the same time, there has been a "dilution" of the light intensity perpendicular to the plane of the screen 25, as shown in the graph in FIG. 4a, which has now been saddle shaped as a result of the "dilution" explained above.

The difference between FIG. 3 and 4 can also be explained as follows: 30 If it is thought that in fig. 3, a light meter is placed in the shaded area between the rays 40 and 41 in the distance e.g. 2 fL from the top of the lens member 25, the light meter will display a value Ex. Repeating the experiment of FIG. 4 and placing the light meter between the rays 40 and 41 and at the same distance from the top of the lens member DK 167944 B1 11 25, the light meter will show a value E2 which is less than Ej.

Since in both cases the lens means 25 are illuminated with the same energy and on the same area, namely 5 between the rays 40 and 41, the energy per surface unit at the same distance from the top of the lens member (2 f1) be lower in FIG. 4 than in FIG. 3, since the light of FIG. 4 is spread over a larger angle β + Δβ.

As explained, the energy is decreased by the light emitting near the optical axis 25 * optical axis, and in turn the energy is increased on both sides for that axis, as can also be seen in FIG. 4a, where the half-angle angle of FIG. 3a has grown from a to a + Δα due to the reduced peak gain. (half value 15 is defined as the half peak gain which in practice can be perceived as the light energy emitted in the direction perpendicular to the screen).

In FIG. 5, the beam passage for an embodiment of the screen according to the invention is shown in horizontal section. In this case, the middle lens member 25 is shown illuminated throughout its width and with parallel light directed toward the rear of the screen perpendicular to its plane.

It will be seen that the top lens portion 45 focuses it through this incident light at the focal point 44 (focal length 25 f3), thereby dissipating this portion of the light. This light is overlaid with the light entering the lens member 25 through the lens portions 51 and 52 located to the side of the top lens portion 45 and focused on the forward facing lens 23 (focal length, for example), thereby producing the lens 30 shown in FIG. 5, the vector array 50 shown, which has, within a wide angular range, an envelope curve which is largely circular in shape and thereby represents a constant luminous energy within a wide angular range.

FIG. 5a shows the light energy after the light has passed through the lens part 25, incl. the top lens portion 45, and it will be seen that the curve is saddle-shaped. FIG. 5b shows the light energy distribution when the light has passed the forward curved lens 23. It will be seen from FIG. 5b, it is seen that the graph is now smoothed out due to the loss in the forward vaulted lens 5 23, at the inside of which reflection will appear, which will be greatest at its lateral portions. Thereby the constant light intensity at different viewing angles is obtained. The focal point of the lens 23 is designated 46, and it will be seen that this is very close to the surface of the top lens portion 45, however slightly inward relative to it. Thus, in the embodiments shown, the lenses 23 have approximately the same focal length as the focal length defined by the lenses 54, 56; 55.57. In other words, the focal points of the forward vaulted 15 lenses 23 would be on the highest portions of the rearward facing lens means 25, provided that these were not provided with the stronger vaulted top lens portions 45. This relationship is highlighted in FIG. 5 and 6, in dotted line, is shown the shape that 20 the rearward facing lens members 25 would have if the top portions 45 were not found.

Computer calculations have shown that the lens portions 51 and 52 compensate for the focal point 44 inside the screen material and that this location does not change the color scheme.

In FIG. 6 and 7, suitable dimensions are shown with respect to an embodiment of the screen according to the invention. In FIG. 6, it will be seen that uniform viewing conditions have been achieved, regardless of whether the screen is viewed directly from the front or at 45 ° angles in the 30 side direction relative thereto. The top lens portion 45 has a radius R3 of 0.35 mm over an angular dimension of 80 °. To each side of the top lens section 45 is a short side lens section 54, 55 with a radius R2 of 0.40 mm and limited in the lateral direction away from the top lens section 45 by radii forming an angle of 90 ° to each other. After parts 54.

DK 167944 B1 13 and 55 follow lateral sections 56 and 57, respectively, whose radius of curvature is 0.84 mm and whose centers are arranged symmetrically with respect to the optical axis of each lens member 25, namely at a distance of 0.30 mm from this one.

Furthermore, these centers are positioned 0.24 mm inward relative to the intersection sites between the present lens members 25. The screen is further provided with forward vaulted lenses 23 with a radius of curvature R4 of 0.40 mm. The pitch of the screen is 1.0 mm and the keys to the innermost portions of the lens side portions 56 and 57 form a peak angle of 38 °. This gives a height of the lens member 25 (including the lens top portion 45) of 0.48 mm, as shown in FIG. 7. The distance between the surface of the top lens portion 45 and the surface of the forward lens 23 is 1.18 mm, and if the screen had been designed in a conventional manner, cf. the dotted line to the right in FIG. 7, the distance between the top of a rear facing lens 25 and the top of a forward facing lens 23 would have been 1.15 mm.

As to the width of the top lens portion 45, it should be noted that its arc length should be less than one-third the arc length of the lens member 25, incl. the arc length 45 of the arc length 45 and it should be greater than one-twentieth of that arc.

With regard to the lengths of the radii mentioned above and R 2 and R 3 above, the following examples may be mentioned in a screen where the pitch is 1.0 mm:

Example 1 R-1 = 0.84 mm; R 2 = 0.40 mm and R 3 = 0.35 mm.

DK 167944 B1 14

Example 2 R-1 = 0.90 mm; R2 = 0.43 mm and R3 = 0.40 mm.

Example 3

Ri = 0.80 mm; R2 = 0.38 mm and R3 = 0.33 mm.

In addition, these radii must satisfy the condition that R2 10 is greater than R3 and less than Rx.

Claims (3)

1. Rear projection screen design, in particular for use in projection devices of the kind having three adjacent television projectors (1, 2, 3) for projecting a red, green and blue television image to the respective screen construction ( 4) back side, which screen structure (4) has a screen (A) which a) has a rearwardly facing vertically extending lens means (25) formed as collecting lenses for rear light (30, 33, 34) ) and b) on its front side have vertical extending masking strips (22) forming between them striped 15 portions (23) arranged against the rearward facing lens means (25), each consisting of a central convex top portion (45). and a pair of convex side portions (54, 56; 55, 57) with a curvature deviating from the top portion (45). 20, characterized in (c) that each top portion (45) occupies a relatively small portion of each of the rearward facing lens means (25) and has a focal length (f3) shorter than the focal length of the side portions (54, 56; 55.57) for example, and d) that the focal length (f3) of the top members (45) is shorter than the distance between the highest portions of the rearward lens members (25) and the strip-shaped portions (23) measured perpendicular to the plane of the screen. Rear light projection screen construction according to claim 1, 30, characterized in that the focal length (e.g.) of the side parts (54, 56; 55, 57) corresponds approximately to the distance between the highest portions of the rear facing lens means (25) and the strip-shaped portions (23). measured perpendicular to the plane of the screen. DK 167944 B1 16 Rear light projection screen construction according to claim 1, wherein the strip-shaped portions are formed as forward-arched lenses (23), characterized in that the focal length of the latter lenses (23) corresponds approximately to the focal length of the 5 side portions (54, 56; 55, 57).