Rear projection screen

Provided is a rear projection screen apparatus having a rear projection screen, light rays from a projecter being incident upon the rear surface of the rear projection screen at steep angles, wherein a plurality of elongated prisms are formed on the rear surface of the screen, each prism having a reflection surface extending therealong, thereby the incedence light rays are internally reflected, and then transmitted through the medium of screen for emerging from the front surface of the screen on the viewing side, and the rear surface of the screen is in a curve shape which is convex toward the viewing side so that the brightness of the screen can be made to be uniform.

BACKGROUND OF THE INVENTION 
The present invention relates to a rear projection screen for use in a 
projection television set and the like. 
As shown in FIG. 15, a rear projector used in a video projection television 
set is adapted to expand light emerging from a light source P, e.g. a CRT, 
by means of a lens system L, project the light from the rear side of a 
screen S and allow a viewer to observe a picture from the opposite side of 
the screen S. However, if the distance from the light source P to the 
screen S is prolonged as shown, the projector becomes large size. In 
practice, therefore, a method is adapted whereby one to three mirrors M 
are combined, and the light is projected after being reflected on them 
once, as shown in FIGS. 16A, 16B, 16C. However, there have been drawbacks 
in that, in the method of FIG. 16A, the height of the projector becomes 
large, while, in the methods of FIGS. 16B and 16C, it cannot be said 
positively that the projectors have been made sufficiently compact in 
terms of the height and depth. 
To overcome these drawbacks, a rear projection screen has been proposed for 
observing an image by introducing light at a sharp angle from the rear, 
characterized in that a plurality of prisms are provided in parallel with 
each other on the light incidence surface, each having a total reflection 
surface, so that the incident light is totally reflected on the total 
reflection surface of the screen and is then emerged from the front 
viewing surface of the screen (refer to Japanese Pat. Application No. 
29964/1984, U.S. Pat. Application Nos. 837,412 filed March 7, 1986, now 
U.S. Pat. No. 4,674,836, issued June 23, 1987 and 925,723). 
Explanation will be made to the abovementioned rear projection screen 
proposed by the abovementioned U.S. Pat. Application Nos. 837,412 and 
925,723 with reference to FIG. 1 in which like reference characters 
designate like or corresponding parts as shown in FIG. 15. 
Light introduced from a light source P is made incident upon the rear 
surface of the rear projection screen S at a relatively large incident 
angle. The angle .theta. of incidence is generally dependent upon the 
distance of light projection, but preferably is in a range from 40to 85 
deg. The distance 1' from the light source P to the rear projection screen 
S can be made to be shorter than the distance 1 shown in FIG. 15 by a 
relatively large incident angle .theta. within a range as mentioned above 
or by making the light from the light source P incident upon the rear 
surface of the screen S at an acute angle to the latter. Since the light 
source P is located obliquely downward, a distance 1' in the direction of 
the depth is: 
EQU 1'=1 cos .theta. 
so that e', can be made relatively small in comparison with e. 
However, since the height of the projection set may not be always made 
sufficiently low, it is desirable in practice to reduce the height by 
employing a mirror M.sub.1, as shown in FIG. 2A, and to reduce the length 
in the direction of the depth as well. In addition, in order to further 
reduce the height so as to make the overall dimensions smaller, two 
mirrors M.sub.2 and M.sub.3 may be combined as shown in FIG. 2B, and the 
light source P may be interposed between the rear projection screen S and 
the first mirror M.sub.2 so as to project the light after being reflected 
twice. 
FIG. 3 illustrates a portion of the rear projection screen mentioned above, 
and, a plurality of prisms having the same configuration are provided on 
the rear surface of the rear projection screen. In other words, these 
prisms are constituted by a plurality of prisms 1 extending along lines or 
concentric circular arcs in parallel and each prism 1 having a light 
incidence surface 1B and a light reflection surface 1A. A total reflection 
surface is formed on the light reflection surface 1A in such a manner that 
the light entering from the light incidence surface 1B is totally 
reflected and is then emerged from a front viewing surface. 
As can be seen in FIG. 1, the light ray incident upon the rear projection 
screen S at the lower edge thereof has an incidence angle smaller than 
that of the light ray incident at the upper edge thereof. That is, the 
higher the incident position of the light ray on the rear projection 
screen, the greater the incidence angle of the right ray becomes, in other 
word, the higher the incident position of the light ray, the lower the 
density of the light rays incident upon the rear projection screen would 
becomes, resulting in that a picture image projected on the screen from 
the light source cannot be uniformly focused over the entire surface of 
the rear projection screen if no correction measure is taken for the light 
source P. However, optical measures or electric measures are proposed to 
overcome the above-mentioned problem. 
However, there is a drawback in the above-mentioned rear projection screen. 
That is, if the incidence angle of the light ray is comparatively small or 
if the relative height of the prisms is small with respect to the pitches 
thereof, that is the apex angle .theta..sub.1 of the prisms is large, a 
part of light rays incident upon the rear surface of the screen S directly 
enters into the medium of the screen S without reflecting upon the light 
reflection surfaces 1A of the prisms. More specifically, referring to FIG. 
4, of the light rays .alpha., the part of light rays .beta. reflects upon 
the light reflection surface 1A of each prism and emerges from the screen 
as effective light rays A to be viewed, but the other part of light rays 
directly enters into the medium of the screen as stray light rays denoted 
by the reference character B, resulting the efficiency of light to be 
viewed at the viewing side of the screen being lowered. 
In order to overcome the above-mentioned drawback, it has been proposed to 
make the apex angle .theta..sub.1 of the prisms small or to make the 
incidence angle .theta. of the light rays incident upon the screen. 
However, it would be impractical to decrease the apex angle .theta..sub.1 
since the manufacture of such a kind of screen is difficult if the apex 
angle .theta..sub.1 of the prisms becomes less than about 50 deg. 
Further, it is undesirable to increase the incidence angle .theta. since 
the above-mentioned uniform focussing problem would be serious if the 
incidence angle exceeds about 50 deg. at the center of the screen, 
resulting in difficulty in taking the above-mentioned correction measure 
at the light source P. 
SUMMARY OF THE PRESENT INVENTION 
The present invention has been achieved to overcome the above-mentioned 
drawback experienced in the rear projection screen proposed by the U.S. 
application Nos. 837,412 and 925,723. 
A main object of the present invention is to provide a rear projection 
screen having a rear surface formed with several parallelly extending 
prisms for refracting and reflecting light rays received from a light 
source or projector, which can have a relatively high efficiency of light 
to be viewed. 
A further object of the present invention is to provide a rear projection 
screen of the type mentioned above, in which the apex angle of the present 
invention can be made to be relatively large so that the production 
thereof can be facilitated. 
According to the present invention, there is provided a rear projection 
screen having a front surface and a rear surface, in a rear projection 
apparatus which emerges light rays to be viewed to a viewing side, and 
which includes a light source with an optical axis along which light rays 
are projected onto the rear surface of the rear projection screen said 
front surface of the rear projection screen confronting the viewing side 
and a rear surface, the rear surface of the rear projection screen having 
a principle line extending thereon and crossing the optical axis of the 
light source at a relatively steep angle and being formed thereon with 
several prisms arranged in parallel with each other and being 
substantially orthogonal to the main principle line at least crossing 
points therebetween, each of the prism having an incidence surface through 
which the light rays from the light source are introduced into the prism, 
and a reflection surface upon which said light rays introduced are 
reflected to be directed toward the viewing surface of the rear projection 
screen, and further the rear surface having a concave cross-sectioned 
shape along the main principle line.

PREFERRED EMBODIMENTS OF THE INVENTION 
Referring to FIG. 5 in which a rear projection screen system of the present 
invention is generally shown, including a rear projection screen S on 
which arcuate prisms 1 coaxially extending in the horizontal direction are 
formed. 
In this embodiment the rear projection screen S receives light from a light 
source P positioned therebelow in the rear thereof so that the light is 
projected onto the rear surface of the screen S, obliquely upward, and 
therefore, the arcuate form of the prisms is convex upward. Meanwhile it 
is possible to project light obliquely downward. However, in this case, 
the arcuate form of the prisms is convex downward. Further, it is noted 
that the prisms may take a straight shape extending horizontal direction, 
being arranged in parallel with each other. 
In the above-mentioned arrangement shown in FIG. 5 assuming that the light 
source for projection, e.g. a CRT, is P and the center of the concentric 
circular arc in a plane F including the screen S is 0, and if a segment OP 
is made perpendicular to the plane F, each point on the same arc becomes 
equidistant from the light source P. Consequently, by making the cross 
sections of the prism 1 along this circular arc identical, the angles of 
emergence from the cross sections of the prism become equal, resulting in 
the design of the rear projection screen becoming less complicated, and, 
furthermore, it is possible to realize a well-balanced screen S which is 
capable of controlling the light not only in the vertical direction but 
also in the horizontal direction. In FIG. 5, reference character O denote 
the center of the screen S. 
Assuming now that horizontal distance between the light source P and the 
center O' of the arcs is x and the vertical distance between the center O 
of the screen S and the center O' of the arcs is y, as shown in FIG. 5, 
the open angle of the prism at a point having distance .gamma.(upwardly 
positive) from the center of the vertical axis passing through the center 
O of the screen is .theta..sub.1, and the inclination of the light 
incidence surface of the prism is .theta..sub.2, then .theta..sub.2 in 
the case of parallel emergence can be obtained by the following Formula 1 
(wherein n is an index of refraction): 
##EQU1## 
If the cross-sectional configuration of the prisms 1 is made into one 
expressed by the above Formula (I) all the rays of light emerging from the 
surface of the screen S become parallel rays of light which are normal to 
the screen S. Hence, it becomes possible to obtain a screen which is more 
compact and displays a more uniform brightness as compared with a 
conventional screen having a Fresnel lens. 
It is convenient for expalaining the general configuration of the arcuate 
prisms formed on the rear surface of the screen to consider a main 
principle line which extends on the rear surface of the screen, passing 
through the center O of the latter and crossing the optical axis OP of the 
light source at the center O. From this consideration the main principal 
line crosses inevitably orthogonal to the above-mentioned arcuate prisms 
at least at the crossing point therebetween. It is noted that the this 
principal line is coincident with the line OO' as shown in FIG. 5 if the 
rear surface of the screen is flat. However, from the general aspect of 
the present invention, the main principal line does not coincide with the 
line OO' in the screen according to the present invention since the screen 
is concavely curved at least on the rear surface thereof. Further, as 
mentioned before, the light rays impinge upon the rear surface of the 
screen at relatively large angles in order to provide a compact 
arrangement to a rear projection screen apparatus. Accordingly, the 
principal line crosses the optical axis of the light source at a steep 
angle. 
Referring to FIG. 7 from which the essential arrangement of this embodiment 
is clearely understandable, the rear projection screen S has a 
cross-sectioned shape which is curved in the lower half section thereof, 
that is convex in the direction toward the viewing side but the rear 
surface of the screen has a concave shape. 
Comparison will be made to the arrangement shown in FIG. 7 with respect to 
the conventional one as shown in FIG. 6. For the purpose of the 
comparison, there were prepared a conventional rear projection screen as a 
comparison example, as shown in FIG. 6 and two reference examples, as 
shown in FIG. 7 and FIG. 8, of the rear projection screen of the present 
invention. Result of the comparison is shown in Table 1. 
COMATIVE EXAMPLE 
A transparent acrylic resin sheet having a refraction index of 1.49 and a 
thickness of 3 mm was prepared, which was subjected to a heating press 
moulding process to produce a rear projection screen having arcuate 
prisms. In this example, the specification and the positional relationship 
with a light source of the prisms were as follows, and its arrangement was 
as shown in FIG. 5. 
Position of the light source: 
x rearward of the screen=570 mm 
y downward of the center of the screen=820 mm 
(Light rays made to be incident upon the center of the screen at an angle 
of 55 deg. with respect to the rear surface of the screen.) 
Apex angle of the prism .theta..sub.1 =45 deg. 
Center of the circular arc of the prism: 820 mm 
downward from the center of the screen 
Pitch of the prism: P=0.5 mm 
Size of the screen: 600 mm (height) 800 mm (width) 
The angle of .theta..sub.2 of inclination of each prism was set to the 
angle calculated from Formula (1). While the above-mentioned conditions 
were all satisfied, this example was arranged such that all light rays 
emerges from the screen surface in the direction normal thereto. 
The thus formed rear projection screen was arranged as shown in FIG. 6 to 
measure the efficiency of light at points U, C and D. As understand from 
the table 1, row .circle.1 , it was confirmed, in this example, that the 
efficiency of light was lowered at the point D. 
REFERENCE EXAMPLE 1 
A similar rear projection screen (S) as explained in the comparison example 
was prepared, excepting that the lower half thereof is curved below the 
point C as shown in FIG. 7 so at to form a curved surface which is convex 
toward the viewing side, having a radius .gamma..sub.1 of curvature of 
1,500 mm, and was arranged as mentioned in the comparison example to 
evaluate the efficiency of light at the points U, C, D. The result of the 
evaluation is denoted in the Table 1, row .circle.2 from which it is 
understandable that the rear projection screen in this reference example 
offers 100% efficiency at either of the points U, C, D. 
EXAMPLE 2 
A rear projection screen similar to the one explained in the reference 
example 1 was prepared, having its lower half section curve so as to have 
a radius of curvature of 1,500 mm and its upper half section also curved 
to have a radius .gamma..sub.2 of curvature of 2,000 mm as shown in FIG. 
8, which is more advantageous in image projection since the curvature of 
the rear projection screen is uniform at several points thereon. Then, the 
efficiency of light was evaluated at the points U, C, D for this rear 
projection screen. The result of the evaluation is shown in the table 1, 
row .circle.3 from which it is understandable that 100% light efficiency 
is obtained at either of these three points U, C, D. 
TABLE 1 
______________________________________ 
EVALUATION ITEMS AND RESULTS 
MEAS- LIGHT 
URING INCIDENCE EFFICIENCY** 
POINT* ANGLE % C 
______________________________________ 
1 COM- U .theta..sub.u = 63 deg. 
100 
ATIVE C .theta..sub.c = 55 deg. 
100 
EXAMPLE D .theta..sub.d = 42 deg. 
74 
.circle.1 
EXAMPLE 1 
U .theta..sub.u = 63 deg. 
100 
.circle.2 
C .theta..sub.c = 55 deg. 
100 
D .theta..sub.d = 55 deg. 
100 
EXAMPLE 2 
U .theta..sub.u = 56 deg. 
100 
.circle.3 
C .theta..sub.c = 55 deg. 
100 
D .theta..sub.d = 55 deg. 
100 
______________________________________ 
NOTE: 
*Measuring points are shown in FIGS. 6 and 8, respectively. 
**Light Efficiency was obtained by the formula: .beta./.alpha. .times. 10 
(%) at each point U, C, D as shown in FIG. 4. 
As mentioned above, by estimating a main principle line (n) extends on the 
rear surface of the rear projection screen S, crossing the optical axis of 
the light source P at the center point of the screen S (refer to FIG. 5), 
at least the rear surface of the screen S is curved in cross-section along 
the main principle line (n) such that it is convex toward the viewing 
side. With this arrangement, the screen S according to the present 
invention can give uniform brightness since the light efficiency with 
respect to the incidence light can be maintained at every point on the 
screen S. 
It is noted that the rear projection screens shown in FIGS. 7 and 8 can be 
formed by a hot-press moulding process although it is also possible to 
forcedly curve them. 
FIGS. 9 to 14 show various embodiments, in part, of the present invention. 
FIG. 9 shows a most basic rear projection screen, in which are formed a 
plurality of prisms 1 each having a total reflection surface 1A and an 
incidence surface 1B on the projection side. FIG. 10 shows a rear 
projection screen in which a lenticular lens surface 1C extending 
vertically is formed on the viewing side in the embodiment shown in FIG. 
9, and this lenticular lens surface 1C imparts the horizontally diffusing 
characteristics of the light. In addition, FIGS. 11 and 12 show rear 
projection screens in which lenticular lens surfaces 1D, 1E having total 
reflection surfaces 1D.sub.1, 1E.sub.1 are similarly formed on the viewing 
side thereof, respectively, thereby making it possible to obtain even 
greater horizontal diffusing characteristics of light, i.e., a greater 
field of vision. 
Further, such total reflection surfaces formed in the lenticular lens 
surface allow the light rays totally reflected thereon to emerge from the 
screen over a wide viewing angle, and therefore to cross each other so 
that it is possible to enhance the mixing of colors in the case of color 
television sets. Since the arrangement and operation of lenticular lens 
surfaces 1D, 1E having total reflection surfaces shown in FIGS. 11 and 12 
are described in detail in Japanese patent application Nos. 51194/1981 and 
90544/1981, (corresponding to U.S. Pat. No. 4,418,986), 91896/1981, 
(corresponding to U.S. Pat. No. 4,469,402), 212584/1981, 29178/1981 
(corresponding to U.S. Pat. No. 4,468,092) and 59389/1982 filed by the 
applicants, description of them will be omitted herein. 
If a lenticular lens surface is formed on the viewing side, as shown in the 
embodiments of FIGS. 10 to 12, a portion which does not transmit light is 
formed on the lens surface, and an external light absorbing layer 1F may 
be formed by making use of this light nontransmissive portion, as shown in 
FIG. 13. In addition, if a lenticular lens surface having a total 
reflection surface is formed, as shown in FIGS. 11 and 12, it is possible 
to ensure that the light does not substantially emerge from this total 
reflection surface, so that an external light absorbing layer can be 
provided to this total reflection surface. In this case, the external 
light absorbing layer 1F may be provided via a reflection layer 1G made of 
a material having a smaller index of refraction than that of a substrate, 
as shown in FIG. 14, so that the function of the total reflection surface 
will not be deteriorated. 
It is noted that all screens mentioned above are formed in a curved 
cross-sectioned shape which is convex toward the viewing side. 
In the afore-mentioned embodiments, prisms 1 are disposed continuously such 
as to extend in a substantially horizontal direction, but the prisms 1 can 
be also arranged such as to extend in the substantially vertical direction 
by being turned by 90 deg. It goes without saying that the projector or 
light source should be disposed in the transverse direction in this case. 
With the rear projection screen according to the present invention, since 
an image is projected diagonally from the rear, a distortion may occur in 
the image and the fading of the image will occur. However, these problems 
can be overcome by taking the following measure in the projection system. 
In other word, the distortion of the image may be corrected by the 
electrical circuit of a CRT by taking account of an amount of distortion 
for each portion. Meanwhile, since the fading of the image results owing 
to a difference in the distance between the lens system and the screen, an 
image introduced from the CRT to the lens system may be given a 
predetermined angle with respect to the optical axis in such a manner as 
to form an equal focal length on the screen. 
As to a material used for the rear projection screen of the present 
invention, acrylic resin is the most suitable. The reason is that acrylic 
resin is particularly superior in terms of optical and fabrication 
characteristics, and fabrication efficiency while having a suitable 
rigidity. However, instead of acrylic resin, it is also possible to use 
polyvinyl chloride resin, polycarbonate resin, olefin resin, and styrene 
resin. When these synthetic resin materials are used, it is possible to 
fabricate a rear projection screen pertaining to the present invention by 
means of extrusion molding, heating press, or injection molding. 
Furthermore, a light diffusing means for further enhancing the light 
diffusing characteristics may be added to the base material the rear 
projection screen of the present invention separate sheet. As for this 
light diffusing means, one or two or more kinds of additives of diffusing 
materials which are not dissolved in or undergo chemical change by a 
liquid synthetic resin syrup and molten medium, including SiO.sub.2, 
CaCO.sub.3, Al.sub.2 O.sub.3, TiO.sub.3, BaSO.sub.4, ZnO, Al(OH).sub.3, 
fine glass powders, and an organic diffusing agent, may be mixed and 
distributed in the medium such as to be dispersed uniformly in a synthetic 
resin constituting the substrate, e.g., acrylic resin, or a layer 
including such diffusing materials may be provided to the screen. 
Furthermore, it is also effective to form a fine matted surface on the 
projection-side screen surface and/or the viewing-side screen surface. If 
such a means for imparting light diffusing characteristics is adopted, the 
diffusion of light in the horizontal and vertical directions of the screen 
can be compensated, thereby enabling enhancement of the uniformity of the 
diffusion of light. 
It has been explained that the total reflection surface is formed in each 
of prisms in order to reflect incident light rays toward the viewing side 
of the screen. Although a remarkable advantage can be expected with the 
above-mentioned arrangement, it is also possible to use a metalic 
reflection surface, etc., for attaining the same function as that of the 
total reflection surface. 
The present invention having the aforementioned arrangements has advantages 
in that, since light introduced onto the rear surface of the screen at a 
sharp angle can be emerged uniformly from the viewing surface with high 
efficiency by virtue of the configuration of the prisms and the action of 
the internal reflection, it becomes possible to locate the relative 
position of a projector serving as a light source diagonally rearward when 
the rear projection screen of the present invention is used, thereby 
making it possible to make the entire projector compact, and that a rear 
projection screen exhibiting a uniform brightness can be provided easily.