Rear projection screen having resistance to change of external humidity

A rear projection screen free from lowering of resolution which is caused when the screen set undergoes separation or undesirably warps due to a change in the external humidity. The rear projection screen is composed of a plurality of sheets, a front surface sheet 13, a lenticular lens sheet 11 and a Fresnel lens sheet 12. The front surface sheet 13 and the Fresnel lens sheet 12 are respectively composed of two layers, an inner layer 13a, 12a and an outer layer 13b, 12b. The inner layer 13a, 12a is made from a material having susceptibility to water higher than that of a material from which the outer layer 13b, 12b is made.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to rear projection screens useful for 
projector TVs. 
2. Related Art 
FIGS. 6A and 6B are views showing one example of projector TVs, in which a 
conventional rear projection screen is employed. 
A conventional rear projection screen 60 is composed of a lenticular lens 
sheet 61 positioned on the observation side of the screen, and a Fresnel 
lens sheet 62 positioned on the projection side of the screen, and imaging 
light is projected by a projector (light source) 70 from the rear of the 
screen. 
The above-described conventional rear projection screen has such a problem 
that the screen set undergoes separation or undesirably warps due to a 
change in the external humidity, resulting in lowering of resolution. 
Specifically, in the case where the screen set absorbs moisture and expands 
when the external humidity is increased, the two sheets constituting the 
screen set warp so that they are separated from each other as shown in 
FIG. 6B. As a result, separation is caused in the screen set. 
Thereafter, the moisture penetrates from the screen surface into the inner 
part of the screen set, and the screen set returns to its original shape. 
However, it takes many hours for this recovery. 
An object of the present invention is to provide a rear projection screen 
free from lowering of resolution which is caused when the screen set 
undergoes separation or undesirably warps due to a change in the external 
humidity. 
SUMMARY OF THE INVENTION 
A first aspect of the present invention is a rear projection screen 
comprising at least one of a front surface sheet, a lenticular lens sheet 
and a Fresnel lens sheet, wherein at least one of the front surface sheet, 
the lenticular lens sheet and the Fresnel lens sheet is composed of 
multiple layers made from materials which are different in susceptibility 
to water. 
A second aspect of the present invention is a rear projection screen 
comprising at least two of a front surface sheet, a lenticular lens sheet 
and a Fresnel lens sheet in combination, wherein at least one of the front 
surface sheet, the lenticular lens sheet and the Fresnel lens sheet which 
is positioned on the projection or observation side of the screen is 
composed of multiple layers including an inner layer and an outer layer, 
the inner layer being made from a material having susceptibility to water 
higher than that of a material from which the outer layer is made. 
A third aspect of the present invention is a rear projection screen 
comprising a lenticular lens sheet and a Fresnel lens sheet in 
combination, wherein at least one of the lenticular lens sheet and the 
Fresnel lens sheet is composed of multiple layers including an inner layer 
and an outer layer, the inner layer being made from a material having 
susceptibility to water higher than that of a material from which the 
outer layer is made, and the lenticular lens sheet and the Fresnel lens 
sheet are in a shape which is convex relative to the projection side of 
the screen. 
A fourth aspect of the present invention is a rear projection screen 
comprising a lenticular lens sheet and a Fresnel lens sheet in 
combination, wherein at least one of the lenticular lens sheet and the 
Fresnel lens sheet is composed of multiple layers including an inner layer 
and an outer layer, the inner layer being made from a material having 
susceptibility to water higher than that of a material from which the 
outer layer is made, and the lenticular lens sheet and the Fresnel lens 
sheet are in a shape which is convex relative to the observation side of 
the screen. 
A fifth aspect of the present invention is a rear projection screen 
comprising a front surface sheet, a lenticular lens sheet and a Fresnel 
lens sheet provided in the mentioned order, wherein at least one of the 
front surface sheet and the Fresnel lens sheet is composed of multiple 
layers including an inner layer and an outer layer, the inner layer being 
made from a material having susceptibility to water higher than that of a 
material from which the outer layer is made, and the front surface sheet, 
the lenticular lens sheet and the Fresnel lens sheet are in a shape which 
is convex relative to the projection side of the screen. 
A sixth aspect of the present invention is a rear projection screen 
comprising a front surface sheet, a lenticular lens sheet and a Fresnel 
lens sheet provided in the mentioned order, wherein at least one of the 
front surface sheet and the Fresnel lens sheet is composed of multiple 
layers including an inner layer and an outer layer, the inner layer being 
made from a material having susceptibility to water higher than that of a 
material from which the outer layer is made, and the front surface sheet, 
the lenticular lens sheet and the Fresnel lens sheet are in a shape which 
is convex relative to the observation side of the screen. 
In the above-described second to sixth aspects of the invention, it is 
preferable that the multiple layers further include an intermediate layer 
between the inner layer and the outer layer, wherein the intermediate 
layer is made from a material having susceptibility to water higher than 
that of the material from which the outer layer is made, and, at the same 
time, lower than that of the material from which the inner layer is made. 
According to the present invention, since a sheet composed of multiple 
layers including an inner layer and an outer layer which are made from 
materials different in susceptibility to water is used to obtain a rear 
projection screen, it is possible to prevent the lowering of resolution 
which is caused when the screen set undergoes separation or undesirably 
warps due to a change in the external humidity.

DETAILED DESCRIPTION OF THE INVENTION 
By referring now to the accompanying drawings, embodiments of the present 
invention will be described specifically. 
First Embodiment 
FIGS. 1A, 1B and 1C are views showing a first embodiment of the rear 
projection screen according to the present invention. 
A rear projection screen 10 of the first embodiment is composed of a 
lenticular lens sheet 11, a Fresnel lens sheet 12 which is provided on the 
projection side relative to the lenticular lens sheet 11, and a front 
surface sheet 13 which is provided on the observation side relative to the 
lenticular lens sheet 11. 
Among the lenticular lens sheet 11, the Fresnel lens sheet 12 and the front 
surface sheet 13, a sheet which is provided on the outer surface of the 
screen (the outer surface on the observation and/or projection side) 
contains an inner layer and an outer layer which are made from materials 
different in susceptibility to water. 
For example, in the case where the screen is composed of three sheets, the 
lenticular lens sheet 11, the front surface sheet 13 and the Fresnel lens 
sheet 12, the lenticular lens sheet 11 being positioned between the front 
surface sheet 13 and the Fresnel lens sheet 12 as shown in FIG. 1B, the 
front surface sheet 13 is composed of two layers of an inner layer 13a and 
an outer layer 13b, the inner layer 13a being made from a material having 
susceptibility to water higher than that of a material from which the 
outer layer 13b is made, and the Fresnel lens sheet 12 is composed of two 
layers of an inner layer 12a and an outer layer 12b, the inner layer 12a 
being made from a material having susceptibility to water higher than that 
of a material from which the outer layer 12b is made. In this case, the 
lenticular lens sheet 11, which is provided at the center of the screen, 
is not necessary to have a multi-layer structure. 
Further, in the case where the screen is composed of two sheets, the 
lenticular lens sheet 11 and the Fresnel lens sheet 12, as shown in FIG. 
1C, the lenticular lens sheet 11 is composed of two layers of an inner 
layer 11a and an outer layer 11b, the inner layer 11a being made from a 
material having susceptibility to water higher than that of a material 
from which the outer layer is made, and the Fresnel lens sheet 12 is 
composed of two layers of an inner layer 12a and an outer layer 12b, the 
inner layer 12a being made from a material having susceptibility to water 
higher than that of a material from which the outer layer 12b is made. 
In this embodiment, since those materials which are different in 
susceptibility to water are used for the inner and outer layers of sheets 
which constitute a screen, it is possible to control the warpage of each 
sheet which is caused due to a change in humidity. The term 
"susceptibility to water" as used herein means at least one of those 
properties which are rate of water absorption, coefficient of water 
absorption expansion, and water absorption. 
In this case, since materials having high water absorption are used for the 
inner layers of sheets which constitute a screen, the screen can restore 
its original condition in a short time due to a small amount of 
penetrating water. 
Further, when the external humidity is decreased, moisture is released from 
the outer layers of the sheets which constitute the screen, and the sheets 
shrink. Warpage is thus caused in the direction in which the multiple 
sheets push each other, and, due to the ill-balanced forces, warpage in 
the undesirable direction can be caused in the rear projection screen. 
Also in this case, since a material having high coefficient of water 
absorption expansion is used at the inner part of the rear projection 
screen, the warpage in the direction in which the sheets push each other 
is reduced in a short time, and the screen can easily restore its original 
shape at the time of the stable condition. 
In particular, the present invention is remarkably effective for large-size 
screens such as screens of approximately 60 inches or more. 
Examples of materials which are different in water absorption include 
acrylic resins (water absorption: 0.3 to 0.4%), polycarbonate (water 
absorption: 0.15%), and styrol (water absorption: 0.03 to 0.1%). It is 
noted that the above water absorptions are obtained by the testing method 
of JIS (Japanise Industrial Standard) K 7209, in which a test piece having 
a thickness of 3.18 mm is tested for 24 hours. The desired effects can 
filly be obtained when the difference between the water absorptions is at 
least approximately 0.1%, and, moreover, no problem occurs even if the 
difference becomes large. 
Second Embodiment 
FIGS. 2A and 2B are views showing a second embodiment of the rear 
projection screen according to the present invention. 
In the following embodiments, those parts which have the same functions as 
in the first embodiment are indicated by the identical reference numerals, 
and descriptions of such parts may be omitted. 
In the second embodiment, a lenticular lens sheet 21 and a Fresnel lens 
sheet 22 are respectively composed of two layers, an inner layer 21a, 22a 
and an outer layer 21b, 22b, wherein the inner layer 21a, 22a is made from 
a material having susceptibility to water higher than that of a material 
from which the outer layer 21b, 22b is made. 
Further, both the lenticular lens sheet 21 and the Fresnel lens sheet 22 
are so shaped that they are convex relative to the projection side of the 
screen. 
The purpose of making the two lens sheets convex relative to the projection 
side of the screen is to surely prevent separation under the normal 
conditions by allowing the lenticular lens sheet 21 to push the Fresnel 
lens sheet 22 having higher rigidity. In this case, in order to stabilize 
the entire screen, the Fresnel lens sheet 22 is also made convex. 
Therefore, in the second embodiment, adhesion between the lenticular lens 
sheet 21 and the Fresnel lens sheet 22 is excellent, and the two lens 
sheets show high stability when they absorb or release water. 
Third Embodiment 
FIGS. 3A and 3B are views showing a third embodiment of the rear projection 
screen according to the present invention. 
In the third embodiment, as in the second embodiment, a lenticular lens 
sheet 31 and a Fresnel lens sheet 32 are respectively composed of two 
layers, an inner layer 31a, 32a and an outer layer 31b, 32b, wherein the 
inner layer 31a, 32a is made from a material having susceptibility to 
water higher than that of a material from which the outer layer 31b, 32b 
is made. 
However, the screen of this embodiment is different from that of the second 
embodiment in that the lenticular lens sheet 31 and the Fresnel lens sheet 
32 are shaped to be convex relative to the observation side of the screen. 
In the case of an LCD or DMD projector which is considered to have a single 
light source, it is possible to make the lenticular lens sheet thick, 
thereby imparting thereto rigidity. Therefore, the third embodiment is 
suitable for preventing separation by allowing the Fresnel lens sheet 
which has been made thin in order to prevent the formation of dual images 
to push the lenticular lens sheet which has been made thick. 
Fourth Embodiment 
FIGS. 4A and 4B are views showing a fourth embodiment of the rear 
projection screen according to the present invention. 
Like the first embodiment, the fourth embodiment is a screen set composed 
of three sheets, a front surface sheet 43, a lenticular lens sheet 41 and 
a Fresnel lens sheet 42, the lenticular lens sheet 41 being positioned 
between the front surface sheet 43 and the Fresnel lens sheet 42, wherein 
the front surface sheet 43 and the Fresnel lens sheet 42 are respectively 
composed of two layers of an inner layer 43a, 42a and an outer layer 43b, 
42b, the inner layer 43a, 42a being made from a material having 
susceptibility to water higher than that of a material from which the 
outer layer 43b, 42b is made. 
Further, the front surface sheet 43, the lenticular lens sheet 41 and the 
Fresnel lens sheet 42 are in a shape which is convex relative to the 
projection side of the screen. 
It is also possible to make these three sheets convex relative to the 
observation side of the screen although such a screen is not shown in a 
figure. 
Fifth Embodiment 
FIG. 5 is a view showing a fifth embodiment of the rear projection screen 
according to the present invention. 
The fifth embodiment is a screen set composed of two sheets, a lenticular 
lens sheet 51 and a Fresnel lens sheet 52, wherein the two sheets are 
respectively composed of three layers, an inner layer 51a, 52a, an 
intermediate layer 51c, 52c and an outer layer 51b, 52b, the intermediate 
layer 51c, 52c being provided between the inner layer 51a, 52a and the 
outer layer 51b, 52b. 
Also in this case, the inner layer 51a, 52a is made from a material having 
susceptibility to water higher than that of a material from which the 
outer layer 51b, 52b is made. 
Even in the three-layer (multi-layer) structure, if the water absorption 
increases toward the inside of the screen, moisture is absorbed and 
released through layers which are thinner than those in the case of the 
two-layer structure, so that the desired effects can be obtained rapidly. 
The above-described embodiments can be changed or modified variously 
without being limited by the above description, and such variant/modified 
embodiments are also within the equivalent of the present invention. For 
instance, only one sheet which is positioned at the outermost of a screen, 
either on the observation side or on the projection side can have a 
structure in which an inner layer is made from a material having 
susceptibility to water higher than that of a material from which an outer 
layer is made. 
EXAMPLE 
The present invention will now be explained more specifically by referring 
to the following Example. 
In this Example, a lenticular lens sheet composed of two layers, an inner 
layer and an outer layer, produced by means of coextrusion, using 
polymethyl methacrylate (PMMA) and styrene copolymer resin having a water 
absorption of 0.2% for the outer layer, and PMMA resin having a water 
absorption of 0.3% for the inner layer was combined with a Fresnel lens 
sheet made mainly from PMMA resin to obtain a rear projection screen 
according to the present invention. On the other hand, in Comparative 
Example (Conventional Example), a lenticular lens sheet whose inner and 
outer layers were made from PMMA and strene copolymer resin having a water 
absorption of 0.2% was combined with the same Fresnel lens sheet as the 
one used in Example to obtain a comparative/conventional rear projection 
screen. The rear projection screen according to the present invention and 
the comparative one thus obtained were subjected to a comparative test. 
The test was carried out in the following manner: the two screens were 
allowed to stand in a highly humid (95% RH) atmosphere for 160 hours, and 
then under the normal condition (43% RH); and a change in separation 
caused in each screen was observed (see FIG. 7). It is noted that an 
amount of separation in FIG. 7 corresponds to a distance of the gap in 
millimeters (mm) between the lenticular lens sheet and the Fresnel lens 
sheet, and that the distance of the gap is measured as a displacement of 
the surface on the observation side of the lenticular lens sheet when the 
center of the lenticular lens sheet is pushed toward the Fresnel lens 
sheet. 
As a result, it was confirmed that the rear projection screen according to 
the present invention showed only a little separation during the 
preservation under both the highly humid condition and the normal 
condition and that it was easy for the screen of the invention to maintain 
the condition desirable as a screen.