Luminescent screen

A luminescent screen includes a screen plate containing a substance capable of emitting luminescent light in response to stimulating light applied thereto, and a regular array of convexes on one surface of the screen plate for trapping emitted luminescent light that is directed into the screen plate and for directing the trapped luminescent light toward a display surface of the screen plate. The brightness of the display surface is therefore increased.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a luminescent screen, and more 
particularly to a luminescent screen having a screen plate containing a 
substance capable of emitting light in response to stimulating light 
applied thereto. 
2. Description of the Prior Art 
There are known display systems having a luminescent screen comprising a 
screen plate which contains a substance (a luminescent substance) capable 
of fluorescence or phosphorescence in response to stimulating light such 
as ultraviolet radiation, visible light, near-infrared radiation, or the 
like, the fluorescent substance comprising a piperidinium tetra 
(benzoyltrifluoroacetone) europium complex, for example. The stimulating 
light is applied to the screen plate to enable the screen plate to display 
an image. 
Heretofore, the screen plate has generally been in the form of a flat 
plastic plate. As shown in FIG. 1 of the accompanying drawings, when 
stimulating light is applied to a plastic screen plate 20, luminescent 
light is emitted from a luminescent substance (indicated by a star-shaped 
symbol) and scattered in every direction as indicated by the broken lines. 
More specifically, the emitted luminescent light is directed toward 
display and reverse surfaces of the screen, and also propagated in the 
screen plate 20. Therefore, The efficiency with which the applied 
stimulating light is utilized is low, and the intensity of luminescent 
light emitted toward the display side of the screen plate is so low that 
the screen as viewed by the viewer is relatively dark. 
SUMMARY OF THE INVENTION 
In view of the aforesaid drawbacks of the conventional luminescent screen, 
it is an object of the present invention to provide a luminescent screen 
which emits luminescent light of an increased intensity toward a display 
side for a greater degree of screen brightness in response to a reference 
intensity of stimulating light applied to the screen. 
According to the present invention, a luminescent screen comprises a screen 
plate containing a substance capable of emitting luminescent light in 
response to stimulating light applied thereto, and a regular array of 
convexes on one surface of the screen plate. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description when 
taken in conjunction with the accompanying drawings in which a preferred 
embodiment of the present invention is shown by way of illustrative 
example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 2 shows a luminescent screen according to an embodiment of the present 
invention. The luminescent screen includes a screen plate 10 made of 
plastics. Stimulating light (indicated by the solid line) such as 
ultraviolet radiation or the like is applied as a spot to the screen while 
the spot is being two-dimensionally scanned over the screen. A luminescent 
substance indicated by the star-shaped symbol is responsive to the applied 
stimulating light for emitting luminescent light as indicated by the 
broken lines. The size of a pixel is governed by the size of the spot 
applied to the screen. 
The luminescent screen shown in FIG. 2 is constructed so as to be 
incorporated in a front projector. The screen includes a regular array of 
convexes such as conical projections 11 disposed on the reverse surface of 
the screen plate 10 which is opposite to the surface thereof to which the 
stimulating light is applied. Preferably, the conical projections 11 
comprise quadrangular pyramids, triangular pyramids, circular cones, or 
the like. The conical projections 11 are spaced at a pitch p which should 
be close to the pixel pitch. The distance d between the display surface 
and bottom surfaces between the conical projections 11 should be as small 
as possible to minimize the propagation of luminescent light into the 
screen plate 10. A zone of the screen plate where luminescent light is 
emitted in response to applied stimulating light is in the vicinity of the 
bottom surfaces between the conical projections 11. Therefore, if the 
distance d was larger, the emitted luminescent light would not easily 
propagate into the screen plate 10, and the luminescent light directed 
toward the conical projections 11 would be reduced. For this reason, the 
distance d should be reduced as much as possible. 
The apical angle 2.alpha. of each of the conical projections 11 is selected 
to meet the following equation: 
EQU sin.alpha.=1/refractive index of the screen plate 10 
Therefore, the apical angle 2.alpha. is selected to be below the critical 
angle of the screen plate 10. For example, if the screen plate 10 is made 
of an acrylic resin, then the angle .alpha. selected to be about 
42.degree.. 
With the apical angle of each of the conical projections 11 being thus 
selected, luminescent light emitted in response to applied stimulating 
light and directed in a direction to pass through the screen plate 10 and 
in directions to be propagated into the screen plate 10 is trapped in and 
reflected by the conical projections 11. Therefore, the intensity of 
luminescent light directed toward the display surface of the screen plate 
10 is increased, and hence the screen brightness is increased. 
The conical projections 11 may be disposed on the display surface (the 
surface to which stimulating light is applied if the screen plate is 
incorporated in a front projector or the surface through which stimulating 
light passes if the screen plate is incorporated in a rear projector) of 
the screen plate 10, or on the reverse surface opposite to the display 
surface. However, it is more advantageous if the conical projections 11 
are disposed on the reverse surface of the screen plate 10. 
The reason for the above positioning of the conical projections 11 is as 
follows: According to the present invention, the angle of total reflection 
(i.e., the critical angle) between the screen plate 10 and air is employed 
to reflect back luminescent light. If stimulating light is applied to the 
screen plate from the flat surface thereof in a front projector, then 
emitted luminescent light that is directed away from the flat surface is 
reflected back by the conical projections, so that luminescent light of a 
large intensity is collected onto the flat surface of the screen plate. In 
a rear projector, luminescent light of a large intensity is also collected 
onto the flat surface of the screen plate. Since the flat surface of the 
screen plate becomes bright in each of the front and rear projectors, the 
conical projections 11 should be positioned on the surface opposite to the 
display surface of the screen plate. 
With the present invention, the luminescent screen includes a regular array 
of convexes on one surface of the screen plate such that luminescent light 
emitted in response to applied stimulating light and directed to pass 
through and into the screen plate is reflected by the conical projections 
back toward the display surface of the screen plate. Accordingly, the 
intensity of luminescent light emitted from the display surface of the 
screen is increased. The screen brightness with respect to a reference 
intensity level of stimulating light is increased to a level which is 
about 2.5 times the conventional screen brightness level. 
Although a certain preferred embodiment has been shown and described, it 
should be understood that many changes and modifications may be made 
therein without departing from the scope of the appended claims.