Self-luminescent light source for liquid crystal display watch

A self-luminescent light source for a liquid crystal display watch characterized by comprising a fluorescent substance layer which has a light reflectivity and which contains radioactive element promethium (.sup.147 Pm) in at least a part thereof, a reflective layer which is disposed on one surface of the fluorescent substance layer and which serves to reflect and scatter fluorescence from the fluorescent substance layer, a case which is disposed on the side of the reflective layer remote from the fluorescent substance layer and which serves to perform shielding from Bremsstrahlung ascribable to beta rays given out from the promethium, a first cover which is disposed on the side of the fluorescent substance layer remote from the reflective layer and which serves to absorb the beta rays given out from the promethium and to transmit the light from the fluorescent substance layer, and a second cover which is disposed on the side of the first cover remote from the fluorescent substance layer and which serves to perform shielding from Bremsstrahlung ascribable to the beta rays given out from the promethium and to transmit the light from the fluorescent substance layer.

BACKGROUND OF THE INVENTION 
This invention relates to a self-luminescent light source which provides a 
night display of a liquid crystal display watch. 
A field effect type liquid crystal display element displays a character 
pattern by reflecting light from the outside with a reflector plate. 
Accordingly, the display function is lost in the night when it is dark 
outside. 
There has been proposed and already known a liquid crystal display watch 
wherein, in order to overcome such a disadvantage, a plurality of 
self-luminescence type tubes containing tritium (.sup.3 H) gas and having 
inner walls coated with a fluorescent material are arrayed underneath the 
liquid crystal display element, thereby to permit the display in the 
nighttime. 
However, the technique of enclosing the tritium gas into a tube is very 
difficult (refer to U.S. Pat. No. 3,817,733). Even when the gas is 
effectively enclosed and the tube is sealed, the gas leaks in due time and 
is feared to incur radioactive contamination. In addition, since several 
tubes are arrayed, the brightness at the boundary area between the 
adjacent tubes becomes low, and the light source exhibits a nonuniform 
brightness as a whole. A transmission type light scattering member may be 
placed on the tubes in order to avoid the drawback, but this inevitably 
renders the self-luminescent light source thick. 
To the end of overcoming such disadvantages, it is considered that 
promethium (.sup.147 Pm) with which the manufacturing stage of work is 
comparatively easy and which is not feared to leak because it is solid is 
applied as the self-luminescent fluorescent material. However, .sup.147 Pm 
gives out high-energy beta rays of 220 KeV. When a substance adapted to 
absorb the beta rays is interposed, bremsstrahlung is generated, and 
danger is sometimes involved in case of use in close contact with the 
human body as in a liquid crystal display wrist watch. 
SUMMARY OF THE INVENTION 
It is accordingly an object of this invention to provide a self-luminescent 
light source for a liquid crystal display watch which is improved in 
relation to the disadvantage of .sup.147 Pm as stated above and is safe 
and which has a uniform brightness. 
To the accomplishment of the above object, a self-luminescent light source 
for a liquid crystal display watch according to this invention is 
characterized by comprising a fluorescent substance layer which has a 
light reflectivity and which contains radioactive element promethium 
(.sup.147 Pm) in at least a part thereof, a reflective layer which is 
disposed on one surface of the fluorescent substance layer and which 
serves to reflect and scatter fluorescence from the fluorescent substance 
layer, a case which is disposed on the side of the reflective layer remote 
from the fluorescent substance layer and which serves to perform shielding 
from bremsstrahlung ascribable to beta rays given out from the promethium, 
a first cover which is disposed on the side of the fluorescent substance 
layer remote from the reflective layer and which serves to absorb the beta 
rays given out from the promethium and to transmit the light from the 
fluorescent substance layer, and a second cover which is disposed on the 
side of the first cover remote from the fluorescent substance layer and 
which serves to perform shielding from bremsstrahlung ascribable to the 
beta rays given out from the promethium and to transmit the light from the 
fluorescent substance layer.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a section of a display element for a liquid crystal display 
watch which is constructed by employing a self-luminescent light source 
according to this invention. Referring to the figure, numeral 1 designates 
the entire display element for a liquid crystal display watch, and numeral 
2 an upper polarization plate. Numeral 3 designates an upper glass plate, 
the lower surface of which is vapor-coated with transparent conductive 
electrodes corresponding to numerals and letters to-be-displayed. Shown at 
4 is a layer of a nematic liquid crystal material. Numeral 5 indicates a 
lower glass plate, the upper surface of which is provided with transparent 
conductive electrodes corresponding to those disposed on the upper glass 
plate 3. Numeral 6 indicates a lower polarization plate. The upper 
polarization plate 2 and the lower polarization plate 6 are arranged so 
that their polarizing directions may intersect orthogonally to each other. 
Shown at 7 is the self-luminescent light source according to this 
invention, the detailed view of which is given to FIG. 2. Referring to 
FIG. 2, numeral 8 denotes a flap plate of a self-luminescent fluorescent 
substance such as ZnS:Zn, ZnS:Cu or ZnS:Ag, which employs .sup.147 Pm as a 
radiation source and which is usually fabricated by an 
application-printing process. Numeral 9 denotes a white reflective 
material which serves to reflect and scatter fluorescence emitted by beta 
rays and for which barium sulfate (BaSO.sub.4) is recommended. The reasons 
for the recommendation are that BaSO.sub.4 exhibits a high light 
reflectivity in a range of from the visible to ultraviolet regions and has 
an irregular reflection characteristic that is nearly ideal. Also, 
BaSO.sub.4 is a stable substance which is difficult to change in quality 
even when it is held in close contact with a fluorescent substance, etc. 
Numeral 10 denotes a transparent high-polymer thin film such as polyvinyl 
film, which serves to absorb the beta rays. Reference numeral 11 
designates a metallic case which is a base plate for the reflective 
material 9 and which also functions to perform shielding from 
bremsstrahlung produced when the beta rays are absorbed by a substance 
(the absorption takes place in the portions of the flat plate of the 
self-luminescent fluorescent substance 8, the reflective layer 9, the 
transparent high-polymer thin film 10, etc.). For the metallic case 11, an 
aluminum sheet is recommended. Numeral 12 indicates a transparent glass 
plate which absorbs the bremsstrahlung and which also serves for causing 
the fluorescent substance plate 8 to function as a reflector. Desirable as 
the glass plate 12 is lead glass whose Pb-content is especially increased 
(e.g. from 0.5 to 20% by weight) so that the bremsstrahlung may be 
sufficiently absorbed even when the glass plate is thin. An acrylic resin 
such as polyacrylonitrile containing Pb is conveniently used because it 
absorbs the bremsstrahlung simultaneously with the beta-ray absorption. At 
the present time, however, the technique of raising the Pb concentration 
is not developed for the acrylic resin to the degree obtained by the lead 
glass. In the future, this technique will naturally be further developed. 
FIG. 3A is a plan view of the flat plate of the self-luminescent 
fluorescent substance 8 shown in FIG. 2. Numeral 14 designates a surface 
which is coated with a fluorescent material ZnS:Cu), while numeral 13 
indicates a surface which is coated with the fluorescent material (ZnS:Cu) 
mixed with promethium oxide (Pm.sub.2 O.sub.3). As apparent from the upper 
glass plate 3 (FIG. 3B) shown in parallel with FIG. 3A, the promethium 
oxide is applied to only a region which corresponds to the time indicating 
portion of the liquid crystal display watch ("46 minutes past 12 o'clock" 
is indicated in the example shown in FIG. 3B). 
The self-luminescent light source according to this invention is 
constructed as heretofore described. Therefore, the self-luminescent 
fluorescent substance plate does not play the role of a light source in 
the daytime and functions as a mere light reflector in which the outside 
is bright, and it functions as a plane light source in the nighttime. 
Since the plate of the .sup.147 Pm self-luminescent fluorescent substance 
is constructed to be flat, the non-uniformity in brightness as in the case 
of using the arrayal of the .sup.3 H-filled self-luminescent fluorescent 
tubes does not appear. Needless to say, any additional element such as the 
transmission type light scattering member for eliminating the 
nonuniformity of the brightness is not required at all. In consequence, 
the thickness of the whole light source becomes small, and the 
self-luminescent fluorescent substance plate becomes suitable for a wrist 
watch. In the embodiment of FIG. 2, self-luminescent light source 7 is 
about 0.9 mm in thickness. And flat plate 8 is about 0.2 mm, reflective 
layer 9 is about 0.1 mm, transparent high-polymer thin film 10 is about 
0.5 mm, metallic case 11 is about 0.2 mm and glass plate 12 is about 0.35 
mm in thickness. Owing to the shape of the flat plate, the 
application-printing is permitted, which is appropriate for mass 
production. Moreover, .sup.147 Pm is mixed with the particles of the 
fluorescent material (ZnS:Cu) in the form of the solid oxide (Pm.sub.2 
O.sub.3). Therefore, even when the self-luminescent fluorescent substance 
plate is damaged, the radioactive element .sup.147 Pm does not diffuse to 
the exterior and cause radioactive contamination as does the .sup.3 H gas. 
Further, the bremsstrahlung from the high-polymer film, the fluorescent 
material and the while reflector which absorb the beta rays of high energy 
are absorbed by the lead glass and the metallic case, and the X-ray 
emission to the exterior is prevented. Therefore, even when the 
self-luminescent light source is used for an appliance to be held in close 
contact with the human body, such as wrist watch, no danger is involved. 
Yet further, even when the self-luminescent light source is transported as 
the single component or assembled into a wrist watch, not only the beta 
rays but also the bremsstrahlung are intercepted by the single component 
itself, and hence, the light source can be safely handled even by an 
assembly manufacturer. 
Moreover, the self-luminescent fluorescent substance is not applied over 
the entire surface of the flat plate of the self-luminescent fluorescent 
substance 8, but it is applied to only the numeral indicating portion 13 
representative of "hour" or "hour and minute" as shown in FIGS. 3A and 3B. 
The remaining portion 14 for indicating, for example, "date", "a.m. or 
p.m." and "second" is coated with the ordinary fluorescent material which 
is of the same substance as that of the time indicating portion 13 and 
which does not contain .sup.147 Pm. With such a construction, even when a 
high concentration of .sup.147 Pm (its radioactive intensity is about 500 
u Ci.) is put into the time indicating portion 13, the radioactive 
intensity of the entire self-luminescent fluorescent substance flat plate 
8 is suppressed within a prescribed value, and besides, the brightness of 
the time indicating portion 13 is enhanced to enhance the constrast of the 
time display. Since the time indicating portion 13 and the remaining 
portion 14 are coated with the same reflective material, they form a 
reflector plate of a reflection factor being uniform over the entire area 
in the daytime, so that a high display quality is attained in both the 
nighttime and the daytime.