Electronic instrument

An electronic timepiece provides photo-electric cells disposed behind a passive electro-optic display cell for recharging a storage battery. The timepiece includes in its display a polarizer, a passive electro-optic device such as a liquid crystal, a quarter-wave plate, a cholesteric liquid crystal reflector and photo-electric cell arranged in superposition upon each other. Ambient light passing through the cholesteric reflector is used to energize the photo-electric cells which recharge the storage battery. Ambient light reflected by the cholesteric reflector is used to indicate the displayed information.

BACKGROUND OF THE INVENTION 
The present invention relates to an electronic instrument, especially a 
timepiece, with at least one storage battery, comprising photo-electric 
cells ensuring the recharging of the storage battery, and an electro-optic 
passive display cell, provided with a polarizer and a diffuser constituted 
by a cholesteric liquid crystal cell. 
Electronic watches comprising photo-electric cells for recharging of their 
storage battery and in which the display is afforded by a liquid crystal 
cell are known. In the watches of this type, however, the display cell and 
the photo-electric cells for recharging the storage battery or batteries 
are juxtaposed, which presents some drawbacks: As a matter of fact, these 
watches are not only not aesthetic, due to the unpleasant appearance of 
the photo-electric cells, but they cannot comprise a display cell of more 
than 3 to 5 digits, due to the large surface area required for the 
photo-electric cells. But the display and photo-electric cells, all have 
to be situated on the front face of the watch since they have either to be 
easy to be observed or to be able to be reached by the surrounding light. 
The purpose of the present invention is to furnish a watch of the above 
mentioned type, the diffusing effect of the liquid crystal cell which will 
be ensured by a cholesteric liquid crystal cell, and in which one does not 
find these drawbacks. 
SUMMARY OF THE INVENTION 
To this effect, the electronic instrument according to the invention is 
characterized by the fact that its photo-electric cells are situated 
behind the cholesteric liquid crystal cell, so that the light which is not 
reflected by the diffuser activates the photo-electric cells.

DESCRIPTION OF THE INVENTION 
The display cell represented comprises two glass plates 1 and 2 between 
which is interposed a coating of nematic liquid crystal 3. In front of the 
cell is arranged a polarizer 4 and, at the rear, a quarter-wave plate 5 
which replaces the second polarizer conventionally used in electro-optic 
display cells having liquid crystals. 
A second cell, constituted of two glass plates 6 and 7 between which is 
interposed a coating 8 of cholesteric liquid crystal is placed behind the 
quarter-wave plate 5. 
At last, a whole array of photo-electric cells 9 is placed behind the glass 
plate 7 of the second cell. 
The device as disclosed and represented operates as follows: 
The light entering into the first cell is polarized vertically by the 
polarizer 4, this cell being suitably oriented. In the areas where no 
electric field is applied to the liquid crystal 3, its molecules are 
oriented helically so that the plane of polarization of the light rotates 
90.degree.. Hence, in this case, the light going out of the liquid crystal 
3 is polarized horizontally. After it passes through the quarter-wave 
plate 5, it is polarized circularly, the vector of rotation rotating to 
the left, with the quarter-wave plate being suitably oriented. The 
cholesteric liquid crystal 8 has a property of reflecting circularly 
polarized light having the same sense of rotation as the cholesteric 
liquid crystal for a band of wave lengths defined by its own 
characterictics. Hence, the circularly polarized light having a rotation 
to the left is partially reflected by the cholesteric liquid crystal 8. 
The reflected portion of light passes again through the quarter-wave plate 
5, but in the reverse sense, polarized linearly in a horizontal plane. The 
light then passes through the coating 3 of the nematic liquid crystal 
where the plane of polarisation rotates 90.degree.. The vertically 
polarized light then passes through the polarizer 4. The areas where no 
field is applied to the liquid crystal 3 will appear then to the user as 
being light, of the color corresponding to the band of wave lengths 
reflected by the cholesteric liquid crystal. The portion of the light 
whose wave length is outside the band of reflection passes through the 
cholesteric liquid crystal and activates the photo-electric cells 9. 
In the areas where an electric field is applied to the terminals of the 
nematic liquid crystal 3, its molecules do not maintain their helical 
structure but move in the sense of the field and orient themselves 
perpendicularly to the plane of the cell. Consequently, the polarization 
of the light is not affected and the light leaves the coating 3 of the 
liquid crystal polarized vertically the same as it entered therein. The 
quarter-wave plate 5 polarizes the light circularly, but, this time, the 
plane of polarization rotates to the right, so that almost all the light 
passes through the cholesteric liquid crystal and reaches the 
photo-electric cells 9. Hence in the areas where an electric field is 
applied, the display cell appears dark. 
Thus, the part of the display cell where there is no electric field applied 
to the nematic liquid crystal appears light to the observer and activates 
only weakly the photo-electric cells, while, where an electric field is 
applied to the nematic crystal, almost all of the light reaches the 
photo-electric cells which serves to recharge the storage battery. The 
efficiency of the photo-electric cells is consequently relatively poor 
when one displays dark characters on a light background. 
On the other hand, the efficiency is better with the reverse arrangement 
where the display is effected in light characters on a dark background. 
Then, across the whole surface of the background, with the exception of 
the activated areas, the light reaches the photo-electric cells and is 
used for recharging the storage battery or batteries. One realises such a 
cell by rotating, for instance, the polarizers or the quarter-wave plate 
90.degree., or by choosing a cholesteric liquid crystal the rotation of 
polarization of which occurs in the reverse sense. 
One must however note that the density of light activating the 
photo-electric cells remains substantially lower than the density which is 
obtained with cells arranged at the side of the display cell. This 
drawback is widely compensated for by the fact that one uses a greater 
surface for the photo-electric cells. 
It is to be noted that the invention could be applied not only to an 
electronic timepiece, but to other electronic instruments, for instance 
computers.