Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1984-10-03
1986-12-16
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250229, 340365P, H01J 516
Patent
active
046298843
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to electromagnetic radiation circuit elements generally of the touch-sensitive key tape.
BACKGROUND ART
British Pat. No. 1,600,556 relates to a touch-sensitive key having a radiation-propagating unit, such as an optical prism, arranged between a radiation source, such as a light-emitting diode, and a radiation detector such as a phototransistor. When an outer surface of the radiation-propagating unit is touched, the conditions for total internal reflection in this surface are changed, resulting in an identifiable change in the radiation from the radiation source which reaches the radiation detector. Incident external radiation does not reach the radiation detector because it is always internally reflected within the said unit so as to leave through a surface remote from the radiation detector.
This arrangement is generally satisfactory. However, practical problems can arise if the key is used with an optical system under water. The refractive index inside the radiation-propagating unit has to be above a certain critical value to ensure ray separation between detector rays (i.e. rays from the radiation source to the radiation detector) and external rays. This critical value is 1/.sqroot.2 where the medium outside the prism is air and about 1.66 where the medium is water. However, a refractive index of the order of 1.7 cannot be readily achieved with inexpensive materials such as ordinary glass or plexiglass, and expensive heavy optical glass may be necessary.
DISCLOSURE OF THE INVENTION
One object of the present invention is to overcome this problem.
According to one aspect of the present invention therefore there is provided an electromagnetic radiation circuit element comprising a radiation propagating unit between a radiation source and a radiation detector, said unit being adapted to produce an identifiable change in the radiation which reaches the detector from the radiation source when a surface of the unit is touched, characterised in that external radiation entering said unit through said surface can leave said unit at a side surface thereof but in a manner substantially insufficient to effect actuation of said detector thereby.
According to a second aspect of the present invention there is provided an electromagnetic radiation circuit element comprising a radiation-propagating unit between a radiation source and a radiation detector, said unit being adapted to produce an identifiable change in the radiation which reaches the detector from the radiation source when a surface of the unit is touched, characterised in that external radiation entering said unit through said surface is permitted to leave said unit at a side surface thereof but at a grazing angle which is substantially insufficient to enable said radiation to reach said detector.
Considering the case where the said surface is at right angles to opposed side surfaces through which the detector ray is arranged to pass between the radiation source and the radiation detector when the said surface is touched, the limiting case for the largest angle of an emerging ray at one said side surface derived from external radiation occurs when said external radiation is parallel to the said touching surface. If the external medium which interfaces with the touching surface has a refractive index n.sub.o and the said propagating unit has a refractive index n, then the largest angle .phi. which an ambient ray in the unit can have with the side surface is given by (cf Snell's Law):
Refracting this ray in the side surface, it will make an angle .alpha. with this surface, where ##EQU1##
This means that at some distance from the side surface and outside certain sectors from this surface there will be no external rays (provided that the refracted rays can be prevented from reaching the detector by some kind of scattering in the neighbourhood of the detector). Inserting values, it will be seen that for a, say, 4 mm thick plate the distance between surface and detector need only be about 2 mm to ensure that n
REFERENCES:
patent: 3621268 (1971-11-01), Friedrich
patent: 4180702 (1979-12-01), Sick et al.
patent: 4254333 (1981-03-01), Bergstrom
patent: 4340813 (1982-07-01), Sauer
patent: 4346376 (1982-08-01), Mallos
Gatto James G.
Nelms David C.
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