Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1988-12-07
1990-03-06
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250577, 25022731, H01J 516
Patent
active
049068458
DESCRIPTION:
BRIEF SUMMARY
Sensors incorporating light guides have been proposed in applications where information may be obtained optically from a remote position. In general, light from a source is transmitted by total internal reflection along a light guide to a position from where information is required, ad light modulated in some way to carry the information is transmitted back along the same or another light guide to a photodetector. One kind of sensor is used as an optical dip-stick. It consists of a glass or plastics rod which has a bevelled tip, or of a bevelled tip, such as a prism, at the end(s) of one of more light guides. Light transmitted down one side of the rod, or down a light guide, is reflected twice at the bevelled tip and returns up the other side of the rod, or up the other light guide, when the tip is dry. When the tip is wet and the angle of incidence is less than the critical angle, light is lost and the returning optical signal is greatly reduced. A variant of this sensor consists of a sinuous flexible optical fibre along which light is transmitted to and from an end of the sensor, light being lost from the curved portions of the optical fibre when the fibre is wetted.
Such sensors have been used in conjunction with liquid level control systems, whereby, for example, a pump relay is operated in dependence upon whether the intensity of light received by the photodetector is above or below a predetermined threshold.
The known sensors suffer from a number of problems, such as the difficulty of manufacturing the sensors with an accurately reproducible response at a precise sensing location, the need for repeated recalibration owing to fouling of the surface of the light guide by deposits or contact with foreign matter, drop retention at the sensing location leading to light loss when the sensor is otherwise dry, excessive sensitivity in that there is almost complete light loss when only a small part of the sensing location is wetted, and complete light loss when the sensing region is wetted so that the wetted state is indistinguishable from a break in the light guide.
In accordance with the present invention, an optical sensor comprises an elongate probe; a light guide extending from a light source, along the probe towards a tip of the probe and back along the probe to a photodetector, the light guide including, adjacent to the tip, a rigid element, which has a radially outer substantially cylindrical surface providing substantially total internal reflection of light passing circumferentially around the probe within the element or loss of light depending on the optical properties of a surrounding medium, and which is mounted in a substantially cylindrical portion of the probe so that the outer surface of the element is substantially flush with the outer surface of that portion of the probe.
Preferably the element is substantially annular and is closely received in a groove in the probe.
The use at the sensing location of the rigid element, the light transmitting and reflecting surfaces of which will be optically smooth, with the element recessed into the probe with only its radially outer surface exposed, overcomes many of the disadvantages of previous sensors. The element, which will normally be a plastics moulding, will be preformed with the necessary precision and with the appropriate geometry for the particular application. Droplet retention may be discouraged by coating the adjacent part of the outer surface of the probe with a hydrophobic material, such as polyurethane or silicone.
Simple assembly may be provided by mounting the element between two parts of the probe which are connected by a spigot and socket coupling, the element surrounding the spigot. Light may then be transmitted into and out of the element by optical fibres, which preferably extend down the probe, protected in their own duct or ducts in the probe, with ends of the optical fibres abutting optical coupling surfaces of the element facing axially away from the probe tip. The surface of the element facing the probe tip may then be provid
REFERENCES:
patent: 4039845 (1977-08-01), Oberhansli et al.
patent: 4468567 (1984-08-01), Sasano et al.
patent: 4713552 (1987-12-01), Denis et al.
patent: 4792699 (1988-12-01), Duncan
patent: 4816670 (1989-03-01), Kitamura
Bellhouse Brian J.
Goodman Stephen
Bellhouse Technology Limited
Nelms David C.
Shami Khaled
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