Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1989-03-24
1991-02-26
Westin, Edward P.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
356364, 250239, G01J 902
Patent
active
049964160
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to an optical detection system for providing information from a light beam, for use with optical systems, for example, laser interferometers.
In such optical detection systems an array of photo-detectors, usually photo-diode detectors, are disposed in an enclosure having an aperture in one of its walls, and are positioned in the path of a light beam entering the aperture, the intensity of which is to be determined. The detectors receive light from the beam incident upon the light sensitive surfaces thereof and produce electrical signals in dependence upon the intensity of the light incident thereon.
A problem associated with such optical detection systems is that in addition to the light from the optical system, the intensity of which is to be measured, ambient light also enters the aperture, so that the electrical signals produced by the detectors are stronger than would otherwise be the case. If the ambient light falling on each detector is at a constant intensity during any given measurement operation, or if all of the detectors in the array are exposed to the same ambient light intensity level, allowances can be made in the electrical signals produced by the detectors for the proportion of the signal which results from the ambient light falling on the detectors. However, this is not normally the case, so that a problem exists as to how to deal with the ambient light content in the signal from the detectors.
SUMMARY OF THE INVENTION
The invention as claimed in the appended claims reduces this problems in a simple and inexpensive manner by ensuring that all of the detectors have the same acceptance cone angle for light passing through the aperture. This can be accomplished by providing an arrangement wherein the optical path length between the aperture and each detector is the same. In this manner, the ratio of signal to ambient light in the portion of the light beam falling on each detector will be the same.
The effect of having all of the detectors viewing the same aperture from the same distance without interference is preferably achieved in accordance with the invention, by using beam splitters to deflect one or more portions of the beam entering the aperture from its path and towards one or more detectors which are offset from the axis.
Where the light beam entering the enclosure is a combined beam having two mutually orthogonally polarised components, a preferred feature of the invention is the provision in the path of the light beam near the aperture of a quarter-wave plate to convert the two orthogonally polarised components of the beam into interfering circularly polarised components.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be more particularly described by way of example only, with reference to the accompanying drawings in which;
FIG. 1 is a simple diagrammatic representation of the arrangement of the optical components of the present invention inside an enclosure, and,
FIG. 2 is a circuit diagram showing part of the electrical circuit for further processing the signals from the photo-diode detectors of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, three photo-diode detectors D1, D2 and D3 are positioned in an enclosure 10 having an aperture 12 through which a signal-carrying light beam 14 is directed. The light beam may be derived from any source e.g reflected illumination from a scale, or, as in the case of the present example, the light beam is a laser beam from a laser interferometer.
It is clear that in the absence of any additional shielding of the detector enclosure, the aperture 12 will also accept ambient light which will affect the intensity of the light at the detectors. In order to minimise the problems caused by the ambient light, two beam splitters 20 and 22 are positioned on the axis of the incoming laser beam. The first beam splitter is positioned at a distance L1 from the aperture 12, and the second beam splitter 22 is positioned at a distance L2 from the bea
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Beck George C.
Renishaw plc
Westin Edward P.
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