Optics: measuring and testing – By particle light scattering – With photocell detection
Patent
1994-10-14
1996-11-26
Turner, Samuel A.
Optics: measuring and testing
By particle light scattering
With photocell detection
356363, G01B 902
Patent
active
055791085
DESCRIPTION:
BRIEF SUMMARY
The present application relates to the detection of the angle of a beam of light.
Systems for detecting the angle of a beam of light are used, for example, in devices for detecting the alignment of two parts of an apparatus or for detecting the flatness of an optical table or other table by measuring the relative alignment between different portions of the table. Similarly, they may be used for measuring the flatness of an optical slideway or of a machine bed.
In such systems, a beam of light is shone onto the surface the alignment of which is to be detected, and the angle of the reflected beam is detected. The angle of the reflected beam will provide a measure of the angle of the surface from which it is reflected. In practice, the beam is normally reflected not from the surface itself but from a mirror placed on the surface, in order to ensure a high quality specular reflection.
In a known system for measuring the angle of a light beam, a parallel-sided light beam is directed to an object and the reflected return beam is captured by a lens. The angle of the reflected return beam is determined by placing a multi-element photodetector spaced from the lens by the focal length thereof. The reflected return beam will be parallel-sided, and as is well known, a parallel-sided beam is focussed by a lens to a spot at a position one focal length from the lens, the position of the spot in the focal plane being determined by the angle of incidence of the parallel-sided beam. Accordingly, this arrangement creates a spot of light on the multi-element photodetector at a position corresponding to the angle of the reflected return beam. The position of the spot can be detected from the output of the photodetector.
While the arrangement described above is highly accurate in theory, it is in practice limited by the quality of the optical elements used, and also by the characteristics of the multi-element photodetector. If the gemoetry is set up in a theoretically perfect arrangement, the entirety of the light from the reflected return beam is focussed on to a single element. The resolution with which the system can detect the angle of the light beam will depend on the pitch of the elements in the detector. In order to provide a good quality output signal, the elements must be able to withstand the light power of the fully focussed beam, and preferably will not saturate at this light intensity. In practice, the apparatus may be set up so that the light beam is not focussed precisely to a spot on the detector, so that the illuminated area covers more than one element. The position of the brightest part of the illuminated area can be detected by comparing the intensities of the signal outputs from all of the illuminated elements. A drawback to this arrangment is that the calculation is very dependent on the quality of the photodetector elements. This system is adversely affected by the failure of any one of the photodetector elements. GB-A-2129245 proposes an arrangement in which a mask with slits allows narrow lines of radiation to fall on an array of rows of photo-sensitive detector elements. The rows extend transverse to the direction of the lines of radiation. Each element is as long as the distance between the slits of the mask and shaped so that for half its length it has a constant width and for the other half it has an exponentially decreasing width. Alternate rows are arranged to face in opposite directions, and the ratio of signal strength from rows facing one way to signal strength from rows facing the other way provides a measure of the position along the respective detector elements of respective lines of radiation. Since this position will change with the angle of incidence of the radiation, this ratio of signal strengths is also a measure of the angle of incidence.
According to an aspect of the present invention there is provided an arrangement for detecting the angle of a light beam in which a repeating waveform pattern is created on a photodetector array, such that a feature of the waveform, for example its s
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patent: 5046843 (1991-09-01), Keren
Merlino Amanda
Rank Taylor Hobson Limited
Turner Samuel A.
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