Patent GB-1355034-A (GB)
Title: Method of and apparatus for gauging inspecting or measuring physical  properties of objects
IPC: G01N21/956
Filed: 1970-02-02
Abstract: 1355034 Photo-electric apparatus SIRA INSTITUTE 28 May 1971 [31 May 1970] 4859/70 Heading G1A In an apparatus for investigating a physical property of an object, e.g. for measuring the width of a gap 20, in which an image of the object is projected on to a detecting device 4, the device 4 comprises at least two relatively displaced units 50, 51 each having a sinusoidal variation in sensitivity - see Fig. 3 - such that the device produces separate electrical signals K, P indicative of the real and imaginary parts of a Fourier transform of the intensity distribution in the image. It is shown in the Specification that by arranging the maxima of one unit to coincide with the minima (in respect of sensitivity) of the other, then the two outputs K and P correspond to the desired real and imaginary Fourier components. The outputs K and P may be applied to the X and Y plates of a CRO. If the image length equals the period of the pattern 50, 51, then the CRO spot will be undeflected. However, if the image is either larger or smaller than this mill setting the spot will be deflected a distance proportional to the modulus M of the Fourier transform, which in turn is proportional to the change in image size. Alternatively, the outputs K and P may be modulated by a carrier signal #, one sinusoidally, the other cosinusoidally. The resulting signals, K' and P', are summed and the signal (K'+ P') is demodulated and M is thereby recovered. The outputs K and P may also be modulated by two squarewaves in quadrature, before demodulation. Another optical arrangement, Fig.7 (not shown) enables the image size to be altered. Positioned between the lens 3 and detecting device 4 are a slit (7), a collimating lens (8) and a beam spreader (9), e.g. a lenticular screen. In operation, the image of the gap is first made larger than the period of the pattern so as to produce a non-zero value of M. The beam spreader 9 and detector 4 are then rotated, thereby altering the image size, Figs. 8 and 9 (not shown), until there is a null output. From the initial value of M, the angle of rotation # and the period of the pattern, the width of the aperture may be determined. The apparatus may be used to measure the diameter of a cold rod (15) Fig.10 (not shown). The optical system is first arranged so that a standard rod produces a null output. The test rod then replaces the standard and the output is indicative of the rod diameter. A hot rod, which does not need an illuminating system, may be measured in a similar way, Fig.11 (not shown). Detecting devices. In general, the detecting device 4 is in the form of an array of photoelectric elements. In one form, the device is constructed on a single silicon chip divided into its different areas by photo-etching, and each area is accurately delineated by aluminium masks. The sinusoidal variation may also be achieved using photographic masks or moirÚ gratings placed over a plane array of photo-cells. Alternatively a mask 260 having the shape shown in Fig. 3 may be placed over a photo-cell array. Fibres or other light guides may convey light from the mask apertures to the photo-cells. Each sinusoidal unit 50, 51 may have associated therewith an elongated, uniformly sensitive array 60, Fig. 3, to provide a reference output. In this case the output of each sinusoidal unit is subtracted from that of its corresponding reference array to provide the output K (or P) - Fig.4 (not shown). Each of the units 50, 51 and reference arrays 61, 62 may be of the "charge storage" type, Fig. 6. To produce the output K, units 50, 61 are charged simultaneously to an upper reference level from a source 70. These units then discharge at rates proportional to the amounts of light falling thereon. When the voltage on either one reaches a lower reference level, sensor 71 is activated and the difference between the voltages produced by the units 50, 61 is sampled and held at 72. Both units are then re-charged to resume the cycle and update the value of K held at 72. Applications. The apparatus may also be used for measuring the optical transfer function of a lens, measuring the quality of a cathode ray tube spot or the output of an image intensifier. Defects in labels may be detected by determining differences in the Fourier transform of the label compared with that of a perfect label. The two labels may be imaged on the detecting device by means of optical systems which scan the images across the device 4. In rod gauging applications, one device 4 may be used for each edge of the rod, and the light from the source 10 may be chopped.