GB1327065A - Method and device for photoelectrically scanning an object by means of an optical beam - Google Patents

Abstract

1327065 Photo-electric scanning apparatus EXATEST MESSTECHNIK GmbH 24 Nov 1970 [27 Nov 1969 2 April 1970] 55816/70 Heading G1A Photo-electric scanning apparatus for determining the position of an edge defined by a transition from non-illumination to illumination at an unknown level utilises the principle of scanning the edge with an auxiliary beam prior to the main scanning beam in order to generate a signal representative of the full unknown illumination which signal is stored and providing a pulse indicative of the edge position as determined by the main scanning beam at the instant that the detector output attains a particular fraction of the stored auxiliary signal above this stored value. The edge may be that of a radiating body e.g. a glowing wire, or of an object illuminated from behind. Thus, when the position of the other edge represented by a transition from illumination to non-illumination is also determined, the width of the body may be measured. In Fig.1, an object 10, e.g. sheet metal, is illuminated by source 12, and its edges imaged by lens 14 in the plane of detector 16. Rotating polygonal mirror 20 scans reflected radiation across slits 18, 19 which define the auxiliary and main beams X, Y respectively. The output of detector 16 is thus a combination of the light intensities reaching it via apertures 18, 19. During scanning of edge 10", the auxiliary beam scans the edge before the main beam and the output waveform of the detector includes a step portion A defined only by the effect of the main beam. When the output has reached half this value as detected in circuit 24, a timing pulse indicative of edge 10" is generated. During scanning of edge 10', the auxiliary beam scans the edge first and provides in step B in the detector output waveform a value of the illumination representative of that outside the edge 10', so as to provide a reference against which the output due to the main beam may be compared. Thus when the detector output reaches 1-5 times the step B, the other edge timing pulse is produced. The circuit 24 includes a differentiator 30, inverter 32, pulse weight discriminator circuit 34, delay circuit 36 and sample-hold circuit 38. Circuit 38 is present essentially to hold and pass on the levels A and B of Fig.2, towards coincidence detectors 46, 48, after amplification at 40. The actual signals fed to 46 and 48 are 0-5A (provided via dividing circuit 42) and the detector output E to device 46 (this being to detect when the detector output reaches the half value of A); and 2/3E (provided by passing detector output through divider circuit 44) and B (this being to detect when the detector output reaches 1À5 times the step B) to device 48. If desired, timing pulses for the main beam itself as provided by the arrangement of Fig.3; need not be used a less accurate measure of width would be determined by use of the time between steps A and B.