Patent GB-1461700-A (GB)
Title: Method and apparatus for measuring the length of a hole in  a substrate
IPC: G01B11/12
Filed: 1974-02-12
Abstract: 1461700 Measuring dimensions photoelectrically EASTMAN KODAK CO 12 Feb 1974 06246/74 Heading G1A The dimensions of a hole in a substrate are measured by illuminating the hole by a beam or beams of radiation at two different angles and measuring the intensity of radiation emerging. The invention is applicable to various shapes of holes and beam cross-sections, using x-rays or visible light radiation, the holes being in a variety of substrates e.g. sheet metal, circuit boards &c. but is described with reference to the measurement of capillaries in a spinneret for extrusion of synthetic fibres. In a first arrangement, Fig. 6, the spinneret 609 is mounted in a holder 608 which is angularly adjustable, so that two sequential measurements may be made of the amount of light from laser 601 passing through each capillary. A photomultiplier 613 is used to detect the light via a diffuser 610 and fibre optic guide 612. A digital display is provided at 615. From the calculated value of the ratio I # /I 0  where I 0  is the intensity detected for a beam parallel to the capillary axis and I #  is that for an angled beam, a value of <SP>L</SP>/ D  (the ratio of the length to the diameter of the hole) is read off a calibration curve, Fig. 5 (not shown). The diameter D is calculated by its direct dependence (D = K#I 0 , where K is a constant) and, thus L also may be obtained. The calibration curves are of an expression Fig. 4 (not shown), the derivation of which is given, relating I # /I 0  and <SP>L</SP>/ D , and including 0 the angle of the beam to the axis in the second measurement. The arrangement of Fig. 7 is designed to provide I 0  and I #  measurements simultaneously so that use of an analog computer provides an immediate value for L and D. A laser beam is split by semireflector 703 to provide a reference beam 704 against which measurement beams are compared to cancel out source fluctuations. Beam-splitting prism 709 provides two beams which are incident at differing angles on the spinneret 719, being reflected by mirrors 714, 715 which are rotated so that each capillary is scanned a number of times as the spinneret is rotated in its own plane to present the capillaries in sequence. The detector circuitry includes peak detectors so that only the maximum signal obtained from the sequence of scans is used in the calculation circuit. One channel of the analog computer obtains D and the other obtains L by combination either with a nominal diameter value or with the accurately measured value D. Separate digital display means for D and L are provided and the values are recorded. The circuitry is detailed in Figs. 10 and 13 (not shown). A futher automatic arrangement, Fig. 14 (not shown), utilizes two (He - Ne) lasers and two reference beams for added accuracy. Detectors are silicon photo-voltaic devices instead of photo-multipliers. Circuitry is detailed in Figs. 15 and 16 (not shown). The printout may indicate capillaries outside a required tolerance. Some discussion of adaptations to the calculation procedure required in cases when the first measurement is not made with the beam parallel to the hole axis e.g. as when a spinneret is used, and is concave.