Patent GB-468448-A (GB)
Title: Improvements in photo-electric methods of automatically examining surfaces
IPC: B07C3/14
Filed: 1936-01-06
Abstract: 468,448. Photo-electric apparatus for examining postal envelopes etc.; valve relaying circuits. GREGORY, H. J., and RADLEY, W. G. Jan. 6, 1936, No. 418. [Classes 40 (iii) and 40 (v)] A method of detecting the presence of a desired mark on an article, for example a postage stamp on an envelope, comprises illuminating the surface of the article, forming an image of a part, or images of parts of the surface which is greater than or are together greater than the area of the mark, allowing light from parts of the image or from the images in succession to illuminate a single photo-electric cell, and causing the alternating currents in the circuit of the cell due to the presence of the mark to actuate an indicating, counting, or other device. In the apparatus shown, for examining envelopes and cards, the latter are carried at a high speed by a conveyer band 2 past illuminating means 3, preferably two mercury vapour lamps suitably connected in a three-phase supply system so as to give uniform emission. A lens system 4, screened from the lamps, forms at a window 6 a reduced image of an area on the envelope or card. The window may be approximately as wide as a stamp image, and twice as deep. The light passing through the window is projected by a lens system 10 on to a photo-electric cell or group of cells 7, and means are provided whereby the two halves of the window are covered in turn, so producing an alternating current in the cell when the light passing the two halves is different. As shown, for this purpose a rotating disc 8 is arranged with teeth or apertures 9 passing the window, the width and spacing of the teeth &c., being such that the area of the window obscured is constant. Instead of the disc 8, an apertured cylinder may be used, or a vibrating plate may be located in front of the window. The A.C. produced in the cell 7 by the presence of a stamp may be utilized through a circuit as in Fig. 4, to operate a relay 24 which actuates a device for indicating or for other purposes such as sorting. As shown, the potentials generated across a resistance 13 in series with the cell 7 are applied to the grid of an amplifying valve 14 through a condenser 15 having a high impedance to low-frequency signals. A high-pass filter 16a between 14 and a second amplifying valve 16 further separates out the wanted signal currents. The secondary of the output transformer 17 is centre-tapped, and is connected to the primary of the input transformer of a valve 18 through rectifiers 19, which are so biassed by a battery 20 as to offer a high impedance to small signals such as arise from writing on the envelopes. The valve 18 is arranged to have a trigger action, such that when the signal applied to its grid exceeds a certain level, the anode current rises to -its full value and operates the relay 24. A gas-filled relay may alternatively, be utilized. An arrangement of an horizontal conveyer band, with vertical guide rollers for the envelopes, and means for sorting the envelopes according to the result of the photo-electric test, is diagrammatically illustrated in Figs. 5 and 6, (not shown).