Patent GB-332318-A (GB)
Title: Process and means for determining differences in brightness
IPC: B07C5/342
Filed: 1929-05-03
Abstract: 332,318. Geffcken, H., and Richter, H. May 3, 1929. Photometric apparatus.-In a process for determining differences in brightness, by causing the brightnesses to be compared to act in rapid succession upon a light sensitive electrical device, the oscillations caused in the electric circuit of the device are separated from the steady current by transformers, condenser, or the like, and conducted to a relay device which only comes into operation when the alternating current component vanishes. Light from the various objects may be diverted to the device by any suitable means, but it is advantageous if the diversions occur at regular periods. In this case a resonance device may be inserted in the indicating circuit as in Fig. 21 where the amplified alternating current from the photo cell 75 acts on the spring tongue 81 which makes contact with the connection 82 when its vibrations have sufficient amplitude. An electrical resonance circuit can also be used. and both devices filter out false variations and increase the sensitivity of the apparatus. Slow changes such as ageing of the light source or light sensitive device and also constant secondary illumination will not affect the readings. In Fig. 1 the light from the source 1 is concentrated by the device 2 on a small area on the object 3 and the beam can be oscillated between the two positions 4 and 5. If the two halves of the object are not of equal brightness an alternating component will appear in the current from the photo cell 6. The light source and the cell may be interchanged. Fig. 3 shows an example of an arrangement for comparing two sources 7 and 8. These illuminate opalescent plates 11 and prisms 9 and 10 give images of the plates which can be compared by the method above. By using two beams from a single source passing through transparent media the transparencies of the media can be compared. The apparatus can also detect the disappearance of bands in an interference photometer. In Fig. 6 the prism 25 combines beams polarized at right-angles by Nicols 23 and 24 from objects for comparison, and the revolving Nicol 26 selects the beam to reach the photo cell. Kerr cells operated by an oscillatory circuit may also be used, all the Nicols being stationary. Fig. 8 shows an arrangement for comparing the colour of an object 34 with a series of colours 33 carried on the rotating disc 35. An arm 38 moves in synchronism with the disc 35 and the alternating currents caused by the light beam moving between points 36 and 37 move the arm axially so that the nose 39 moves the pointer 40. When the colours match the current ceases and arm 40 is disconnected and indicates the matching point. An indicating device for high speed work is also described in which the length of the cathode glow in a discharge tube measures the current. The apparatus can also operate sorting arrangements where different paths are opened as each colour on disc 33 passes the object and the object is driven down one when the colours match. The light may be diverted by a pierced shutter on a spring kept in vibration by electromagnetic means, or, as shown in Fig. 13, a disc 54 with a gap 55 of periodic nature relative to the circle 53 may rotate behind the opening in the plate 57. In systems in which the light source is fixed in order to avoid having to move the light sensitive device, a device of large aperture is used with a. making device which in known manner covers one object to the same degree it uncovers the other. Fig. 14 shows an example in which the plate 60 vibrates on the spring 61 in front of the objects 58 and 59. The masking screen may be coloured to serve itself as the comparison object and rotating devices may be used as well as vibratory.