Patent GB-974965-A (GB)
Title: Improvements in or relating to colorimeters and the like
Inventor: HAMBLETON JAMES
IPC: G01J3/50
Filed: 1960-03-08
Abstract: 974, 965. Photo-electric colorimeters. JOYCE, LOEBL & CO. Ltd. May 2, 1961 [March 8, 1960], No. 8128/60. Heading G1A. In a photo-electric colorimeter provision is made for replacing a comparative standard, or standards, by an electric signal, or signals simulating the response of the photo-electric detector to such standard, or standards. The Specification first describes a colorimeter in which light from a lamp 10 is directed through a standard colour filter 11 and then split into two beams B and C. Beam B passes through an adjustable attenuator 12 and is split into two equal beams B1 and B2, the latter passes through a 95% transmission filter 12a before reaching a MgO reflectance standard 15 whilst the former falls directly on a similar standard 14. Beam C is also split into two equal 'parts C1, C2, which fall respectively on to the colour sample 16 and a colour standard 17. A rotating shutter 13 allows beams B1, C1 to fall on photo-cell 18 alternately with beams B2, C2. If attenuator 12 is adjusted to give zero A.C. output from the cell then the percentage transmission of the attenuator multiplied by the absorption of filter 12a gives a value for the reflectance of the colour sample for the light defined by filter 11. To avoid the difficulties inherent in the use of MgO standards 14 and 15 the system shown in Fig. 2 simulates the signals produced by their use by electrical signals. For this purpose a stabilized power supply 20 is used, feeding adjustable attenuator 21. Alternately conducting transistors 22, 23 pass the outputs, which are made slightly different in amplitude by offsetting the earth point E1, for combination with signals from photo-cell 30. Light is alternately directed on to this cell after reflection by sample 16 or colour standard 17 in synchronism with the switching of transistors 22, 23. The combined signals are amplified and fed to phase sensitive detector. This may serve for manual or retroactive control of potentiometer 21 to give an arrangement analogues with that of Fig. 1. In a more elaborate arrangement (Fig. 3, not shown) switched compensating resistors are associated with a photomultiplier detector to compensate for the variations in gain of the colour filters used and a transistor impedance matching circuit couples the detector to a rectifier bridge fed also with the standard signals. Calibration may be carried out using a MgO standard. It is also possible to use a single beam system having electrical references previously set equal to the signals that would be derived from the colour standards. Fig. 4 shows an optical system for a single beam arrangement in which light from a lamp L passes through adjustable aperture L1 and is directed by collimating lens L2 across integrating sphere S to sample (or calibrating standard) holder H. The photomultiplier VI views the sphere in a direction at right-angles to the incoming light path A circuit is given (Fig. 5, not shown) in which the D.C. signal from a single beam system is balanced in turn against adjustable proportions of three predetermined potentials equivalent to colour standard signals.