Patent GB-884377-A (GB)
Title: Apparatus for measuring automatically the colour density of materials
IPC: G01N21/31
Filed: 1959-01-29
Abstract: 884,377. Photo-electric measurement of density. TATE & LYLE Ltd., HILL, S., RUNDELL, J. T., and PALMER, B. M. Jan. 29, 1960 [Jan. 29, 1959], No. 3160/59. Class 40(3).  In photo-electric apparatus for measuring the colour density of a material containing an impurity which scatters light passing therethrough a first measurement is made to determine the ratio of the intensities of light transmitted through a sample of the material and a standard in a first narrow band of frequencies in the visible or near visible spectrum where the absorption of light due to the colour of the material is small or a minimum and the transmitted light intensity is therefore determined almost wholly by the effects of scattering. A second similar measurement is then made to determine the ratio of transmitted intensities in a second narrow band of frequencies where the absorption of light due to the colour of the material is large or a maximum. By subtracting absorbancy determined by the first measurement from that determined from the second, there is obtained the absorbancy of the material due to its colour density alone. In practice the last step is effected by determining the ratio of the ratios found by the two measurements and displaying the result on an indicator with a logarithmic scale. The invention is stated to be applicable to measurements of liquids, gases, vapours and semi-transparent solids, e.g. films of synthetic resin, but is described only as applied to measurements of sugar solutions where the first and second narrow bands are respectively 700 Š and 150 Š wide and centred on 7000 Š (red) and 4000 Š (blue). As described the bands are selected by bandpass filter, although reference is also made to the use of low-pass and high-pass filters in conjunction with photo-electric cells with appropriately limited response curves. By using light sources producing outputs in the two bands, the use of filters may be avoided. In Fig. 1, the sample and standard (which may be distilled water) are contained in tubes 1 and 3 with transparent ends, the tubes being mounted on a moveable carriage 2 so that either may be positioned in the light path A-A between a white light source 12 and a photo-electric cell 11. The carriage is moved by rotation of a shaft 9 carrying a cam 8 against which the carriage is urged by springs 5. The shaft also carries cams 26 and 27 by which a shutter 23 and one or the other of two filters 24 (red) and 25 (blue) may be positioned in the light path. Further cams 49, 50, 51, 59, 60 control switches 46, 47, 48, 54, 56 in an associated circuit (see below). The shaft 9, which may be rotated manually by a knob 28, or automatically by a motor, occupies in succession six positions in which the apparatus is first zeroed with the shutter in the light path, next transmission measurements are made on the sample and standard with red light, then transmission measurements on the standard and sample with blue light, and finally the shutter is reinserted in the light path. In the associated circuit, Fig. 2, photo-electric cell 11 operates in a self balancing system in which the cell output, which appears across a resistor 58, is balanced by a voltage from a potentiometer network 41, 42, 52. The difference potential is detected by an amplifier 31 whose output operates a relay 32 and thereby causes motors 35, 36 to rotate the shaft of potentiometers 41, 42 to reduce the difference to zero. Motor 36 also positions the drum 62 of an indicator having a logarithmic scale of density in any suitable units. A further potentiometer 40 controlled by motor 34 is employed in zeroing the apparatus. 29 is a source of stabilised D.C. potential for the potentiometer network 41, 42, 52. Potentiometer 53 is preset to furnish the same potential as potentiometer 42 when the indicator is located on the zero of its scale. In operation, after zeroing, the circuit is first balanced for the red measurement on the sample by potentiometer 41 and is then balanced for the red measurement on the standard by potentiometer- 42. This establishes the first ratio referred to above. Thereafter potentiometer 41 is. readjusted to balance for the blue measurement of the standard and finally potentiometer 42 is adjusted to balance for the blue measurement of the sample. This establishes the second ratio referred to above. It is demonstrated in the specification that the final position of potentiometer 42 represents the ratio of the two ratios and that the indicator, being logarithmic, gives the difference of the measured absorptions.