GB1298517A - Densitometer - Google Patents

Abstract

1298517 Densitometer CIBA-GEIGY A G 2 March 1970 [28 Feb 1969] 9949/70 Heading G1A A two path arrangement including a logarithmic element 7 Fig.1, for determining the optical density of a sample 3 from the light reflected from the sample 3 and a reference 4, includes a servo-amplifier 9 to maintain signals produced in a photo-diode 6 at a substantially mean value corresponding to the mean of the intensities in the two paths so that the working point of the logarithmic element and of the other circuitry is maintained substantially constant. The servoamplifier may control the brightness of source 1 or the amplification of photo detector 6. A polarity reverser 10 produces a signal dependent upon the difference in intensity in the two paths which is indicated in a meter 11 and provision is made at 19 for the introduction of an adjustable reference value by a gate 18 opening only during the reference value pulses. The optical path includes polarization filters 14,15 a chopper 2, and a filter wheel 5 for colour investigation which may be controlled by a unit 20 to drive the filters to a position of maximum detector signal. In a further arrangement the voltage value differences formed by the polarity reverser are passed to a store (23) Fig.3 (not shown) by means of a distributor (25). The drive motor for the filter disc 5 is not controlled by the control voltage detector unit 20 as above but runs in synchronism with the distributor. The control voltage detector unit 20 detects which filter produces the maximum control voltage and on completion of a rotation of the filter wheel causes a gate circuit (24) to pass to an indicator (11) the pair of reference and measurement voltages which cause the maximum control voltage to be produced. Further similar embodiments are described with multiple indicators (one for each location in the store) and with multiple reference bodies.