Patent GB-1087990-A (GB)
Title: Measurement of absorption by a substance of radiation
Inventor: FITZSIMMONS ALAN EDWARD
IPC: G01N21/25
Filed: 1965-05-03
Abstract: 1,087,990. Photo-electric light absorption measurement. VICKERS Ltd. April 6, 1966 [May 3, 1965], No.18606/65. Heading G1A. [Also in Divisions G3 and H3] An apparatus for measuring the concentration of a constituent of a sample, particularly haemoglobin in blood, by light absorbtion is calibrated to give zero and full-scale meter deflection corresponding to the absorbtion of two reference samples respectively, and automatically re-adjusts the calibration before each measurement if these deflection values drifts. A haemoglobinometer is described in which the light from a lamp passes through one of two sample cells which are mounted upon a movable carriage so as to place only one of them in the light path, and through a mercurygreen light passing filter to be received by a photo-resistor. In calibrating the apparatus as in the intensity of the lamp is adjusted to give full-scale deflection with a first reference sample in position; thereafter feedback from the meter current to the lamp supply controls the brightness of the lamp to maintain the meter current constant and prevents a drift in the full-scale reading. Before the second reference is introduced the direct feedback signal is replaced by a standing capacitor voltage which maintains the brightness of the lamp constant at the former value irrespective of the photo-resistor current. With the sect nd reference sample in position a backing-off current is supplied to the meter to reduce the meter deflection to zero. By reintroducing the first sample and resumption of direct meter current feedback a new value of feedback voltage adjusts the intensity of the lamp to once again give full-scale deflection. By repeating these steps the zero and full-scale readings approach the absorbtion values of the reference samples. An unknown sample to be measured takes the place of the second reference sample. The first reference sample is used to re-adjust the feedback before each measurement to a correct for drift in the full-scale reading due to variation of supply voltage or blackening of the lamp glass. The reference samples may be glass or gelatine filters. In the circuit shown in Fig. 2 a photo-resistor 6 which is fed with a stabilized rectified supply from secondary winding 11, receives light from the lamp 1 after transmission through a sample. With the first reference sample in position the brightness of the lamp 1 is controlled and maintained constant by feedback from resistor R9 through a D. C. amplifier 8 to the cascaded emitter followers TR7, TR8 and TR9 which supply current to the lamp. Brightness of the lamp is adjusted independently of the feedback by potentiometer P1 in a circuit fed from secondary winding 12 and providing a stabilized variable supply voltage to the amplifier 8 to give full-scale deflection of the meter. Upon insertion of the second reference sample the direct feedback voltage is broken at switch MS1 but its former value is maintained by capacitor C6. A stabilized backing-off voltage from potentiometer P2 gives zero deflection for the second reference sample. Re-introduction of the first reference sample closes switch MS1 to adjust the brightness to bring the current back to a value to give full-scale deflection. These steps are repeated and the measurement of an unknown sample made as previously indicated. The D. C. amplifier 8 has a temperature compensating resistor 12 and the balanced pair of transistors TR4, TR5 provided to reduce any feedback from this resistor to the input of TR3. The pair of transistors are balanced by potentiometer P3 upon each adjustment of the backing-off voltage. The amplifier has a high input impedance to reduce the leakage from capacitor C6.