Patent GB-1176902-A (GB)
Title: Spectrophotometer
IPC: G01J3/42
Filed: 1968-02-13
Abstract: 1,176,902. Spectro-photometers. SMITH KLINE & FRENCH LABORATORIES. Feb. 13, 1968, [March 2, 1967], No. 7133/68. Heading G2J. [Also in Divisions G1 and H3] A spectro-photometer comprises means for direction a beam of light alternately through a reference path and a sample cell and thence to photo-electric detecting means, means varying the response of the detecting means in accordance with the intensity of the beam from the reference path, means providing an output responsive to the output of said photo-electric detecting means, and means for monochromating said beam before and after it passes through said reference path or sample cell. Infrared, visible or ultraviolet light from a lamp 2, Fig. 1, passes through a spectral wedge filter 150 and is directed alternately on to sample and reference cuvettes 20, 12 by a rotating sectored mirror 24. Light passing through the cuvettes is reflected back under the cuvettes by prisms 14 and 18 and passes through the filter 150 at a point different from but having the same spectral transmission as the point of its first traverse. The light is then led by a guide 32 to a photo-multiplier in housing 28. A knob 106 is used to select the wavelength of the test by driving the filter 150 by cord 88, the wavelength being indicated by an indicator 89. Commonly used wavelengths may be selected by push buttons 104. The filter 150 may be replaced by a grating or prism monochromator, Figs. 12 and 13 (not shown). The reference and sample pulses are of 120 degrees duration of the rotation of mirror 24 and separated by 60 degrees dark periods. A disc 26 co-operates with light guides 108, 110 and photo-diodes 118 and 120 to produce 90 degrees duration gating signals centered on the reference and sample pulses. Electrical circuit. The output pulses from the photo-multiplier 188 Fig. 10 are passed via a field effect transistor 196 and amplifying transistors 200, 218 to the gating transistors 254, 256. Stabilizing negative feedback is applied via resistor 228. The sample pulses are passed by the sync. pulses on photo-diode 118 to a meter or recorder 68 where they provide an indication of the sample absorption smoothed by capacitor 346 and linearized by a shunting resistor/diode network 348-372. The reference pulses are passed by the sync. pulses on photo-diode 120 to charge a capacitor 278. The voltage on capacitor 278 is compared with a reference voltage from potentiometer 280 and any difference is fed back through a field effect transistor 274 and a Darlington pair 290, 292 to vary the amplitude of an oscillator 314, 316 supplying power to the photo-multiplier 188. During the intervals between pulses, diode 232 is forward biased and passes a signal through transistors 234 and 240 indicative of the photomultiplier dark current. This signal is fed back to the gate of field effect transistor 196 to stabilize the dark level of the signal at this point. Range change resistors 264, 266 and 268 are selectable by switches 44, 46, 48 operable by a knob 34, Fig. 1 which simultaneously inserts different attenuators 64 in the reference path and makes a different scale visible in the windows 70 of the meter 68.