Patent GB-688214-A (GB)
Title: Improvements in or relating to recording spectrophotometers
Inventor: HOLIDAY ENSOR ROSLYN
IPC: G01J3/32
Filed: 1949-09-06
Abstract: 688,214. Photo-electric absorption spectrometers. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Oct. 4, 1950 [Sept. 6, 1949], No. 23120/49. Class 40 (iii). In a photo-electric absorption spectrometer the speed of examination is reduced as the absorption increases. Measurement of absorption.-As shown, light from a source 1 is split by an optical system 3 into two light beams 4, 5 alternatively chopped by a rotating perforated disc 6. This disc is driven by a motor 8 with a control to maintain its speed constant.. One beam then passes through a vessel 15 containing a solution of the substance to be analysed while the other passes through a vessel 14 containing only the solvent. The beams are then combined to the same path 17 and dispersed by a prism 18, the prism being slowly rotated by a servomotor 21 so that each part of the spectrum in turn reaches a photomultiplier 20 by way of a diaphragm 19. The servomotor also drives a recording drum 25 through gearing 24 which ensures a linear relation between drum rotation and frequency, and adjusts a non-linear potentiometer 28 which controls the size of the aperture in plate 19 so that the same range of wavelengths is always examined. This is effected by a servomotor 29 which "backs off" the output of the non-linear potentiometer with that from a linear potentiometer 29a. The signal output of the photo-multiplier 20 is applied through a cathode follower 30 to an amplifier 31, a further amplifier 32, gain control 33, amplifier 34 and a anode follower 35. This error signal resulting from the unbalance of the two light paths is fed to an amplifier 36 and thence to a phase discriminator 53 while its second harmonic is selected by amplifier 37, rectified in 38, phasecorrected in 39, averaged in 40 and applied through a long time constant circuit as gain control to amplifier 31, thus determining the level about which the signal fluctuates. Phase discriminator 53 receives a reference signal generated by chopping disc 6 and a further photo-cell 50 and supplies through a cathode follower an output signal of magnitude and polarity corresponding to the error signal this signal being applied through a sensitivity control 55 and an amplifier 57 to a servomotor 56. The servomotor differentially adjusts the path length of the light in the two cells by moving plungers in and out, the cells being connected to filling reservoirs. At the same time the servomotor adjusts a potentiometer 62 to control the galvanometer 63 which reflects light marking a photographic surface on the recording drum. The servomotor also drives a tacho-generator 58 to provide a rate voltage for stabilizing purposes and adjusts control 55 to reduce the servo sensitivity as the error signal increases. In a modification, instead of adjusting the relative path lengths in the two cells a variable stop may be adjusted in one of the light paths. Adjustment of speed of examination of spectrum.-The signal fed through sensitivity control 55 to the servomotor 56 is also fed to a modulus extractor 71 which always renders the error signal of the same sign, is amplified at 70 and used to reduce the speed of servomotor 21 rotating the dispersing prism 18. Poteiitiometer 72 may provide a backing off voltage so that servo speed is not reduced until the error signal exceeds a certain value. Adjustment of printing light intensity.- Servomotor 21 develops a rate voltage by means of a tacho-generator 21a and this voltage is fed to a circuit 66 controlling the intensity of the galvanometer lamp so that as the servo rotates the recording drum 25 quickly the light is brightened, ensuring that each part of the photographic surface of the drum receives the same exposure. Similarly servo 56 which con-. trols the galvo to deflect the light spot axially of the drum feeds a signal proportional to its movement to brighten the lamp.