Patent GB-753890-A (GB)
Title: Improvements in or relating to a spectrophotometer
IPC: G01J3/42
Filed: 1954-03-31
Abstract: 753,890. Photo-electric spectrometer. BECKMAN INSTRUMENTS, Inc. March 31, 1954 [April 1, 1953], No. 9419/54. Class 40 (3). In a photo-electric spectophotometer of the type in which a radiation beam passes alternately through a sample and a reference to a radiant energy responsive device the beam is shifted by an oscillating support to which are attached a pair of spaced mirrors to receive the beam and transmit the radiation to the sensitive device the sample and reference being included in the path between the two mirrors. As shown, light from a source 2 is reflected to a monochromator 1 where a particular wavelength is selected which is reflected from mirrors 10, 11 on an oscillating mounting (to be described) to the cathode of a photomultiplier tube 12. The mirrors oscillate between the position t3 (shown in full lines) and t1 (shown dotted). When the mirrors are in position t3 the light passing between them is intercepted by a cell 16 containing, for example, pure water, while in the position tl the light is intercepted by the sample. In the intermediate position t2 an opaque screen 18 cuts off light from the photo-cell entirely. A switch 22 operated in synchronism with the oscillations of the mirrors connects the output of the multiplier, amplified at 20, to one of three circuits according as the mirrors are in the position t1, t2 and t3. In position t1 the output of the amplifier is fed to indicator 24 with a series resistor 25 both shunted by condenser 26, in position t2 to earth and in position t3 to a reference circuit. The reference circuit adjusts the sensitivity of the multiplier tube in accordance - with the transmissivity of the reference so that meter 24 indicates the transmissivity of the sample as a proportion of that of the reference. The reference circuit comprises a capacitor 32 and resistor 33 having values equal to those present in the sample unit and connected to earth, and a biased diode 31 connected through a further capacitor 34 to one terminal of an amplifier 35. The diode passes the impulse peaks of the reference signal provided their amplitude is greater than the bias, these peaks being amplified at 35 rectified at 38 and used to control the bias of triode valve 40 connected in series with a potential source 44 and the resistance chain, from which are tapped the potentials for the dynode of the electron multiplier. Thus the sensitivity of the multiplier is varied in dependence on the output signal of the reference circuit so that meter 24 is made direct reading. This meter may be such as to mark a record driven in synchronism with the movement of the prism of the monochrometer. So as to ensure that the correcting signal applied to the dynodes persists during the sample measurement the dynodes are shunted by a reactive valve acting as a capacitance. In a modification the photo-multiplier tube is replaced by a photo-cell negative feed-back being applied to amplifier 20. Oscillating mirror mechanism (Fig. 2). The mirrors 10, 11 are mounted on platform 53 carried by a parallelogram linkage consisting of side pieces 55, 56, each supported at the top and bottom by blade springs 57 in that as the supporter is moved sideways it is maintained approximately level. Side piece 56 is driven by a link 64 oscillated by a similar arrangement to that employed in a moving coil loud-speaker, in which a moving coil 59 fed with an alternating or interrupted direct current supply moves in the field of a permanent magnet. Member 56 also carries an arm closing contacts performing the function of switch 22 (above).