Patent GB-532755-A (GB)
Title: Improvements in spectrophotometers
IPC: G01J3/42
Filed: 1939-09-01
Abstract: 532,755. Photo-electric spectrophotometers. CALCO CHEMICAL CO., Inc. Sept. 1, 1939, No. 25086. Convention date, Sept. 7, 1938. [Class 40 (iii)] [Also in Group XX] In a spectrophotometer in which a sample and a standard are illuminated by beams flickering in opposite phase, the relative intensity of the beams being adjusted until the sumtotal of the light reflected or transmitted by sample and standard shows no fluctuation at flicker frequency, means are provided to split the beam normally directed on the sample into two parts, one of which is directed on to the sample whilst the other is directed on to the standard in addition to the beam normally directed on to it. As explained below, the tendency of an automatic recording instrument to be driven away from, instead of into balance, when examining a specimen having a substantially zero reflectance or transmission, is avoided. In the apparatus shown, light of a narrow spectral band passes from the slit 48 of a monochromator through a Rochon or Nicol prism 1, where it is polarized, and thence through a Wollaston prism 2, by which it is separated into two diverging beams polarized at right angles to one another. After passing through an inclined parallel-sided plate 4, by rotation of which any inequality in the two beams due to the optical elements may be corrected, the beams pass through a half-wave plate 7 rotated by a synchronous motor. The flickening beams thus produced pass through a stationary polarizing prism 10 and thence through a Wollaston prism 14, where each beam is split into two diverging beams polarized at right angles to each other. The four beams thus produced are focused by a lens 15 on to four slits 16-19. The three beams reaching the slits 17, 18, 19, that is both components of one of the original two beams together with one component of the other, are directed by a lens 21 on to apertures 23, 26 in a light-integrating sphere 25, and the remaining component of the other original beam is directed by a lens 22 to apertures 24, 27. The specimen to be tested for light transmission or reflectance is placed respectively in the aperture 23 or 26, and a standard body with which it is to be compared is placed in the corresponding aperture 24, 27. The flickering light transmitted or reflected by both the specimen and the standard is integrated within the sphere and activates a photo-electric cell 29 connected to an amplifier 31, the output of which passes to the commutator 33 of a synchronous motor which drives a cable 39 operating an indicating device 38 and a cam 44 which adjusts the angular position of the prism 1 through an arm 46. The rotation of this prism sets the relative intensities of the beams and continues until the transmitted or reflected beams from the specimen and standard balance one another, when the photo-electric cell gives no output and movement ceases. Assuming that 1 per cent of the sample beam is diverted to the standard, and that the sample examined has zero transmission or reflectance, the standard beam must be sufficient to neutralize this beam to give a zero reading. In other words, the zero setting allows 1 per cent of the standard beam to reach the standard instead of completely cutting off this beam, so that no danger occurs of the photometric prism being turned beyond the setting for zero illumination with consequent reverse action of the motor to rotate the prism 1 away from the balancing position. Specifications 432,306 and 438,511, [both in Group XX], and U.S.A. Specifications 1,806,199 and 2,126,410 also are referred to.