Patent GB-1150531-A (GB)
Title: Spectrophotometer
IPC: G01J3/457
Filed: 1967-06-21
Abstract: 1,150,531. Spectrophotometer. BARRINGER RESEARCH Ltd. June 21, 1967 [June 23, 1966], No.28686/67. Heading G1A. [Also in Division G2] A spectrophotometer is provided with a mask 19 having a pattern of lines, alternately transparent and opaque, representing the spectrum of a substance of interest, vibrating means e.g. plate 16 and vibrator 17, for oscillating a spectrum formed from the light beam entering the spectrometer, across the mask, and a detector 21, the amplitude of the A.C. output of which is measured. The light beam entering the spectrometer may previously have been subjected to absorption by the substance of interest and if so, the spectrum formed by the spectrometer will contain lines or bands characteristic of this substance. These lines or bands will, in the mean position of the vibrating means, be superimposed on the corresponding lines of the mask 19, producing maximum transmission of radiation to the detector. Oscillation of the spectrum across the mask is carried out to produce an A.C. detector signal, whose amplitude is representative of the amount of the substance of interest in the area viewed by the spectrometer, this measurement being free from the effects of other interfering substances. The detector output is amplified in preamplifier (22, Fig.5 5 not shown), and then passed through main amplifier (23) tuned to the frequency of oscillation of the vibrating means in the case of a single line, or an irregularly spaced spectrum, e.g. Fig.4 (not shown), or to a harmonic of this frequency in the case of a regularly spaced spectrum, Figs.2, 3 (not shown), when the displacement of the spectrum over the mask by the vibration means is larger. A synchronous detector (23) receiving the amplifier output and a reference signal from the drive for the vibrating means, senses the amplitude of the A.C. signal, and an integrator (27) integrates it, before indication on meter 28 or recording. The D.C. level of the detector, a photo-multiplier, a photo-resistive or -voltaic cell or a photo-diode, is monitored by an AGC loop comprising a low pass amplifier (29), a comparator comparing the A.C. level with a reference signal, controlling the supply of high voltage to the detector. The vibrating means of Fig. 1 (the oscillating parallel-sided plate 16 mounted in front of the diffraction grating 15) may be replaced by a variety of suitable devices for oscillating the spectrum with respect to the grid 19. In one alternative, Fig. 6 (not shown), two refracting plates mounted on one tine of a tuning fork, are used. In a Littrow spectrometer, Fig. 7 (not shown) the grid itself is oscillated across the image plane of the spectrum. In a further arrangement, the grid 19 is oscillated back and forth along the optical axis from a position in the image plane to one a small distance away. Alternatively, the grid is fixed and the entrance slit to the spectrometer, Fig. 9 (not shown), is oscillated back and forth through a small angle. Besides investigation of absorption spectra, emission spectra may be analyzed. In each case the grid 19 is prepared previously to any investigation of an unknown sample or sample volume by producing a photographic plate of the spectrum of a known sample of the particular substance of interest, the vibrating means being absent. The mask may be made more efficient after developing the plate, by inking over the darker lines to increase the contrast. Alternatively a mask may be prepared completely by drawing. The mask may have opaque or transparent lines corresponding to those of the spectrum, with a contrasting background.