Patent GB-1132394-A (GB)
Title: Improvements in or relating to optical systems for spectrophotometers
IPC: G01J3/12
Filed: 1966-04-05
Abstract: 1,132,394. Spectrometer. BECKMAN INSTRUMENTS Inc. April 5, 1966 [April 29, 1965], No.15037/66. Heading G2J. A spectrophotometer comprising a radiation source 10, a beam splitter 11, two concave mirrors 13, 14 arranged to direct radiation from the beam splitter 11 through a sample area 18 to two further concave mirrors 19, 20 which in turn reflect the radiation to a beam recombiner 22 so that the radiation is directed along a common path to detector means 26 is characterized in that intermediate images A, B of the source 10 are formed in the separate beam paths at unit magnification and a further image C of the source is formed at unit magnification in said common path. In a particular embodiment, the concave mirrors 13, 14, 19, 20 have a common focal length #. The source 10 is arranged at a distance 2# along the optical paths from each of the mirrors 13, 14 so that the intermediate images A, B are formed at a distance 2f from the mirrors 13, 14 at unit magnification. The mirrors 19, 20 are positioned at a distance 4f from the mirrors 13, 14 respectively so that the images A, B will be re-imaged by the mirrors 19, 20 at unit magnification at a distance 2# along the optical paths. Monochromator. A variable monochromator 23 is positioned at the image C and comprises an annular variable multi-layer interference filter formed in two sectors (50), (52) and mounted on a rotatable wheel (49). A slit which permits passage of radiation through the filter is defined by two jaws which are automatically moved towards or away from each other on rotation of the wheel (49) in order to vary the slit width as required. The slit height is defined by the radial height of the annular filter which dimension varies linearly with rotation of the wheel (49). The filter comprises a plurality of coatings of alternate high and low refractive index material applied to a suitable annular substrate, the difference in refractive indexes being small. The thickness of each layer varies linearly around the annulus so that the waveband transmitted by a small segment of the filter will also vary substantially linearly around the filter. Control circuit. A chopper 28 driven by a motor 29 provides alternate reference and sample beam pulses which, after wavelength selection by the monochromator 23, are focused on the detector 26. The output signal from the detector 26 is passed through a transformer 31 to an amplifier 32. The A.C. signal received is converted to a D.C. signal having an amplitude and polarity which varies as a function of the difference in energy in the reference and sample beams. The D.C. signal is applied to a servo-amplifier 40 which drives a servomotor 41 coupled by a drive band 44 to an optical attenuator 43 positioned in the reference beam. As the motor 41 is driven, the attenuator 43 is moved to balance the reference beam energy with that of the sample beam and thus reduce the D. C. signal to zero. A pen 46 attached to the drive band 44 moves over a chart 47 which is driven in synchronism with the wavelength selection element of the monochromator 23, the resulting record being the transmittance of the sample as a function of wavelength.