A NEW PHOTOMETRIC SYSTEM FOR AN ULTRAVIOLET SPECTROPHOTOMETER
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Publisher:Ohio State University
Several photometric systems for the automatic recording of ultraviolet transmittance are discussed. Optical balance systems are held to be less advantageous than electrical balance systems for instruments of high accuracy and wide wavelength range because of difficulties with available optical attenuators. Electrical balance systems employing space or time separation of the sample and reference signals are shown theoretically to have a better signal-noise ratio than systems employing phase or frequency separation when operated with multiplier phototubes. The space separation systems, which employ separate phototubes for the sample and reference radiation, have an advantage in simplicity, but encounter difficulties in work demanding the highest accuracy because of drift in the relative sensitivities of the two photometric system was chosen for further development. The resulting system is described in detail. The optical system employs a mirror rotating at 30 cps which directs the monochromatic radiation alternately through the sample and reference cells to the single phototube. A shutter on the mirror shaft produces a dark interval while the transfer takes place. The phototube signals are passed through an amplifier whose de bias is adjusted twice each cycle so that the output during the dark interval is zero. At the output of the amplifier, the reference signal is separated from the sample signal by means of a synchronous switch. The ratio of the separated signals is taken by an exponential slidewire associated with circuits which drive the recorder pen motor so as to balance the slidewire. An auxiliary circuit drives a slit control motor so as to maintain the reference signal at a constant level. A dual pen recorder with a 1.0 second period is used which gives absorbance recordings in the ranges 0-1.1 and 1.0 to 2.1 on an 11-inch chart. In these ranges, the over-all system reproduces .001 absorbance units and appears to be accurate within .003 units.
Author Institution: Applied Physics Corporation
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