GB909445A - Improvements relating to optical null systems - Google Patents

Abstract

909,445. Photo-electric comparison of light beams. PERKIN-ELMER Ltd. Dec. 23, 1960 [Jan. 18, 1960], No. 1801/60. Class 40 (3). Means for comparing the intensities of two light beams include a logarithmic attenuator in one beam which is automatically adjusted until the intensities are equal. Light from a source 1, Fig. 1, is directed by a rotating chopper 10, Fig. 2, comprising reflecting, transparent and opaque segments 12, 16 and 20, along paths 14 and 18 alternately to a detector 30. The output of detector 30 at the chopping frequency fo, proportional to the intensity difference I<SP>1</SP>-I, is demodulated in the rectifier 36 and used to adjust the position of the logarithimic attenuator 22. It is shown that, with a logarithmic attenuator, to maintain constant gain in the servo system, I<SP>1</SP>kc must be kept constant where k is the sensitivity of detector 30 and the preamplifier 32 and c is the gain of units 34, 36 and 40. The output of detector 30 at a frequency f 1 = 2f 0 is proportional to I<SP>1</SP>k. This output which appears at amplifier 52 may be made to control the source intensity or the width of slits 2, 3 to keep I<SP>1</SP>k constant, or rectified and applied to the grid of a pentode, Fig. 3 (not shown), to the cathode of which is applied a reference potential. Variations from the reference potential cause variations in the anode potential which are applied through a cascade stabilizer to control the dynode potential of a photomultiplier constituting detector 30. In these systems, the gain of amplifier 40 must also be kept constant. The output of amplifier 52 may alternatively control the amplifier 40 so that c is inversely proportional to I<SP>1</SP>k. In the Provisional Specification a variable Á valve is included in amplifier 40 for this purpose.