. The Bell System technical journal . variations in the photocell output take place at the rate of abouttwo cycles per second. These are recorded on a moving chart by aninstrument similar to a high speed level recorder,^ differing from ilchiefly in having a linear instead of a logarithmic scale. ^ A High Speed Level Recorder, Wente, Bedell and Swartzel, Jour. Acous. , vol. 6, p. 121, January 1935. AN OPTICAL HARMONIC ANALYZER 411 The present instrument has been designed to take records of f{x)which are from one-sixteenth to five-sixteenths of an inch long and nohigher than their lengt
. The Bell System technical journal . variations in the photocell output take place at the rate of abouttwo cycles per second. These are recorded on a moving chart by aninstrument similar to a high speed level recorder,^ differing from ilchiefly in having a linear instead of a logarithmic scale. ^ A High Speed Level Recorder, Wente, Bedell and Swartzel, Jour. Acous. , vol. 6, p. 121, January 1935. AN OPTICAL HARMONIC ANALYZER 411 The present instrument has been designed to take records of f{x)which are from one-sixteenth to five-sixteenths of an inch long and nohigher than their length. The focal length of the enlarging lens inches. The collecting lens is placed quite close to the cosinescreens, and forms an image of the enlarging lens on the plate of thephotocell. With this arrangement the patterns of both f{x) and thecosine screens are well diffused on the photocell plate, so that surfacevariations in sensitivity of the plate are unimportant. The illumina-tion is uniform across the field to ±2 per Fig. 3—The optical harmonic analyzer. The cosine screens w^ere made on photographic plates, by printingfrom variable density motion picture sound track negatives containingrecords of pure single frequencies. The pattern thus produced isabout 1X2 inches. The increase in width of the track from aboutone-tenth of an inch in the negative to one inch on the plate wassecured by making a contact print with negative and plate slightlyseparated, and moving the plate sideways under the negative whileprinting. The important requirements for the screens are good wave form, 412 BELL SYSTEM TECHNICAL JOURNAL uniformity in modulation and average transmission, and accuracy inwave-length. In the present instrument it was found possible to keepthe harmonic content of the screens down to 5 per cent. The modula-tion varied from 79 per cent to 94 per cent in different screens, and theaverage transmission from 20 per cent to 24 per cent. Variations inthe wave-length o
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