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Densitometry of Silver Sulfide Sound Tracks
By ROBERT C. LOVICK
Silver sulfide deposits have spectral density characteristics which tend to make densitometry less reliable than density measurements of silver deposits. Interference filters may be useful in restricting the bandwidth of response of electronic densitometers so that densitometry of silver sulfide deposits will have increased significance.
A HE SOUND TRACK on Kodachrome film and other multilayer, reversal, color films is commonly a deposit of silver sulfide. Silver sulfide images are used because no satisfactory solution has yet been obtained to the problem of producing a good silver image in the sound track area and pictures free of silver in the adjacent area.
The curves of Fig. 1 show density as a function of wavelength for a silver deposit on 16mm Eastman Fine Grain Release Positive Film, Type 7302, and for a reversal silver sulfide deposit on Kodachrome Duplicating Color Film, Type 5265. Physical densitometers usually have maximum sensitivity in the visual region at about 525 m/z. In this wavelength region, the major errors in the measurement of the density of photographically deposited silver are the result of the geometry of the instrument. Differences in spectral response of the receptor have practically no effect on the calibration of the densitom
Presented on April 24, 1952, at the Society's Convention at Chicago, 111., by Robert C. Lovick, Color Control Div., Eastman Kodak Co., Rochester 4, N.Y.
eter. The accuracy of density determination may be adequately stated for silver deposits. There is a tendency to assume that the same reliability applies to silver sulfide deposits.
The density of silver is minimum at 525 mju and increases moderately with wavelength. The density of silver sulfide varies much more with wavelength. Assuming equal densities at a wavelength of 800 mju, the slope of the spectral density curve for silver sulfide is over six times greater than the slope of the curve for a silver deposit.
The density of the silver deposit measured at 800 mju is 14% greater than the density at 525 mju. The density of the silver sulfide deposit measured at 800 mju is only 40% of the density at 525 m/i. Consequently, silver sulfide sound tracks are visually denser than the density effective in a reproducer with a phototube having an S-l response. As a result, reversal silver sulfide sound tracks are often seriously overexposed in attempts to give them the appearance of silver sound tracks.
Measurements of sound tracks to obtain indications of the density effective in a reproducer require that heat
August 1952 Journal of the SMPTE Vol. 59
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