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Fig. 3. Shot-noise power vs. h/\c.
where the height of the scanning beam is small compared to the wavelength of the film modulation, the signal amplitude will be proportional to the scanningbeam height. Thus, in this frequency range, the ratio of signal amplitude to shot-noise amplitude will be proportional to the square root of the slit height. The ratio of signal amplitude to thermal noise will be directly proportional to the slit height.
At frequencies where the slit height is an appreciable fraction of the modulation wavelength, the signal amplitude will also be a function of frequency.7 Since the overall frequency response has been specified, the product of the frequency discrimination due to the slit height and the frequency discrimination due to amplifier compensating equalizers must remain fixed. The necessary amplifier-frequency characteristics, plotted with decibel ordinates, are shown in Fig. 2. These equalizer characteristics distort the frequency spectra of shot and thermal noises. Because of the fre
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Fig. 4. Thermal-noise power vs.
quency characteristics of the amplifier equalizers, both noise levels will be functions of slit height. The rms noise voltage must be determined by integration of noise power per cycle over the frequency band of the amplifier. This may be done graphically by plotting the equalizer characteristics as the amplitude squared versus the frequency, measuring the areas under these curves and taking the square roots of the areas. Each such calculation gives the noise voltage, either shot or thermal, associated with a particular ratio of slit height to wavelength. Signal-to-noise ratios versus slit height-to-wavelength ratio can now be plotted. Figure 3 is a plot of shot noise in decibels versus the ratio of slit height to wavelength at cutoff. Figure 4 is a similar curve for thermal noise. In both figures, the noise levels are referred to an arbitrary fixed signal level at the amplifier output. Minimum noise level occurs at slightly different h/\c ratios on the two curves of Figs. 3 and 4. The minimum
Grimwood and Horak: Reproducer Slit Height
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