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26 PIERRE MERTZ January
of this, for a variety of values of Bs as indicated. The plot is continued in each case to the approximate boundary of subjective black.
The curves from the Moon and Spencer formula have been compared with other determinations of the deviations from the WeberFechner law.7 The agreement is not always too satisfactory, although this is undoubtedly in part caused by differences in viewing conditions, which are quite varied. In a general way, one can say that in the present state of knowledge the form of the formula proposed is reasonably adequate, but the constants used in it may be subject to some later revision.
The Moon and Spencer formula represents an important step forward in understanding the perception of contrast in the viewing of scenes and images. It, together with the Holladay principle, traces in a simple form the variations in this perception with picture content and highlight luminance level. The general qualitative facts of this are common knowledge, but the formula presents them in a compact expression.
The formula, of course, describes the perception of contrasts of areas of about photometric size, rather than grains of the size found in random noise. It has been found, however, that the vision of fine lines and areas is largely describable in terms of contrast perception in larger areas.8 Thus the information on areas of photometric size is at least illustratively valid, and probably even more, in the consideration of random noise perception.
3. INFLUENCE OF SIGNAL TO LUMINANCE TRANSLATION
It is clear that the susceptibility to noise in the over-all system is dependent only upon the characteristics of the system beyond the point at which the noise is introduced, up to the ultimate viewing. Hence the only portion of the characteristic influencing the susceptibility of the system to the noise is the transfer characteristic between the electrical signal at this point and the final image luminance, with the subjective characteristics appropriate to the image viewing conditions. This is illustrated in two forms of presentation, parts (A) and (B) of Fig. 16.
In (B) the electrical signal is plotted in terms of db below the maximum signal. In (A) the electrical signal is plotted as the arithmetical ratio to the maximum signal. The luminance is plotted, in both cases, in terms of a hypothetical photographic density by which maximum