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1950 TV RANDOM NOISE 21
nance of the test field BB. It approaches its minimum value of zero for a surround which is all physically black except for a small area of luminance Bs, and the value further falls rapidly as this area is removed from the line of sight. Some intermediate fields are illustrated in Fig. 10.
The value of b is 0.05 for a field of luminance Bs entirely surrounding the circle VI, the space within this, to the test field boundary, being physically black. Single areas of luminance Bs at the spots III, IV, and V give values of b respectively 0.01, 0.001, and 0.0001. Fig. 10 also shows the outline of a picture field, viewed at D = 1 with aspect ratio of 3 (height) to 4 (width).
Moon and Spencer specify the luminance of a test object enough lower than that of the test field to be just perceptible, and tell how this is influenced by the adaptation luminance. The test object subtends an angle of one degree, indicated by field I in Fig. 10. The formula breaks up into two cases according to whether the adaptation luminance is greater or less than the test field luminance.
In the first case, where
BA = aBB + bB8 ^ BB, (17)
the threshold is reached at a value indicated by the empirical equation BB Be = A£ = c (A -f VaBB + bBs}\ (18)
where B0 = luminance of test object in millilamberts; A = a constant = 0.255; c = a constant = 0.143.
In the second case, where
BA = aBB + bB8 < BB, (19)
(20)
A plot of the formula is illustrated in Fig. 11 in terms of the Fechner fraction (that is, AB/J5) as a function of the luminance BB of the test field, for a variety of values of bBs. The curves are in general given for integral powers of 10 for bBs, but hi one case b is given its maximum value (0.077) for a maximum surround luminance Bs = 10,000 millilamberts. The asymptotic value reached for bBs = 0 is also illustrated.
As in the case of the Weber-Fechner law, it is possible to integrate the Moon and Spencer formula to obtain the total cumulation of perceptible steps in luminance, say from some maximum luminance