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I IXKMATOGRAPHIC ANNUAL
Unless there is a variation in gamma with printer point, the shift of printer point is equivalent merely to shifting the range of negative densities to cover successive (and overlapping) portions of the whole printer curve. To obtain Fig. 3, the print visual density was plotted against the negative visual density for each printer point separately, and the plots were brought into register by appropriate shifting along the axis of negative visual density. Fig. 3 gives no indication of a variation in gamma with printer point; this result has been confirmed on every printer so far examined, provided accurate density measurements are made on the negative loop and on the positive prints.
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CIRCLES PONT 18
DOTS POINT 12
CROSSES POINT 6
DAYLIGHT TIME SCALE Y = 1.81 PRINTER APPARENT 1 = 208 PRINTING COEFF OF NEC = 1.21 PRINTER TRUE T 1.72
PRINTERS (TRUE I DAYLIGHT TIME SCALE V
DETERMINATION OF PRINTER CURVE : FOX LAB.
N£G. SENS. STRIP PRINTED AT 18,12,1 6
( PRINTS DEVELOPED IN SAME BATH WITH TIME SCALE SENSITOMETER STRIP )
0.4 OB 1.2 16 2£>
NEGATIVE VISUAL DENSITY ZERO ARBITRARY
Fig. 3. Apparent printer characteristic.
Study of Fig. 3 shows that a change of 3 printer points is equivalent to a shift of 0.10 along the axis of negative visual density. This is confirmed by all the prints made of this negative: the print at point 6 registers with that at point 18 when shifted 0.40 in visual density. It is known from illumination measurements at the printing gate of the Bell & Howell back-shutter printer that between points 3 and 1 8 there is a linear logarithmic relation between printer point and metercandles at the gate; a change of 12 printer points means a change of 0.48 in log meter-candles. A change in visual density of 0.40, therefore, is equivalent to a change of 0.48 in log effective exposure; the printing coefficient of this negative then is 1.2.
While this determination lacks precision both in the illumination measurements and in the registrations of the different print curves, it checks closely with the relation previously stated between the daylight time scale gamma and the printer intensity scale gamma. It will be recognized that a curve plotted between print visual density and negative visual density will give an apparent printer gamma higher than the true, if the P.C. of the negative is greater than unity. The gradations of negative density should be multiplied by the nega