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Sensitometry

light, plate, negative, density, speed, series and opacity

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SENSITOMETRY (Pr., Sensitomitrie ; Ger., Empfindlichkeitsmessung) Soon after the introduction of the gelatine dry plate, it was usual to express the speed of the emulsion as " x times," which meant that it was x times the speed of a wet collodion plate. This speed was no fixed quantity, and the expression consequently meant but little. Warnerke intro duced a sensitometer, consisting of a series of numbered squares with increasing quantities of opaque pigment. The plate to be tested was placed in contact with this, and an exposure made to the light emanating from a tablet of luminous paint, excited by burning magnesium ribbon. After development and fixation the last number visible was taken as the speed of the plate. The chief objections to this method were that practically no two numbered tablets agreed, that the pigment possessed selective spectral absorption, and that the luminosity of the tablet varied considerably with the lapse of time between its excitation and the exposure of the plate. Various other methods were pro posed, but none found any practical use. In 189o, Hurter and Driffield published a series of papers on the subject of speed determination, and proposed a method of exposing a plate to a series of lights of known intensities and measuring the densities obtained on develop ment. This method has become very general in England, though possibly it is not strictly adhered to by all its users. Notwithstanding various attacks, the main principles of the H. and D. system, as it is briefly termed, remain uncontroverted. On the Continent, another system, known as Scheiner's, has been elab orated by Eder, and in this a rotating sector wheel is used, the steps of which bear a ratio of z : After exposure, the last number visible, when the negative is placed film down wards on white paper, is taken as the speed of the plate.

Chapman Jones has introduced a modified Warnerke tablet containing a series of twenty five graduated densities, a series of coloured squares, and a strip of neutral grey, all five being of approximately equal luminosity, and a series of four squares passing a definite portion of the spectrum ; finally, there is a square of a line design, over which is superposed a half-tone negative. This " plate tester," A, is used with a

standard candle as the source of light, and is useful for rough tests of both plates and print ing papers. Definitions of the leading terms employed must here be given. " Opacity " is the suppression or absorption of light by the silver image. " Transparency " is the remnant of the original light which passes through the negative image. " Density " is the relative quan tity of silver deposited per unit area. The exist ing confusion is well shown by the term " a very dense negative," when really what is meant is that the " opacities " of the silver deposited are so great that they possess very little deposited transparency ' ; that is to say, the negative absorbs the greater portion of the incident light. It is true that the " denser " a negative the greater the opacity, but it must not be forgotten that, as defined above, density is the quantity of silver deposited. Density is the logarithm of the opacity ; thus, a negative which has an opacity of roc has a density of 2'00, as this is the com mon logarithm of zoo, and it has a transparency of lia• The usual mathematical expression of the above facts is as follows, but in the following pages, as far as possible, mathematics will be excluded and everyday working instructions given :— Transparency, T = Intensity of light transmittedI Intensity of incident light Opacity, 0 — Intensity of incident lightIo I Intensity of transmitted light — I T Density, D = T = log, 0 O.

The last term is also frequently expressed as D = – log. T = log. 0, Napierian, instead of common, logarithms being used.

Hurter and DriEeld pointed out that in a perfect negative the opacities of the different gradations were strictly proportional to the light reflected by those portions of the subject which they represented when the plate had received correct exposure, and that a true repre sentation of the tones of the original is only possible when the density or the quantity of silver is proportional to the logarithm of the light in tensity. To use the H. and D. system correctly, it is essential to have a standard light, an expos ing instrument, and a photometer or instrument for measuring the densities.

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