In 189o, F. Hurter and V. C. Driffield (Jour. Soc. Chem. Industry, vol ix., p. 455, May 7, 1890) published a classic paper entitled "Photo-Chemical Investigations and a New Method of Determination of the Sensitiveness of Photographic Plates," in which they studied systematically the relation between exposure, development, and the deposit of silver produced in the photo graphic process. They first defined the photographic density D as being the logarithm of 1/transparency, or the logarithm of the opacity, which was defined as the inverse of the transparency.
The following table of relations between density and transmis sion will be convenient: Hurter, Driffield and others showed experimentally that the density D of a given silver deposit was proportional to the mass of silver per unit area contained in the deposit. It has recently been shown by Sheppard and Ballard that the relation is only approximate and that there may be considerable departures from true proportionality with variations of exposure and development. A deposit transmitting approximately one-tenth of the incident light, that is, having a density of 1, is given by about i/io mg. of silver per square centimetre of the film.
Basing their studies on their definition of density, Hurter and Driffield exposed photographic plates for definite times to a standard candle by means of a rotating wheel with cut-out sectors of various angles. The plates were developed in a non-bromided developer, fixed, washed, dried, and the densities plotted on a chart with the logarithms of the exposure time (logioE) as abscissae and densities as ordinates, as is shown in the accom panying diagram. (See fig. ii.) This shows what is known as the characteristic curve of an emulsion. There are three fairly well defined regions of the curve. Thus, from A to B we have the initial part convex to the log E axis, which may be termed the region of "underexposure"; between B and C, known as the period of "correct exposure," the increase of density is practically constant for each increase of exposure, being arithmetical for each geometric increase of exposure; in the third region, from C to D, this arithmetical increase falls until the density becomes constant; this is the region of overexposure.
By prolongation of the straight-line portion of the curve, the log E axis is cut at a point which Hurter and Driffield termed the "inertia," which, when divided into a factor, gives the "speed" of the plate.
Hurter and Driffield adopted the factor 34 and provided that the "inertia" should be expressed in "candle-metre-seconds"; that is, seconds of exposure at one metre distance from one British standard spermaceti candle. The speeds so obtained were appli cable to an exposure metre called the "Actinograph," which they designed. It is common in England to publish so-called "H. and D." speeds for photographic materials. Such speeds are not, however, always determined in accordance with the specifications of Hurter and Driffield. At the present time exposures are made to an electric lamp. There is no general agreement as to a satis factory method for evaluating speeds, although the question is being discussed internationally.
The spermaceti candle being quite obsolete as a standard light source, the character of the source to be adopted is of great importance, since its photographic effect will depend upon its colour. One candle power of daylight has more than ten times the photographic effect of one candle power of light from a spermaceti candle. As most plates are exposed to daylight, it might be assumed that this should be the standard adopted ; but the practical difficulties of ensuring a constant illumination of constant spectral composition are insuperable. The standard candle is notably poor in ultra-violet, violet, and blue radia tions, being distinctly yellow in colour. The same objection applies to the Hefner-Alteneck amyl acetate lamp, which was used in Germany as standard.
It is impossible to examine in detail all light sources suggested, but it has now been agreed to use a lamp standardized to work at a colour temperature of 2,360° K with colour filters to reduce its spectral composition to the equivalent of sunlight.
The spectral composition of the light is all important in view of the ever-increasing employment of colour-sensitive materials.