Here follows the table of incident light, one actino, first appearance times for various colored surfaces, as measured with any factor 8 tinting medium, as citrate, disco, solio, sunrae, etc.
As has already been explained the above numbers may also be considered as the actin icities of these different colored surfaces when their incident light measures 1 second with a slow medium. The actinic factors or relative actinicities in the same light are given as it may be necessary to refer to them later.
It must not be lost sight of that the effective actinicity as well as the luminosity of any sur face is greatly influenced by the direction from which it is photographed or seen. This follows in accordance with well known laws of reflection. In the analysis of subjects practically, as will be explained later, these differences are allowed for in such a way as to make it unnecessary further to consider them, except in certain special lightings in portraiture, as for example in the so called "line" lighting. In this and similar cases the narrow edge or "line" of light is greatly increased in intensity by the angle from which the face is photographed. Here however the incident light is measured when the window is arranged as for an ordinary front view lighting, and, while preserving this in tensity as the basis of exposure, about half of the incident light is screened away from the face with some opaque screen, being careful that this screen be placed far enough from the window not to interfere with the light on the reflector which is used for raising the actinicity on the shady side of the face.
To further illustrate, suppose it is desired to photograph some red roses, and the first ap pearance time of the light incident upon them (always in the case of incident light it should be understood that a factor 8 tinting medium is used, unless otherwise stated) is found to be 4 seconds; what is their actinicity, and what will be the normal exposure in photographing them with a 4 unit diaphragm and using a plate having a speed of 128 seconds or 2 minutes? It will be noted from the accompanying table that in an 8 second incident light, a red surface has an actinicity of one actino. Now since
the incident light is twice as strong as this, measuring 4 instead of 8 seconds the actinicity of the roses will of course be 2 actinos. Or, by the contrary method, the table shows that for red a 1 second time indicates an intensity of 8 actinos, and a time of 4 seconds, being only 4 as strong, indicates only i of 8, or 2 actinos as before. Now it has already been explained (p. 17) that the speed of a plate is the exposure time with diaphragm number one, unit scale, when the actinicity of the surface photographed is one actino. The surface being 2 actinos in intensity it will require, with the same, or unit diaphragm, one half of the plate speed exposure, 128 seconds, which is 64 seconds. Since, how ever, the 4 unit diaphragm is employed instead of the unit cone, the exposure is : of 64 or 16 seconds. Or, if the 64 unit diaphragm be used the exposure will be 1 second, etc.
Again, a newly painted white building is in bright sunlight when the sun is high and the day clear; what is its actinicity? In the paragraph dealing with bright sunlight conditions (p. 151) it will be seen that when the sun is high the first appearance time in the open with p.o.p. is i of a second and that the light value is never greater than is indicated by this time. It will be remembered from the table that a one second time indicates for white an actinicity of 128 actinos and since a time of i second indicates eight times as strong a light the actinicity of the building will be 8 times 128, which is 1,024 or 1M actinos.
Rule: Divide the actinicity indicated by a one second first appearance time for any colored surface by the time as measured; the quotient will be the intensity of that surface in actinos. Or, divide the characteristic one actino, incident light, first appearance time for any surface by the first appearance time as measured; the quo tient will be the actinicity of that surface in actinos.