International photographer (Jan-Dec 1934)

Thirty-four The INTERNATIONAL PHOTOGRAPHER July, 1934 Patterns of Illumination By F. Morris Steadman N my forthcoming book, illumination is classified into: Spherical, Hemispheric and Fractional. (This last, as with suns, flames, openings, etc., when part of the hemispheric field before each illuminated surface molecule is total darkness.) The three types are subdivided into about thirty sub-types or patterns. In the December installment of this series I promised to describe a pattern of illumination in which the intensity varies inversely to the distance and not to the square of the distance. This type is the long, narrow light source, the law applying for distances close to the source. The noted scientist, Sylvanus P. Thompson, in his speech of acceptance of the presidency of the first Illuminating Engineering Society, in London, unjustly ridiculed two men because they were making an effort to discover the law of intensity variation for the long luminous tube. This tube is a certain type of light source and functions by its specific law, Thompson and the Point Source theory notwithstanding. To arrive at this law: If we wrap a pure white paper around such a luminous tube, each of its contacting surface molecules will receive an energy influx from the tube in a full hemisphere of directions and the white paper will be made as intense as the tube itself. Here both the length and width of the tube occupy 180° relative to each point on the paper. If we now lay a carpenter's square over the tube, the latter will occupy but 90° (by its width) relalative to that point of distance where the two sides of the square come together. The length of the tube, being so great, remains practically at its former angle of 180° and the tube therefore now occupies one-half of the hemispheric field before the points of a small section of the paper placed at that distance. The paper here is therefore half as bright as the tube. Now if angles are cut in pieces of card-board with 45, 22^2 and 11/4 degrees and these be laid over the width of the tube, the distances will be located where the tube will function at *4> Y% ar>d 1/16 hemispheric, the alteration in the angle subtented by the length of the tube at these short distances, being negligible. At each increase of distance, only one of the tube's dimensions, its width, varies. Therefore, at each step the tube becomes one-half as large in the hemispheric space field, instead of one-fourth as for a more symmetrically formed light source. And since it is this change in solid angle that alters the intensity, we have the intensity varying also inversely to the distances, and not to the squares of the distances. No matter what type of illuminant is considered and no matter whether the distance is zero or greater, the basic law is: Intensity varies directly with the solid angle of the light pencil which illuminates independent molecules. There is no constancy or variation of light intensity, either in natural, artificial or optical illumination, which does not rest squarely on the above fundamental law. This law has been used and partially understood, since photography was discovered, in marking lens stops: In lenses of one inch and eight feet focal length, the F/8 stop has diameters of l/% of an inch and one foot, respectively. The exposures under otherwise like circumstances, would be the same. Again: We are distant from the sun 108 of the sun's diameters. Shining on a certain surface, the intensity of an image also cast on that surface would be of the same intensity, when the lens functions with a stop F/108 therein. This is because the optical and the natural light pencils would correspond in solid angle convergence. Another observation : A grain of white chalk in the sun will rest at zero distance, will be hemispherically illuminated, (on each of its sides or faces,) and will therefore be as intense as the sun itself. Also: If the sun could be spread out in the form of a hemisphere so as to fill our whole sky extension, and at the same time retain its intrinsic intensity, a grain of white chalk on the earth would also be made as intense as the sun, in spite of the fact that now this spread out sun would rest, not at zero distance, as above, but at its present distance of 93,000,000 miles. The change of distance is simply not a factor of the brightness produced. The unchanged intensity is due to the hemispheric light convergence under both the conditions given. Another phase of these basic laws at work reveals the fact that a pure white surface can not escape from the intensity, or the average intensity of the hemispheric space field which confronts it. To illustrate. The full sky shining at its maximum brightness of 512 Actinos, creates that same brightness on a horizontal white surface on the earth. Now if half of the sky extension should be completely darkened, leaving only half of it to function, the white surface would be reduced to half its former value, or to a brightness of 256 Actinos. But we also see that the average brightness of the whole sky expansion is now also 256 Actinos, from the fact that half of it has a brightness of 512 Actinos while the other half is at zero brightness. The brightness there, spread, in the imagination, over double the extension, gives the average brightness of the sky extension, as 256 Actinos, and the white surface has not escaped, therefore, from the average brightness of its confronting field. If this is true for this rather symmetrical, "50-50" condition in the sky, then it is also true for all the complicated conditions of luminosity which exist about us in nature. Hold a pure white surface anywhere you choose and although there may be a thousand different light values confronting it, the surface will be as bright as the average brightness of all of them. From this truth the student can make himself a light measuring tool from a little note book and some slips of tinting paper that will enable the intensity of any subject before the camera to be measured in Actinos of brightness, and the correct exposure found by simple division. Students need this working knowledge of light intensity. More than half the students in our schools have cameras but they are limited to "snap shooting" by their present ignorance of light values. Any reader who is interested in this reform should write the Editor of this magazine of that interest. The photographic lens also works according to a certain light pattern, which is usually spoken of as a "point to point" condition. Each point on the surface of objects before the lens spreads out its light before it in all directions. The lens front catches upon itself some of this wave and turns each ray in a different direction, with the result that these rays all pass through a specific point in space at a pre-determined distance behind the lens. If the light is caught on a ground glass, film or plate, at that precise Please mention The International Photographer when corresponding with advertisers.