American cinematographer (Jan-Dec 1926)

Twenty-six AMERICAN CINE MATOG RAP HER December, 1926 fied, is the influence exerted by the obliquity of the incident ray. If we stand on the shore of a large body of water, such as a tranquil lake, we can readily observe that at high noon, when the sun is approximately at the zenith of the lake surface, we can look at the water without being disturbed by any glaring reflection. As the sun nears the horizon, we will notice that a sufficient amount of sunlight is reflected by the water, so as to bother the sensitiveness of our eye. This will happen when the incident and reflected rays form equal angles with the normal to the water surface. The more the sun continues to approach the horizon, the greater is the obliquity of the incident ray and the greater becomes the intensity of the reflected ray, to such an extent that if the eye is placed at the proper angle, the glare effects it almost to the same extent as if the sun was looked at directly. The smoothness of the reflecting surface is obviously an important factor when a maximum of reflected light is desired. The roughness of a surface is formed by a conglomeration of small surfaces facing the incident light under different angles. In viewing such a surface, our eye will only be struck by the reflected rays striking the surfaces whose position is such that they answer the laws of reflection. Only a portion of the surface will thus respond to these requirements, and the intensity of the reflected light is then diminished in proportion to the degree of roughness of the surface. The infinite number of conditions in which light can be reflected render impractical an average calculation of the intensity of reflected light. As a reference, taking the incident ray perpendicular to the reflecting surface, it has been found that: Mercury reflects 374 of the incident light; Silver reflects 3 5 of the incident light when its surface is smooth and highly polished; Glass reflects 1/25 of the incident light; Water reflects 1/50 of the incident liirht. LET us note here, that in the study of light, when mention is made of a mirror, a distinction must be made between the well known object of everyday use and an optical mirror. ONE of the uses made of reflected light by the photographer is to reflect the light of a luminous body in order to more strongly illuminate the object he desires to photograph. Of this order are the various kind of reflectors used by the cinematographer in outdoor work, and the white surfaces of the interior of the art lamps and backing the mercury tubes in studio photography. Reflection of light is applied in the Graflex type of cameras, in which the image formed by the photographic objective is reflected by a 45° inclined mirror, to a focusing ground glass. An interesting application of reflected light is the prismatic reflection and side reversal of the image formed on the focusing ground glass and the prismatic reflection and side reversal of the rays emanated by the subject, before entering the photographic objective. This reversal of the image is extensively used in the photography of object destined to be reproduced in printing, in the half-tone color process. (To Be Continued Next Month) In optics the reflecting surface alone is called a mirror, while in a glass mirror the glass is merely the support and the protection to the silver coating which is the real optical mirror. In fact, in a glass mirror the front surface of glass and the silver coating are two distinct reflecting surfaces and this is proven by the following experiment. Place a lighted match in front of a glass mirror and look at it obliquely. Two very distinct images of the match are seen; one reflected from the silver coating, and the other from the front surface of the glass. Furthermore each one of these images become a luminous object in respect to either one of the two reflecting surfaces, and other images of the match are seen which would multiply to an infinite number were it not for the absorption and scattering of light that takes place at each reflection. These images are seen to gradually diminish in intensity, until the eye is not any more affected by their luminosity.