Motion Picture News (Mar-Apr 1923)

1202 Motion Picture New/ Fundamentals of Light for Cinematography Continuation of Article on Relation of Principles of Physics to Photography Extracts from " Motion Picture Photography " by Carl L. Gregory, published by New YorkInstitute of Photography Chapter III [Continued) REFRACTION— When light passes from one medium to another of different density it is refracted or bent, as shown in diagram Xo. 5. The different colored rays Fig. 5. When a ray of light strikes another medium of greater or lesser density than the one it is leaving then, unless it strikes exactly perpendicular to the surface of the new medium, it will be bent or refracted. Figure 5 shows a ray passing through a block of glass and suffering two refractions, one upon entering and one upon leaving. In this case the two surfaces being parallel, the first refraction is neutralized by the second and the light ray continues in its original direction slightly displaced but parallel to its original course being refracted or bent in different degrees. Upon this principle depends the construction of lenses. Dispersion is shown in diagram Xo. 6. that is, light in passing through a glass prism is separated into its component parts, and in case of white light into the spectrum colors violet, indigo, blue, green, yellow, orange and red. Absorption — When light falls on an object which neither reflects, refracts nor transmits, the light is said to be absorbed. Xo known substance is an absolute absorber of light; that is, an absolute non-reflector. A flat or matte black surface comes the nearest to being a total absorber of light, but it is not possible to paint an object so black but what sufficient light will be reflected from it to reveal its details when brilliantly illuminated. Thus we see that what we call blackness is not caused by no light reaching the eye but when very little does. The blackest object looks gray in comparison to what is called Chevreul's black, which is the darkness of the mouth of a dark cavern or a hole in a large box lined with black velvet. If the object reflects only red all the other colors are absorbed ; if only yellow is reflected, then all others are absorbed. Again, if we use, as our incident light, any particular color of light which happens to be wholly absorbed by the object, that object will appear black; if, for example, we look at a yellow and a blue flower by the yellow flame of a spirit lamp with common salt in the wick, the yellow flower appears distinctly yellow, for it does not absorb yellow light on reflection, but the blue flower looks black, for it absorbs all the yellow light and reflects none of it. We have briefly discussed four qualities of light. The entire science of optics is embraced under these four sub-heads, and the better we understand these properties of light the more intelligently will we be able to know how to illuminate a scene and what lenses to use, in order to obtain any photographic result that we wish. We have already found that light is propagated outwardly in straight lines in every direction from a luminous object. When it strikes a smooth reflecting surface, such as a mirror or a pool of still water, it is reflected back from the reflecting surface at the same angle at which it strikes, or in more scientific terms, the angle of reflection is equal to the angle of incidence, as shown in Figure -8. As we have become accustomed to visualizing objects as being in a straight line before us, since light always travels in straight lines, when we look into a mirror we do not see the mirror itself but the image which it reflects, and the reflected image appears to be behind or beyond the mirror, since our habit of sight perceives the reflected object in that direction. If, however, the rays of light fall upon an object which is not perfectly smooth, each tiny particle which composes its surface presents a different angle to the light rays than its neighbor, so that the light will be reflected at a different angle from each of these particles. This light reflected from the rough surfaces has thus had its direction broken so that it travels in many different directions. This is shown in exaegerated form in Figure Xo. 9. Such light is called a diffused light, thus. on a cloudy or hazy day the light of the sun is diffused by its many reflections and rereflections from the particles of watery vapor I in the atmosphere. On a clear day the direct rays of the sun cast a dark shadow when any I object is interposed between the sun and any surface upon which its rays fall, but when the light is diffused the reflected rays from many directions fall beneath the object, since the object is not in line with these reflected rays, and illuminate the surface beneath the object and we are not able to distinguish any perceptible shadows. Practically all interior illumination is dif Figure 6 Fig, 6. Production of the spectrum by means of a prism haracter of a primary color. Figure 7 Fig. 7. Showing the elementary Primary colors cannot be further resolved into other colors Fig. 8 When light strikes a smooth reflecting surface such as a mirror or a pool of still water it is reflected back at the same angle at which it strikes or in more scientific terms the angle of reflection N, C, B in figure 8 if equal to the angle of incidence A, C, N, both angles being measured from a line perpendicular to the reflecting surface at the point where the reflection takes place. These two angles always lie in the same plane with the perpendicular line -which is always at right angles to the reflecting surface fused light, for we can only have direct illumination where the sun shines through a window or other opening. We find it necessary to diffuse the light in interior scenes in order to make them appear natural, for it is not yet possible in the majority of cases to obtain sufficient illumination in an actual interior to act upon a photographic film with sufficient intensity in the short time of the exposure necessary with the motion picture camera. We have to build our interior sets in a studio leaving them open to the light at the top, and generally upon two sides, thus allowing a flood of light to enter. If the stage is an open platform, or if the studio is not of ground or ribbed glass, which of itself diffuses the light, it becomes necessary to suspend screens of thin white cloth called diffusion or halation screens above the set, to break up and diffuse the direct rays of the sun. We can all recall witnessing, even very recently, interior scenes taken in the direct sunlight where the pictures hung on the wall cast long oblique shadows and the characters, as they went through their actions on the screen, were each accompanied by a funereal silhouette which mocked every gesture in grotesque distortion upon the floor or wall. Happily, such scenes have now passed into the limbo of fading memories. When artificial lights are used, such as arc lamps, the light is diffused by ground or ribbed glass screens or with tracing cloth or similar material. The tubes of Cooper-Hewitt lights cover such an area that it is not usually necessary to use a screen for (Continued on page 1204>