International projectionist (Oct 1931-Sept 1933)

HALATION: ITS CAUSE, EFFECT AND THE REMEDY H. Parker and J. I. Crabtree RESEARCH LABORATORIES, EASTMAN KODAK COMPANY THERE has been considerable discussion recently on the subject of halation and methods for its prevention. Halation is the blur or halo of light which is sometimes seen in photographs around the edges of bright highlights, and although it may be present throughout the area of the highlight, where it has a deleterious effect on the quality, it is usually noticed only when the blur of light extends into the dark area surrounding a highlight. In most cases halation is undesirable because it tends to destroy detail, and this is particularly true in the case of the motion picture because of the high degree of magnifipation on the screen. In order to attempt to eliminate or reduce this spreading of light around the highlights, it is necessary first to understand the cause. Cause of Halation When a ray of light falls on the photographic emulsion, it does not. pass straight through, but is scattered in all directions by the silver bromide grains which it encounters in the sensitive layer. This scattering, which causes a spreading of the edges of the ray, is called "irradiation," and results in a slight but perceptible blurring around the edges of the image. When these scattered rays reach the back of the emulsion and pass into the film support, they are travelling in all directions, and so strike the rear surface of the support at all angles. Most of these rays pass on out into the air, but some strike the rear surface at such an angle that, because of the difference in the refractive indexes of the support and the air, they cannot escape but are totally reflected back into the emulsion, thus causing a ring of light around the image. This ring of light reflected from the base is the true cause of halation. The diagram of a cross-section of film shown in Figure 1, with a narrow incident ray and its scattered and reflected rays, illustrates the manner of formation of both these effects, the irradiation and the halation proper. Means For Solution It can be seen readily that the nature and severity of the halation will depend to a large degree on the physical characteristics of the negative material, because the transparency of the emulsion determines the fraction of the light transmitted and the degree of scattering; while the tendency for reflection at the rear surface of the support will determine the quantity of light reflected and the thickness of the support will determine the size of the ring. Many means have been devised for reducing halation, either by increasing the effective opacity of the emulsion layer so that less light could reach the support, or by reducing the tendency for reflection from the rear surface. Light absorbing dyes have been incorporated in the emulsion itself, or in a substratum over which the emulsion is coated, but this often involves an unnecessarily prolonged period of washing in order to remove the dye for printing. It was found, however, that excellent results were obtained by coating the fast riegative over a slow opaque emulsion, such as a lantern slide emulsion. This acted in two ways to reduce the halation: first, because the bottom emulsion being quite opaque, only a small proportion of the light could pass through, and, secondly, since it was relatively insensitive, the light which did get through to be reflected produced very little effect. Many means have been tried to reduce the reflection from the rear surface of photographic glass plates. If some material having a refractive index equal to that of glass is coated on the back of the plate, the reflecting surface is moved to the rear surface of the coating. If then, the coating is colored with a dye or a pigment, such as lamp black, all the light is absorbed before it can be reflected, and halation is eliminated. Many backing materials, colored in various ways, have been used with more or less success. Some of the backings were Figure 1