Sometimes in a photograph there appears to be a blurring of the bright parts over the dark parts of the picture, and if lamps or other very bright lights are included they may appear in the print as bright spots surrounded by a dark ring beyond which is another bright ring. This curious effect, which is called "halation" is well illustrated in the photograph shown in Fig. 98.
Halation is caused by light which passes completely through the emulsion and also through the glass on which the emulsion is coated and is then reflected back into the emulsion from the back of the glass. The simplest form of such reflection is shown by the diagram, Fig. 99, where we see a ray of light falling on the emulsion at A. Most of this light is absorbed by the emulsion but some of it passes through to the glass and is reflected from the back of the glass, so that it reaches the emulsion again at B.
But this simple diagram does not account for the appearance of the lights in Fig. 98, because if a ray of light had fallen on the plate squarely at right angles and had passed through the emulsion at right angles it would be reflected straight back and the halation would not be spread beyond the image, whereas, the halation is just as bad in the center of the picture where the light fell squarely on the emulsion as at the edges. Also, it does not account for the ring which is shown around the lights.
Fig. 98. Halation in Print.
Fig. 99. Simplest Form of Reflection.
As a matter of fact, light falling on a photographic plate does not go straight through in this simple way. When a narrow ray of light falls on the grains of silver bromide it is reflected from them and scattered about.
So we must imagine that if we could examine a magnified section through the plate, we should see the light falling on the emulsion scattered in all directions, so that a narrow beam of light is spread out into a kind of blur, the size of the blurring being very minute but still appreciable, Fig. 101; this effect of the light spreading in the film is called irradiation.
We see then that the light which passes through the emulsion of the photographic plate is traveling in all directions, whatever may have been its direction before it reached the emulsion, and if we follow the light into the glass, we shall find that most of the rays pass out of the glass again into the air but that some of them are reflected back into the emulsion.
In order to understand this we must look at the way in which different rays of light travel through glass. (See chapter III (About Lenses).) When a ray of light passes from air into a block of glass, it is bent by the glass which is a medium of different density, and when it leaves the glass again it is bent back so as to travel along a path parallel to that along which it entered the glass, but if a ray leaving the glass meets the surface at too big an angle, it cannot go out and it will be totally reflected back again. See Fig. 102. It is these totally reflected rays which produce the ring of halation.
Fig. 100. Scattered Reflections.
Fig. 101. Irradiation.
Fig. 102. Rays in a Block of Glass.
When the image of the lamp falls on the emulsion and enters it, the rays are spread out by irradiation, so that we get a small spot at the center of the lamp, then this scattered light passes into the glass of the plate, and the rays which are near the center pass out into the air from the glass and we get a dark ring, but when suddenly the angle of the rays to the surface of the glass gets too big to get out they are reflected back and produce a sharp ring of halation around the center of the image, and then as they go farther and farther from the image the light gets weaker and the halation fades away again. Thus we can account completely for the rings of light shown in the picture.
If we coat the back of the glass with some substance into which the rays would pass directly from the glass and which would completely absorb them, we should wholly prevent the halation and if we choose this "backing", as it is called, so that it is of the right kind and almost completely absorbs the light, allowing very little of it to be reflected, then it will be quite effective in reducing halation, but in practice it is not altogether easy to get a satisfactory backing and to apply it correctly. The photographer tried a "backed" plate, but although he got rid of the sharp rings of halation his lights are still obscured by irregular blotches of light reflected from the back of the glass. (Fig. 105).
Fig. 103. Rays in Photographic Plate.
Fig. 104. Complete Diagram of Halation.
The best way of avoiding halation is not to have any glass at all. If we take the photograph on film, the support is sothin that the light has very little room to spread and we get only a very small spreading of the light rays. This spreading in fact is no greater than that necessary to give a correct representation of the effect of the light on the eye since there really is a spreading of the light in the eye and we do not actually see a bright light on a dark night as perfectly sharp, but as having a small amount of blur around it. So that in Fig. 106, which was taken on Kodak film, we get a result which gives a very good idea of the scene as it appeared.
Fig. 105. Print from Backed Plate Negative.
Fig. 106. Print From Negative Taken on Film.