The formation of a halo is illustrated in Fig.
149, which represents a magnified section of an emulsion coated on to a transparent support, which we will suppose for the moment to be glass. Owing to the turbidity of the emulsion, each strongly illuminated point of the coating (e.g. the point receiving the image from the lamp), will itself radiate light in all directions. A fraction of the incident light is diffused from the face of the sensitive coating, and is absorbed by the black surfaces of the camera. Those of the rays which are diffused in directions almost parallel to the surface of the emulsion cause irradiation ( 205), viz. a spreading of the image on to neighbouring portions, which we have seen to be negligible in ordinary photographic work.
The luminous rays diffused towards the back traverse the support and reach the back surface of the latter. Those of them which are least inclined pass through the surface into the air, and are absorbed by the black surface of the negative holder ; the rays that reach the back surface of the glass at an angle equal to or greater than 4i° (the limiting angle for glass of mean refractive index 1.5) cannot emerge, and are totally reflected, thus forming the halo round the true image.' The dimensions of the halo depend entirely on the thickness of the support and its refractive index. While it is practically negligible with thin film as long as the image is not appreciably it becomes a disturbing factor in photography on film in miniature sizes and in cinematography, where there is considerable magnification. The intensity of the halo depends on the turbidity, the thickness, and the colour of the emulsion, as well as on the method of development. A thick or a yellow emulsion gives less halo than a thin or uncoloured one. Finally, the halo increases in relative intensity with increase in exposure.
Contrary to what has sometimes been stated, halation occurs with at least equal intensity on plates or on film exposed to light by their dorsal side.
232. The halo does not exhibit the geometric form which we have considered, except in the very rare cases in which separate bare lights appear against a dark background, as in the case of the photography of roads in towns at night, where, to obtain detail, there is, perforce, considerable over-exposure of the light-sources. In the case of a highly illuminated surface included in the field, the sum of the individual halos of each point of this surface causes its image to overlap the images of adjoining darker bodies. It is in this way that the image of the
window, in the photograph of an interior, is spread out and distorted, the rectangle becoming curved and the image of the cross-bars dis appearing more or less completely. In landscape photography the image of the sky shades off into that of the ground and of the trees or buildings which appear in outline against the sky, the slender branches of trees disappearing more or less completely. In the copying of a pen-and-ink drawing or letterpress, the image of the white paper encroaches more or less on to the images of the lines, and can, in certain cases, completely obliterate the finer details.' Even when the halo does not show as an alteration of the shape of the image, it appears as a fog, which is in addition to the fog due to reflection of light in the lenses (§ 57).
233. Prevention of Halation. The only effec tive preventives' are (a) The application to the back of the glass of an absorbent layer (Marlow, 1861), in optical contact with the support,' having a refractive index at least as great as that of the glass (Carey-Lea, 1865; Cornu, 1890).
(b) The interposition of a light-absorbent between the emulsion and the support.
This material may be either an opaque salt or a dye. It is eliminated during the course of de- velopment or fixation, or by special treatment.' In the case of plates the anti-halation coating is usually in the form of a layer intermediate between the glass support and the emulsion, for a coat on the back runs the risk of being scratched when the plates are placed in the sheaths or dark-slides. In films it is generally combined with the coating that most films have on the back, this coat being sometimes covered with a very thin coating of wax to facilitate A very ingenious method for the measurement of the anti-halation quality of a sensitive material (ratio of the quantities of light giving rise respectively to halation and the first trace of a developable image) has been devised by R. Mauge (1926) 234. One of the first methods employed to protect photographic plates against halation had the further advantage that it also protected them against over-exposure and reversal by solarization ( J. T. Sandell, 1892). The plate is successively coated with two or more emulsions of increasing sensitivity (§ 203).