HALATION (Fr., Halation, Aureolage ; Ger., Licht fleck) A halo-like, blurred effect, frequently seen surrounding a brightly lit portion of a photo graphic image, caused either by reflection from the back of the glass plate or by lateral spread ing of light in the film. The defect has been observed from the earliest days of photography, but it became more common on the introduction of glass plates. It is believed that the first men tion of the word in print occurred in the year 1859. Halation is usually seen in its most aggravated form in a photograph of a dark interior which includes a bright window. The latter will not be clearly outlined, and all round it there will be fog. Halation also appears sometimes when the roof of a house or a tree cuts against the sky ; around a white collar or dress in a portrait ; and, in fact, in all subjects where a very bright line comes in sharp contrast with a deep shadow. The chief cause of hala tion is the reflection from the back of the plate of the bright beam of light that reached it. In the illustration, the plate is shown in a ver tical position, A being the film side and B the plain glass side or back of the plate. E F repre sents a ray of light coming from the lens and striking the film at r, part of it being reflected along the line F G and being dispersed in the camera. The light being strong and the sensi tive film translucent, some of the light passes through the film and strikes the back of the plate at j. If all the light continued its way and went out at the back, it would do little or no damage, but instead at j another reflection takes place, as at j a, and the light passes again through the film and causes fog on the other parts. Assuming that a bright window in a dark interior is being pictured on the plate, and that the part below P represents the window, and the part above F the dark wall of the room, some of the light rays Er from the window would pass through the plate to j and be reflected to Er, thus causing the sharp outline of the win dow to be blurred. When the bright object comes at the side of the plate, the halation is usually worse than when it comes in the centre, because rays of light which strike the plate almost at right angles to its surface at e D are reflected back through the film at practically the same places through which the light first passed.
As the degree of obliquity increases, the amount of reflection increases also, until at a certain angle, varying with the refractive index of the glass, the whole of the light is reflected. The thicker the glass plate the greater will be the extent of the halation, because of the wider angle formed by the reflected rays ; this explains in part why the thin celluloid films do not show appreciable halation. A contributory cause of halation is under-exposure and forced develop ment, conditions that are more often responsible for the defect than are the direct causes. As a rule, the quicker a plate is developed the less will halation show.
It will be seen that the principal causes of halation are (a) the translucency of the emul sion, and (b) the reflective power of the back of the glass. Some remedies are at once obvious. (1) To replace the glass with paper or other non-reflecting support. (2) To stain the film or in other ways to make it so opaque as not to allow the rays to penetrate the glass. (3) To prevent the back reflection by coating the plain glass side with a non-reflecting or absorbing medium. Others have also been suggested. The third remedy, that of coating the back of the plate before exposure with some non-actinic and non-reflective substance, is in general use, and has been found the most convenient and effective in practice. Formulw for suitable mixtures are given under the heading " Backings, Plate." In process work, it is found advisable to back dry plates with caramel or other medium to prevent halation, which would rob the half-tone dots of their sharpness. A sharp dot is an essen tial in half-tone work.
A commercial size of dry plate and printing paper, measuring 6+ in. by 42- in., and largely used for " cabinet " portraits and occasionally as a stereoscopic size.