Photographic Journal, Sept 1911 p. 302
Halation, the effect produced in the negative by light which has passed through the sensitive surface and been reflected back by the inner surface of the glass or other support, is of minor importance compared with irradiation, or scatter of light, in the gelatine film. Circumstances conducive to halation proper are inimical to irradiation, though the remedies suited to the one defect are frequently advocated for both.
Transparency of the sensitive film reduces irradiation, but, as more light passes through the film, the effect of reflection, unless the plate is properly backed with a light absorbing material, is greater than with a more opaque emulsion. Lantern plates are invariably more transparent than negative plates; but how many people trouble to back them? Chloride plates are particularly transparent, and it is essential that they be backed to obtain good results. There are two ways of avoiding halation : (1) To use a sensitive film which is transparent, and to absorb all the rays passing through it with a perfect backing; the Lippmann film offers advantages in this direction, but it has the defect of slowness: (2) to work with a film through which light would not penetrate, a thick film physically and optically; this would give every chance to irradiation. There is a compromise, probably useful, but here I do not write from experience, in which the sensitive film is coated on a light-absorbing layer. In this case halation should be avoided, but irradiation remains.
Modifications of development are frequently suggested as curing halation, but it is quite open to question if they have any value whatever. My experience in a set of experimental exposures was that halation was proportional to density. I should expect a plate that had received surface development and subsequent intensification to exhibit as much halation as a negative simply developed up to that same density. Halation is easy to obtain experimentally on plates. A photograph of an incandescent filament lamp shows it as well as anything. The result then is seen to be quite distinct from irradiation. It is quite possible to detect a secondary line, following the line of the filament, where the maximum reflection has taken place; irradiation merely blurs the image. Halation, in my opinion, is seldom or never met with in ordinary photographic practice, but irradiation is generally miscalled halation.
The connection between over-exposure and irradiation is a subject that would be worth investigation.