by DR. V. B. SEASE
DuPont, Pathe Film Mfg. Corp.
HALATION is defined as a halo of light sometimes seen around the image of a bright object in a photographic positive print. In a negative it appears as a flare of silver along the edges of the bright object. It is most likely to occur in photographs of bright incandescent lamps, of strongly back lighted objects, of interior scenes including sunlit windows, or of the outline of a building against a blue sky.
What Causes Halation Photography and Cinematography Effects
It is clearly a phenomenon of scattered light. It is known that the crystals of silver halide in the sensitive layer may scatter light, but in the fine grained high speed emulsions of the present day it is doubtful if this scattering effect is ever responsible for more than a dulling of definition. True halation is caused by the reflection from the back of the film or plate of a certain amount of light which has passed through the sensitive coating. Sensitive coatings may be quite translucent. This is evidenced by the fact that in a bipack film a very strong image is recorded on the rear film by the light which has passed through the front orthochromatic emulsion.
The scattering which the light receives in passing through a sensitive coating means that some of the rays will strike the rear surface of the film or plate obliquely and will be reflected back into the emulsion at some distance from the edge of the image.
The History of Halation: From Collodion Plates to Modern Film
Halation is not a new phenomenon. It dates back to the days of the collodion plate. As early as 1867 we find an American inventor securing a patent on a black backing for collodion plates. Early workers encountered serious trouble with halation. They sought to remedy it in various ways. They recognized the fact that the rear surface of the glass or celluloid support was an excellent reflector of light, since the index of refraction of these materials is quite different from that of air. The surface of the support on which the emulsion is coated disappears because of the fact that the gelatine of the emulsion has approximately the same index of refraction as the support, so the early experimenters attempted to eliminate the rear reflective surface. They coated the back of plates with caramel, or other substances whose index of refraction is similar to that of glass. Dyes, lamp black, or other pigments were put into the caramel to absorb the light. Such back coatings accomplished the desired results. They were temporary and very easily washed off in the developer or in running water.
Other backing compositions such as asphaltum in benzene or such as pigments or dyes in collodion, gelatine, etc. were successfully used. However, a little more effort was necessary for the removal of those backings which were not water soluble.
Preventing Halation: Historical Approaches and Patent History
There are many references in the literature to attempts at incorporation of dyes into the emulsion to prevent penetration by light While the idea was fundamentally good, it was found that the dyes generally had a deleterious effect upon the photographic quality of the emulsion.
Substratum Coatings and Dye Absorption Methods
A number of schemes have been revealed for putting a substratum coating under the emulsion, capable of absorbing the light to which the emulsion is sensitive. An English inventor as early as 1893 disclosed a dyed gelatine coat between the emulsion and the plate. He claimed that the color might remain without serious annoyance in printing. Seyewitz recommended a coating of transparent red, orange or green dye
between the emulsion and the plate. He noted that only a few dyes such as congo red and rosolic acid could be used, as most dyes diffused into the sensitive coating and reduced sensitivity. He preferred that the dye should be capable of being readily bleached so as not to affect the printing from the finished negative. The dyed substratum coating was indeed a beautiful theory but was hard to put into practice
on account of the tendency of dye to diffuse into the sensitive layer.
The Double-Coated Plate: Origin of Anti-Halation Technology
While searching for suitable dyes to try in a substratum coating, Sandell recalled that lantern emulsions were orange colored, and he conceived the idea of coating fast negative emulsion upon a lantern slide plate. His anti-halation results were remarkable. Thus originated the double coated plate which has enjoyed a wide popularity. Such plates owe their anti-halation quality, however, not to the color of the undercoat but rather to the opacity of the fine grained undercoat and its ability to absorb light. Undoubtedly this has proved to be one of the most satisfactory methods for prevention of halation, as the anti -halation coating disappears in the hypo bath and leaves a perfectly clear negative without any extra operations of removing backings or discharging dyes or pigments.
Manganese Compounds and Chemical Anti-Halation Treatments
There have been numerous patents for brown manganese compounds in an undercoat. These compounds are capable of being destroyed in a strongly acid fixing bath. A German firm covered molybdenum ferrocyanide in the same manner. The alkali of the developer was supposed to destroy the ferrocyanide.
Tinted Film and Colored Supports as Anti-Halation Methods
Other workers have described the use of a colored support to prevent halation. In 1894 an English inventor defined certain permanent colors for either support or sensitive layer. A Belgian inventor revealed the application of a violet tint under the substratum or on the back of celluloid film. Celluloid tints are not easily bleached out, but a violet or blue tint offers very little interference in making prints. Workers with tinted film like those with dyed undercoats, have confined their efforts to the use of colors which absorb in the spectral region in which the emulsion is sensitive. Any yellow or red dye is effective as an anti-halation tint with orthochromatic film but panchromatic film requires a tint secured with two or more dyes whose composite absorption curve has approximately the same shape as the spectral sensitivity curve of the emulsion.
Halation in Modern Panchromatic Film Emulsions
Thus it is seen that the problem of producing anti-halation emulsions has been tackled during the past from every angle. Early workers probably had more difficulties than the photographer of today. Although they had much slower emulsions, these emulsions were coarse grained and, therefore, had low opacity which allowed much light to pass through their coatings. The modern fine grained emulsions with their high opacity have done much to relegate halation to a position of secondary consideration.
Early workers also had emulsions whose sensitivity was mainly in the blue end of the spectrum, while modern emulsions are becoming more and more sensitive toward the red end. A gelatine-silver halide layer has a good absorbing power for the blue, or short wave lengths of light, but it is much more easily
penetrated by the yellow and red rays. So the tendency to increase panchromatic speed more and more has called the attention of the photographic industry again to halation. But with a background of much experience in combating halation, it is certain that this industry will not allow the quality of present day photography to suffer on account of this defect.
From the American Cinematographer July 1931