An anti-halation backing is a layer found in many

photographic film Photographic film is a strip or sheet of transparent film base coated on one side with a gelatin photographic emulsion, emulsion containing microscopically small light-sensitive silver halide crystals. The sizes and other characteristics of the ...

s—and almost all film intended for motion picture cameras—usually a coating on the back of the

film base A film base is a transparent substrate which acts as a support medium for the photosensitive emulsion that lies atop it. Despite the numerous layers and coatings associated with the emulsion layer, the base generally accounts for the vast majorit ...

, though it is sometimes incorporated between the light-sensitive

emulsion An emulsion is a mixture of two or more liquids that are normally Miscibility, immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloi ...

and the base. Its purpose is to absorb light that passes through the emulsion, thus preventing any light from being reflected back through the emulsion from the rear surface of the base, or from anything behind the film, such as the pressure plate of the camera. This prevents a

halo HALO, halo, halos or haloes may refer to: Most common meanings * Halo (optical phenomenon) * Halo (religious iconography), a ring of light around the image of a head * ''Halo'' (franchise), a sci-fi video game series (2001–2021) Arts and en ...

-like effect (halation) from forming around bright points or edges in the image. Still cameras, which handle less film and thus contend with less wear, typically hold their film in the gate with components painted or treated to be black, so reflections are less of an issue and few still films made use of anti-halation backings. The notable exception was Kodak's

Kodachrome Kodachrome is the brand name for a color reversal film introduced by Eastman Kodak in 1935. It was one of the first successful color materials and was used for both cinematography and still photography. For many years, Kodachrome was widely used ...

, which incorporated such a backing to aid with a very sensitive innermost layer. The anti-halation layer is rendered transparent or washed out during processing of the film, for example, the K-14 process for Kodachrome still film and the Eastman Color Negative (ECN2) process for color motion picture film both have steps which remove this layer. The lack of an anti-halation layer in

Kodak High-Speed Infrared Kodak High-Speed Infrared film, also known as Kodak HIE, was a popular black-and-white infrared photographic film from Kodak. The film was sensitive to the visible light spectrum (with decreased green sensitivity), infrared radiation up to 900nm i ...

(HIE) film caused the ethereal "glowing" effect often associated with

infrared photography In infrared photography, the photographic film or image sensor used is sensitive to infrared light. The part of the spectrum used is referred to as near-infrared to distinguish it from far-infrared, which is the domain of thermal imaging. Wav ...

, rather than an artifact of IR itself.

Motion pictures

This effect is particularly pronounced in motion picture cameras. These cameras are subject to the constant motion of film being dragged through the film gate, so most motion picture cameras have film movements made or plated with wear-resistant alloys such as hard chrome. Given such a relatively reflective pressure plate behind the film, many motion picture films use an anti-halation (and anti-static) backing. Different kinds of effects can be achieved by removing the anti-halation backing. Halation is one of the properties unique to analog film stock and isn't found in digital footage unless modified in

post-production Post-production, also known simply as post, is part of the process of filmmaking, video production, audio production, and photography. Post-production includes all stages of production occurring after principal photography or recording indivi ...

. Despite anti-halation backing, most film stock still renders a slight red halo around the brightest elements in a picture, where the incoming light is so strong that it cannot fully be absorbed by the anti-halation backing, and instead is scattered back into the red layer of the stock, creating additional, halo-like exposure in that particular layer, before it gets fully absorbed. Halation can be digitally imitated to some degree, as shown below:

References

Photography equipment {{Photo-stub