Halation is the soft glow that surrounds bright highlights in a film image, caused by light passing through the emulsion, striking the film’s backing or pressure plate, and scattering back through the emulsion from behind. Each highlight effectively becomes a small secondary source, and the reflected light exposes the silver halide grains around the original highlight, creating a halo. The effect is strongest at high-contrast edges (window frames, neon signs, headlights, sun glints) and falls off softly into the surrounding tone.
The phenomenon depends on the structure of the film. Black-and-white and color emulsions are coated with an antihalation backing (typically a dye layer that absorbs stray light and washes out during processing) to suppress the effect, but the backing is never perfect. Some stocks, especially Cinestill 800T (which is repurposed motion picture film with the antihalation layer removed for still-camera development), produce dramatic red halation around any bright source, since the remjet layer that normally absorbs that light has been stripped before sale.
Halation is almost entirely absent in digital sensors. The microlens array, color filter array, and silicon photosite stack are designed to deliver photons cleanly to each pixel, and the absence of a translucent emulsion means there is nothing for light to scatter back through. What digital does produce is blown highlights with sharp edges, bloom around very intense sources (a different optical effect), and lens flare, none of which look like film halation.
Because of its strong nostalgic association, halation is heavily emulated in modern grading and film-look profiles. Software like Dehancer, FilmConvert, FilmBox, and Wes Anderson-flavored Lightroom presets simulate the effect by blurring the highlights, tinting the bloom slightly red or orange, and adding a small amount of grain. The result is recognizable but rarely identical to film, since true halation tracks the spatial layout of each highlight rather than applying a global blur. Cinematographers use halation as one of the easy tells that an image was shot on film rather than digitally with a LUT.
Practical use of halation in color grading is straightforward but easy to overdo. A subtle red glow around streetlights, taillights, and neon sells a night street scene as filmic; the same effect cranked up makes faces look like they are radiating fire. Most grading plugins offer separate controls for halation intensity, color, and threshold, and the best results sit just below the level where you actively notice them.
Halation is distinct from grain, gate weave, and reticulation, though all four are bundled into most film-emulation packages. It is also distinct from chromatic aberration, which originates in the lens rather than the recording medium. Understanding it as a specific optical artifact of layered emulsion (rather than just a creative effect) helps a digital photographer apply it convincingly, and recognize it as a stylistic signature of stocks like Cinestill, Portra 800 pushed, and motion picture film in general.