Halation, bloom and glow — what is what
The soft red-orange halo around bright lights in film footage is one of the most recognisable film artefacts and one of the most often faked badly. It has a specific physical cause, appears only in specific conditions, and looks wrong the moment it is applied everywhere.
What causes halation
Light passing through the emulsion does not all stop there. Some continues into the clear plastic base, reflects off the back surface, and returns into the emulsion a small distance from where it entered — exposing it a second time in a ring around the bright area. Because the red-sensitive layer sits deepest and red light scatters furthest, the returning light is predominantly red-orange. Most film had an anti-halation backing to suppress this; the effect is strongest in stocks where that layer was thin or removed, which is why certain stocks are prized for it.
Why it is not the same as bloom
Bloom is a lens phenomenon: light scattering inside the glass elements, spreading a general veil around bright sources. It is roughly neutral in colour and affects the whole frame's contrast. Halation happens in the film itself, is coloured, and stays local to the bright area. Digital sensors produce a third thing entirely — clipping and sometimes blooming into neighbouring photosites, which reads as a hard white smear rather than a soft coloured ring. Mixing these up is why some 'film look' presets glow in a way that does not resemble film at all.
Where it should and should not appear
Halation needs a bright source against a darker surround: practical lights at night, a window in an interior, sun through leaves, a highlight on wet skin. It does not appear across an evenly lit daylight scene, and it does not wrap every edge in the frame. The most common mistake in simulation is applying a glow to everything above a brightness threshold, which produces a soft, foggy image rather than a film one. Restrict it to genuine highlight sources and keep the radius small.
Tuning it so it reads as film
Three parameters matter: threshold, radius and colour. Set the threshold high enough that only real light sources trigger it. Keep the radius tight — halation is a halo of a few pixels at normal viewing size, not a general haze. And keep the colour in the red-orange range rather than white, since white glow reads as lens bloom and instantly looks digital. If you are unsure whether you have too much, look at a night frame with streetlights: the lights should have a warm edge, not a mist.
Frequently asked questions
Why is halation red and not another colour?
Because red light penetrates the emulsion layers most deeply and scatters furthest before reflecting off the film base, so the light that returns is weighted towards red and orange. The exact hue varies by stock and by how the anti-halation layer was designed.
Do modern films still show halation?
Much less, because anti-halation layers are effective. The stocks people specifically seek out for the effect are usually ones where that layer is thin, absent or removed during processing, which is why the look is associated with certain films rather than film in general.
Can I get halation from a lens instead?
Not the same effect. Lens filters and vintage glass produce bloom and flare, which are neutral and spread wider. They can be lovely, but they read differently — halation is specifically a film-base phenomenon.
Should halation be visible at all in daytime outdoor shots?
Barely, if ever. Unless the sun sits directly in frame or there is a strong reflection off glass or water, an evenly lit daylight scene is just as foreign to noticeable halation as real film is; if it shows up constantly, the threshold is set too low.
See it on your own camera. CAMCAM runs the film simulation as a live shader on your phone's camera. Nothing is uploaded — the look is computed on the device, frame by frame.
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