Where the glow comes from
Film is a stack of light-sensitive layers on a transparent base. Most light is absorbed by the emulsion, but a very bright highlight sends more than the layers can absorb, and the surplus passes through to the base, reflects off the back surface, and scatters back up into the emulsion around where it entered.
The result is a soft ring of extra exposure surrounding the bright area. It is coloured because the layers are ordered by wavelength sensitivity and red penetrates deepest, so what comes back is predominantly red. That is the whole mechanism, and it explains every property of the look: it is tight, it is warm, and it only appears where the highlight was strong enough to overwhelm the emulsion.
Manufacturers fought it with anti-halation backing layers, which is why the effect is far stronger on older stocks and on reversal film than on a modern negative. Cinematographers who want it now often shoot stocks or formats where it survives.
Telling it apart from its neighbours
Three effects get conflated, and grading them as if they were the same produces the generic hazy look that reads as a filter.
- Lens flare. Scatter inside the lens assembly. Often has structure: streaks, polygonal ghosts, a veiling wash across the frame.
- Bloom. A broad neutral glow, usually a rendering or sensor artefact, and typically the thing game engines mean by the word.
- Halation. Coloured, tight, and film-specific. It hugs the highlight.
The distinguishing test is radius and colour. A warm glow within a few pixels of a bright source is halation. A neutral wash across half the frame is flare or bloom.
Adding it convincingly
The technique is restraint, and the main failure is applying it globally.
The correct approach isolates the brightest part of the image first, using a highlight-only selection rather than the whole frame. Blur that isolated area slightly, tint it warm toward red, and composite it back at low strength. Because the source was only the highlights, shadows and mid-tones stay untouched, which is exactly what film does.
The common mistake is to add a soft glow layer over the entire image and reduce its opacity. That lifts the blacks and softens everything, which is why so many attempts at a film look end up flat and misty rather than filmic. Real halation coexists with deep, uncompromised shadows.
Prompting it
Models know the word, and they know it in company: asking for halation alone often returns a mild general glow, while asking for it as part of a coherent film signature works better.
Qualify the strength and the location: gentle halation with a warm red bleed around the brightest highlights only. The word only is doing real work there, because it discourages the global wash.
Give the scene something to glow. A frame with no bright source has nothing for the effect to attach to, so name the sources: a bare bulb and a neon sign in frame. Night interiors, neon exteriors and backlit windows are where it reads most clearly.
Then protect the shadows explicitly: deep shadows with no lift. This one clause is what separates a convincing film look from the hazy version, and it is worth including whenever you ask for any glow effect at all.