Halation happens because film's base is transparent: light that survives the emulsion keeps traveling, hits the base-air interface, and some of it bounces back up to re-expose the emulsion from underneath, red-shifted because the shorter blue wavelengths scattered away on the trip. What is halation covers that mechanism in full. What's worth a closer look is the layer manufacturers put there specifically to stop it — and what happens when that layer fails partway.
Two Versions of the Same Fix
Most modern still-film stocks carry an anti-halation backing built from dye, coated onto the back of the base, tuned to absorb light at the wavelengths most likely to cause trouble. Some motion picture stocks — a number of Kodak's cinema films historically among them — use a different approach called remjet: a black, carbon-based layer bonded to the back of the base rather than a dye suspended in it. Both do the same job by the same principle, absorption instead of reflection, but they're not interchangeable materials, and remjet in particular needs its own removal step in processing (an alkaline pre-bath) before the film can go through standard development, which is part of why remjet stocks need labs equipped to handle it.
Either version is sacrificial by design. The backing's only function is to be there during the exposure and gone by the time development is finished — it plays no role in forming the image, and if it survived onto the final negative it would simply be an opaque layer blocking the picture. So it's built to dissolve or lift away chemically during processing, its job already finished by the time you'd ever need to look at it.
What a Scratch Reveals
A uniformly intact backing produces a uniformly suppressed halation response — whatever glow gets through does so more or less symmetrically around a bright point, however that point's own shape and the lens aperture render it. That symmetry is the tell for what's actually happening: the absorbing layer is uniform, so the light it fails to catch escapes in whatever pattern the optics alone would have produced.
Scratch or wear through that backing in one spot, and the missing patch doesn't behave like a smaller version of the same glow. It behaves like a gap. Light traveling to that exact patch of base finds no absorber waiting and reflects freely, while light hitting the intact backing around it keeps getting suppressed as designed. The result is a halo that follows the shape of the damage — a line if the damage is a scratch, a dot if it's a pinhole — rather than the round, aperture-shaped bloom you'd expect from an evenly weak or entirely absent backing. Damage location and image content also have nothing to do with each other, so a scratch produces its line of reflected light regardless of whether anything bright happens to be near it in the frame; it's a property of the film, not of the scene.
That's the practical signature worth knowing: a soft, symmetrical halo tracks back to the amount of backing present overall, uniform or absent. A directional, sharp-edged halo — especially one repeating in the same position across multiple frames from the same roll — tracks back to physical damage on one specific patch of base, not to anything about the light source itself.