Guide · Updated
What is halation
The red glow round lights on film.
Halation is the red-orange glow round bright lights in a film photo. Light goes through the film, reflects off the back and comes into the bottom, red-sensitive layer again, a little to the side: a red halo, orange right next to a strong light. Every color film has a faint one, seen mostly round lights against dark. CineStill 800T, a cinema film sold without its anti-halation backing, has a strong one.
A phone photo shows a clean edge round a light instead, or a sharpening halo. Editors add halation as a blurred, red-tinted copy of the highlights; a film camera app can build it into the film.
01 · How it happens
What causes halation
A color film has three layers sensitive to blue, green and red, with the red one at the bottom, next to the film’s base. Light that gets through all three reflects behind the emulsion and comes back up into the red layer, spread a little to the side. Where it lands, the film records light that was never there: a glow round anything bright.
Film makers put a dark anti-halation layer behind the emulsion to soak that light up. It is “not absolutely black” (Dehancer), so some always comes back. Sources disagree on where the reflection happens, at the back of the base or at the camera’s pressure plate; they agree on its color and where it shows.
- How much
- Andrea Volpato’s spectral film model uses 3 % of the light coming back in red, 0.3 % in green and 0.1 % in blue.
- How far
- A spread (sigma) of 200 µm in the same model: about 0.6 % of a 35 mm frame’s 36 mm width, or about 23 pixels on a 12 MP photo (our arithmetic).
- What kind
- Added exposure, so the film’s curve shapes it like any other light. It shows most where the surroundings are dark.
02 · Color
Why halation is red
The reflected light reaches the bottom layer first, and the bottom layer records red. Right beside a strong light, enough comes back to reach the green layer above it too, so the glow turns orange there and red further out. The light itself stays white.
03 · Films
Which films have halation
All of them a little; how much depends on what stops the light behind the emulsion.
| Film | What stops the light | Halation |
|---|---|---|
| Color negative (Portra 400, Gold 200, Ektar 100, Superia X-TRA 400) | An anti-halation layer (on Portra 400, inside the film, clearing in processing) | Faint, seen round lights against dark |
| Color slide (Kodachrome 64, Velvia 100) | A backing (Kodachrome’s was removed in K-14) or an undercoat that clears | Faint |
| Cinema negative (Kodak Vision3 500T) | Rem-jet, a dark backing the cinema lab (ECN-2) removes | Faint, as other color films |
| CineStill 800T | Vision3 500T with the rem-jet taken off so a C-41 minilab can run it | Strong red rims round every light |
| Black and white (Tri-X 400, HP5 Plus 400) | One silver layer, on a gray base (Tri-X) or a backing that clears in development (HP5 Plus) | No red halo: there is no red layer |
CineStill 800T
Kodak makes Vision3 500T for cinema: “balanced for exposure with tungsten illumination (3200K)”, on “an acetate safety base with rem-jet backing”, processed in ECN-2 (Kodak’s datasheet). The rem-jet is a dark coating that the cinema lab washes off. CineStill takes it off before selling the film, so an ordinary C-41 minilab can run it. With nothing dark behind the emulsion, the red rims round lights are strong, and that glow is what the film is known for.
04 · Glows
Halation, bloom and sharpening halos
Three glows round bright things that are easy to mix up:
| Quality | Halation | Diffusion filter | Sharpening halo |
|---|---|---|---|
| Where it happens | Inside the film, behind the emulsion | In glass in front of the lens | In the phone’s processing |
| Color | Red, orange right beside the light | The light’s own color | A bright and a dark rim, no color of its own |
| Spread | Narrow: about 0.2 mm on the film, half a percent of a 35 mm frame’s width* | Wide: a few percent of the frame in Clack’s model | A pixel or two |
| Where it shows | Lights and bright edges against dark | Every bright thing; shadows next to them lift | Every edge |
* Volpato’s 200 µm spread on a 36 mm frame; our arithmetic.
05 · Editing
How to add halation to a photo
In any editor with layers and a blur (Photoshop, Affinity Photo, GIMP, DaVinci Resolve, After Effects or Premiere Pro), the steps follow what the film does:
- Copy the highlights. On a new layer, keep only the brightest parts: lights, bright windows, specular highlights. Clack counts a pixel as a light from about 72 % brightness, fully at white.
- Blur the copy. For an ordinary film, about half a percent of the image width (20 to 25 pixels on a 4,000-pixel-wide photo). For a CineStill look, about twice that.
- Tint it red. Mostly red with a little green, which reads as red-orange near the light. Clack uses red 1, green 0.18, blue 0.05.
- Add it as light. An additive blend: Linear Dodge (Add) in Photoshop, Add in DaVinci Resolve’s Layer Mixer. Do it before the main color grade, since on film halation is exposure that the film’s curve then shapes.
- Keep it off the light itself. The glow belongs round the light, not on it: the light stays white. Mask the light out of the glow layer if it tints it.
- Keep it faint. Every color film’s halo is a few percent of the light. Only a film without its anti-halation backing glows strongly.
Lightroom has no layers to blur, so halation there comes from a preset or plugin, or a round trip to Photoshop. Whatever the tool, the common giveaways are a glow on the light itself, a glow round everything rather than round the bright parts, and pink or magenta instead of red-orange.
06 · Clack
How Clack does halation
Clack is a film camera app for Android and iPhone. Halation is part of each film, not a filter on top: the finder shows it live and the saved JPEG has it.
- In film’s order
- The halo is added as light before the film’s curve, so the curve shapes it as a print does. It is drawn round the light, never on it, in red with a little green.
- Faint by default
- Every color film gets the faint halo, spread about 0.6 % of the frame’s long side, close to Volpato’s 200 µm on 35 mm. The black and white films get none.
- Anti-halation layer
- On the lab slip of each color film: On, or Removed for the strong red rim, twice as wide. Vision3 500T comes with it removed, as a still photographer buys that film. The strong setting was tuned by eye on the phone’s frames, not measured.
The film base, where the lab slip lives, runs on Android phones whose camera accepts a tone curve of its own. On other phones Clack’s films run on the phone’s own frame, with the faint halo on the color films and the strong one on Vision3 500T.
Android · iPhoneComing soon to Google Play and the App Store. More about Clack
Questions
- What is halation in photography?
- A red-orange glow round bright lights on film. Light goes through the emulsion, reflects off the back of the film and exposes the red-sensitive layer a second time, a little to the side of where it first landed.
- Why is halation red?
- The red-sensitive layer is the bottom one, nearest the base, so it is the layer the reflected light reaches first. Right beside a strong light some reaches the green layer too, which makes the glow orange there.
- Does all film have halation?
- Every color film has a faint halo: anti-halation layers are not perfectly black, so a little light comes back. It shows mostly round lights against dark. A film without its anti-halation backing, like CineStill 800T, has a strong one.
- Why does CineStill 800T glow red?
- CineStill 800T is Kodak Vision3 500T cinema film with its rem-jet backing removed so ordinary C-41 labs can process it. Without that dark backing, light reflects off the base back into the red layer: strong red rims round street lights and lamps.
- Is halation the same as bloom?
- No. Halation happens inside the film and is red. A diffusion filter (such as a Black Pro-Mist) spreads light in front of the lens, wider and in the light’s own color.
- Does black and white film have halation?
- Not a red one: black and white film has a single silver layer, so there is no red layer to glow. Tri-X has a gray base and HP5 Plus an anti-halation backing that clears in development. Clack draws no halation on its black and white films.
Sources
- Dehancer on halation
- Andrea Volpato, spectral film simulations from scratch
- spektrafilm, digitized film data (CC BY-SA 4.0)
- Kodak VISION3 500T 5219 / 7219 technical data (H-1-5219)
- CineStill (Wikipedia)
- CineStill 800T and Kodak Vision3 explained (PhotographyTalk)
The figure is rendered with Clack’s halation code, ported to the desktop, over a drawn target. Pixel sizes are our arithmetic from the published figures.