The lamp
Film scanning light source: narrowband RGB or white?
For camera scanning, the lamp decides what your camera actually measures. Both kinds work; they differ in how much has to be worked out afterwards.
What the choice changes
Under a white lamp the camera reads the negative through its own red, green and blue filters, which are wide and overlap — each channel carries some of the other two dyes, and the orange mask sits under all of it. One exposure per frame, and the software has to un-mix the channels afterwards.
Under a narrowband RGB lamp you light the negative with one narrow band at a time — deep red, green, blue — so each exposure measures one dye layer and almost nothing else. Three exposures per frame, and nothing left to un-mix. That is how a minilab scanner reads film. Measured over a full 36-frame roll on one station, the white-light route lands about five counts a channel away from the three bands. How trichromatic (narrowband RGB) scanning works goes through the why.
What to look for in a narrowband RGB lamp
Three bands you can light one at a time, each with its own brightness. On the empty gate each band is set so the sensor fills to between 75 and 90 % of its range, and the three settings end up nowhere near each other — on one measured rig the three colours ran at 190, 485 and 937 counts per unit of setting.
Wavelengths that stay put. The comparisons on this site were shot at 665, 525 and 455 nm. Bands somewhere else change the measurement itself: the app still develops the roll, but the colour will not land where these frames landed until that lamp is measured.
An even field, and a steady one. Falloff towards the corners and dust on the light path both show in a shot of the empty gate, and every density is divided by that light — so it is shot once per session under each band, and again whenever the lamp, the lens or the aperture changes. Lamps also drift: one was measured moving 1.3 % between two consecutive exposures at the same setting, which is why the 75–90 % window above stops short of the sensor's ceiling.
What to look for in a white lamp
The same even, steady field, with one exposure of the empty gate per session. Set it so the brightest colour fills between 60 and 92 % of the sensor's range; the three colours will not read alike, because green collects roughly twice what red and blue do through a camera's filters. Under white light you also name the film, from a table of twenty measured emulsions.
The rest of the bench — the camera, the stand, the exposure — is in how to scan colour negatives with a camera.
A holder matters under either lamp: one that shows the frame and the clear film around it. That border is unexposed film base, and it is where the roll's own base density is read.
Three ways to shoot it, one program.
Trichroma Lab is camera scanning software for whichever lamp you end up with — choose the way in under LIGHT in MERGE.
| Lamp | Per negative | The light, measured by |
|---|---|---|
| Narrowband RGB | Three exposures, one under each band — the app works out which three belong together | Three shots of the empty gate per session, one under each band |
| Narrowband RGB ×1 | One exposure under all three diodes, separated into the three bands | A calibration in RGB · ONE SHOT — or, without one, the app asks which lamp it was: Scanlight, Lento Dock, or an unknown lamp |
| White light ×1 | One exposure, and the film named | One shot of the empty gate per session |
Already have a film scanner instead? Its own files — a 16-bit linear TIFF, or the scanning program's raw DNG — come into the same app. See the quick start.
The lamp these pages were shot under
A Scanlight — an open-source narrowband light source, hardware and firmware both published, so you can build one or have one built. Trichroma Lab has no connection to that project beyond being a grateful user of it. Six negatives shot under it, beside a Frontier SP‑3000's own scans.
Shooting under another lamp, or another camera? Tell me about your rig and we measure it together.