Green screen lighting guide

A chroma key is only as good as the light on the backdrop. Light the screen evenly by itself, light the subject separately, keep the two apart, and stop the green bouncing back onto the person. This page is about what to fix before you record — and it ends with what lighting cannot fix.

Why the light matters more than the keyer

The key measures how far each pixel sits from the key colour in the Cb/Cr plane and ignores brightness, which is why a shadow falling across a screen often still keys out. “Often” is the whole problem. A green screen at 65% of its full brightness is still close enough to the key colour to be removed; the same screen at 60% is not. On a synthetic brightness ramp, the backdrop keys cleanly all the way down to 0.65 of full brightness at the shipped tolerance of 25, and flips to fully solid at 0.60 — its chroma distance from the key reaches about 33.5, past the roughly 29.5 radius that tolerance allows. One global threshold, one screen, and half of it stops working.

That is what uneven lighting does. It is not a subtle quality difference; it is the difference between a pixel being transparent and being solid. A screen lit flat from two sides has one colour; lit from one side, a gradient of them.

The rule: two separate lighting jobs

Lighting a green screen is two jobs, not one, and the usual mistake is doing both with the same lamp.

• Job one: the backdrop. Its own lights, aimed only at the screen, even from edge to edge.
• Job two: the subject. Their own light, aimed only at them, shaped the way you would light them anyway.
• Then keep them apart. Distance is what stops the subject casting a shadow on the screen and stops green light bouncing back into their hair and shoulders.

With one light, put it on the subject and light the screen as evenly as you can, then raise tolerance instead. The honest trade: more tolerance forgives an uneven screen and eats into fine detail.

Setting it up

1. Get the screen flat first. A wrinkle is a shadow you cannot light away, and every fold is a different brightness — the same failure as the ramp above. Stretch it, steam it, or weight the bottom edge.
2. Light the screen from both sides. One light from each side, aimed at the backdrop at roughly 45°, feathered so the two overlap in the middle. Two matched lights are ideal; two different ones are fine as long as the screen reads as one colour.
3. Do not let the subject light touch the screen. Flag it off, or move the person forward. That spill is what makes the bright patch behind one shoulder.
4. Push the subject away from the screen, as far as the room allows: their shadow falls behind them instead of on the backdrop, the green bounce falls off, and the screen keeps its own light.
5. Expose the screen sensibly. Under-expose it and you get noise in the matte; blow it out and the green desaturates towards white, which cannot be separated from anything else of the same brightness.
6. Block the bounce. Green light wrapping onto the subject is spill. The Spill slider pulls some of it back out of hair, shoulders and light clothing, but every bit you stop with distance or a flag is a bit it does not have to guess at.

Check it before you shoot

Take one still with the subject in place and drop it into the tool. Toggle the transparency view and look at the corners against the centre: if the screen keys in one place and not in another, you have a lighting problem, not a slider problem. Look at the outline of the subject for a green rim — that is bounce reaching them. And check the auto-detected key colour: on a shadowed screen it reads darker than the real backdrop, in our fixture run #00a23b where the screen is #00b140, a largest-channel difference of 15. Small enough to key fine; for the exact colour, pick it by hand off a lit part of the screen.

How much does fixing it actually buy you? On the two synthetic fixtures that carry a shadow, the shipped tolerance of 25 scores a mean IoU of 0.618 and 0.623 — roughly two thirds of the backdrop left in the frame. Raising tolerance to 40 takes them to 1.000 and 0.966. Same images, no code change: the shadow was a lighting artefact the keyer was being asked to compensate for.

Those are generated fixtures — smooth ramps with no camera noise, no fabric texture and no compression — so read them as the shape of the problem, not a promise about your shot. The full measurement is on the chroma key benchmark page, the CSV files are in the reference repository, and the write-up is archived at doi.org/10.5281/zenodo.22916767.

When this does not work

Lighting fixes evenness, spill and shadow. It does not fix these, and no page on this site should pretend otherwise:

• Semi-transparent detail. Flyaway hair, fur, veils, smoke, glass, anything with motion blur. A colour key produces one matte; these need a hand-drawn mask or a rotoscope. There is no brush here.
• A subject that is itself green. Green clothing or a green prop gets keyed out with the screen, and more tolerance makes it worse. Shoot on blue instead — see the blue screen page.
• Specular highlights. A shine blown out to white on a shirt or on the screen itself sits at the neutral centre; a chroma key cannot separate it from any other white. Flags, diffusion and a polariser help by removing the highlight, not by making the key smarter.
• Wrinkles you cannot get out. Every fold is a different brightness. Steam it, stretch it, or accept a manual mask.
• A room too small for distance. With no separation, the shadow lands on the screen and the bounce lands on the subject. A blue backdrop reflects less light than a green one, which is one reason people switch — a mitigation, not a cure.
• Mixed or drifting colour temperature. Daylight through a window next to a tungsten lamp gives you two different greens, and the whole shoot drifts as the sun moves. Close the blinds and light it yourself.
• A screen that is too dark overall. Noise at the edge is not something a threshold can read. Add light, or raise the camera’s exposure.
• Shooting through heavy compression. A highly compressed recording smears colour across the boundary before the keyer ever sees it. Lighting cannot undo that; the video page covers what to feed the tool instead.

Then key it locally

Once the screen is lit, drop the clip or the still into the tool and the backdrop is keyed out in the browser tab — no upload, no account, no watermark. The no-upload page says what keeping the file on your machine avoids.

Open the tool