RGB Lumens vs White Lumens: Why You Can't Compare Them

The short version

What a lumen is

Brightness to a human eye

Lumens weight every color by how sensitive your eye is to it, and your eye cares most about green.

Eye-weighted

Why RGB scores low

Red and blue get discounted

The same light output earns far fewer lumens in deep blue or red than in white or green.

Up to 12x less

What to compare

Dimming and color

For light painting, how low and how smoothly a light dims matters more than its peak lumen number.

Low end first

What a lumen actually measures

A lumen isn't a count of how much light comes out of a flashlight. It's a count of how bright that light looks to an average human eye. To get it, you take the light's real power at each wavelength and multiply it by the eye's sensitivity curve, known as V(lambda), then add it all up.

That curve peaks in yellow-green at 555nm. Light there counts at 100%. Move toward red and the weighting drops fast: about 27% at 630nm. Move toward blue and it drops even faster: about 4% at 450nm, the deep blue most RGB LEDs use.

In numbers, one watt of light at 555nm is worth 683 lumens. One watt of 630nm red is worth about 180. One watt of 450nm blue is worth about 26. A typical white LED lands around 300 lumens per watt of light, because its spectrum is mostly in the colors the eye likes best.

Why an RGB light looks weak on paper

Put those numbers side by side. The same amount of real light that scores 300 lumens as white scores about 26 as deep blue. That's more than ten times lower, for the same energy leaving the lens.

So when a color light lists a low lumen figure, especially for blue or red, it usually doesn't mean the light is weak. It means the lumen scale is built for eyes, and those colors are the ones eyes discount most. A 40 lumen blue and a 500 lumen white can be putting out roughly the same amount of actual light.

White made by an RGB light is its own case. It's built by running red, green and blue together, and that mix scores differently again from a single phosphor white LED. That's why lumen claims for RGB lights are often quoted for the white mix only, and why they can't be lined up against a white flashlight's rating and read as a fair comparison.

Why it matters more for a camera

Your camera doesn't see through the V(lambda) curve. Its blue pixels respond well to deep blue, so a color your eye calls dim can fill the sensor's blue channel quickly. On top of that, a pure color lands almost entirely in one channel, while white spreads across all three. Put together, a colored light can overexpose a light painting sooner than a white one with a much bigger lumen number.

That's the reason colored trails so often come out white in the middle. We cover what's happening and how to fix it in why colored light trails turn white.

What to compare instead of lumens

• How low it dims. For light painting you'll spend far more time at the bottom of the range than the top. A light that goes very dim gives you room to shoot at sensible apertures without blowing out.
• How it dims. Many lights dim by flickering fast, called PWM, which can show up as dotted trails on camera. Smooth, analog-style dimming draws clean lines.
• Color range and control. How many colors you can pick, how finely you can set them, and whether the saturated colors look truly saturated rather than washed out.
• Beam and accessories. What the light does with tubes, blades, fiber and other tools matters more for the final photo than a raw number off the box.

The takeaway

Lumens are a fair way to compare two white flashlights. They're a poor way to compare a color light with a white one, and between colors they mostly tell you how your eye feels about each color. When you shop for a light painting light, check how low and how cleanly it dims first, then the colors, and treat the lumen figure as the least useful number on the page.

For the settings side of the same problem, see our light painting camera settings guide.