A Field Guide to Color: Beyond RGB

Lightness

Last but certainly not least, we have lightness. Physically, lightness comes from how much light a surface emits, either by reflecting it (like paint on a wall with color you can only see when the lights are on) or by emitting it directly (as the lights behind the screen you're reading this on!)

There's a lot more to the story. There is a difference of roughly 15 orders of magnitude between the dimmest light you can detect and a light bright enough to cause optic damage. If we perceived the sun going behind a cloud as causing a tenfold reduction in lightness, even if that's what happens physically, it'd be quite difficult for us to process the world around us. As such, we have a very sharp eye for small differences between shades of black that then tapers off dramatically. There's no magic formula for this, but I've graphed one attempt to describe this behavior opposite.¹ A gray that reflects 50% of the available light, when placed in a sequence by humans, will be around 18% of the way from black to white.¹

Perceived lightness depends on surroundings and many other factors, as many optical illusions demonstrate. We won't focus on that too much right now.