50% Gray in RGB, Lab ang Gray-gamma 2.2: why are different?

Started Jan 15, 2013 | Discussions thread
technoid Senior Member • Posts: 1,986
Here's Why

The basic problem is that the idea of 50% gray is one of perception. It is not objective. For intance, the so-called 18% Gray card has a Lab L=50. That is approximately percieved as being a middle gray but only 18 photons out of every 100 are reflected which is where the 18% comes from.  If you look at a Gray patch that reflects 50% of the light hitting it it looks like an extremely bright gray and appears much closer to white than black even though it is technically right in the middle.

The sRGB value that corresponds to a patch that reflects half the light hitting it is (288,288,288). The purpose of the Gamma curve is to approximate the human perception of light which is highly compressed.

The relatively small variations that occur between sRGB, Gamma 2.2 (which isn't the same as the sRGB tone curve), and the more complex Lab are in large part because there is no precise way to measure human perception of brightness. They are all approximations.

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