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

Started Jan 15, 2013 | Discussions thread
technoid Senior Member • Posts: 2,144
Re: Here's Why

Vernon D Rainwater wrote:

technoid wrote:

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.

I really should not post in this thread due to my limited level of understanding the details.

The OP evidently has listed RGB Values as reported by a Photo Editing software such as Photoshop -- However, you have discussed Human perceptions of brightness. The RGB readings listed (some being different and of course three sets of RGB Values are listed but how are the measurements being different related to your mentioned "human perception of brightness" since the differences are the above measured values rather than opinions of one's vision of these three.

Also, I don't see any mention of the OP using an 18% Gray Card to create these different patches so are you effectively indicating the Patches that were created as explained by the OP can (or do) only reflect 18% out of a possible reflection of 100 %.

The 50% Gray fill used by the OP simply sets the luminosity at 50% of max in the current colorspace. Since the tone curves for his three examples are different the 50% point on each of these tone curves will produce differing amounts of light. Similarly, any given patch of 50% gray will yield different results when converted to a colorspace with a different Gamma.

Since there is no technically valid way of exactly specifying how much reflectance represents half way between black and white, as percieved, the commonly accepted method is to simply set the value at whatever the colorspace's tone curve produces at half of maximum.  For Lab colors, that happens to be 18% reflectance at L=50. For  sRGB it's around 20% reflectance. For ProPhoto RGB, easily the furthest from the pack, it's about 25% reflectance.

An 18% Gray card has an L value of 50 and is approximately the grayness of other "50%" gray patch surfaces which is the OP's topic.  The point is that unlike measuring light magnitude, where it is just a question of engineering precision as to how accurate something can be measured, the percentage of reflected light that corresponds to what people percieve as 50% gray is not. Some may call a card with 22% reflectance middle gray while others might say the same for 15% or 25%. Because of this variation there is no one tone curve or Gamma that exactly matches human perception. The non-linear tone curve, like the Gamma=2.2 that aRGB uses, that sRGB approximates, and Lab deviates somewhat more from, all have slightly different luminosities at the "50%" gray settings.

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