If you have a linear R,G, or B value in sRGB primaries, the equation to find the value in nonlinear sRGB space are:

if R ≤ 0.00304,

R

_{nl}= 12.92* R

if R>0.00304,

R

_{nl}= 1.055* R

^{1/2.4}-0.055

repeated for Green and Blue of course.

So, save for the multiplication with 100 to get percentages, Lightroom uses this exact math to calculate the percentage values in linear prophotoRGB to the sRGB tonecurve modified prophotoRGB value display space (I'll call that the Lightroom Value Space or LVS). This tonecurve looks like the following on a double-log plot.

In Red the sRGB tonecurve and in Blue the curve for a simple 2.2 gamma. You can see that for values below 0.05 in the linear space, the deviation from a 2.2 gamma is larger than 5%. Conversely, in the non-linear LVS space, values below 10% are significantly different from when you would assume a 2.2 gamma. This means that the values I published before for the values for the MacBeth colorchecker color patches in the LVS system are all wrong. Here are the correct values:

Sorry for the graphic, I still haven't figured out how to make tables appear correctly in blogger. You can clearly see that the values are significantly different. Hope this is useful for somebody!

Do you have a current update for the relationship of Lightroom RGB % values to color-checker RGB or Photoshop RGB ColorPicker Values?

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