Yes I did, and he responded. I thought I had posted his reply, but apparently not so here is his answer:

*"The L* value of the WhiBal gray is 75. So in PS it is 75 in l*a*b* mode and in Lightroom 75%"*

Deriving various RGB color space values of an L* value of 75 yields the following:

Pro Photo RGB (16 bit): 170/170/170

Adobe RGB (1998) (8 bit): 184/184/184

sRGB (8 bit): 185/185/185

Yes I did, and he responded. I thought I had posted his reply, but apparently not so here is his answer:

*"The L* value of the WhiBal gray is 75. So in PS it is 75 in l*a*b* mode and in Lightroom 75%"*

Deriving various RGB color space values of an L* value of 75 yields the following:

Pro Photo RGB (16 bit): 170/170/170

Adobe RGB (1998) (8 bit): 184/184/184

sRGB (8 bit): 185/185/185

I don't want to appear pedantic, but I do not think Lightroom readouts in per cent correspond to L* but rather to normalized sRGB values expressed as percentages. To normalize sRGB, one divides the 8 bit value (0..255) by 255.

This graph shows sRGB vs L* from calculated values according to the formulas for the two spaces. The red line is the identity line. The sRGB values are close to the L* over the entire range, but deviate most prominently in the mid-range.

I created files in L*a*b in Photoshop and filled them with L* values of 50 and 75 and noted the sRGB values in Photoshop. I then imported the files into Lightroom and read out the luminance values in per cent. The results are shown below:

Note that L* of 75 is 185 in sRGB, or 0.725 when normalized. The value in Lightroom is 72.6%, not 75%. For L* of 50, the sRGB values are 119 and 0.467 and LR gives 46.7%, not 50%.

Regards,

Bill