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I have a CIELab file that I am looking at with Photoshop. I would like to obtain the mean value of the image's L*, a*, and b* values. However, when I open the Histogram tool and select the individual Lab channels, the displayed mean value is different than then Lab* values displayed by the Info tool, which are correct. For example the mean L* value is listed as 251 when it should be 99 based on the Info tool, the a* mean value is listed as 127 when it is should be -1, and the mean b* value is 128 when it should be 0. The listed values might make sense if PS is taking the mean of the unsigned 8-bit values which I got from this link, instead of the mean of the Lab* values displayed by the Info tool. Is that right? Or, is there a way to convert the incorrect mean Lab* value into the correct mean Lab* value?

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  • \$\begingroup\$ What are you trying to accomplish? \$\endgroup\$
    – mattdm
    Aug 14, 2018 at 18:43
  • \$\begingroup\$ I have an image containing Lab values. I want to know the mean Lab value for the image. What Photoshop is displaying as the mean Lab value is wrong, but can I correct it to get the correct mean Lab value? I edited the question to be (hopefully) clearer. \$\endgroup\$
    – KAE
    Aug 14, 2018 at 19:25
  • \$\begingroup\$ What are you trying to accomplish by calculating the mean values? And why are you using Photoshop for this? \$\endgroup\$
    – mattdm
    Aug 14, 2018 at 19:35
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    \$\begingroup\$ BTW, to get good "info" readings, set the info dialog precision at 32bits for any colorspace. This will show Lab* values to fractional precision. Be sure to set zoom factors at multiples of 100% (100, 200, etc). Otherwise Photoshop uses interpolated values which appear to be what it generates for the display and they can be off. I've seen as much as 0.4 variation from actuals. \$\endgroup\$
    – doug
    Aug 15, 2018 at 2:35
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    \$\begingroup\$ I'm voting to close this question as off-topic because it is not about photography but rather about decoding technical information from an image file in a non-photographic context. \$\endgroup\$
    – mattdm
    Aug 15, 2018 at 13:38

1 Answer 1

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Photoshop's histogram provides means(averages) for Lightness, a, and b.

They are not normalized but shown in the LAB PCS space used by ICC profiles which go from 0 to 255 in unsigned, 8 bit values.

To convert these to real Lab* values, divide the lightness reading by 2.55 to get the actual L*. Subtract 128 from the a and b values to get the true, CIE a* and b* values.

If you are interested, the details of conversion of Lab* into the LAB PCS system can be found at www.color.org and is detailed in the specification for ICC profiles.

I don't know for sure but guess that Photoshop's histogram was hard coded for RGB values from 0-255 and they didn't do the work necessary to scale it properly for Lab*. They also scale their histogram 0-255 even for 16 bit RGB spaces.

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  • \$\begingroup\$ Thanks! Where can I find the Info dialog settings in PS that you mention in your comment? I don't see it in Edit > Preferences, nor in Edit > Color Settings. Am I missing it there? I am using CS6 \$\endgroup\$
    – KAE
    Aug 15, 2018 at 11:59
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    \$\begingroup\$ I believe Adobe added this after they enhanced Photoshop to include super high dynamic range 32 bit images. It's been in PS CC for quite a while but I'm pretty sure it wasn't available in CS6. \$\endgroup\$
    – doug
    Aug 15, 2018 at 14:55

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