How do I interpret Photoshop histogram mean values for Lab images?
Asked 8/14/2018
1 views
2 answers
0
In Photoshop, I opened an image in Lab color mode and want the mean L*, a*, and b* values for the image. The Histogram panel shows mean values that do not match the Lab values shown by the Info panel. For example, the histogram might show L=251, a=127, b=128, while the expected Lab values are about L*=99, a*=-1, b*=0. Is Photoshop reporting the channel means in a different encoding, and if so, how can those histogram values be converted to standard CIELab values?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
2
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.
Originally by user58107. Source · Licensed CC BY-SA 4.0
user58107
7y ago
0
Generated from our catalog & community — verify before relying on it.
Yes. In Photoshop, the Histogram panel for Lab channels is typically showing the ICC Lab/PCS-style encoded channel values, not the displayed CIELab values you see in the Info panel.
Use these conversions:
- L*: divide the histogram mean by 2.55
- a*: subtract 128 from the histogram mean
- b*: subtract 128 from the histogram mean
So for your example:
- L = 251 → 251 / 2.55 ≈ 98.4
- a = 127 → 127 - 128 = -1
- b = 128 → 128 - 128 = 0
That matches the expected Lab values closely.
In other words, Photoshop is using an unsigned 0–255 encoding for the histogram, rather than directly reporting standard Lab ranges. This is consistent with Lab values as used in ICC profile connection space encoding.
Recommended products
UniqueBot
AI7y ago
Your Answer
Related Questions
Why can CIELAB values with L*=0 convert to non-black RGB?
What does Photoshop’s RGB histogram show, and how is it different from luminosity?
What illuminant and observer does SilverFast use for CIELab values?
How does gamma encoding preserve shadow detail if the display applies gamma again?
Why does Lightroom show red/blue clipping overlays when the histogram triangles aren’t white?