8 Replies Latest reply on Mar 30, 2017 3:28 AM by c.pfaffenbichler

# What does the Lab color values in Photoshop mean?

I am using Adobe Photoshop CS6 in a Windows 7 environment. I am familiar with the L*a*b* color system and color values as, for example, described here Understanding Photoshop color modes

I just opened a photo in RAW format in Photoshop and chose Image > Mode > Lab Color. I looked at the Histogram for each color channel and it does not show the values I expected. For example:

Lightness has a mean value of 109.61 > not between 0 and 100

a has a mean value of 130.67 > not between -128 and 128

b has a mean value of 140.98 > not between -128 and 128.

So my questions: What does these values mean and why are they not within the expected range? How are the values calculated? Is there any way to convert the values to the expected range?

• ###### 2. Re: What does the Lab color values in Photoshop mean?

I expect all three are given as »0-to-255« and not as percentages or -128 to 128 (which would seem (edited) incorrect in an 8bit file as it would necessitate 257 values).

• ###### 3. Re: What does the Lab color values in Photoshop mean?

Thank you so much for your reply, that is very helpful. But is there any way I can convert between the Photoshop values and -128 to 128? I need to compare my values to values that are displayed as -128 to 128.

• ###### 4. Re: What does the Lab color values in Photoshop mean?

The histogram shows the standard 8 bit 256 values, even in Lab mode where the traditional 8-bit scale is 0-100 (as it also is in CMYK).

The same for a and b channels, the middle point in the readout is 128, not 0.

Don't ask me why it's done this way. But it should be easy to convert if you need it.

• ###### 5. Re: What does the Lab color values in Photoshop mean?

c.pfaffenbilcher has given the correct answer. The histogram is using 0-255 instead of -127 to +128 for a and b and 0-255 instead of 0-100 for L

If you look at a plain white document with the eyedropper the Lab values in the Info panel show 100, 0, 0 as expected.

The mean values in the histogram for the same document show, 255, 128 ,128

Dave

• ###### 6. Re: What does the Lab color values in Photoshop mean?

We are like buses - you wait ages for one - then three come at once

Dave

• ###### 7. Re: What does the Lab color values in Photoshop mean?

Thanks everyone who replied! This was very helpful, and I know what to do now!

Until next time!

• ###### 8. Re: What does the Lab color values in Photoshop mean?

One could create a Script that translates the values or even a custom Panel (I could not, but I assume it is possible in principle), but you have to decide whether the effort is justified.