What do the RGB percentages in Lightroom mean?

[Kent Shafer]

<p>In the Lightroom 3.6 Develop module, when you hover the cursor over a spot in an image, R, B, and G percentage numbers are displayed below the histogram. What do these mean? Is there any way to display good old 0–255 RGB numbers instead?</p>

<p>For example, I have an image that includes a gray card. The Lightroom display shows approximately 65% for each of the three colors. When I open the image in CS5 and place a color sampler pointer on the gray card, the Info palette shows:</p>

<ul>

<li>8-bit: R=149, G=150, B=149</li>

<li>CMYK: C=37%, M=30%, Y=31%, K=0%</li>

</ul>

<p>I can't seem to find any way to make the CS5 Info palette display numbers corresponding to Lightroom's 65% and can't figure out how to interpret the Lightroom values.</p>

<p>Thanks for any thoughts.</p>

[qalam]

65% of 255 is 165.75

[digitaldog]

<p>They don’t mean anything more or less useful than the 0-255 scale. What does a mile mean if you only understand kilometers or have no conversion? <br>

In either system, there are important items to understand. Zero is the blackest black there is, 255 (or 100%) is the lightest. Doesn’t 100% lightness make more sense than 255? The other item is that when each value is the same (25/25/25), it is neutral in these RGB working spaces. <br>

As Ben points out, 65% translates into 165 but is that useful to edit the image? Not really. </p>

<p>You point out that:</p>

<ul>

<li>8-bit: R=149, G=150, B=149</li>

<li>CMYK: C=37%, M=30%, Y=31%, K=0%</li>

</ul>

<p>Without a color space associated to each, the numbers are ambiguous. R=149, G=150, B=149 in sRGB is a different color than R=149, G=150, B=149 in ProPhoto RGB. Just like if you ask me the distance between your home and mine, and I write 1200, without specifying a scale, the number is meaningless (1200 miles, meters, leagues?). </p>

<p>CMYK in 24 bit has the same encoding as RGB yet the values are not 0-255, they are in percentages. Confusing right? Just like teaching someone the metric system when they only understand the US Standard system. </p>

[Kent Shafer]

<p>That is helpful. Thank you.</p>

<p>I was a bit puzzled about the different readings in Lightroom and Photoshop. The example image I described was opened in Photoshop in the ProPhoto RGB color space, which I thought was the same color space used by Lightroom. Since 65% of 255 is 165.75, it would seem the 8-bit readings in Photoshop should be in the neighborhood of 166 rather than 150. It's not a huge difference I admit, but it isn't trivial either. (It wasn't sampling error. The gray card was evenly lit, and both Lightroom and Photoshop gave very consistent readings all over the card.)</p>

<p>Thinking about Andrew's color space point, I tried converting the Photoshop image to the sRGB color space, and — voilà — the readings changed to 166, 166, 166. Thus I conclude that the percentages displayed in Lightroom relate to the sRGB color space.</p>

<p>This makes me wonder whether Lightroom does in fact use the ProPhoto RGB color space. If so, displaying the percentages in sRGB seems to be something of an anomaly.</p>

 

[digitaldog]

<p>The percentage values in LR are based on ProPhoto RGB chromaticity values but with an sRGB TRC gamma curve. <br>

You can build your own ICC profile to use as a soft proof in LR4 (for RGB values in 0-255) or within Photoshop. Just go into the Color Settings. Select ProPhoto RGB from the list. Then select Edit. Alter the Gamma to 2.2, save out the profile and use it wherever you wish. </p>

[Kent Shafer]

<p>Thanks again Andrew. I will try that after installing Lightroom 4. (The CD's sitting on my desk.)</p>

[Kent Shafer]

<p>A further thought: I've noticed that most of the time when I adjust an image so it looks good in Lightroom, then open it in Photoshop, it looks a bit drab and needs a gentle curve to brighten the midrange and perk it up again. I suppose the difference in gamma is the reason.</p>

[digitaldog]

<p>The two should look identical. Check your color settings in Photoshop and export settings in LR. Always evaluate color at 1:1 in Develop. </p>

[Kent Shafer]

<p>Andrew, thanks for all your help.</p>

[photomark]

Kent, if you are getting different results in LR and Photoshop, you might double check that your version of

the camera raw plugin is up-to-date. There is normally a warning when you have version mismatch, but if

you've turned it off in the past you may be using an old version without being aware.

[twmeyer]

<p>Be sure your sampling (color checker) isn't set to point source... Andrew can tell you in detailed fashion how this affects your readings.</p>