From the December 2011 issue

The miracle of Lab Color

February 2012: Once you separate an image's color and luminance data, you can really enhance its appearance.
By | Published: December 27, 2011 | Last updated on May 18, 2023
As you continue to practice techniques in Photoshop, you’ll certainly notice “Lab Color.” What’s so special about this concept anyway?

From the viewpoint of this short tutorial, Lab Color allows you to increase an image’s color saturation without adding the noise that you would encounter if you just moved the “Saturation” slider to the right.

Lab Color works in both 8-bit and 16-bit mode, so you can use it during any stage of your processing. What makes Lab Color special is that it separates the pure color data from the luminance data. And when you increase the color saturation, you will work only with the color. Here are my steps to make Lab Color work for you.

Step 1: Have your image open in Photoshop and select “Image,” then “Mode,” and then “Lab Color.” Note that your image started as “RGB Color,” but by clicking on “Lab Color,” it is now in that color mode.

What a difference Lab Color makes! The image of the famous Orion Nebula (M42) on the left exhibits low color saturation. After following the steps the author outlines in this column, he transformed that image into the highly saturated one on the right. Both images: Tony Hallas
Step 2: You might think that nothing happened — at least at first glance. In the next step, however, you will see something strange. Open up “Curves,” and instead of the usual R, G, and B, it now displays “Lightness.” Something indeed has happened. If you open the drop down menu under “Lightness,” you will also see a and b. What happened is that Photoshop separated your image into its luminance (lightness) and color (a + b).

Step 3: It is now possible to only adjust the contrast and density of the image in the “Lightness” mode, but what we are really interested in is the two parts that make up the color: a and b. The a color is the green and magenta hues, and the b color is the yellow and blue ones. It’s easy to remember which is which: b = blue.

Step 4: To make something happen, remember that Lab Color increases saturation when you increase the contrast of either the a or b “Curves” line. To increase the contrast with “Curves,” simply make an S-shaped curve.

It is critical to have each half of your S be equal. If you don’t, you will change the color balance of your image. The easiest way to make sure that they are even is to use the grid lines in the “Curves” box. Pull down and anchor on the first line, and then pull up and anchor on the third line when the middle of the curve intersects the center of the graph. (See the image above.) Nothing could be easier!

In Photoshop‘s “Curves” box, you must have each half of your S curve equal the other half. To do this, pull down and anchor Point A. Then, pull up and anchor point C when you center point B. Astronomy: Roen Kelly
Step 5: Note how the color has increased in your image. A mild S shape increases it just a little, and a strong S shape will increase it a lot. If you went too far, you do not have to start over. Remember that you are using “Curves,” so you can go to “Fade Curves” under the “Edit” menu and adjust the saturation back to where you want it.

Step 6: If you do want to change the overall color balance, all you have to do is move the midpoint of the curve in the respective color (a or b). Experiment with this, and it will soon become clear.

Step 7: When you have increased the saturation to your satisfaction, convert back to RGB Color by going to “Image,” then “Mode,” and then “RGB Color.” You will now have gorgeous color saturation, and you won’t have added a lot of noise. Also, as a longtime photographer, I can attest that this works equally well on non-astronomical subjects like landscapes and flowers.