Lab Lightness

Lab color mode can be used for brightness, contrast, and color enhancements in somewhat different ways than RGB mode. Where RGB mode provides a composite channel (RGB) that is composed of red, green and blue channels, Lab mode provides a composite channel (Lab) that is a composite of Lightness, “a” and “b.” The a and b channels encode the color information, while the Lightness channel, as it name suggests, encodes the grayscale values.

Image of a man, showing Lab and lightness channels

Sample image with Lab and lightness channels

The Lab channel and the RGB channel both display the same color values; however, the Lightness channel is not exactly the same as an RGB channel rendered as grayscale. It can can be adjusted without affecting color information in the “a” and “b” channels. You can adjust brightness and contrast in the Lightness channel rather than using the composite Lab channel. The Lightness channel can be unsharp masked without affecting color.

Here’s a basic tutorial on enhancing contrast and unsharp masking in the Lab mode:

  1. First open an image. You can use the sample image for this tutorial, if you like.
  2. From the Image menu, select Mode > Lab Color. The image is changed to Lab mode. It won’t look any different.
  3. To use the Lab channel to enhance a color image, it is really handy to have two windows open: one for the color composite channel (Lab) and one for the lightness channel. From the Window menu, select Arrange > New window for filename. A new window opens for the file. Drag it off to the side of the first window.
  4. Open the channels palette. You will see that it displays four channels: Lab, Lightness, a, b. Click on the Lightness channel. A grayscale image is displayed.
  5. There are several ways to adjust brightness and contrast. The Levels and Curves dialogs provide the greatest level of control. From the Image menu, choose Adjustments > Curves… or type command-M to open the Curves dialog. Choose Medium Contrast from the popup menu in the Curves dialog and then adjust the points on the curve manually if you want finer control. Click OK to accept the changes.
  6. Still operating on the Lightness channel, from the Filters menu choose Sharpen > Unsharp Mask…. The Unsharp Mask dialog opens. The illustration below shows some recommended values for the sample image:
Unsharp Mask Dialog from Photoshop

Unsharp Mask Dialog

The values shown in the the Unsharp Mask dialog work well for printed images: a high level of sharpening (144) is applied on pixel or subpixel level (0.9) to provide good edge enhancement while avoiding halos that large radius values produce. The relatively high Threshold value prevents small, low contrast details from being overly sharpened and becoming grainy: it masks low contrast edges. In this case, it prevents the man’s face from looking like coarse sandpaper. Most images don’t require quite so high a threshold.

Image of a man's face before and after contrast and sharpening

Before and after contrast and sharpening in Lab mode

Once you have finished editing the image in Lab mode, change it back to RGB mode to save it. Lab images can only be saved in a limited number of file formats.

Lab color can also be used for brightness, contrast and color enhancements using the Image > Apply Image… command. We’ll take a look at those techniques in the next tutorial.

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