Using Photoshop’s Lab color mode, you can perform a number of simple image enhancements. For some of these enhancements there are similar RGB operations; however, the results are subtly (and sometimes not-so-subtly) different. The international standard L*a*b* color space from which the Lab mode in Photoshop was derived was constructed to capture the range of human vision. It was based on statistical evaluations of the range of color vision (the “a” and “b” channels) and of just-perceptible differences in brightness (the Lightness channel).
Several techniques are illustrated by Photoshop actions that you can download. Explanations and a few tips on how to perform the actions manually follow.
Several of these actions use the Image menu > Apply Image… command with different blending modes. This technique was mentioned by Scott Kelby, author of some popular books on Photoshop, among them The Adobe Photoshop CS4 Book for Digital Photographers. This was certainly one of the more useful books I read on Photoshop. The Apply Image command can also be used in RGB mode, with slightly different results.
The Apply image command can be used to enhance color in Lab color mode. The Blending mode should be set to Soft Light or Overlay or possibly Hard Light–it all depends on the image and the desired degree of color saturation and contrast. Generally you will not want to apply the blending at 100%. Similarly, you can use Multiply blending mode to darken an image. Screen blending mode will lighten the image. The addition of an alpha mask can isolate specific areas of the image. The Lightness channel is particularly useful as a mask to constrain blending to the light tones in an image. Invert the mask to constrain blending to the dark tones. The “Lab apply saturate,” “Lab apply brighter,” and “Lab apply brighter mask” actions illustrate this technique. Really, you can use any one of them and just change the Blending mode to suit your purpose.
Of the remaining actions, “Lab USM lightness” applies an unsharp mask to the Lightness channel and “Lab Lightness to RGB luminosity” extracts the Lightness channel and loads it as an RGB layer in luminosity blending mode. Unsharp mask can applied to Lightness channel in Lab color mode to sharpen an image without affecting its color. Depending on the image, you may be able to apply more sharpening than in RGB mode. The Lab Lightness channel is not the same as a black and white rendition of a color image. It is generally lighter in overall tonality. It can be used as RGB luminosity to shift the brightness and contrast of a color image. It is especially useful for modifying a black and white image derived from a color image, as shown in the example.
If you want to do the USM manually, here are step-by-step instructions.
- From the Image menu, select Mode > Lab color. If you want exquisite accuracy, especially when you shift back to RGB, you can use 16 bit color, though this may be overkill (if not for image processing, at least for printing).
- Open the Channels palette (Windows > Channel. The image appears in color and there are eye icons in all the checkboxes in the Channels palette, but only the Lightness channel is selected–it should be the only one highlighted in a different color. Now you can see the color image while only applying sharpening to the Lightness channel.
- Select Filter > Sharpen > Unsharp Mask. The Unsharp Mask dialog appears. Just which USM values are optimal for an image depends mightily on the image resolution and output resolution. It’s a huge topic, and clearly one for a another post or three. For output to inkjet printer for a high resolution image, an Amount of 50%-200% and a Radius of 1.0 down to 0.3 should work well–you want to sharpen the image without producing halos. Use a Threshold of 1 to 12 if there are texture details in the image which you don’t want to sharpen.
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