The purpose of sharpening is to increase the contrast between tonal (brightness) boundaries in an image. This increases perceived sharpness. It doesn't ACTUALLY improve resolution in an image, but it makes the image appear crisper by making the demarkation of detail more visible.

Some of the "detail" in an image is actually noise, not true detail. In most digital cameras, noise usually appears as fine little speckles of color. The overall tone in a fairly noisy image can be fairly constant. That is, if you look at the image purely in terms of brightness, the speckles disappear.

Let's take an extreme example--an image that has no details in it at all, just color noise:

http://www.pbase.com/image/23601967.jpg

If you open this image up and use "image/mode/grayscale" to convert it to a black-and-white image, you would find that it is a perfectly smooth, middle gray. All the speckles would disappear.

Likewise, if you converted it to LAB mode and looked at just the luminosity channel, all the noise disappears. Again, in terms of tone, this is a perfectly smooth image.

If I apply sharpening to the RGB image using settings of 150%, 0.8, 0, then the following image results:

http://www.pbase.com/image/23601969.jpg

The speckles (noise) have been emphasized! I didn't just sharpen up my image detail, I actually made the noise look worse. If I convert this image to grayscale, it no longer looks completely smooth.

Now instead, let's convert the image to LAB mode, select just the luminosity channel, and apply the same unsharp mask. Here is the resulting image:

http://www.pbase.com/image/23601970.jpg

The image looks just like the original! I haven't sharpened any detail, because there was no detail to sharpen. But at least I haven't made the noise any worse.

This is why many people recommend applying unsharp mask to the luminosity channel rather than to the full RGB image; you avoid sharpening the color noise.

- Jared

Isca wrote:
> What is it? I want to use the luminosity channel for sharpening but
> also want to know why I'm doing it!!!!
>
> G.