Photoshop

Historical Posting – Originally published in 2004 – Since this is an archived story – products mentioned may not be available or even made any more – Let us know if you have any questions.

Sharpening color images with Unsharp mask applies to the full version of Photoshop

Have you ever tried to sharpen an image and wound up with color fringing on high contrast lines in your image? Try this when sharpening images the next time: first make a copy of an image your want to experiment with. Instead of sharpening your image in RGB color first convert the image to Lab color {Image – Mode – Lab }. Next using the channel picker {Window – Show Channels} choose the lightness layer by clicking on it. The image on the screen will appear in grayscale since you are only looking at the lightness and darkness components of the image and none of the color information. Now click on the filter menu and choose { Sharpen – Unsharp Mask }. This is a visual decision but you may want to start by setting the filter to 150%, Radius 2, Threshold of 1, then click ok to apply the filter. Next go to the Channels pallet and choose click the Lab box in the channels pallet, the image will now appear in color. Finally convert the image back to RGB by { Image – Mode – RGB } If you’d like to compare the two methods, make a copy of the original image again apply the same amount of sharpening to the RGB copy of the image and compare the two side by side. What do you think?

Here’s why this trick works. When you sharpen an image in RGB you are essentially sharpening 3 different image layers or channels; red, green and blue. This means you are working with three-color images, color fringing occurs in areas where there is high contrast edges or transitions from color to color. In its worst case, this appears like a red, green and blue edge. Working in Lab color, the color information is separated from the light and dark tonal information that makes up the image. So when you sharpen the Lightness channel you are doing nothing with the color of the image and for this reason no color fringing is introduced into your image. Using this approach you will find that you can apply a great deal of sharpening without degrading the image. Good Luck!