'Lab' is an acronym that spells Lightness, channel A, and channel B. In 'Lab mode', greyscale channels are translated into the following instead of the more common RGB mode.

• Lightness (luminance): or simply known as brightness channel.
• Channel A: consist of colors from red to green, and
• Channel B: consist of colors from blue to yellow

Conversion to Lab mode is useful for preserving the original color value while enabling change to certain aspects of the image. In terms of sharpening an image, lesser artifacts and noise would be produced, whilst ensuring color data are unaffected. This ultimately contributes to better quality prints during post production.

Example:
Image below may already contain subtle artifacts and noise. We want to sharpen objects in an image without increasing the effect or visibility of those unwanted elements and also in a way that does not affect the original colors.

A brief note on Unsharp Mask before advancing:
Unsharp Mask is a filter that sharpens objects in an image, or in other words increase edge contrasts to objects in an image.

Amount (%): Determines the amount of edge contrast.
Radius (pixels): Increase depth of contrast.
Threshold (pixels): Smooths the sharpening effect.

Let's begin with this image.
Original Image - Image ID: 778937 © Stephen Orsillo 123RF.com

Go to Image > Mode > Lab Color. Please ensure layer is flattened.

Ctrl+Click on Lightness layer and inverse the selection Shift+Ctrl+I (it will automatically select the darker areas of the image). Perhaps we can introduce Feather to the selection > 0.5~2 pixels to further smoothen the sharpening process.

Next, apply Filter > Sharpen > Unsharp Mask.


Lastly, convert the image back to RGB mode (Image > Mode > RGB color)

 

Comparisons:
*For step 2) and 3), a higher level of Unsharp Mask and of the same settings have been used to make the effect more evident for comparison purposes.

Original image:

1) High pass sharpened image (check out our High Pass Sharpening tutorial here):

I’ve tried to recapture the same depth of sharpness, to enable comparison between the next 2 methods. Color changed slightly, and artifacts are now more visible, not feasible for processing photographs or scans.

2) Unsharp Mask in RGB mode (without lightness selection):

Color still show changes and edges are too contrasting. White edges seem to extrude from edges of objects and dark edges are overly thick.

3) Unsharp mask in Lab mode (with Lightness selection):

The same setting of unsharp mask filter has been applied but the drawbacks or concerns of previous comparisons are not visible. It is able to produce a sharpened effect, albeit a more balanced one. Therefore Lab mode sharpening is more favorable.

You might want to minimize in-camera sharpening or avoid sharpening the image in your RAW file converter. Instead, sharpen the image during post production using Photoshop's Lab mode sharpening method. Although it is not much of a difference to the naked eye (especially when the image is small), the computer reads every pixels. It is the tiny details like this that collectively increase the overall quality of an image.

Note: This sharpening method works paticularly well on scenic images!

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