Lab color mode is a mode in Photoshop. Lab is different from RGB color mode in that it separates the luminosity information into it’s own channel. Lab mode has three channels: L, a and b.
The L Channel contains the image’s luminosity information. This allows you to manipulate the image’s luminosity without affecting it’s colors. The L channel is similar to black and white film in that it only records the image’s luminosity.
The “a” and “b” channels contain color information. The a channel controls the red/green axis. The b channel controls the yellow/blue axis. The a and b channels you to manipulate the color of the image without affecting it’s luminosity.
Black and White in Lab
The simplest way to convert an image to black and white using Lab color mode is to delete the a and b channels. However, you won’t be able to make any other adjustments until you change the mode back to RGB after deleting the a and b channels.
- Go to the Channel palette and select the Lightness channel
- Change the mode to Grayscale by selecting
- Change the mode again to RGB by selecting
From here you can apply the usual adjustments such as levels, curves and toning.
One sequence of adjustments even has it’s own name. The Gorman conversion, made famous by photography Greg Gorman, combines the Lab L channel with solid color toning and high pass filter sharpening.
Wikipedia article on Lab Color Space