TechNote (Archived)

The Lab Color Mode in Photoshop

What's Covered

The Need for Device-Independent Color

The Lab Color Model

Mode Conversion In Photoshop

Working With Photo CD Images

Selective Color Correction

The Lab color mode allows you to work in a device-independent color space (the goal of device-independent color is to create consistent color regardless of the device used to reproduce it). This document provides a brief overview of device independence and the Lab color mode, and offers suggestions on using the Lab color mode in Adobe Photoshop.

The Need for Device-Independent Color

Different devices produce colors differently. Color may vary not only with the type of monitor, printer, or slide imager, but may even vary with different units of the same manufacturer and model.

Colors can also differ on the same device, depending on the image's color model. Color must be represented mathematically in a color model for the computer to interpret it. Each color model can reproduce a different range of colors. Certain color models, such as the RGB and CMYK models, comprise a subset of the visible spectrum. The key to device-independent color is a standardized color model that encompasses all colors; such a model can then provide a system for translating color from device to device.

The Lab Color Model

The Commission Internationale d'Eclairage (CIE), an international organization formed to standardize color measurement, developed a color model in 1931 based on the way the human eye perceives color. In 1976, the CIE proposed two additional color systems based on their original model. One of these systems is CIE L* a* b*.

In the Lab color model, L defines the lightness of the color, and a and b define the color along a red/green and blue/yellow axis, respectively. Like other CIE color models, the Lab model comprises all colors in the visual spectrum and is device-independent. It is useful for converting colors between other color models -- for example, from an RGB model to a CMYK model. It also can preserve original color values from one color reproduction device to another.

Mode ConversionIn Photoshop

Photoshop uses color modes to determine the color model used to display and print images. Photoshop bases its color modes on established color models, such as Lab. To use a color model in Photoshop, choose the corresponding mode command.

Internally, Photoshop uses the Lab color mode when converting color values from one mode to another. Because the Lab color mode provides a system for defining color values in all modes, using Lab as an intermediate mode for color conversions ensures that in-gamut (i.e., those colors that can be reproduced on a specified device), colors are not altered in the conversion process. For out-of-gamut colors, Photoshop 5.x allows you to choose the conversion method that best suits your needs.

For example, when converting an RGB image to CMYK mode, Photoshop first converts the RGB color values to Lab mode using the information in the RGB Setup dialog box (essentially an ICC profile) in Photoshop 5.x or the Monitor Setup dialog box (Photoshop 4.0.1 or earlier). Photoshop then uses information in the Built-in option of the CMYK Setup dialog box (Photoshop 5.x) or the Printing Inks Setup and Separation Setup dialog boxes (Photoshop 4.0.1 or earlier) to build a color table and convert the image to CMYK mode. In Photoshop 5.x, you can specify a CMYK profile, a color engine, and a rendering intent to perform the conversion using the ICC option of the CMYK Setup dialog box.

Once the image is in CMYK mode, Photoshop must reconvert the color values to RGB to be displayed on an RGB monitor. To do this, Photoshop converts the CMYK values back to Lab (using the same color table or profile and rendering intent) and then back to RGB (again using the RGB Setup or Monitor Setup dialog box).

Working With Photo CD Images

When opening Photo CD images in Photoshop, try opening the image in Lab color mode instead of RGB mode. Opening a Photo CD image in Lab color mode preserves all colors in the image. The native color space of Photo CD images, Photo YCC, is another implementation of a device-independent CIE Color model. After you open the image in the Lab color mode, you can convert the image to RGB mode for editing, or convert the image to CMYK mode for separations.

In Photoshop 5.x, you can choose an RGB color mode with a large gamut, such as AdobeRGB(1998) or WideGamut RGB in the RGB Setup dialog box, then save that as a destination profile to use when opening Photo CD images.

Selective Color Correction

Because the Lab color mode separates the lightness component (channel L) from the other color components (color channels a and b), you can use this mode to edit the lightness values, the red/green component, or the yellow/ blue component of an image individually. For example:

- To create a grainy quality in a color image, or the appearance of speckled color, apply the Add Noise filter to adjust the L channel of an image in Lab color mode. Noise in the L channel does not expand the pixels for a speckled appearance.

- Run the Unsharp Mask filter on the L (lightness) channel to sharpen a color image without creating color artifacts.

- To create painterly effects in a grayscale image, convert the image to Lab color mode. Create a horizontal gradient fill from black to white in the a channel and a vertical gradient fill from black to white in the b channel. This creates a blend in the color channels of the image without affecting the image detail in the lightness channel.