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Be true to your colors: Adjust your images in Lab color mode

Added on Tuesday 28th of March 2006 01:04 am EST
 

Application:

Adobe Photoshop 7/CS/CS2

Operating Systems:

Macintosh, Microsoft Windows

 

Adjusting an image is more of an art than a science, and any one technique that attempts to address all color and tonal image corrections will eventually come up short. However, there are some basic procedures, such as converting to Lab color, which can help you keep true to an image’s color and help ensure it prints as you intend it to for the majority of your design projects.

 

To shed some light on Photoshop’s Lab color mode and help you master tonal correction, we’ll:

     Define Lab color, its relationship to RGB and CMYK, and the benefits you’re missing if you haven’t used Lab color.

     Describe the benefits of using adjustment layers and blending modes with your Lab image so you don’t obliterate the pixels in the original.

     Explain how to enhance your Lab images by adjusting opacity and lightness levels while your colors stay true.

 

Performing tonal and color corrections to an image isn’t usually a one-step process. And, unfortunately, when you adjust the tonal range, sometimes the colors shift. By converting your image to Lab color mode, you can make your necessary adjustments and still maintain the image’s color integrity.

 

What’s so great about Lab color?

Lab color is used in Photoshop to address the differences in, and shortcomings of, the RGB (projected light) and CMYK (reflected light) color gamuts. But do we really need another color gamut? Why not just have one gamut containing all the colors in the universe?

Well, the Lab color model (and the resulting Lab color mode in Photoshop) is the result of Adobe trying to answer these very questions. And although Lab color doesn’t encompass “all” colors, it does include more than either the RGB or CMYK models, as illustrated in Figure A.

 

 

Color Gamuts

RGB

Lab

CMYK

A

 

 

 

What’s a gamut?

The simple definition of gamut is all the colors contained within a specific color model (such as RGB, CMYK, Lab); or the restricted colors a device can display or print. A full explanation involves a lot more physics than we’re going to delve into (a PhD in Color Science awaits, if you’re interested).

 

All the colors of the rainbow … plus magenta

Creating a color space that includes “all” colors is more difficult than you might imagine (although the CIE color space, which was developed by the Commission Internationale de l’Eclairage (CIE) to mathematically define all colors, is successful for most practical purposes). But even defining all possible colors isn’t entirely quantitative. For example, where does one color end and another begin? How many colors are there between sky blue and indigo? (Don’t forget, color spaces are three dimensional.) And where does magenta come from? (You won’t find that hue in a rainbow.)

Actually, this last question we can answer: Take the spectrum of visible light that we’re all familiar with from high school physics courses or design tutorials (the one with the reds at one end and the violets at the other), wrap it around like a ribbon to form a circle, and between the reds and violets—allowing from an easy gradation—you’ll find magenta. At least, something very similar to magenta

 

 

Work in the lab

The color mode you work in is important because it defines (and restricts) the colors your image uses. Computer monitors live primarily in the world of RGB, and printers CMYK, but the color mode you use also affects the results of filters and other imaging effects in Photoshop—including levels.

Some designers like to apply filters while in RGB mode, and then convert to CMYK to fine-tune their color levels for print. However, if you convert to Lab mode prior to doing any adjustments, you can still apply filters. You also have a better chance of retaining your image’s true colors primarily because of the way Lab color defines color with its three channels, as illustrated in Figure B.

 

 

B

 

Lab color mode’s three channels:

        L (Lightness or Luminance). Allows you to adjust levels from 0 to 100 (Black to White) without affecting color information, which is contained within Lab’s other channels.

        a. Represents color values ranging from Green (negative) to Red (positive).

        b. Represents color values ranging from Blue (negative) to Yellow (positive).

 

To convert an image to Lab color: