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Digital Photography Workflow

Color Management—Color Models and Color Spaces

 
Click to explore how sRGB and Adobe RGB color spaces compare when it comes to the number of colors they can capture.
 
 
Click to explore how red, green and blue can create full color images.
 
 
Click to explore how cyan, magenta and yellow can also create full color images.
 
 
If you align the letters CMY under RGB, you have a guick guide to how CMY works. Cyan creates red (directly above it) by absorbing the other two colors, green and blue; Magenta creates green by absorbing red and blue; and yellow creates blue by absorbing red and green.
 
 
Here is the sRGB color space superimposed over the larger Adobe RGB color space (shown here ghosted). You can see how much smaller its gamut is.
 
 
sRGB's gamut plotted against the larger CIE LAB.
 
 
CMYK uses cyan, magenta, yellow (and black) to form all other colors.
As you've seen, the image sensor in a digital camera captures just red, green and blue (RGB) light. RGB is what we call a color model and it's based on the way the human eye perceives full-color images by blending various proportions of red, green and blue light. The RGB model used in display devices creates an image with transmitted red, green, and blue light; and the CMY model used in printers creates it with cyan, magenta, and yellow inks that absorb colors so only red, green and blue are reflected.

Color Models

Color models are fairly basic in that all they tell you is what amount of each color needs to be blended to create a third color.
  • RGB specifies the amount of each color in units between 0-255.
  • CMY specifies the amount of each color as a percentage between 0-100%.
For example, if you start with pure red its RGB values would be R:255 G:0 B:0 indicating that the color's red component is 256 (remember, we count from 0, not 1) and both green and blue components are zero. This sounds like a detailed description of a color but it isn't because it doesn't refer to a specific color as you would perceive it, it just tells a device such as a display screen or printer to generate all of the red it's capable of. (One expert calls these RGB values "input signals"). The most fully saturated red could be bright and vibrant on one device and dull and muddy on another. It's as if a driver's manual told you that to reach a specified speed you press the accelerator down 1 inch. However, if you follow this instruction in a Ford, you might go 35 miles per hour, while following it on a Ferrari would have you going 135 miles per hour. In digital photography what's needed is a way to make the values refer to a very specific color and that's where color spaces come in.

Color Spaces

A color space plots each of the millions of possible colors on a three-dimensional chart in such a way that their positions and spacing show how they relate to one another—often called scaling. Each color can be specified or located by its coordinates in this space.

One of the key features of a color space is its gamut— the range of colors it represents. Different color spaces have different gamuts as do different devices. It's not at all uncommon to have a color in an image that is within the gamut of the display but not of the printer and vice versa. When a color falls outside of a color space's gamut in this way, it can't be reproduced by the device and is called out—of gamut. In the next section you'll see how a color management system can bring such colors back into the gamut of a device.One thing to keep in mind is that a wider gamut doesn't mean more colors. The only way to do that is to capture images in the RAW format rather than JPEG. A wider gamut just spreads out the available colors.

In digital photography you will find references to a variety of RGB color spaces. Here are the most common:

  • sRGB has the smallest gamut of the spaces discussed here, but is ideal for images that will be displayed on a screen or projected. Almost all cameras assign this as the default space for JPEG images. Almost all browsers and display devices are set to display this color space most accurately.
  • Adobe RGB has a wider gamut than sRGB and is often used when the goal is making high-quality prints. One drawback is that images using this color have subdued colors when displayed on a display because almost all displays are based on the sRGB color space. However, if you use this space, Photoshop, Lightroom and other products can convert it to sRGB without any loss in quality.
. In color management, the term "space" is so widely used that it's lost any specific meaning other than the distribution of RGB or CMYK colors into a three dimensional chart that shows their relationships.

. The CMY color model is more often called CMYK with the "K" standing for black. Black is needed because although mixing CMY at 100% should create black, it's a muddy black.
  • ProPhoto is the largest color space currently used in digital photography and the only one that has a gamut that includes all of the colors a camera can capture. This color space can cause problems when used with 8 bit JPEG images. There are so many fewer levels of tones (256 versus a RAW image's 65,536) that if you make any large adjustments to the image you may get banding, noticeable transitions in what should be smooth gradations.
  • CIE LAB (pronounced "see-lab"), and its very closely related CIE XYZ, are different but important kinds of color spaces although you don't interact with them directly. Unlike the other color spaces CIE LAB arranges colors based on how we perceive them rather than on any particular device. For this reason this color space is called "device-independent" (although if there were a device, it would be the human eye). The space contains almost all of the colors a human can perceive. (Interestingly, the actual CIE LAB color model can't be accurately printed or displayed because no device has all of the required colors). In the section that follows on color management you'll see how this color space plays an important role in moving images from the camera to the screen and then to the printer while keeping colors constant on all of these different devices.
  • Working space. When editing an image, the editing application lets you select a working space so the colors are what you expect to see when you display or print the image. The working space can be sRGB, Adobe RGB, Pro-Photo RGB or any other space supported by the application.

Although a color space is initially embedded in a JPEG image by the camera that captured it there are two ways to change the color space.

  • Attaching a new color space changes the look and colors of the image without changing the color values of each pixel.
  • Converting to another color space keeps the image looking the same, but converts the color values of each pixel to fit into the new color space.
When using Photoshop you can assign different profiles and watch the image change appearance as you do so. This is a good way to find the space that works best for a particular image. The widest possible gamut isn't always the best choice. A smaller gamut such as sRGB has smaller spaces between the colors so smooth gradations, such as those found in skin tones, are reproduced more faithfully. However, if the center of interest in the image is a fluorescent orange traffic cone, most of its colors might be outside of the sRGB's gamut so ProPhoto would be a better choice.


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