Profiles And Color Spaces

 

Computers store color information as numeric codes.  Because of the various needs of graphic artists, web designers, publishers, photographers, etc. different sets of codes have been devised to serve their particular needs.  These different sets of codes are known as color spaces.  A color space is intended to reproduce a certain range of colors.  The range of colors in a color space is called its color gamut.
 

Color Space

Color space is a predefined set of visible colors.  Specifically, a given set of numeric codes used to reproduce a specific color gamut is called a color space.  Some of the color spaces you may have heard of are sRGB, Adobe RGB (1998), LAB, etc.

A good analogy can be seen in Figure 1.  Figure 1 shows two sets of colored pencils.  The set on the left has 24 different color pencils.  The set on the right has 12.  We can consider these two packs of pencils as representing two different color spaces because they have a different set of predefined colors.
 

Color Gamut

Color gamut is the specific range of colors in a color space.  Using Figure 1 again, the 24 pencil color space can produce twice as many colors as the 12 pencil color space.  Therefore, the 24 pencil color space has a wider color gamut than the 12 pencil color space.
 

Color Model and Mode

Color models are a way to categorize color spaces.  For example, sRGB and Adobe RGB (1998) are two different color spaces that have different color gamuts but they belong to the same RGB color model.  CMYK is another color model as well as Grayscale and Lab.  An image's color model is referred to as its color mode.

 

Color Profile

There are two kinds of color profiles: device profiles and image profiles.  Device profiles are separate computer files that tell the hardware and software how to reproduce color.  Image profiles are part of the image file and tell all devices the numeric code set to use when interpreting and reproducing an image's colors.  Image profiles define color.  Device profiles reproduce color.

 

In digital photography, there are many devices involved in the storage and reproduction of color.  For example, digital cameras, scanners, computers, monitors and printers.  When a monitor is calibrated, a profile is created and stored on your computer.  Your hardware and software uses the monitor's profile to reproduce colors on the monitor.  Device profiles are commonly called by the hardware they are associated with.  For example, a printer profile or a monitor profile or a scanner profile.  But they are all device profiles.  The device profile takes the numeric codes in the image and translates them into color based on the image's color profile.  Device profiles are not associated with any single image.

 

When digitally capturing an image or when editing it, we can tag an image with a profile.  Tagging means denoting in the image file's properties what color space should be used to interpret the numeric codes.  This color space information is referred to as an embedded profile and these profiles are not separate files.  Embedded profiles tell the hardware and software what color space is associated with an image.

So, an image profile tells the hardware and software what color space the image is in.  And the device profiles use this information to reproduce the colors.

 

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Color Space Comparison

There are a wide variety of color spaces. Only the common ones used in digital photography are discussed here.
 

Color Space	Color Model	Number of Channels	Typical Usage
sRGB	RGB	3	Web, monitor and projector display.  Even though it is impossible to create an image that will look identical on all monitors and web browsers, the sRGB color space is supported by the web environment more than most other color spaces. Total color gamut is smaller than Adobe RGB (1998).  While there are colors in sRGB that are not part of Adobe RGB (1998) and vice versa, in total, the Adobe RGB color space contains more colors than sRGB.
Adobe RGB (1998)	RGB	3	A well supported color space. Commonly used in workflows intended for photo inkjet printers.  Has one of the larger color gamuts of the RGB color model.
CMYK	CMYK	4	Commercial process color printing.  Smaller color gamut than RGB.
Grayscale	Grayscale	1	Only color space that Photoshop allows to be used with their duotone, tritone and quadtone features.
Lab	Lab	3	Device independent color space.  The color gamut is the entire visible spectrum.  Stores luminance on a separate channel from the hue and saturation information.  Green and red information are stored on the a channel.  Blue and yellow information are stored on the b channel.  Hence the name, Lab. The only color mode (in Photoshop) that separates color from lightness, or tone. Often used by color space conversion routines as an intermittent step.  For example, when converting a RGB image to CMYK (and vice versa), Photoshop first converts the image to Lab and then converts it to the destination color space.

Table 1.  Color Space Comparison

 

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Getting Into Trouble

One of the quickest ways to become frustrated in digital photography is not to use a good color managed workflow.  For example, assume we capture an image in the sRGB color space.  Then in Photoshop we assign the Adobe RGB (1998) profile to it.  What we just did was tell all devices that will process that image, including Photoshop, to interpret the sRGB numeric codes using the Adobe RGB (1998) code set.  Since the numeric codes for sRGB are not the same as the codes for Adobe RGB (1998), then the incorrect colors will be reproduced.  If an image's color space needs to be changed, the color space has to be converted from the existing color space to the desired color space.
 

Converting Color Spaces

Converting an image from one color space to another is easy.  In Photoshop, click Edit > Convert to Profile.  Select the desired destination space and click OK.  Save the image so the new profile is recorded.

Be sure you select Convert to Profile and not Assign Profile.  Convert to Profile is not the same as Assign Profile.  Converting takes the numeric codes in the existing color space and converts them to the numeric codes of the desired color space.  Assign Profile does not convert the numeric codes.  It tells the software to interpret the existing numeric codes using the newly assigned profile.  And if the numeric codes are not for the assigned profile, that is how we get into trouble as described above.

Be sure to read the Photoshop Color Settings page to learn about setting up Photoshop to work with profiles and color spaces.