Color Spaces
Basics
In technical terms, color is defined as a spectrum of wavelength that is reflected from a given object when it is hit by (white) light. White light itself is the combination of all wavelengths of the (visible) spectrum.
To express this phenomenon in numbers, a color can be represented using abstract mathematical figures. A specific way to interpret a color is called a Color Model. Such Color Models commonly use 3 to 4 values to represent any given color. While an entire Color Model represents the entirety of possible colors, device that reproduce color (Such as monitors or printers) usually can only display a subset of the possible colors within their color model. To clarify such limitations, certain named color spaces (called gamuts) have been created and standardized, e.g. Adobe RGB and sRGB both represent different subsets of the RGB color model.
In the following, the terms “CMYK color space” (or just “CMYK”), “RGB color space” (or just “RGB”), and “Lab color space” (or just “Lab”) are used as generic terms for any RGB, CMYK, or Lab color space.
CMYK
The CMYK model is based on absorbtion of ink on paper. If we imagine white light hitting a specific color, part of the waves are absorbed and part are reflected back to the eye of the viewer. The combination of the reflected light makes up the color we actually see.
Theoretically, a pigment mixture of pure Cyan (C), Magenta (M), and Yellow (Y) absorbs all of the light and creates black; thus, they are called subtractive colors. But, since in reality inks for printing also contain impurities this mixture results in a dirty brown and has to be mixed with black ink (K) to to create actual black. Mixing the inks for printing to reproduce all other colors is called four-color printing.
Note: subtractive (CMY) and additive (RGB) colors are complementary colors. Two subtractive colors at a time create an additive color and vice versa.
To define colors in CMYK mode, a percentage value for each print color is assigned to each pixel of a CMYK image. The lightest shades of a color (lights) have low percentage values whereas darker shades (depths) have higher percentage values. A bright red e.g. may be built by 2% Cyan, 93% Magenta, 90% Yellow and 0% Black. In CMYK images, pure white is created when all components have a value of 0%.
The CMYK mode is used when an image is prepared for four-color printing. The therefore necessary conversion process from RGB to CMYK is called color separation.
Today´s large format printers often do not print with CMYK only, but also with lighter dilutions of the basic inks (Light inks) and additional full tone inks such as Orange, Green, Blue, Violet, etc. These additional inks are used to reduce the amount of ink needed (e.g. when using an orange ink instead of mixing magenta and yellow) or to increase the gamut of the printer when the color tone of the additional ink cannot be achieved by just mixing the basic inks.
RGB
The RGB model describes what kind of light needs to be emitted to produce a given color.
A part of the visible color spectrum can be represented by mixing the three basic components of colored light. These components are the primary colors Red, Green and Blue (RGB). Overlapping these three primary colors creates the secondary colors Cyan, Magenta, and Yellow.
Because the primary colors together create white, they are called additive colors. When all colors are joined together white will be created – that is, light is completely reflected to the eye. Additive colors are used for illumination, in video technology, for slide exposure, and for monitors. The monitor creates colors by emitting light through red, green, and blue phosphor particles.
In RGB color images, an intensity value between 0 (black) and 255 (white) for each RGB component is adjusted to each pixel. Bright red may have a value of 246 for R, 20 for G and 50 for B. When the value for all components is the same, gray will be created. When the value of all components is 255, pure white is created; black is built by a value of 0 for all components.
As an example, 8 bit RGB images use three colors to represent about 16.5 million colors on the monitor. They are 3 channel images each channel having 255 variants of the color tone; they contain 24 (8x3) bit per pixel.
RGB is standard for monitors, scanners, digital cameras, and any other device that displays colors using light waves.
sRGB
The displayable RGB colors always depend on the device on which they are displayed. The sRGB color space, a standard RGB color space, was created by Hewlett-Packard and Microsoft Corporation for use on (CRT) monitors, RGB printers, and the internet. Since the gamut of the sRGB color space can be displayed with almost all monitors, the sRGB color space is a common color space for the internet.
Modern non-CRT hardware, such as LCD monitors, digital cameras, and RGB printers, might have a larger gamut but - since sRGB turned out to be a standard for RGB devices - are built to obey the sRGB standard, with the exception high-end professional equipment which sometimes uses larger color spaces. In most cases though it is safe to assume that an RGB image was created using the sRGB color space.
Lab
The Lab color model is based on the color model that the “Commission Internationale d‘Eclariage” (CIE) declared in 1931 to be the international norm for measuring colors. This model was improved in 1976 and named CIE Lab.
Lab colors are device independent; that is, they create constant colors independently from certain devices like monitors, printers, or computers that are used to create and print images.
Lab colors are built by a luminosity or brightness component (L), and two chromatic components: the a-component ranges from green to red; the b-component from blue to yellow.
The Lab model is often used when editing high-professional images or modifying luminosity and color values in an image independently from each other.
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