Understanding Different Color Models

> Two of the many good books available on the subject of color management are Color Confidence by Tim Grey ([15]) and Real World Color Management by Bruce Fraser ([14]).

* Some formats even allow for 32 bits per primary color - a technique used for producing HDR (high dynamic range) images.

> As of the CS2 version, Photoshop also supports 32-bit (per channel) color data. There are various 32-bit file formats, such as PSD, PSB, Radiance, PFM, OpenEXR, and TIFF.

Let's start with the various color models (or modes) supported by Photoshop. A color model defines the way colors are described in a technical, mathematical way - i.e., the basic primary colors or primaries that make up a color and how those components are interpreted and arranged within color data. A color model also describes the amount of data required to record this information.

Photoshop supports several different color models, and the main color models used by photographers are:

Photoshop also provides additional color models that are rarely used by photographers, including Bitmap for pure black-and-white (bitonal) images, and Index mode, used primarily for Web graphics GIF files (if you can live with fewer than 256 different color values). Duotone is used with grayscale images and allows the addition of a second color, giving a print more depth and feel.

Color depth: In a color model, you can use either 8 bits (one byte) or 16 bits (2 bytes)* to describe a single color channel, so you can store your image in 8-bit or 16-bit mode. Different bit depths are possible but are not supported by most applications. Using 16-bit color depth doubles the disk space necessary to store values, but it gives you more headroom when it comes to differentiating color values. It also allows for more precise calculations with fewer rounding errors. In 8-bit mode, the value of a single component can vary from 0 to 255 (using integers). (For the technicians among you: Integer values are used for 8-bit as well as for 16-bit data per color channel. Floating point numbers are used to describe some 32-bit formats.

In 16-bit mode, the single channel range increases from 0 to 65,535 (although Photoshop only uses 15 bits, reducing the maximum value to 32,767). We recommend using 16-bit whenever possible, but it is still common practice to use 8-bit values.

For most color management issues, it doesn't matter which mode you use. You will need to reduce your image to an 8-bit format when you are producing output, as most printers are not capable of reproducing more than 256 different shades of a color. The various combinations of the three primary RGB colors add up to 16,777,216 (256 x 256 x 256) different reproducible colors. Our eyes can only differentiate between 120-200 hues of a particular color, depending on illumination, contrast, viewing distance, etc. 16-bit mode is nevertheless better for performing color optimization where color shifts, transformations, and resampling processes are performed.

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