The photo CD developed by Kodak, specified in the Orange Book, works according to the YIQ color model or the YCC color model and stores all photos in various resolutions with a color depth of 24 bits. The luminance is stored in 8 bits and each of the two color difference signals for the chrominance is also stored in 8 bits. The following resolutions are stored simultaneously: 192 x 128 pixels, 384 x 256 pixels, 768 x 512 pixels, 1,536 x 1,024 pixels, 3,072 x 2,048 pixels and 6,144 x 4,096 pixels. For resolutions of 1,532 x 1024 pixels and higher, the Photo CD uses compression.
The file format from the Photo-CD can be read by many graphics programs, but cannot be written by any other program. The file extension of the Photo-CD is *.pcd. The representation with the lowest resolution is called thumbnail.