16bit

In digital images, 16 bits per each color channel (red, green and blue), or 48 bits per pixel, is used for accurate processing. Such images do not look much better than 24-bit images, but the existence of more shades between colors (65,536 as opposed to 256) means that more operations can be performed on the image without risk of noticeable banding or w:posterization[*]. (from wikipedia[*])