From LightWiki

Coloured render	Corresponding depth map

A type of image map that stores the distance from the camera to the scene per pixel. Can be normalised and be in the range of 0, black, closest distance, to 1, white, furthest distance, or if using 32-bit maps the actual distance can be stored per pixel. Useful for compositing, for overlaying effects while respecting depth, and for applying filters based on depth such as fog or depth of field.