— C o l o u r — luminance information

Press the coloured buttons to restrict the visual information in the photograph.
H: hue only L: luminance only S: saturation only
-H: uniform hue -L: uniform luminance -S: uniform saturation

Explanation

The human brain processes luminance information and chromatic information in separate parts of the visual system.¹

Luminance is perceived at a higher resolution—an image may appear to be more blurred if every pixel is made to have the same luminance, even if the hue and saturation remain unchanged.−L The recognition of edges, where luminance changes, is critical for the perception of position—you can usually see clearly where everything is in a monochrome photograph.L

Chromatic information helps with object recognition. It’s easy enough to recognise objects in the original image.HLS It’s harder in an image where every pixel has an arbitrary constant hue, even if the luminance and saturation are retained.−H On it’s own, hue can be difficult to interpret.H (Similar comments apply to saturation.)

Some observations

Chromatic information is initially processed using red-cyan and blue-yellow opponent colours.¹ Other chromatic qualities, such as saturation, are deduced from this information at a higher level of the visual system.
Saturation provides an added complication because it cannot be fully separated from luminance. If the luminance of a colour is close to 0 or close to its maximum, the colour will be almost black or almost white, so it can only have a low saturation.
The illustration can only be a rough guide to these perceptual effects. Human perception is complex and variable, and the adjustment of computer monitors is rarely accurate. No attempt has been made at theoretical accuracy—ad hoc approximations are used in the Java applet.
Some image compression techniques exploit the differences in human responses to luminance and chromatic information. The chromatic information in an image can be stored at lower resolution with very little perceived loss of quality, as long as the resolution of the luminance information is maintained.