HSI stands for Hue, Saturation, and Intensity. When humans view a color object it is described by its hue, saturation, and brightness.

Hue is a color attribute that describes a pure color (yellow, orange, or red).

Saturation gives a measure of the degree to which a pure color is diluted by white light.

Brightness depends upon color intensity, which is key factor in describing color sensation. The intensity is easily measurable and the results are also easily interpretable.

Thus the model that is used to describe a color object is the HSI model.

The HSI model decouples the intensity from color-carrying information (hue and saturation) in a color image.