Illumination Model - Polygon rendering and ray tracing methods
Theoretically illumination is exposure of a thing to the light that contributes to light reflected by an object to our eyes and such phenomenon in turn fins out the color perceived by a thing. Hence, if white light is incident on an object then if such object absorbs blue and green light so we shall perceive this as being red. The colour of the light incident on the surface will find out the colour perceived via the viewer, for illustration, if you notice red rose in blue light so this will appear black since all blue rays are absorbed through the object and nothing is reflected consequently it appears black. As the same, it is the reflectance of the object surface which determines such an object will appear shining or dull; whether the object absorbs a high percentage of the light incident on it so it will show dull whereas if it reflects a huge percentage of the light falls on it so it will show as shiny or glossy. For instance, the surface would be perceived as black because a red surface absorbs green and blue, if green light were to shine on a red surface.
Thus, to produce realistic computer-generated images of solid opaque objects the diverse interactions of light along with a surface have to be accounted for, in several form of reflected light and for such the Illumination Model is the gift to Computer Graphics from Physics, that will us assist to attain realism in any graphic scene. An illumination model is also termed as lighting model and sometimes considered to as shading model that is utilized to compute the intensity of the light i.e. reflected at a specified point on surface of an object.