An illumination model /light model /shading model are used to calculate intensity of light that we should see at a given point on the surface of an object. A surface rendering algorithm uses intensity calculations from an illumination model to determine the light intensity for all projected pixel positions for the various surfaces in a screen.

Illumination is a technique to intensify (brightness will be added) an object or point by applying certain model in order to get a realistic image.

The Basic Illumination Models are:-

1. Ambient Light.
2. Diffuse Illumination.
3. Point-source Illumination.
4. Specular Reflection.

Diffuse Illumination model:-
An objects illumination is as important as its surface properties in computing its intensity. The object may be illuminated by light which does not come from any particular source but which comes from all directions. When such illumination is uniform from all directions, the illumination is called diffuse illumination. Basically the diffuse illumination is a background light which is reflected from walls, floor, and ceiling.

when we assume that going up, down, right and left is of same amount then we can say that the reflections are constant over each surface of the object and they are independent of the viewing direction. Such a reflection is called diffuse reflection. In practice, when object is illuminated, some part of light energy is absorbed by the surface of the object, while the rest is reflected. The ratio of the light reflected from the surface to the total incoming light to the surface is called coefficient of reflection or the reflectivity. It is denoted by R. The value of R varies from 0 to 1. it is closer to 1 for white surface and closer to 0 for black surface.

The diffuse reflections from the surface are scattered with equal intensity in all directions, independent of the viewing direction. Such surfaces are sometimes referred to as ideal diffuse reflectors or Lambertian reflector, since radiated light energy from any point on the surface is governed by Lambert’s cosine law. This law states that the reflection of light from a perfectly diffusing surface varies as the cosine of the angle between the normal to the surface and the direction of the reflected ray. This is illustrated in figure (a) below

Thus if the incident light from the source is Perpendicular to the surface at a perpendicular point, that Point is fully illuminated.On the otherhand, as the angle of illumination moves away from the surface normal,the brightness of the point drops off; but the Points appear to be squeezed closer together, and the net effect is that the brightness of the surface is unchanged. This is illustrated in figure (b). In other words we can say that the reduction in brightness due to cosine of angle gets cancelled by increase in the number of light-emitting points within the area of view.

light or dark scenes or gray shade objects.

Point-source Illumination model:-

Point sources emits rays from a single point and they can approximate real world sources such as a small incandescent bulbs or candles. A point source is a direction source, whose all the rays come from the same direction, therefore, it can be used to represent the distant sun by approximating it as an infinitely distant Point source. The modelling of point sources requires additional work because their effect depends on the surface's orientation. If the surface is normal (perpendicular) to the incident light rays, it is brightly illuminated. The surfaces turned away from the light source (oblique surfaces) are less brightly illuminated. This is illustrated in figure (c) below

For oblique surfaces, the illumination decreases by a factor of cos I, where I is the angle between the direction of the light and the direction normal to the surface plane. The angle I is known as angle of incidence as shown in figure (d).

The factor cos I is given as

cos I= N. L

Where L is the vector of length 1 units pointing towards the light source and N is the vector of length 1 in the direction normal to the surface Plane.

Considering both diffuse illumination and point source illumination, the shade of the visible surface of an object is given as

Where ka Ia, is the intensity of light coming from visible surface due to diffuse illumination,

I1 is the intensity of light comes from the point source, kd is the diffuse reflectivity coefficient and vector dot product (L.N) gives the cosine of the angle of incidence.