The illumination model is also known as the lightning or shading model. It is used to calculate the intensity of light which reflects at a given point on the surface. Here light sources are considered to be point sources. Calculations are based on the optical properties of surfaces and lighting conditions. 

The examples of illumination models are:

• Metal surfaces
• Gold and silver coated surfaces. There are three factors on which light effect depends they are

Light Source:  It is a light emitting source, generally three types of light sources are there: 

          a. Point sources: This emits rays in all directions.  Example: A bulb in a room.

          b. Parallel sources: It can be considered as a point source that is far from the surface. Example: The Sun.

          c. Distributed sources: Here the rays originated from a finite area. Example: Tube Light.

Their shape, position, and electromagnetic spectrum determine the lightning effect.

Surface:  Suppose a light falls on the surface, a part of it is reflected and part of it is absorbed. Surface structure decides the amount of reflection and absorption. The position of the surface and all nearby surface positions also determine the lightning effect.

Observer: The sensor spectrum and observer’s position also affect the lighting effect. 

Types of models:

1. Ambient Illumination: Let’s consider that you are standing on a road, facing a building with a glass exterior and sun rays are falling on that building reflecting back from it and falling on the object under observation. This can be known as Ambient Illumination. In simple words, Ambient Illumination is the one where the source of light is indirect.
2. Diffuse Reflection: It generally, occurs on rough or grainy surfaces, here the brightness of a point of reflection depends upon the angle made by the light source and the surface.

Specular Reflection: When light falls on any shiny or glossy surface most of it is reflected back, such reflection is known as Specular Reflection.

Illumination model in computer graphics: https://www.geeksforgeeks.org/basic-illumination-models/