Basic Illumination Models

Illumination model, also known as Shading model or Lighting model, is used to calculate the intensity of light that is reflected at a given point on surface. There are three factors on which lighting effect depends on:

1. Light Source : Light source is the light emitting source. There are three types of light sources: Their position, electromagnetic spectrum and shape determine the lighting effect.
   1. Point Sources - The source that emit rays in all directions (A bulb in a room).
   2. Parallel Sources - Can be considered as a point source which is far from the surface (The sun).
   3. Distributed Sources - Rays originate from a finite area (A tubelight).
2. Surface : When light falls on a surface part of it is reflected and part of it is absorbed. Now the surface structure decides the amount of reflection and absorption of light. The position of the surface and positions of all the nearby surfaces also determine the lighting effect.
3. Observer : The observer's position and sensor spectrum sensitivities also affect the lighting effect

1. Ambient Illumination

Assume you are standing on a road, facing a building with glass exterior and sun rays are falling on that building reflecting back from it and the falling on the object under observation. This would be Ambient Illumination,In simple words, Ambient Illumination is the one where source of light is indirect. The reflected intensity I_amb of any point on the surface is:

2. Diffuse Reflection

Diffuse reflection occurs on the surfaces which are rough or grainy. In this reflection the brightness of a point depends upon the angle made by the light source and the surface. The reflected intensity I_diff of a point on the surface is:

3. Specular Reflection

When light falls on any shiny or glossy surface most of it is reflected back, such reflection is known as Specular Reflection. Phong Model is an empirical model for Specular Reflection which provides us with the formula for calculation the reflected intensity I_spec.