A spotlight is similar to a point light source, but emits light in a specific direction in the shape of a cone.




Source: Unity3d

A spotlight radiates light within a cone.  In addition, light is modeled so that it is brightest along the direction the spotlight is pointing.

For this we required three additional light properties: 

• a direction the spotlight is pointing
• a cutoff angle, used to define a cone around the direction vector
• an exponent, describing how light falls off from the centre

When rendering a point, we first check to see if it’s inside the cone.  If it is not inside the cone, we just return the ambient term.

If the point is inside the cone, we determine the intensity of the spotlight at the point based on the form

High