# How does the Modified Phong Lighting Model from the Phong Lighting Model?

How does the Modified Phong lighting model also known as the Blinn-Phong differ from the Phong Lighting Model?

How can I distinguish the two?

For example, if $$\vec{V}$$ is the viewing direction, $$\vec{L}$$ the incoming light direction and $$\vec{R}$$ the perfect specular reflection direction for $$\vec{L}$$, then the specular response is $$\text{max}(\vec{V}.\vec{R},0)^p$$, where $$p$$ is the exponent that controls the fall-off of the specular response.
The Blinn-Phong model uses a half vector $$\vec{H}$$, which is computed as $$\vec{H} = \frac{\vec{L}+\vec{V}}{|\vec{L}+\vec{V}|}$$, which is then used to compute the specular response as $$\text{max}(\vec{H}.\vec{N},0)^p$$, where $$\vec{N}$$ is the surface normal. The idea is that when the eye vector $$\vec{V}$$ is aligned perfectly with the perfect mirror direction $$\vec{R}$$, the half vector $$\vec{H}$$ would be exactly aligned with the surface normal $$\vec{N}$$. Hence the Blinn-Phong can get away with computing an expensive mirror direction $$\vec{R}$$ everytime by using the half vector which can model the specular response.