It is based on Phong's informal observation that shiny surfaces have small intense specular highlights, while dull surfaces have large highlights that fall off more gradually. It describes the way a surface reflects light as a combination of the diffuse reflection of rough surfaces with the specular reflection of shiny surfaces. Phong reflection is an empirical model of local illumination. Phong's methods have proven popular due to their generally efficient use of computation time per rendered pixel. Phong's methods were considered radical at the time of their introduction, but have since become the de facto baseline shading method for many rendering applications. It was published in conjunction with a method for interpolating the calculation for each individual pixel that is rasterized from a polygonal surface model the interpolation technique is known as Phong shading, even when it is used with a reflection model other than Phong's. The Phong reflection model was developed by Bui Tuong Phong at the University of Utah, who published it in his 1975 Ph.D. In 3D computer graphics, it is sometimes referred to as 'Phong shading', particularly if the model is used with the interpolation method of the same name and in the context of pixel shaders or other places where a lighting calculation can be referred to as “ shading”. The Phong reflection model (also called Phong illumination or Phong lighting) is an empirical model of the local illumination of points on a surface designed by the computer graphics researcher Bui Tuong Phong.

Equation of the intensity of light