The Bidirectional Reflectance Distribution Function (BRDF) is a mathematical model that describes how light is reflected off a surface. It defines the relationship between the incoming light direction and the outgoing light direction, providing a way to quantify how much light is reflected at various angles. This function is essential in fields like computer graphics, remote sensing, and optical engineering. BRDF is typically represented as a four-dimensional function, where two dimensions correspond to the incoming light direction and the other two to the outgoing direction. Understanding BRDF helps in accurately simulating materials and surfaces, making it crucial for applications involving 3D rendering, photometry, and material science.