This chapter discusses physics-based rendering at a high level, defines the units and terms used in the area, and provides a brute force “path tracing” algorithm that can produce very accurate images very slowly. A simple one would be a simple array of sensors and a box with a small hole in it so it acts like a pinhole camera. The pinhole camera will produce very sharp images just like a real pinhole camera will. But it will need long exposures because very few photons are lucky enough to make it through the pinhole before being absorbed. Something usually done in graphics is to reverse time and send rays from the camera and record them when they hit the light. The most important quantity of radiance, but most graphics people if they do learn what radiance is map it to the intuitive concepts of brightness/color/intensity, and in practice, the works 99% of the time.