 
Summary: Transfer Equations in Global Illumination \Lambda
James Arvo
Program of Computer Graphics
Cornell University
Abstract
The purpose of these notes is to describe some of the physical and mathematical properties
of the equations occurring in global illumination. We first examine the physical assumptions
that make the particle model of light an appropriate paradigm for computer graphics and
then derive a balance equation for photons. In doing this we establish connections with the
field of radiative transfer and its more abstract counterpart, transport theory. The resulting
balance equation, known as the equation of transfer, accounts for largescale interaction
of light with participating media as well as complex reflecting surfaces. Under various
simplifying assumptions the equation of transfer reduces to more conventional equations
encountered in global illumination.
1 Introduction
Global illumination connotes a physicallybased simulation of light appropriate for synthetic
image generation. The task of such a simulation is to model the interplay of light among
largescale objects of an environment in order to approximate the quantity and quality of
light reaching the eye of an observer. The problem is global in that all objects can interact
with one another by means of scattered light. To properly simulate the lighting of a room,
