ABSTRACT
This chapter discusses the practical issues of measuring light, usually called radiometry. The terms that arise in radiometry may at first seem strange and have terminology and notation that may be hard to keep straight. However, because radiometry is so fundamental to computer graphics, it is worth studying radiometry until it sinks in. The chapter also covers photometry, which takes radiometric quantities and scales them to estimate how much "useful" light is present. For example, a green light may seem twice as bright as a blue light of the same power because the eye is more sensitive to green light. Photometry attempts to quantify such distinctions. It is useful to estimate a rate of energy production for light sources. This rate is called power, and it is measured in watts, which is another name for joules per second.
