# 5 Color and Radiometry

In order to precisely describe how light is represented and sampled to compute images, we must first establish some background in radiometry—the study of the propagation of electromagnetic radiation in an environment. Of particular interest in rendering are the wavelengths () of electromagnetic radiation between approximately 380 nm and 780 nm, which account for light visible to humans. The lower wavelengths () are the bluish colors, the middle wavelengths () are the greens, and the upper wavelengths () are the reds.

In this chapter, we will introduce four key quantities that describe electromagnetic radiation: flux, intensity, irradiance, and radiance. These radiometric quantities are each described by their spectral power distribution (SPD)—a distribution function of wavelength that describes the amount of light at each wavelength. The Spectrum class, which is defined in Section 5.1, is used to represent SPDs in pbrt.