7.1 Introduction This chapter deals with the numerous measurements for which we use radiometers and spectroradiometers. This is the true meaning of the word radiometry, the measurement of radiant energy. First, we describe the types of measurements made. Next is a discussion on errors, their sources, and treatment. The generalized measurement equation and several derived range equations follow. An introduction to the philosophy of radiometric calibration is presented next, and a discussion of calibration configurations completes the chapter. 7.2 Measurement Types Radiometric measurements may be classified into four general types. They are: (1) detector and radiometer characterization, (2) optical radiation source measurement, (3) material properties measurement, and (4) temperature measurement. A fifth measurement type is calibration, which will be discussed later. Table 7.1 subdivides the first four categories into what is not an exhaustive list. 7.3 Errors in Measurements, Effects of Noise, and Signal-to-Noise Ratio in Measurements A measurement of any kind is incomplete unless accompanied with an estimate of the uncertainty associated with that measurement. The term error implies a difference or deviation from a “true” value, while the term uncertainty means an estimate characterizing the range of values within which the true value of the measured quantity lies, including all sources of error. Errors come in two primary flavors, random and systematic, as we will see through consideration of Fig. 7.1. Systematic (type B) errors are readings that vary in a predictable, hopefully detectable, way. Systematic errors are repeatable and consistent, with a fixed bias, the difference between the measured value of x (mean of N measurements) and the true value of x.