OVERVIEW

Radiometry is the study of creation, transport, and absorption of electromagnetic radiation, and the wavelength-dependent properties of these processes. The term is also often used to include the detection and determination of the quantity, quality, and effects of such radiation. The term photometry describes these phenomena for the visible portion of the spectrum only. Photometry and its terms and dimensions are a result of normalizing (or attempting to normalize) the measurement of light to the response of the human eye.

William Herschel (1738 1822) not only discovered infrared radiation, he attempted to draw the first distribution of thermal energy as a function of wavelength and thus can be considered the farther of radiometry. Johann Lambert (1728 1777) noted that the amount of radiated (and in some cases reflected) energy in a solid angle is proportional to the cosine of angle between the emitter and receiver. Incidentally, Lambert also proved that ? is an irrational number and introduced the hyperbolic functions sinh and cosh. A few decades later, Gustav Kirchoff (1824 1887) discovered that the emissivity of a surface is equal to its absorptivity and that the total of reflection, absorption, and transmission of a material always equals 1. Later, Austrian physicist Josef Stefan (1835 1893) determined the total radiant exitance from a source from all wavelengths to be equal to the emissivity multiplied by a constant (the Stefan-Boltzmann constant) times its temperature raised to the fourth power. In 1866, Langley used a crude bolometer to study the radiation of carbon...