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Chapter 1:
Introduction to Radiometry
1.1 Definitions Consider the following definitions a starting point for our study of radiometry: radio- [<L. radius] a combining form meaning ray, raylike -metry [Gr. -metria < metron] a terminal combining form meaning the process, art, or science of measuring radiometer (rä'dë-óm´í-ter) n. [radio- + -meter], an instrument for detecting and measuring the intensity of radiant energy, by exposing to sunlight a set of vanes blackened on one side and suspended on an axis in a vacuum and measuring their speed of rotation (i.e., the mechanical energy into which the radiant energy has been converted) (See Fig. 1.1) radiometry ( rä'dë·óm´í-tri), n. the use of the radiometer: the measurement of radiation These definitions are taken from Webster's New World Dictionary, and may be satisfactory for the general nonscientist. The definitions are not satisfactory, however, for scientists and engineers pursuing the art of radiometry. So let's get technical: radiometry (rä'dë·óm´í-tri) n. the measurement of optical radiant energy The practical electromagnetic spectrum extends from dc to frequencies greater than 1020 Hz. The optical portion of the spectrum covers the five-decade frequency range from 3 × 1011 to 3 × 1016 Hz, corresponding to the wavelength range from 10 nm to 1000 μm, as shown in Fig. 1.2. This range includes the ultraviolet, visible, and infrared regions. Shorter wavelengths are called x rays and gamma rays, while longer wavelengths are microwave and millimeter radio waves.
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