Abstract
This chapter discusses the light intensity measurement and characteristics of light sources. There are three basic types of radiant power measurements and each measurement can be expressed in two distinct types of units—radiometric or photometric units. The three types of measurements and corresponding radiometric or photometric terms are: (1) total radiant power emitted from a source in all directions, (2) radiant power emitted from a surface at that surface, and (3) radiant power incident on a surface at some distance from the source. Light intensity measurements are made by using a variety of techniques that often are applied to unrelated disciplines. The light measurements are expressed in photometric units which are applicable only to the visible portion of the electromagnetic spectrum. Another type of light measurement employs a detector with a response independent of wavelength. The requirements for light sources in photosynthesis and nitrogen fixation studies are dependent on whether the light source is to be used as an analytical light source for spectroscopy measurements or as an actinic light source for photoreaction studies. Filters and/or monochromators are used to select wavelength bands of light. Each method has certain advantages and limitations.
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