Proceedings PaperThe Nature And Measurement Of Light
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Light is radiant power in a narrow band of the radiation spectrum; it is the basic material of vision. Both quantitative and qualitative human responses vary with wavelength, as do the optical properties of materials, so the spectral luminous efficiency (Vλ) is a fundamental relationship in the measurement of light. Light suffers only two kinds of change between its source and our eyes; it may be re-directed many times (from one straight line to another) or it may be partially absorbed. Vision depends on these changes, always in terms of ratios rather than differences. Photometry, which depends on the measurement of radiant power, is traditionally on a linear scale. The units of light are complex, because light has long been a dominant feature of human experience. They may be unnecessarily confused if non-metric units of length are used. They are: Flux - the total luminous power (measured in lumens) Intensity - the angular concentration of flux (candelas) 2 Illuminance - the surface density of incident flux (lux or lumen/m2 ) Luminance - the intensity emitted per unit area (candela/m2 ) Other terms such as transmittance, absorbance or colour temperature are briefly discussed. The SI standard of light is the candela, defined physically in terms of luminance, but sometime in the future the standard will probably be the lumen,defid in terms of an efficacy of 683 lumens per watt for monochromatic radiation whose frequency is 540 x 1012 hertz (wavelength 555 nanometres).