Glossary - WXYZ

watt† the unit of power (rate of doing work). In electrical calculations, one watt is the power produced by a current of one ampere across a potential difference of one volt. The symbol often used in equations is "P," although "W" is also acceptable.

wavelength† the distance between two successive points of a periodic wave, in the direction of propagation, at which the oscillation has the same phase. The three commonly used units are listed in the following table:

The use of the terms micron and angstrom is deprecated.

Weber's fraction† See luminance contrast.

wide-angle diffusion that in which flux is scattered at angles far from the direction that it would take by regular reflection or transmission. See *narrow-angle diffusion*.

wide-angle luminaire a luminaire that concentrates the light within the cone of a comparatively large solid angle. See narrow-angle luminaire.

width line (roadway lighting) the radial line (the one that makes the larger angle with the reference line) that passes through the point of one-half maximum intensity on the lateral intensity distribution curve plotted on the surface of the cone of maximum intensity.

Wien displacement law an expression representing, in a functional form, the spectral radiance of a blackbody as a function of the wavelength and the temperature:

The two principal corollaries of this law are

which show how the maximum spectral radiance Lm and the wavelength λm at which it occurs are related to the absolute temperature *T*. See *Wien radiation law.* Note The currently recommended value of b is 2.8978 × 10−3 m × K or 2.8978 × 10−1 cm × K. From the Planck radiation law, b' is found to be 4.0956 × 10−14 (W × cm−2 × sr−1 × µm−1 × K−5).

Wien radiation law an expression representing approximately the spectral radiance of a blackbody as a function of its wavelength and temperature. It commonly is expressed by the formula

This formula is accurate to 1% or better for values of λT less than 3000 µm × K.

wing clearance lights a pair of aircraft lights provided at the wing tips to indicate the extent of the wing span when the navigation lights are located an appreciable distance inboard of the wing tips.

working standard a standardized light source for regular use in photometry.

workplane the plane on which a visual task is usually done, and on which the illuminance is specified and measured. Unless otherwise indicated, this is assumed to be a horizontal plane 0.76 m (30 in.) above the floor.

Z zonal-cavity interreflectance method a procedure for calculating coefficients of utilization, wall exitance coefficients, and ceiling cavity exitance coefficients, taking into consideration the luminaire intensity distribution, room size and shape (cavity ratio concepts), and room reflectances. It is based on flux transfer theory.

zonal constant a factor by which the mean intensity emitted by a source of light in a given angular zone is multiplied to obtain the lumens in the zone. See Chapter 2, Measurement of Light and Other Radiant Energy.

zonal factor interreflection method a procedure used for calculating coefficients of utilization, based on integral equations, that takes into consideration the ultimate disposition of luminous flux from every 10° zone from luminaires.

zonal factor method a procedure for predetermining, from typical luminaire photometric data in discrete angular zones, the proportion of luminaire output that would be incident initially (without interreflections) on the workplane, ceiling, walls, and floor of a room.

zonal multipliers multipliers for the flux in each 10-degree conical zone from 0° (nadir) to 90° (horizontal) from a luminaire, expressing the fraction of that zonal flux that is directly incident on the floor of a room cavity. These multipliers are a function of the room cavity ratio and are used to determine the direct ratio.