Glossary - F

far (long-wavelength) infrared† the region of the electromagnetic spectrum extending from 5000 to 1,000,000 nm.

far ultraviolet† the region of the electromagnetic spectrum extending from 100 to 200 nm.

fay light a luminaire that uses incandescent parabolic reflector lamps with a dichroic coating to provide "daylight" illumination.

fenestra method a procedure for predicting the interior illuminance received from daylight through windows.

fenestration any opening or arrangement of openings (normally filled with media for control) for the admission of daylight.

field angle the angle between the two directions for which the intensity is 10% of the maximum intensity as measured in a plane through the nominal beam centerline. For beams that do not possess rotational symmetry, the beam angle is generally given for two planes at 90°, typically the maximum and minimum angles. Note that in certain fields of applications the angle of the 10% of maximum directions was formerly called beam angle.

fill light illumination added to reduce shadows or contrast range.

film (or aperture) color† the perceived color of the sky or a patch of color seen through an aperture.

filter a device for changing, by transmission or reflection, the magnitude or spectral composition of the flux incident upon it. Filters are called selective (or colored) or neutral, according to whether or not they alter the spectral distribution of the incident flux. Alternatively, a component of an electronic dimmer used to control electromagnetic or radio-frequency interference.

filter factor the transmittance of black light by a filter. Note The relationship between glow factor and filter factor is illustrated by the following formula for determining the luminance (L) of fluorescent materials exposed to black light:

where

E = [fluorens × m−2] fg = glow factor ff = filter factor.When integral-filter black-light lamps are used, the filter factor is dropped from the formula because it already has been applied in assigning fluoren ratings to these lamps.

finsen† a suggested practical unit of erythemal flux density equal to one E-viton per square centimeter.

fixed light a light having a constant luminous intensity when observed from a fixed point.

fixture† See luminaire.

flashing light a rhythmic light in which the periods of light are of equal duration and are clearly shorter than the periods of darkness. See *group flashing light, interrupted quick-flashing light,* and *quick-flashing light.*

flashtube a tube of glass or fused quartz with electrodes at the ends and filled with a gas, usually xenon. It is designed to produce high-intensity light flashes of extremely short duration.

flattery index (of a light source), *R*f† a measure appraising a light source for appreciative viewing of colored objects, for promoting an optimistic viewpoint by flattery (making the view more pleasant), or for enhancing the perception of objects in terms of color. Also sometimes called color preference index (CPI).

flicker fusion frequency (FFF) the frequency of intermittent stimulation of the eye at which flicker disappears. It also is called the *critical fusion frequency* (CFF) or *critical flicker frequency (CFF).*

flicker index a measure of the cyclic variation in output of a light source, taking into account the waveform of the light output. It is the ratio of the area under the light output curve that is above the average light output level to the total area under the light output curve for a single cycle. See Chapter 6, Light Sources.

flicker photometer† See visual photometer.

floodlight a projector designed for lighting a scene or object to a luminance considerably greater than its surroundings. It usually is capable of being pointed in any direction and is of weatherproof construction. Note The beam spread of floodlights can range from narrow field angles (10°) to wide ones (more than 100°). See *beam angle, field angle, heavy-duty floodlight, general-purpose (GP) floodlight, ground-area open floodlight,* and *ground-area open floodlight with reflector insert.*

floodlighting a system designed for lighting a scene or object to a luminance greater than its surroundings. It can be for utility, advertising, or decorative purposes.

floor cavity the cavity formed by the workplane, the floor, and the wall surfaces between those two planes.

floor cavity ratio (FCR) a number computed by using the distance from the floor to the workplane (*h*f) as Heightcavity in the equations given in cavity ratio. See Chapter 9, Lighting Calculations.

floor lamp a portable luminaire on a high stand suitable for standing on the floor. See torchère.

fluoren† a unit of black-light flux equal to one milliwatt of radiant flux in the wavelength range 320 to 400 nm.

fluorescence the emission of light as the result of, and only during, the absorption of radiation of shorter wavelengths (time scale less than approximately 10−8 s).

fluorescent lamp a low-pressure mercury electric-discharge lamp in which a fluorescing coating (phosphor) transforms some of the UV energy generated by the discharge into light. See instant-start fluorescent lamp, preheat (switch-start) fluorescent lamp, and rapid-start fluorescent lamp.

flush-mounted or recessed luminaire a luminaire that is mounted above the ceiling (or behind a wall or other surface) with the opening of the luminaire level with the surface.

flux transfer theory a method of calculating the illuminance in a room by taking into account the interreflection of the light flux from the room surfaces based on the average flux transfer between surfaces.

fog (adverse-weather) lamps units that can be used in lieu of headlamps or in connection with the lower-beam headlights to provide road illumination under conditions of rain, snow, dust, or fog.

follow spot (light) any instrument operated so as to follow the movement of an actor. Follow spots are usually high-intensity, controlled-beam luminaires.

footcandle, fc a unit of illuminance equal to 1 lm/ft2 or 10.76 lx.

footcandle meter† See illuminance (lux or footcandle) meter.

footlambert, fL a lambertian unit of luminance equal to 1/*π* candela per square foot. This term is obsolete, and its use is deprecated.

footlights a set of striplights at the front edge of the stage platform used to soften face shadows cast by overhead luminaires and to add general toning lighting from below.

form factor, *f*1→2 the ratio of the flux directly received by surface 2 (and due to lambertian surface 1) to the total flux emitted by surface 1. It is used in flux transfer theory.

Also, the ratio of the average illuminance on surface 1 to the causative exitance of lambertian surface 2:

Note In the literature, this quantity is also called the *angle factor, configuration factor, geometrical factor, I-factor, illumination factor,* and *shape modulus.*

formation light a navigation light especially provided to facilitate formation flying.

fovea a small region at the center of the retina, subtending about 2°, that contains cones but no rods and that forms the site of most distinct vision.

foveal vision† See central (foveal) vision.

Fresnel spotlight a luminaire containing a lamp and a Fresnel lens (stepped flat lens with a textured back) that has variable field and beam angles obtained by changing the spacing between lamp and lens (flooding and spotting). Produces a smooth, soft-edged, defined beam of light.

fuselage lights aircraft aeronautical lights, mounted on the top and bottom of the fuselage, used to supplement the navigation lights.