Glossary - F

F 

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. 

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. 

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

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.

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. 

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.