Glossary - T

table lamp a portable luminaire with a short stand, suitable for standing on furniture. tail lamp a lighting device used to designate the rear of a vehicle by a warning light. talbot, T† a unit of light equal to one lumen-second. tanning lamp an ultraviolet lamp that emits a significant portion of its radiative power in the UV-A band (315 to 400 nm) or UV-B band (280 to 315 nm).

task ambient lighting a combination of task lighting and ambient lighting within an area such that the general level of ambient lighting is lower than and complementary to the task lighting.

task lighting lighting directed to a specific surface or area that provides illumination for visual tasks.

taxi-channel lights aeronautical ground lights arranged along a taxi channel of a water airdrome to indicate the route to be followed by taxiing aircraft.

taxi light an aircraft aeronautical light designed to provide necessary illumination for taxiing.

taxiway centerline lights taxiway lights placed along the centerline of a taxiway except on curves or corners having fillets. These lights are placed a distance equal to half the normal width of the taxiway from the outside edge of the curve or corner.

taxiway edge lights taxiway lights placed along or near the edges of a taxiway.

taxiway holding-post light a light or group of lights installed at the edge of a taxiway near an entrance to a runway, or to another taxiway, to indicate the position at which the aircraft should stop and obtain clearance to proceed.

taxiway lights aeronautical ground lights provided to indicate the route to be followed by taxiing aircraft. See *taxiway centerline lights, taxiway edge lights*, and *taxiway holding-post light*.

temperature radiator an ideal radiator whose radiant flux density (radiant exitance) is determined by its temperature and the material and character of its surface and is independent of its previous history. See *blackbody* and *graybody*.

thermopile† a thermal radiation detector consisting of a number of thermocouples interconnected in order to increase the sensitivity to incident radiant flux.

threshold the value of a variable of a physical stimulus (such as size, luminance, contrast, or time) that permits the stimulus to be seen a specific percentage of the time or at a specific accuracy level. In many psychophysical experiments, thresholds are presented in terms of 50% accuracy, or accurately 50% of the time. However, the threshold also is expressed as the value of the physical variable that permits the object to be just barely seen. The threshold can be determined by merely detecting the presence of an object, or it can be determined by discriminating certain details of the object. See *absolute luminance threshold, brightness contrast threshold, luminance threshold*, and *modulation threshold.*

threshold lights runway lights so placed as to indicate the longitudinal limits of that portion of a runway, channel, or landing path usable for landing.

top light illumination of a subject directly from above, employed to outline the upper margin or edge of the subject. torchère an indirect floor lamp that sends all or nearly all of its light upward.

tormentor lighting luminaires mounted directly behind the sides of the stage arch.

total emissivity† See *spectral-total directional emissivity* and *spectral-total hemispherical emissivity*.

total internal reflectance (TIR)† total reflection of a light ray at a surface of a transmitting medium occurs when the angle of incidence exceeds a certain value whose sine equals n2/n1, the ratio of indices of refraction, or when sin r = 1, where r equals the angle of reflection.

touchdown zone lights barrettes of runway lights installed in the surface of the runway between the runway edge lights and the runway centerline lights to provide additional guidance during the touchdown phase of a landing in conditions of very poor visibility.

traffic beam† See *lower (passing) beams*.

train the angle between the vertical plane through the axis of a searchlight drum and the corresponding plane in which this plane lies when the searchlight is in a position designated as having zero train.

transient adaptation factor (TAF) a factor that reduces the equivalent contrast due to readaptation from one luminous background to another.

transition lighting in roadway lighting, lighting gauged to compensate for visual adaptation between regions of high and low light level, as when entering tunnels.

translucent† transmitting light diffusely or imperfectly.

transmission a general term for the process by which incident flux leaves a surface or medium on a side other than the incident side, without change in frequency. Note Transmission through a medium is often a combination of regular and diffuse transmission. See diffuse transmission, regular transmission, and transmittance.

transmissometer a photometer for measuring transmittance. Note Transmissometers can be visual or physical instruments.

transmittance τ = ①t/①i the ratio of the transmitted flux to the incident flux. It should be noted that transmittance refers to the ratio of flux emerging to flux incident; therefore, reflections at the surface as well as absorption within the material operate to reduce the transmittance. Transmittance is a function of

1. Geometry

	1. of the incident flux
	2. of collection for the transmitted flux
2. Spectral distribution

	1. characteristic of the incident flux
	2. weighting function for the collected flux
3. Polarization

	1. of the incident flux
	2. component defined for the collected flux.

Notes (i) Unless the state of polarization for the incident flux and the polarized component of the transmitted flux are stated, it should be considered that the incident flux is unpolarized and that the total transmitted flux (including all polarization) is evaluated. (ii) Spectral transmittance depends on the beam geometry and the character of the transmitting surfaces and media (and polarization). In addition, luminous transmittance is a function of the spectral distribution of the incident beam. (iii) If no qualifying geometric adjective is used, transmittance for hemispherical collection is meant. (iv) In each case of conical incidence or collection, the solid angle is not restricted to a right circular cone but can be of any cross section, including a rectangle, a ring, or a combination of two or more solid angles. (v) These concepts must be applied with care if the area of the transmitting element is not large compared to its thickness, due to internal transmission across the boundary of the area. (vi) For all of the following geometrical quantitites--biconical transmittance, bidirectional transmittance, bihemispherical transmittance, conical-directional transmittance, conical-hemispherical transmittance, directional-conical transmittance, directional-hemispherical transmittance, hemispherical-conical transmittance, and hemispherical-directional transmittance--it is assumed that the radiance (luminance) is isotropic over the specified solid angle of incidence. Otherwise, the property is a function of the directional distribution of incident radiance (luminance) as well as the beam geometry and the character of the transmitting surfaces and/or media. The following breakdown of transmittance quantities is applicable only to the transmittance of thin films with negligible internal scattering, so that the transmitted radiation emerges from a point that is not significantly separated from the point of incidence of the incident ray that produces the transmitted ray(s). The governing considerations are similar to those for application of the *bidirectional reflectance distribution function* (BRDF), rather than the bidirectional scattering-surface reflectance distribution function (BSSRDF).

transparent† having the property of transmitting rays of light through its substance so that bodies situated beyond or behind can be distinctly seen (opposed to opaque and usually distinguished from translucent).

transverse roadway line (TRL) any line across a roadway that is perpendicular to the curb line.

tristimulus values of a light, X, Y, Z the amounts of each of three specific primaries required to match the color of the light.

troffer a long recessed lighting unit usually installed with the opening flush with the ceiling. The term is derived from "trough" and "coffer"

troland a unit of retinal illuminance that is defined as the product of object luminance (candela per square meter) and pupillary aperture area (square millimeters), that is, one troland is the retinal illuminance produced when the luminance of the distal stimulus is 1 cd/m2 and the area of the pupil is 1 mm2. The troland can be photopic or scotopic. Note The troland makes no allowance for interocular attenuation or for the Stiles-Crawford effect.

tube† See *lamp*.

tungsten-halogen lamp a gas-filled tungsten filament incandescent lamp containing a certain proportion of halogens in an inert gas whose pressure exceeds 3 atm. Note The tungsten-iodine lamp (U.K.) and quartz iodine lamp (U.S.) belong to this category. Obsolete U.S. term.

Turn-signal operating unit that part of a signal system by which the operator of a vehicle indicates the direction a turn will be made, usually by a flashing light.