Glossary - UV

ultraviolet lamp a lamp that emits a significant portion of its radiative power in the ultraviolet (UV) part of the spectrum; the visible radiation is not of principal interest. ultraviolet radiation for practical purposes, any radiant energy within the wavelength range 100 to 400 nm is considered ultraviolet radiation. See *regions of the electromagnetic spectrum*. Note On the basis of practical applications and the effect obtained, the ultraviolet region often is divided into the following bands:

There are no sharp demarcations between these bands, the indicated effects usually being produced to a lesser extent by longer and shorter wavelengths. For engineering purposes, the black light region extends slightly into the visible portion of the spectrum.

units of luminance† the luminance of a surface in a specified direction can be expressed as luminous intensity per unit of projected area of surface or as luminous flux per unit of solid angle and per unit of projected surface area. Note Typical units are cd/m2 [lm/(sr × m2)] and cd/ft2 [lm/(sr × ft2)]. The luminance of a surface in a specified direction is also expressed (incorrectly) in lambertian units as the number of lumens per unit area that would leave the surface if the luminance in all directions within the hemisphere on the side of the surface being considered were the same as the luminance in the specified direction. A typical unit in this system is the footlambert (fL), equal to 1 lm/ft2. This method of specifying luminance is equivalent to stating the number of lumens that would leave the surface if the surface were replaced by a perfectly diffusing surface with a luminance in all directions within the hemisphere equal to the luminance of the actual surface in the direction specified. In practice no surface follows exactly the cosine formula of emission or reflection; hence the luminance is not uniform but varies with the angle from which it is viewed. For this reason, this practice is denigrated.

unrecoverable light loss factors† See *nonrecoverable light loss factors*.

upper (driving) beams† one or more beams intended for distant illumination and for use on the open highway when not meeting other vehicles. Often referred to as high beams. Formerly country beams. See lower (passing) beams.

upward component that portion of the luminous flux from a luminaire emitted at angles above the horizontal. See *downward component*.

utilance† See *room utilization factor*.

V vacuum lamp an incandescent lamp in which the filament operates in an evacuated bulb.

valance a longitudinal shielding member mounted across the top of a window or along a wall (and is usually parallel to the wall) to conceal light sources, giving both upward and downward distributions.

valance lighting lighting comprising light sources shielded by a panel parallel to the wall at the top of a window.

values of spectral luminous efficiency for photopic vision, V(λ) values at 5-nm intervals (see Chapter 1, Light and Optics) were provisionally adopted by the CIE in 1924 and were adopted in 1933 by the International Committee for Weights and Measures as a basis for the establishment of photometric standards of types of sources differing from the primary standard in spectral distribution of radiant flux. Note These standard values of spectral luminous efficiency were determined by observations with a 2°photometric field having a moderately high luminance. Photometric evaluations based upon them consequently do not apply exactly to other conditions of observation. Watts weighted in accord with these standard values are often referred to as light watts.

values of spectral luminous efficiency for scotopic vision, V'(λ) values at 10-nm intervals (see Chapter 1, Light and Optics) were provisionally adopted by the CIE in 1951. Note These values of spectral luminous efficiency were determined by observation by young dark- adapted observers using extra-foveal vision at near-threshold luminance.

vapor-tight luminaire a luminaire designed and approved for installation in damp or wet locations. It also is described as enclosed and gasketed.

VASIS (Visual Approach Slope Indicator System) the system of angle-of-approach lights accepted as a standard by the International Civil Aviation Organization, comprising two bars of lights located at each side of the runway near the threshold and showing red or white or a combination of both (pink) to the approaching pilot, depending on his or her position with respect to the glide path.

veiling brightness a brightness superimposed on the retinal image that reduces its contrast. It is this veiling effect produced by bright sources or areas in the visual field that results in decreased visual performance and visibility.

veiling reflection regular reflections that are superimposed upon diffuse reflections from an object that partially or totally obscure the details to be seen by reducing the contrast. This sometimes is called *reflected glare*. Another kind of veiling reflection occurs when one looks through a plate of glass. A reflected image of a bright element or surface can be seen superimposed on what is viewed through the glass plate.

vertical plane of a searchlight the plane through the axis of the searchlight drum that contains the elevation angle. See horizontal plane of a searchlight.

visibility the quality or state of being perceivable by the eye. In many outdoor applications, visibility is defined in terms of the distance at which an object can be just perceived by the eye. In indoor applications it usually is defined in terms of the contrast or size of a standard test object, observed under standardized viewing conditions, having the same threshold as the given object. See *visibility (meteorological)*.

visibility (meteorological) a term than denotes the greatest distance that selected objects (visibility markers) or lights of moderate intensity on the order of 25 candles (25 cd) can be observed and identified under specified conditions of observation. The distance can be expressed in kilometers or miles in the United States until the metric system becomes more widely used.

visibility level (VL) a contrast multiplier to be applied to the visibility reference function to provide the luminance contrast required at different levels of task background luminance to achieve visibility for specified conditions relating to the task and observer.

visibility performance criteria function (VL8) a function representing the luminance contrast required to achieve 99% visual certainty for the same task used for the visibility reference function, including the effects of dynamic presentation and uncertainty in task location.

visibility reference function (VL1) a function representing the luminance contrast required at different levels of task background luminance to achieve visibility threshold for the visibility reference task consisting of a disk that subtends 4 minutes of arc exposed for 0.2 s.

vision† See *central (foveal) vision, mesopic vision, peripheral vision, photopic vision,* and *scotopic vision*.

visual acuity a measure of the ability to distinguish fine details, measured with a set of optotypes (test types for determining visual acuity) of different sizes. Quantitatively, it is the reciprocal of the minimum angular size in minutes of the critical detail of an object that can just be seen.

visual angle the angle that an object or detail subtends at the point of observation. It usually is measured in minutes of arc.

visual approach slope indicator system† See VASIS.

visual comfort probability (VCP) the rating of a lighting system expressed as a percent of people who, when viewing from a specified location and in a specified direction, will be expected to find it acceptable in terms of discomfort glare. Visual comfort probability is related to the discomfort glare rating (DGR).

visual field the locus of objects or points in space that can be perceived when the head and eyes are kept fixed. Separate monocular fields for the two eyes can be specified or the combination of the two. See *binocular portion of the visual field, central visual field, monocular visual field*, and *peripheral visual field.*

visual perception the interpretation of impressions transmitted from the retina to the brain in terms of information about a physical world displayed before the eye. Note Visual perception involves any one or more of the following recognizing the presence of something (object, aperture, or medium); identifying it; locating it in space; noting its relation to other things; and identifying its movement, color, brightness, or form.

visual performance the quantitative assessment of the performance of a visual task, taking into consideration speed and accuracy.

visual photometer one in which the equality of brightness of two surfaces is established visually. See physical photometer. Note The two surfaces usually are viewed simultaneously side by side. This is satisfactory when the color difference between the test source and comparison source is small. However, when there is a color difference, a flicker photometer provides more precise measurements. In this type of photometer the two surfaces are viewed alternately at such a rate that the color sensations either nearly or completely blend, and the flicker due to brightness difference is balanced by adjusting the comparison source.

visual range (of a light or object) the maximum distance at which that particular light (or object) can be seen and identified.

visual surround includes all portions of the visual field except the visual display used in performing a task.

visual task conventionally designates those details and objects that must be seen for the performance of a given activity, and includes the immediate background of the details or objects. Note The term *visual task* as used is a misnomer because it refers to the visual display itself and not the task of extracting information from it. The task of extracting information also has to be differentiated from the overall task performed by the observer.

visual task evaluator (VTE) a form of visibility meter that measures the level of contrast of a given visual display above the threshold of visibility. The ratio of the contrast of a display to its threshold contrast represents its level of visibility. Used to evaluate the visibility level (VL).

vitrine† a transparent enclosure of glass or acrylic around an artifact, usually the top of a display case.

volt† the difference in electrical potential between two points in a circuit. It is also called the electromotive force. The symbol often used in equations is "E" (from the latter term), although "V" is also acceptable. voltage-to-luminaire factor the fractional loss of illuminance due to improper voltage at the luminaire.