Glossary - V

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

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

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. 

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). 

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

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. 

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.