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.
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