What is a topic without its own jargon?  

The definitions in this glossary are adapted from "Recommended Practice of Daylighting" (I.E.S., 1979), and the "IES Lighting Handbook; Reference Volume" (I.E.S., 1981).

the angular distance of a heavenly body measured on the great circle that passes perpendicular to the plane of the horizon through the body and through the zenith.  It is measured positively from the horizon to the zenith, from 0 to 90 degrees.

 lighting throughout an area that produces general lighting.

 the ratio of the total envelope area (roof and walls) to the fenestration area.  Can also be used for the ratio of the area of one surface to its aperture area.

a single opague or translucent element to shield a source from direct view at certain angles, or to absorb unwanted light.

the subjective attribute of any light sensation giving rise to the precept of luminous intensity; a subjective sensation.  (The photometric, or measurable term is luminance.)

The Si unit of luminous intensity; one candela is one lumen per steradian.

 a sky that has less than 30 per cent cloud cover

that part of a building rising clear of the roofs or other parts and whose walls contain windows for lighting of the interior.

the light from the sun and the sky.

the ratio of the flux leaving a surface or medium by diffuse reflection to the incident flux.

 the process by which incident flux is re-directed over a range of angles.

lighting provided on the work-plane or on an object, that is not predominantly incident from any particular direction.

glare resulting from high luminances or insufficiently shielded light sources in the field of view.  It usually is associated with bright areas, such as luminaires, ceilings and windows which are outside the visual task or region being viewed.

 glare resulting in reduced visual performance and visibility.  It often is accompanied by discomfort.

 glare producing discomfort.  It does not necessarily interfere with visual performance or visibility.

any opening or arrangement of openings for the admission of light.

a low-pressure mercury electric-discharge lamp in which a fluorescing coating (phosphor) transforms some of the ultraviolet energy generated by the discharge into light.

the unit of illuminance when the foot is taken as the unit of length.  It is the illuminance on a surface one square foot in area on which there is a uniformly distributed flux of one lumen, or the illuminance produced on a surface all points of which are at a distance of one foot from a directionally uniform point source of one candela.

lighting designed to provide a substantially uniform level of illumination throughout an area, exclusive of any provision for special local requirements.

the sensation produced by luminance within the visual field that is sufficiently greater than the luminance to which the eyes are adapted to cause annoyance, discomfort, or loss in visual performance and visibility.

the density of the luminous flux incident on a surface; it is the quotient of the luminous flux by the area of the surface when the latter is uniformly illuminated.

lighting by luminaires distributing 90-100 per cent of the emitted light upward.

SI unit of luminous flux.  Radiometrically, it is determined from the radiant power.  Photometrically, it is the luminous flux emitted within a unit solid angle (one steradian) by a point source having a uniform luminous intensity of one candela.

a complete unit consisting of a lamp or lamps together with the parts designed to distribute light, to position and protect the lamps and to connect the lamps to the power source.

the quotient of the luminous flux at an element of the surface surrounding the oint, and propagated in directions defined by an elementary cone containing the given direction, by the product of the solid angle of the cone and the area of the orthogonal projection of the element of the surface on a plane perpendicular to the given direction.  The luminous flux may be leaving, passing through, and/or arriving at the surface.  Formerly, photometric brightness.  

the time rate of flow of light.

the SI unit of illuminance.  One lux is one lumen per square meter.

one from which the reflection is predominantly diffuse, with or without a negligible specular component.

SI unit of illuminance equal to one candela per square meter.

one that has 100 per cent cloud cover; the sun is not visible.

source of radiant energy of dimensions negligible compared with the distance between source and receptor.

pertains to the distribution of luminance in a visual environment.  The term is used in a positive sense and implies that all luminances contribute favorably to visual performance, visual comfort, ease of seeing, safety, and aesthetics for the specific visual task involved.

energy in the form of electromagnetic waves or particles.

the ratio of the light reflected by a body to the incident light.

glare resulting from specular reflections of high luminances in polished or glossy surfaces in the field of view.  It is usually associated with reflections from within a visual task or areas in close proximity to the region being viewed.

a general term for the process by which the incident flux leaves a surface or medium from the incident side, without change in frequency.

the total reflection factor of a layer of material of such a thickness that there is no change of reflection factor which increase in thickness.

shading coefficient
(SC)

visible radiation from the sun redirected by the atmosphere.

the coherent relection of light (as though from a mirror.)

direct visible radiation from the sun.

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

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.

that portion of light through a diffusing window shade assumed to be contributed by the sky, for the purposes of interior illumination calculation.

a luminance superimposed on the retinal image which 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.

regular reflections superimposed upon diffuse reflections from an object that partially or totally obscures the details to be seen by reducing the contrast.  This is sometimes called reflected glare.

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 is usually defined in terms of the contrast or size of a standard test object, observed under standardized view-conditions, having the same threshold as the given object.

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 4 minute disk exposed for 1/5 second.

a measure of the ability to distinguish fine details.  Quantitatively, it is the reciprocal of the minimum angular separation in minutes of two lines of width subtending one minute of arc when the lines are just resolvable as separate.

the locus of objects or points in space that can be perceived when the head and eyes are kept fixed.  The field may be monocular or binocular.

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

includes all portions of the visual field except the 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 object.

the plane at which work usually is done, and on which the illuminance is specified and measured.  Unless otherwise indicated, this is assumed to be a horizontal plane 0.76 meters (30 inches) above the floor.