Glossary - D

dark adaptation the process by which the retina becomes adapted to a luminance less than about 0.034 cd/m2 = 2.2 × 10−5 cd/in.2 = 0.01 fL.

daylight availability the luminous flux from sun plus sky at a specific location, time, date, and sky condition.

daylight factor a measure of daylight illuminance at a point on a given plane, expressed as the ratio of the illuminance on the given plane at that point to the simultaneous exterior illuminance on a horizontal plane from the whole of an unobstructed sky of assumed or known luminance distribution. Direct sunlight is excluded from both interior and exterior values of illuminance.

daylight lamp a lamp producing a spectral distribution approximating that of a specified daylight.

densitometer a photometer for measuring the optical density (common logarithm of the reciprocal of the transmittance or reflectance) of materials.

dichroic filter† a filter that transmits certain wavelengths and reflects those not transmitted; the absorption is small.

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

Note Provision for the exclusion of regularly reflected flux, which is nearly always present, must be clearly described.

diffuse reflection that process by which incident flux is redirected over a range of angles.

diffuse transmission that process by which the incident flux passing through a surface or medium is scattered.

diffuse transmittance the ratio of the diffusely transmitted flux leaving a surface or medium to the incident flux.

Note Provision for the exclusion of regularly transmitted flux must be clearly described.

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

diffuser a device to redirect or scatter light from a source, primarily by the process of diffuse transmission.

diffusing panel a translucent material covering the lamps in a luminaire in order to reduce the brightness by distributing the flux over an extended area.

diffusing surfaces and media those surfaces and media that redistribute at least some of the incident flux by scattering. See complete diffusion, incomplete diffusion, narrow-angle diffusion, perfect diffusion, and wide-angle diffusion.

digital display† See alphanumeric display.

dimmer a device used to control the intensity of light emitted by a luminaire by controlling the voltage or current available to it.

direct component that portion of the light from a luminaire that arrives at the work plane without being reflected by room surfaces. See indirect component.
direct glare glare resulting from high luminances or insufficiently shielded light sources in the field of view. It is usually associated with bright areas, such as luminaires, ceilings, and windows, that are outside the visual task or region being viewed. A direct glare source can also affect performance by distracting attention.

direct-indirect lighting a variant of general diffuse lighting in which the luminaires emit little or no light at angles near the horizontal.

direct lighting lighting involving luminaires that distribute 90 to 100% of the emitted light in the general direction of the surface to be illuminated. The term usually refers to light emitted in a downward direction.

direct ratio the ratio of the luminous flux that reaches the floor of a room cavity directly to the downward component from the luminaire.

directional-conical reflectance, p(ei, φi; ror) ratio of reflected flux collected through a conical solid angle to essentially collimated incident flux.

Note The direction of incidence must be specified, and the direction and extent of the cone must be specified.

directional-conical transmittance, τ(ei, φi; rot) ratio of transmitted flux collected through a conical solid angle to essentially collimated incident flux.

Note The direction of incidence must be specified, and the direction and extent of the cone must be specified.

directional-hemispherical reflectance, p(ei, φi; 2π) ratio of reflected flux collected over the entire hemisphere to essentially collimated incident flux. Note The direction of incidence must be specified.

directional-hemispherical transmittance, τ(ei, φi; 2π) ratio of transmitted flux collected over the entire hemisphere to essentially collimated incident flux. Note The direction of incidence must be specified.

directional lighting lighting provided on the workplane or on an object. Light that is predominantly from a preferred direction. See *accent lighting, key light, and cross light.*

disability glare the effect of stray light in the eye whereby visibility and visual performance are reduced. A direct glare source that produces discomfort can also produce disability glare by introducing a measurable amount of stray light in the eye.

disability glare factor (DGF) a measure of the visibility of a task in a given lighting installation in comparison with its visibility under reference lighting conditions, expressed in terms of the ratio of luminance contrasts having an equivalent effect upon task visibility. The value of the DGF takes account of the equivalent veiling luminance produced in the eye by the pattern of luminances in the task surround.

discomfort glare† glare that produces discomfort. It does not necessarily interfere with visual performance or visibility.

discomfort glare factor the numerical assessment of the capacity of a single source of brightness, such as a luminaire, in a given visual environment for producing discomfort (this term is obsolete and is retained only for reference and literature searches). See *glare* and *discomfort glare.*

discomfort glare rating (DGR) a numerical assessment of the capacity of a number of sources of luminance, such as a luminaire, in a given visual environment for producing discomfort. See *discomfort glare factor*. See also Chapter 9, Lighting Calculations.

distal stimuli in the physical space in front of the eye one can identify points, lines and surfaces, and three-dimensional arrays of scattering particles that constitute the distal physical stimuli that form optical images on the retina. Each element of a surface or volume to which an eye is exposed subtends a solid angle at the entrance pupil. Such elements of solid angle make up the field of view, and each has a specifiable luminance and chromaticity. Points and lines are specific cases that have to be dealt with in terms of total intensity and intensity per unit length. Distal stimuli are sometimes referred to simply as lights or colors.

distribution temperature (of a light source) the absolute temperature of a blackbody whose relative spectral distribution is most nearly the same in the visible region of the spectrum as that of the light source.

dominant wavelength (of a light), λd the wavelength of radiant energy of a single frequency that, when combined in suitable proportion with the radiant energy of a reference standard, matches the color of the light. See *complementary wavelength.*

downlight a small direct lighting unit that directs the light downward and can be recessed, surface- mounted, or suspended.

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

driving beam† See upper (driving) beams.

dual headlighting system two double headlighting units, one mounted on each side of the front end of a vehicle. Each unit consists of two sealed-beam lamps mounted in a single housing. The upper or outer lamps have two filaments that supply the low beam and part of the high beam, respectively. The lower or inner lamps have one filament that provides the primary source of light for the high beam.

dust-proof luminaire a luminaire so constructed or protected that dust does not interfere with its successful operation.

dust-tight luminaire a luminaire so constructed that dust does not enter the enclosing case.