Discover the world of lighting with our comprehensive guide! Learn about accent lighting, color rendering, ballasts, and more. Adapt to the light Interior Lighting Design!

Key lighting

Directional lighting that emphasizes a particular object or draws attention to a part of the field of view. (See directional lighting.)


The process by which the retina of the eye becomes accustomed to more or less light than it was exposed to in the immediately preceding time period. This causes changes in the eye’s sensitivity to light.

Essential Terms for Effective Interior Lighting Design

Ambient light

General or background lighting in a space, such as the overall illumination in a retail area.

Average rated life

The average rated operating time of light sources when initially installed in a lighting system and 50% have reached the end of their life. At this point, typically, the remaining 50% of light sources will fail at an increasingly faster rate.


A single opaque or semi-translucent element that shields or absorbs unwanted light from a certain viewing angle.


An auxiliary device used in conjunction with all gas discharge light sources (including fluorescent and high-intensity gas discharge lamps) to obtain the necessary circuit conditions (voltage, current, and waveform) for the light source to start and operate. Dimmable ballasts are special rectifiers that change the light output of the light source when used with a dimming controller. (See electronic ballast.)

Color rendering

The effect of a light source on the color appearance of objects compared to a reference standard light source.

Color rendering index (CRI)

A numerical index assigned to a light source (lamp) that shows its relative color accuracy compared to a reference standard light source. CRI is only valid when compared to light sources with similar color properties.

Color temperature

The absolute temperature at which a black body must operate when its color appearance is the same as that of the light source.

Diffuse lighting

Light that does not enter predominantly from any one direction.

Diffuse light source

A wide-surface luminous light source, such as a fluorescent lamp, providing soft and broad multi-directional illumination, but typically difficult to focus or aim precisely.

Direct glare

Glare caused by insufficient shielding of high-brightness or light sources in the field of view, or high-brightness reflection areas. Typically, it is associated with bright areas outside the visual task or observed area, such as luminaires, ceilings, and windows.

Direct lighting

Lighting provided by luminaires that direct 90-100% of their emitted light in the overall direction of the illuminated surface. Also includes reflector lamps that may be used without luminaires. This term usually refers to light emitted downward. (See key lighting.)

Directional lighting

Lighting primarily performed on the working surface or object from a single direction.

Disabling glare

Glare that reduces visual performance and visibility. It is often accompanied by discomfort.


A luminaire that distributes light downward onto a horizontal plane. Downlights typically contain a compact 45 to 50° optical cutoff angle.


See luminaire efficiency and light source luminous efficacy.

Enhanced reflection

Reflection that enhances appearance, described with terms such as glitter and sparkle.

Feature lighting

Lighting that emphasizes or draws attention to objects with higher brightness than general or background lighting (2:1 to 3:1 ratio relationship). Feature lighting with a ratio greater than 3:1 is typically considered key lighting.

Fluorescent lamp

A low-pressure mercury discharge lamp, typically tubular in shape, with a fluorescent coating (phosphor) inside that converts ultraviolet energy into visible light.


Directing or aiming light at a specific area or object within a space. Also refers to the physical act of directing or aiming an adjustable luminaire and/or light source within an adjustable luminaire.

Foot-candle (FC)

A unit of illuminance: The illuminance on a square meter surface evenly distributes the luminous flux of first-class light (1lm/ft2 or 10.76 lux) (see Illuminance).


A glass or plastic element inside a luminaire used to change the direction of light and control its distribution.

Low Voltage

A voltage ranging from 6V to 24V, typically 12V, supplied to a luminaire and/or light source. A transformer is used in a single luminaire or at the common end to convert line voltage (120-277V) to the low voltage required by the luminaire and/or light source.


A complete lighting unit consisting of one (or more) light sources and electrical components (when required), along with parts designed to distribute its light output, position and protect the light source, and connect the light source to a power supply.

Luminaire Efficiency

The ratio of the total luminous flux (lumens) emitted by the luminaire to the total luminous flux emitted by all the light sources (one or more) used within the luminaire.

Luminous Efficacy of a Light Source

The total luminous flux emitted by a light source divided by the total power of the light source. It is expressed in lumens per watt (lm/W).

Note: In the past, the term luminous efficiency was widely used for this concept.

Lux (lx)

The SI unit of illuminance: illuminance on a surface of one square meter uniformly distributed with a luminous flux of one lumen (1lm/m²).

Matte Surface

A dull, non-glossy surface opposite to a glossy (specular reflective) surface. Light reflected from a matte surface is diffused.

Three-Dimensional Effect

The effect of creating shape through the use of highly directed light, shadows, and highlights.

Perimeter Lighting (Boundary Lighting)

Lighting aimed at vertical surfaces such as the perimeter walls of commercial retail areas.

Point Light Source

A light source using a filament or arc tube that produces a very small emitting point, allowing for easy focusing and/or directing of its light.

Lighting Quality

Pertains to the distribution of illuminance in the visual environment. The term is used in a positive sense, implying that all brightness levels are conducive to the visual performance, visual comfort, ease of viewing, safety, and aesthetics of the specific visual task involved.


The ratio between the light reflected by a surface and the light incident on that surface.

Reflected Glare

Glare caused by high brightness specular reflections on polished or smooth surfaces in the field of view.


A device that changes the direction of light by the process of reflection.

Shading Device

An umbrella term for all devices used to block, diffuse, or redirect light, including baffles, louvers, diffusers, lenses, and screens.

Specular Reflective Surface

A glossy, highly polished surface where the angle of reflected light is equal to the angle of incident light.

Narrow Beam

A narrow beam of light, typically between 12 and 15°, produced by a light source or luminaire with an optical focusing system.


A device that converts line voltage to the higher or lower voltage required by a specific type of light source. Step-up transformers create the higher voltage required for neon lights or cold cathode tubes. Step-down transformers create the 6 to 24V voltage required for most low-voltage lighting systems.

Track Lighting

A lighting equipment system consisting of an electrified track (rail) and detachable luminaires (lamp holders). The system allows for maximum lighting flexibility.

Veiling Reflection

Reflection that reduces visibility of details by lowering contrast, either partially or completely.

Wall Washer

A luminaire with an asymmetric distribution designed to direct a large portion of its total luminous flux towards a vertical surface, such as a wall.

Work Surface

A plane where work is typically carried out and on which illuminance is specified and measured. Unless otherwise specified, this plane is assumed to be a horizontal surface 0.76 meters (30 inches) above the ground.

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