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Lighting Glossary

The ultimate dictionary of Lighting terms to empower you to understand Lighting System

Accommodation is the process by which the eye locates and focuses on an object. The nearer the object, the more convex the lens of the eye will be. The farther away the object, the flatter the lens. Prescription glasses compensate for the inability of the lens to change shape sufficiently to provide clear vision.


Adaptation involves the size of the pupil opening and sensitivity of the retina. The pupil of the eye opens wide in low levels of light and gets smaller as the light level increases. A change also occurs in the photochemical substances of the retina. It takes longer to adapt from light to dark - like going into a movie theatre in the daytime than it does to adapt from dark to light.

Beam Angle

The Beam Angle is the angle at which the luminous intensity of the luminaire falls to half the maximum luminous intensity.

Colour Rendering Index

Color Rendering Index is a measure of the quality of color rendering properties of light emitted by a lamp, compared to a reference source that has a CRI 100.

In simple words, it is the ability of a light source to show the true color of the object. The higher the CRI of a light source, the more "natural" colors will appear under it. Light source with a low CRI will distort colors. Note that we say "natural", meaning as seen in daylight or sunlight.

Colour Temperature

Imagine a piece of metal heated to a high temperature-it glows. At high enough temperature, it will give off light (or incandescence)

The higher the temperature, the whiter the light. We can use this idea to describe the color of a light source. It is given by its Color Temperature. Light sources with a color temperature of up to 3000 K give a warm impression, while at 4000 K, the impression is neutral and at 5000 K and above, it’s cool.


Illuminance indicates the amount of luminous flux ( ) from a light source falling on a given surface. It is the ratio of the luminous flux to the area of this surface. The unit of illumination in SI system is Lux, and in the FPS system it is foot-candle. One foot-candle is 10.76 Lux. Unit: Lux (Lm) Symbol: E

Inverse square law

As a surface, illuminated by a light source, moves away from the light source, the surface appears dimmer. In fact, it becomes dimmer much faster than it moves away from the source. The inverse square law, which quantifies this effect, relates illuminance ( E) and intensity (I) as follows: E=I/d^2 Where d = the distance from the light source. For example, if the illuminance on a surface is 40 Lux at a distance of 0.5 meters from the light source, the illuminance decreases to 10 Lux at a distance of 1 meter, as shown in the figure above.


It is the luminous intensity of any surface in a given direction per unit area of that surface as viewed from that direction (brightness of the surface). Unit: Candela/ sq.m Symbol: L

Luminous Efficacy

It is the ratio of the total luminous flux emitted to the total input lamp power (lamp wattage). Luminous efficacy = Luminous flux/ Wattage

Luminous Flux

Luminous flux is the total light quantity emitted by a light source. This is the power of the light supplied by a source and it expresses the quantity of energy given off in the unit of time in the form of visible radiation. Unit: Lumen Symbol:(Lm)

Luminous Intensity

Luminous intensity describes the amount of light emitted from a light source in a particular direction. Unit: Candela (Cd) Symbol: I


When standardizing the light sources and luminaries, using them and in assessing the adequacy of lighting for various tasks, it has been necessary to develop a scientific system for the measurement of light which is known as Photometry. Thus Photometry is a basic, indispensable branch of illumination engineering, ranging from the very exact measurements carried out in the laboratory by standard institutes while calibrating reference sources, to routine measurements in research and quality control of sources and for field measurements, which sometimes can be very exact, but very often only needed to give a global impression of existing lighting levels.


Reflection of light is the most familiar property of light, since it is what enables us to see the objects around us.

Light from a source, such as the sun, or a lamp, travels in a straight line until it strikes an object, at which point, it may be either absorbed, transmitted, or reflected. Silver or white surfaces reflect the most light. A highly, smooth and flat silver surface acts as a mirror, reflecting perfect image of the world around it.


Light that is transmitted through a medium will usually be deviated somewhat from the straight path it was previously following.

The phenomenon is familiar with transparent objects such as glasses and lenses - objects seen through them appear larger, smaller or distorted, place a stick partially into water and it appears to be bent at the surface.