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Lighting Terminology

Lighting terminology can be simple or complex, but its incorrect usage can create confusion for facility managers and owners/operators investigating a lighting upgrade. Understanding a few basic terms can increase your effectiveness in dealing with lighting designers and vendors. The following are a few basic terms to get you started.

Lamp vs. Bulb - There is a difference between a lamp and a bulb. A lamp produces light, while the bulb is merely that lamp's glass housing.

Lamp Descriptions - Most often, lamps are described by their shape and diameter (in 1/8-inch increments). A  T-12 fluorescent lamp's bulb is tubular... T=tubular... and the 12 means that it is 12/8 of an inch in diameter. Therefore a T-8, means the lamp is tubular and 8/8 or 1-inch in diameter.

Ballasts - Fluorescent lighting does not use the power coming in directly, as do incandescent lamps. Transformers or Power Supplies for fluorescent lamps are called ballasts.

Fixture - A fixture includes all the items you think of as a "light"... it contains the lamps and ballasts and directs light using reflectors, lenses and louvers.

Lighting Controls – Controls that turn lights on and off either manually or automatically. Automatic controls can be time, motion, sound or light-level activated.

Understanding Lighting Concepts

The following are a few characteristics to be understood, when evaluating a present or proposed lighting system:

Color Temperature – A light bulb looks like a certain color because of the Color Temperature. It is not actually how hot the light is but how the radiator reaches a certain temperature to produce the color of the bulb. The radiator is a laboratory term for a blackbody. The blackbody will change different colors as the temperature increases from red to orange to yellow, white, blue-white and finally blue. This is why at night time when you look out at all of the lights from a distance some will appear to be different colors, some more yellow than others. This is because the lamp's color temperature causes the color variation.

Light Output – There are several ways to measure light output:

Candlepower (cp) – Luminous intensity expressed in candelas.

Lumens – International System of Units (SI) unit of luminous flux. Radiometrically, it is determined from the radiant powder as in luminous flux. Photometrically, it is the luminous flux emitted within a unit solid angle (1 sr ) by a point source having a uniform luminous intensity of 1 candela (cd).

Illuminance – The areal density of the luminous flux incident at a point on a surface.

Brightness (of a perceived aperture color) – The attribute by which an area of color of finite size is perceived to emit, transmit, or reflect a greater or lesser amount of light. No judgment is made as to whether the light comes from a reflecting, transmitting or self-luminous object.

Luminaire Efficiency – The ratio of luminous flux (lumens) emitted by a luminaire to that emitted by the lamp or lamps used therein.

Quality of Light: The following terms refer to the optical qualities of lighting.

Luminous Efficacy of Radiant Flux – The quotient of the total luminous flux by the total radiant flux. It is expressed in lumens per watt.

Color Rendering Index (CRI) – This is how you perceive the color of an object under a light fixture. Not all bulbs will produce the same type of light. Below is an index of CRI and will indicate which bulb will have the least amount of color shift. It is rated on a scale of 0-100 with 100 being the best:

Some Definitions from The IESNA Lighting Handbook-Reference & Application - written by the Illuminating Engineering Society of North America. Editor-in-Chief - Mark S. Rea, Ph.D., FIES.