The traditional industry standard name given to the Correlated Color Temperature of 6500 kelvin.


See Direct Current.


A unit of sound measurement. Used when measuring the sound being made by a transformer/ballast.


  • Emitting less light than the light source should.
  • The act of intentionally dimming one or more light sources.


  • Able to be dimmed, regardless of the bulb type and/or application and the type of dimmer, transformer and/or ballast that is required. Generally however this is applied to bulbs and not a lighting system. A lighting system that dims is generally called a dimming lighting system. All of the auxiliary gear (whether mounted in side or outside the fixture) and the fixture itself that operates one or more bulbs is a lighting system. All of the wiring devices that come before a fixture, ballast, driver or step down transformer used to for example power low voltage bulbs are not generally considered part of a lighting system. An exception to this rule may be certain lighting controls. Examples of lighting controls would be a DMX system for stage & studio lighting, dimmers and photocontrols (dusk to dawn). An electrical relay used to switch lighting on and off however would not be considered part of the lighting system for the purposes of this definition.
  • Dimming transformers, ballasts and drivers are often only compatible with certain dimmers. A compatibility list is generally available manufacturers. Dimmable bulbs also are only compatible with specific dimming ballasts and/or dimmers. Dimming ballasts, drivers, transformers, dimmers themselves and dimmable bulbs all have separate compatibility lists and each must be taken into consideration when troubleshooting a dimming application.

    Note: Simply because a particular piece of auxiliary gear or a particular bulb is not listed on a compatibility list does not mean that they are not compatible! If a particular combination isn't listed on a compatibility list that only means that that combination wasn't tested by the manufacturer and does not imply any particular reason for not choosing other combinations. Manufacturer only choose to test the combinations they feel are likely to be encountered for no reason other than simple efficiency.


  • The act of intentionally dimming one or more light sources.
  • The description of a lighting system that is intentionally designed to have the capability of being dimmed.
  • If a bulb is dimmed certain aspects of its output such as color temperature, x and y chromaticity values and CRI may be affected.

Direct Replacement

For LED fluorescent retrofit bulbs this means that the bulb must be operated by an appropriate fluorescent ballast and may not be powered directly by the line voltage.

Double Contact (Base)

A bayonet base that, has two electrical contacts on the bottom as opposed to a single electrical contact like on a single contact bayonet base. On a double contact base the outer shell of the base is not used as an electrical contact.
See also: Single Contact.

Double Envelope

This phrase is applied to halogen bulbs that have an outer envelope around the halogen capsule such as PAR bulbs. An outer envelope will have a lower temperature than the inner capsule simply because of its larger size and distance from the filament. However, it will still be very very hot. An outer envelope will protect against accidental oil deposits from touching the capsule.

Photo: single envelope (left), double envelope (right).

Drift Speed

The average speed at which electrons travel through a conductor as a current. Contrary to common thought, electrons do not travel in a straight line through a conductor but actually move in a zig-zag pattern as they run into other electrons which in turn pushed those electrons into others and so on and so forth. Electrons move surprisingly slow. Through copper wire with a load of 10A they move approximately 1 millimeter every 4 seconds.