Efficacy (LPW): The overall system efficiency of a luminaire, which is the total usable light output (given in lumens) divided by the real electrical power input (given in watts). Commonly referred to as “Lumens Per Watt,” or LPW.

Correlated Color Temperature (CCT): The temperature of the black body, or Planckian radiator, whose perceived color most closely resembles that of a given light source at the same brightness and under specified viewing conditions. Given in Kelvin (K).

Color Rendering Index (CRI): The ability of a light source to reproduce the color of various objects faithfully in comparison with an ideal or natural light source.

LED Lifetime: The number of operating hours that the LED can maintain >70% of its original light output for a given set of temperature and light flux conditions. Lifetime is given as L70. (LEDs do not catastrophically fail like incandescent or fluorescent lights. Instead, they get progressively dimmer over a long period of time as they are operated.)

Spectral Power Distribution (SPD): The light power per unit area per unit wavelength of an illuminating source. Given as a two dimensional graph, with the x-axis being wavelength and the y-axis being relative light power per unit area.

Power Factor (PF): The ratio of real power to apparent power. Commonly given as a percentage (%) or a two decimal number (0.00).

Dimming Range: Ratio of minimum to maximum measured light, usually expressed as a percentage (%).
Definitions

Driver: Auxiliary device(s) needed to operate and vary the intensity of light output from LED lamp source(s) by regulating the voltage and current powering the source.

Measured Light: Stated light level defined by a manufacturer.