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The density of luminous flux, i.e., the ratio between luminous flux and the illuminated surface area, expressed in lux (lumens per square meter) or footcandles (lumens per square foot).  Illuminance can be measured with a light meter or calculated according to characteristics of the lighting system.

Initial illuminance is the light output rating of a lamp before applying the light loss factor.  This rating applies when the lamp is first installed.

Maintained illuminance is the light output rating of a lamp after applying the light loss factor and corresponds to the average illuminance level expected over the long term.

Lamp efficacy
The ratio between luminous flux and the electrical power consumed by a lamp, expressed in lumens per watt (lm/W).

The luminous flux of a light source decreases over time and is known as lamp lumen depreciation.  Illuminance calculations must take into account the initial and maintained lamp efficacy.  In general, manufacturers supply the maintained value of luminous flux of a bulb as measured halfway though its rated life.

Light loss factor
This factor is used to calculate average maintained illuminance, or simply “average illuminance”.  Several variables account for reductions in luminous flux over time: lumen depreciation of the bulb, dirt deposits on the luminaire, leaking of the seal, decay of the ballast, etc.  To obtain the maintenance or light loss factor, the light output of a lamp is allowed to decrease until it corresponds to the long-term output desired.  In practice, one multiplies the actual light output by the light loss factor to obtain average illuminance.  Light loss factors typically range from 0.6 to 0.9.

Luminaire efficiency
The ratio between the luminous flux of a luminaire to the luminous flux of the bulb, expressed as a percentage.

Luminous flux
The total amount of light emitted by a bulb in all directions as perceived by the human eye, measured in lumens (lm).  This value is marked on bulb packages as “light output”.

Luminous intensity
The amount of luminous flux emitted by a light source per unit solid angle, measured in candelas (cd) or lumens per steradium steradian (lm/ sr).  1 cd = 1 lm/ sr.

Photometric report

A technical report describing the principal characteristics of a luminaire, especially how the light is emitted. The report contains data relating to the manufacture of the luminaire, including:

  • the percentage of luminous flux emitted above the horizon;
  • the percentage of luminous flux illuminating the ground;
  • the IESNA cutoff class;
  • the horizontal distribution of luminous flux around the luminaire;
  • other information (iso-illuminance contours, isocandela curves, coefficient of utilization, etc.).