How to Read Photometrics: Part 1 - Photometric Terms

Photometric Terms

Lumens: Also known as Luminous Flux, which is measured in lumens (lm), is the total amount of light produced by a light source without regard to direction. The luminous flux is provided by lamp manufacturers and common lumen values are included in the lamp matrix.

Candela: Luminous intensity, measured in candela (cd), is the amount of light produced in a specific direction. Graphically, this information is compiled into polar formatted charts that pinpoint the intensity of light at each angle away from 0° lamp axis (nadir). The numeric information is also available in tabular form.

Footcandles: Illuminance, measured in footcandles (fc), is the measure of the quantity of light that arrives on a surface. Three factors that affect illuminance are the intensity of the luminaire in the direction of the surface, the distance from the luminaire to the surface, and the angle of incidence of the arriving light. Although illuminance cannot be detected by our eyes, it is a common criterion used in specifying designs.

Candelas/meter2: Luminance measured in candelas/meter2 (cd/m2) is the quantity of light that leaves a surface. It is what the eye perceives. Luminance will reveal more about the quality and comfort of a design than illuminance alone.

Cutoff: The cutoff angle of a light is the angle between its vertical axis and the line of sight, where the brightness of the source or its reflected image is no longer visible. This is the deciding factor which lighting designers use to determine the visual comfort in a lighting system. Deep cutoff optics provide low brightness luminaires, allowing the eye to see more effectively. The shielding angle is the complementary angle to the cutoff angle.

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