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Luminous Efficacy

The visible light produced by light sources

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Luminous Efficacy is a measure of how well a light source produces visible light and the ratio of luminous flux (lumens, lm) to power (W). Luminous efficacy can be expressed as

η = Φ / P                   (1)

where

η = luminous efficacy

Φ = luminous flux - the quantity of light emitted by a light source (lumens, lm)

P = power (W)

Light Types and typical Luminous Efficacy:

LightLuminous Efficacy
- η -
(lumens/W)
Fluorescent lamp 45 - 75
Halogen lamp 16 - 24
High pressure sodium vapor lamp 85 - 150
LED lamp 30 - 90
Mercury vapor lamp 35 - 65
Metal halide lamp 75 - 100
Tungsten incandescent light bulb lamp 12 - 18

Luminous Intensity describes the quantity of light that is radiated in a particular direction. This is a useful measurement for directive lighting elements such as reflectors. It can be expressed as

I = Φ / Ω                       (2)

where

I = luminous intensity (lm/sr, candela, cd)

Φ = luminous flux (lumen, lm)

Ω = solid angle (the amount of the field of view from some particular point that a given object covers) into which luminous flux is emitted (steradians, sr)    

Example Luminous Efficacy - Power required to a LED Lamp vs. a Tungsten Incandescent Lamp

A specific application requires 500 lumens of light.

The required power to a tungsten incandescent lamp with luminous efficacy 15 lm/W can be calculated by modifying (1)

P = Φ / η

  = (500 lm) / (15 lm/W)

  = 33 W

The required power to a LED lamp with luminous efficacy 70 lm/W can be calculated by modifying (1) to

P = Φ / η

  = (500 lm) / (70 lm/W)

  = 7.1 W

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