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Luminous Efficacy

Light sources production of visible light

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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).

η = Φ / P                   (1)


η = luminous efficacy

Φ = luminous flux (lumens, lm)

P = power (W)

Light Types and typical Luminous Efficacy:

LightLuminous Efficacy
- η -
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

Example - 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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