Luminous Efficacy

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 (lumen, lm)

P = power (W)

Light Types and typical Luminous Efficacy:

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

Temperature ^oC
K
^oF

Length m
km
in
ft
yards
miles
naut miles

Area m²
km²
in²
ft²
miles²
acres

Volume m³
liters
in³
ft³
us gal

Weight kg_f
N
lbf

Velocity m/s
km/h
ft/min
ft/s
mph
knots

Pressure Pa
bar
mm H₂O
kg/cm²
psi
inches H₂O

Flow m³/s
m³/h
US gpm
cfm