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:
| Light | Luminous 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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