Lighting efficiency

Lighting efficiency: Artificial light sources are usually evaluated in terms of luminous efficacy of the source, also sometimes called wall-plug efficacy. This is the ratio between the total luminous flux emitted by a device and the total amount of input power (electrical, etc.) it consumes. The luminous efficacy of the source is a measure of the efficiency of the device with the output adjusted to account for the spectral response curve (the luminosity function). When expressed in dimensionless form (for example, as a fraction of the maximum possible luminous efficacy), this value may be called luminous efficiency of a source, overall luminous efficiency or lighting efficiency.

The main difference between the luminous efficacy of radiation and the luminous efficacy of a source is that the latter accounts for input energy that is lost as heat or otherwise exits the source as something other than electromagnetic radiation. Luminous efficacy of radiation is a property of the radiation emitted by a source. Luminous efficacy of a source is a property of the source as a whole.

The following table lists luminous efficacy of a source and efficiency for various light sources. Note that all lamps requiring electrical/electronic ballast are unless noted (see also voltage) listed without losses for that, reducing total efficiency.

Category	Type	Overall luminous efficacy (lm/W)	Overall luminous efficiency
Combustion	Candle	0.3	0.04%
Combustion	Gas mantle	1–2	0.15–0.3%
Incandescent	15, 40, 100W tungsten incandescent (230 V)	8.0, 10.4, 13.8	1.2, 1.5, 2.0%
Incandescent	5, 40, 100W tungsten incandescent (120 V)	5, 12.6, 17.5	0.7, 1.8, 2.6%
Halogen incandescent	100, 200, 500W tungsten halogen (230 V)	16.7, 17.6, 19.8	2.4, 2.6, 2.9%
	2.6W tungsten halogen (5.2 V)	19.2	2.8%
	Halogen-IR (120 V)	17.7–24.5	2.6–3.5%
	Tungsten quartz halogen (12–24 V)	24	3.5%
	Photographic and projection lamps	35	5.1%
Light-emitting diode	LED screw base lamp (120 V)	Up to 102	Up to 14.9%
	11W LED screw base lamp (230V)	138	20.3%
	21.5W LED retrofit for T8 fluorescent tube (230V)	172	25%
	Theoretical limit for a white LED with phosphorescence color mixing	260–300	38.1–43.9%
Arc lamp	Carbon arc lamp	2–7	0.29–1.0%
	Xenon arc lamp	30–50	4.4–7.3%
	Mercury-xenon arc lamp	50–55	7.3–8%
	Ultra-high-pressure (UHP) mercury-vapor arc lamp, free mounted	58–78	8.5–11.4%
	Ultra-high-pressure (UHP) mercury-vapor arc lamp, with reflector for projectors	30–50	4.4–7.3%
Fluorescent	32W T12 tube with magnetic ballast	60	9%
	9–32W compact fluorescent (with ballast)	46–75	8–11.45%
	T8 tube with electronic ballast	80–100	12–15%
	PL-S 11W U-tube, excluding ballast loss	82	12%
	T5 tube	70–104.2	10–15.63%
	70–150W inductively-coupled electrodeless lighting system	71–84	10–12%
Gas discharge	1400W sulfur lamp	100	15%
	Metal halide lamp	65–115	9.5–17%
	High-pressure sodium lamp	85–150	12–22%
	Low-pressure sodium lamp	100–200	15–29%
	Plasma display panel	2–10	0.3–1.5%
Cathodoluminescence	Electron stimulated luminescence	30	5%
Ideal sources	Truncated 5800 K black-body	251	37%
Ideal sources	Green light at 540 THz (maximum possible luminous efficacy by definition)	683	100%

Sources that depend on thermal emission from a solid filament, such as incandescent light bulbs, tend to have low overall efficacy because, as explained by Donald L. Klipstein, “An ideal thermal radiator produces visible light most efficiently at temperatures around 6300 °C (6600 K or 11,500 °F). Even at this high temperature, a lot of the radiation is either infrared or ultraviolet, and the theoretical luminous [efficacy] is 95 lumens per watt. No substance is solid and usable as a light bulb filament at temperatures anywhere close to this. The surface of the sun is not quite that hot. At temperatures where the tungsten filament of an ordinary light bulb remains solid (below 3683 kelvin).

SI photometry units


Quantity		Unit		Dimension	Notes
Name	Symbol	Name	Symbol	Symbol	Notes
Luminous energy	Q_v	lumen second	lm⋅s	T⋅J	The lumen second is sometimes called the talbot.
Luminous flux, luminous power	Φ_v	lumen (= candela steradians)	lm (= cd⋅sr)	J	Luminous energy per unit time
Luminous intensity	I_v	candela (= lumen per steradian)	cd (= lm/sr)	J	Luminous flux per unit solid angle
Luminance	L_v	candela per square metre	cd/m²	L⁻²⋅J	Luminous flux per unit solid angle per unit projected source area. The candela per square metre is sometimes called the nit.
Illuminance	E_v	lux (= lumen per square metre)	lx (= lm/m²)	L⁻²⋅J	Luminous flux incident on a surface
Luminous exitance, luminous emittance	M_v	lux	lx	L⁻²⋅J	Luminous flux emitted from a surface
Luminous exposure	H_v	lux second	lx⋅s	L⁻²⋅T⋅J	Time-integrated illuminance
Luminous energy density	ω_v	lumen second per cubic metre	lm⋅s/m³	L⁻³⋅T⋅J
Luminous efficiency (of radiation)	K	lumen per watt	lm/W	M⁻¹⋅L⁻²⋅T³⋅J	Ratio of luminous flux to radiant flux
Luminous efficiency (of a source)	η	lumen per watt	lm/W	M⁻¹⋅L⁻²⋅T³⋅J	Ratio of luminous flux to power consumption
Luminous efficiency, luminous coefficient	V			1	Luminous efficacy normalized by the maximum possible efficacy

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