Light Sources


Fluorescent tube is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. The typical luminous efficacy of fluorescent lighting systems is 50–100 lumens per watt, several times the efficacy of incandescent bulbs with comparable light output.

Fluorescent lamp requires a ballast to regulate the current through the lamp, but contain mercury, many fluorescent lamps are classified as hazardous waste. Environmental Protection Agency recommends that fluorescent lamps be segregated from general waste for recycling or safe disposal, and some jurisdictions require recycling of them.

Light-emitting diode (LED)

A light-emitting diode (LED) is a semiconductor light source that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (corresponding to the energy of the photons) is determined by the energy required for electrons to cross the band gap of the semiconductor. White light is obtained by using multiple semiconductors or a layer of light-emitting phosphor on the semiconductor device.

luminous efficacy

Luminous efficacy is a measure of how well a light source produces visible light. It is the ratio of luminous flux to power, measured in lumens per watt in the International System of Units (SI).  

Light SourceTypeOverall luminous efficacy (lm/W)Overall luminous efficiency
Incandescent5, 40, 100 W tungsten incandescent (120 V)5, 12.6, 17.50.7, 1.8, 2.6%
Light-emitting diode (LED) Theoretical limit for a white LED with phosphorescence color mixing260-30038.1–43.9%
Ideal sourcesTruncated 5800 K black-body25137%
Light-emitting diode21.5 W LED retrofit for T8 fluorescent tube (230 V)17225%
Light-emitting diode11 W LED screw base lamp (230 V)13820.3%
Gas discharge1400 W sulfur lamp10015%
Gas dischargeLow-pressure sodium lamp100-20015-29%
Gas dischargeHigh-pressure sodium lamp85-15012-22%
FluorescentPL-S 11 W U-tube, excluding ballast loss8212%
FluorescentT8 tube with electronic ballast80-10012-15%
Fluorescent70–150 W inductively-coupled electrodeless lighting system71-8410-12%
FluorescentT5 tube70-104.210-15.63%
Gas dischargeMetal halide lamp65-1159.5-17%
Fluorescent32 W T12 tube with magnetic ballast609%
Arc lampUltra-high-pressure (UHP) mercury-vapor arc lamp, free mounted58-788.5-11.4%
Arc lampMercury-xenon arc lamp50-557.3-8%
Fluorescent9–32 W compact fluorescent (with ballast)46-758-11.45%
Halogen incandescentPhotographic and projection lamps355.1%
Arc lampXenon arc lamp30-504.4-7.3%
Arc lampUltra-high-pressure (UHP) mercury-vapor arc lamp, with reflector for projectors30-504.4-7.3%
CathodoluminescenceElectron stimulated luminescence305%
Halogen incandescentTungsten quartz halogen (12–24 V)243.5%
Halogen incandescent2.6 W tungsten halogen (5.2 V)19.22.8%
Halogen incandescentHalogen-IR (120 V)17.7-24.52.6–3.5%
Halogen incandescent100, 200, 500 W tungsten halogen (230 V)16.7, 17.6, 19.82.4, 2.6, 2.9%
Incandescent15, 40, 100 W tungsten incandescent (230 V)8.0, 10.4, 13.81.2, 1.5, 2.0%
Arc lampCarbon arc lamp2-70.29–1.0%
Gas dischargePlasma display panel2-100.3–1.5%
CombustionGas mantle1-20.15–0.3%
Light-emitting diodeLED screw base lamp (120 V)Up to 102Up to 14.9%

Color Rendering Index (CRI)

A color rendering index (CRI) is a quantitative measure of the ability of a light source to reveal the colors of various objects faithfully in comparison with an ideal or natural light source.
Color rendering: Effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant
The CRI of a light source does not indicate the apparent color of the light source; that information is given by the correlated color temperature (CCT). The CRI is determined by the light source’s spectrum.

Correlated color temperature (CCT)

The correlated color temperature (CCT, Tcp) is the temperature of the Planckian radiator whose perceived color most closely resembles that of a given stimulus at the same brightness and under specified viewing conditions