The encyclopedia of LEDs
Light-emitting diode
History, physics, types and applications of the LED, from the first glowing crystal to MicroLED
A light-emitting diode (LED) is a semiconductor device that emits light when an electric current passes through it. Inside, electrons and holes recombine at a p–n junction and release energy as photons, a process called electroluminescence. The color is set by the semiconductor's band gap. White LEDs usually combine a blue chip with a yellow phosphor.
LEDs went from dim red indicator lights in the 1960s to the world's dominant light source in the 2020s. They now light homes, streets, stadiums and operating rooms, form every pixel of modern screens, signal from cars and runways, grow food indoors, disinfect water and carry data through optical fibers. LEDlights.org explains all of it in depth: the science, the history, every type of LED and application, plus free calculators.
- 1907First light from a solid-state device (SiC)
- 1962First visible LED (red GaAsP, GE)
- 2014Nobel Prize for the efficient blue LED
- >200 lm/WBest commercial white LEDs today
- 75–90%Energy saved vs. incandescent
- >50%Share of global lighting sales (2022)
Explore the encyclopedia
LEDlights.org is organized into five sections: History (1907 to MicroLED), How LEDs work (the physics), Types of LEDs (every package and display technology), Applications (from homes to hospitals) and Calculators. Seven in-depth guides compare SMD, COB and display types and explain CRI, drivers, lifespan and bulb bases.
1907 to today
Round's carborundum, Losev's light relay, Holonyak's red LED, the Nobel-winning blue LED, white light and MicroLED.
- Interactive timeline · 20 milestones
How LEDs work
P–n junctions, band gaps, materials and forward voltage, white light, lumens, CRI, CCT, heat and drivers.
- P–N junction, spectrum and CCT/CRI diagrams
Every type of LED
DIP, SMD 2835/5050, COB, CSP, filament, high-power, OLED, QLED, Mini-LED, MicroLED, IR, UV, WS2812B.
- Footprint and cross-section diagrams · comparison table
Where LEDs are used
Homes, offices, streetlights, stadiums, screens, cars, aircraft, boats, grow rooms, hospitals and sensors.
- Lux, PPFD, beam angles and IP ratings
LED calculators
Series resistor with E24 values and color bands, plus a lighting-equivalent and energy-savings calculator.
- Runs in your browser
Master comparison table
Efficacy, lifespan, CRI, heat and cost of 25 LED and legacy light sources, sortable and filterable.
- Includes incandescent, CFL, HPS, MH
What is an LED and how does it work?
An LED is a semiconductor diode that turns electric current directly into light. Electrons from the n-type side and holes from the p-type side meet at the junction and recombine, releasing photons. The photon energy equals the band gap, so the material sets the color: λ (nm) ≈ 1240 / Eg (eV).
An LED is a tiny chip of semiconductor crystal, often smaller than 1 mm², made of two regions. The n-type region has extra electrons and the p-type region has "holes" where electrons are missing. When you apply a voltage in the forward direction, from anode to cathode, electrons and holes are pushed together at the junction. Each time an electron falls into a hole, it gives up a fixed packet of energy, and in the right materials that energy leaves as a photon of light.
Because the photon's energy equals the material's band gap, the color is built into the crystal. Indium gallium nitride gives blue and green, aluminium indium gallium phosphide gives red, orange and amber, and gallium arsenide gives infrared. No filament is heated, so LEDs light instantly, switch millions of times per second, and waste far less energy as heat. They do need careful current control and good heat sinking. Read the full physics explanation →
A brief history
LED history runs from H. J. Round's glowing carborundum in 1907 and Oleg Losev's studies in the 1920s to the first practical infrared and red LEDs in 1961–62. The efficient blue LED of 1993 enabled white LEDs in 1996 and won the 2014 Nobel Prize; LEDs now dominate lighting and displays.
In 1907 Marconi engineer H. J. Round noticed a yellow glow from a carborundum (silicon carbide) radio detector. In the 1920s the Soviet physicist Oleg Losev studied the effect in detail, showed it was not heat, and patented a "light relay" for signalling. The physics was explained in 1951, once semiconductor theory caught up.
Practical LEDs arrived in 1961–62: infrared gallium arsenide diodes from Biard and Pittman at Texas Instruments, and the first visible red LED from Nick Holonyak Jr. at General Electric. Mass production of red indicators and calculator displays followed in 1968. George Craford added yellow and brighter reds in 1972, and Thomas Pearsall created LEDs for fiber optics in 1976.
Blue was the hardest color. Isamu Akasaki and Hiroshi Amano made the first p-type gallium nitride in 1989, and Shuji Nakamura announced a bright InGaN blue LED in 1993, followed by the white LED in 1996. All three shared the 2014 Nobel Prize in Physics. Since then, efficacy has passed 200 lm/W, governments have phased out incandescent and fluorescent lamps, and LEDs have taken over displays as OLED, Mini-LED and MicroLED. Explore the interactive timeline →
How LEDs make white light
No semiconductor emits white light directly. More than 95% of white LEDs are phosphor-converted: a 450 nm blue chip excites a yellow YAG:Ce phosphor, and the blue-plus-yellow mix looks white. Red phosphor adds warmth and color rendering, RGB chips mix colors, and violet-pumped LEDs give full-spectrum light.
Adding red phosphor to the classic blue-plus-yellow recipe produces warm 2700 K light and high color rendering (CRI 90+). Mixing separate red, green and blue chips is used for color-changing lights and displays, and violet-pumped full-spectrum LEDs closely match sunlight. More on white light, or read the CRI and CCT guide →
Why LEDs replaced other light sources
LEDs replaced incandescent, halogen, fluorescent and HID lamps because they are 5–15 times more efficient than incandescent, last 25,000–100,000 hours, contain no mercury, switch on instantly and dim smoothly. They are also directional, compact, robust and available at specific wavelengths for plants, medicine and sensing.
| Light source | Efficacy (lm/W) | Life (h) | Instant on | Dimmable | Mercury |
|---|---|---|---|---|---|
| Incandescent | 10–17 | ~1,000 | Yes | Yes | No |
| Halogen | 15–24 | 2,000–4,000 | Yes | Yes | No |
| CFL | 50–70 | 8,000–10,000 | Slow warm-up | Limited | Yes |
| Fluorescent tube | 80–100 | 20,000–36,000 | Mostly | With special ballast | Yes |
| Metal halide / HPS | 65–140 | 10,000–24,000 | No (minutes) | Limited | Yes |
| LED | 80–220+ | 25,000–100,000 | Yes | Yes, to <1% | No |
Beyond efficiency, LEDs are directional (less light wasted inside fixtures), small (new form factors such as strips and filament bulbs), robust (no glass or filament to break), controllable (color tuning, smart dimming, data transmission) and available at specific wavelengths for plants, medicine and sensing. Use the energy calculator to see what switching saves you.
The main LED families
The main LED families are through-hole DIP indicators, surface-mount SMD packages such as 2835 and 5050, chip-on-board (COB) and chip-scale (CSP) modules, filament LEDs, high-power emitters, display technologies (OLED, QLED, Mini-LED and MicroLED), invisible infrared and ultraviolet LEDs, and addressable RGB pixels such as the WS2812B.
In-depth guides
Seven in-depth guides go deeper than the main articles: SMD 2835 vs. 5050 vs. 3030, COB vs. SMD vs. CSP, and OLED vs. QLED vs. Mini-LED vs. MicroLED for types; CRI and CCT, drivers and flicker, and L70 lifespan for physics; and bulb shapes and bases for buyers.
Glossary of LED terms
This glossary defines the terms used most often in LED datasheets, lighting standards and product listings, from band gap and electroluminescence to CRI, CCT, L70, binning, PPFD and IP ratings. Each entry gives a short, precise definition that matches how the term is used across this encyclopedia.
- Light-emitting diode (LED)
- A semiconductor diode that emits light when current flows forward through its p–n junction. The color is set by the band gap of the semiconductor, or by a phosphor in white LEDs.
- Electroluminescence
- Light emitted by a material in direct response to an electric current or field, rather than heat. In LEDs it comes from electrons and holes recombining at a junction.
- p–n junction
- The boundary between p-type (hole-rich) and n-type (electron-rich) semiconductor. Forward bias pushes both carriers into it, where they recombine and, in an LED, emit photons.
- Band gap (Eg)
- The energy between a semiconductor's valence and conduction bands, in electron-volts. It sets the photon energy and therefore the wavelength: λ (nm) ≈ 1240 / Eg (eV).
- Forward voltage (Vf)
- The voltage across a conducting LED: about 1.2–1.5 V for infrared, 2 V for red, 3 V for blue and white, and 5–7 V for UV-C.
- Quantum well
- A semiconductor layer only 2–3 nm thick that traps electrons and holes so they recombine efficiently. Modern LEDs stack several of them (multiple quantum wells) in the active region.
- Phosphor (pc-LED)
- A luminescent material, such as YAG:Ce, that absorbs blue or violet LED light and re-emits longer wavelengths. Phosphor-converted LEDs produce more than 95% of white LED light.
- Lumen (lm)
- The unit of luminous flux: the total visible light a source emits, weighted by the eye's sensitivity. A 60 W-equivalent bulb gives about 800 lm.
- Luminous efficacy (lm/W)
- Lumens produced per watt of electrical power. White LEDs reach 80–230 lm/W commercially, compared with 10–17 lm/W for incandescent bulbs.
- Wall-plug efficiency (WPE)
- The share of electrical input power that leaves an LED as light. The best blue LEDs exceed 60%; UV-C LEDs reach only a few percent.
- Efficiency droop
- The fall in LED efficiency at high current density, caused mainly by Auger recombination. It is why efficient designs run more LEDs at lower current.
- Green gap
- The dip in LED efficiency between about 530 and 590 nm, where neither InGaN nor AlInGaP performs well, and a main reason white LEDs use phosphors instead of RGB mixing.
- Correlated color temperature (CCT)
- How warm or cool white light looks, in kelvin, matched to a glowing black body: 2700 K is warm white, 4000 K neutral and 6500 K cool daylight.
- Duv
- The distance of a light's color point above (greenish) or below (pinkish) the black-body curve. Good white LEDs stay within about ±0.006.
- Color rendering index (CRI, Ra)
- A score up to 100 for how accurately a light renders eight pastel test colors compared with a reference source. Ra 80 is the commercial minimum; 90+ counts as high CRI.
- R9
- The CRI score for a saturated red test color, which is not part of Ra. Low R9 makes skin, wood and food look dull; look for R9 above 50.
- TM-30 (Rf, Rg)
- The IES method that scores color fidelity (Rf) and gamut (Rg) with 99 color samples. It is more informative than CRI.
- MacAdam ellipse (SDCM)
- A region of the color diagram used to measure color differences in steps. LEDs within a 1–2 step ellipse look identical; at 5 steps or more, differences can be visible.
- Binning
- Sorting LEDs after manufacture into groups of similar flux, chromaticity and forward voltage, so that the LEDs in one product match in brightness and color.
- Junction temperature (Tj)
- The temperature of the light-emitting region inside the chip. Higher Tj lowers light output, shifts color and shortens life.
- Thermal runaway
- A loop in which current heats an LED, lowering its forward voltage, which draws more current and more heat until it fails. Resistors and constant-current drivers prevent it.
- L70 lifetime
- The operating hours until an LED's light output falls to 70% of its initial value. Quality LED luminaires are rated 50,000–100,000 h L70.
- LM-80 and TM-21
- IES LM-80 is the method for measuring LED lumen maintenance over at least 6,000 hours; IES TM-21 projects lifetime from that data, to at most six times the test duration.
- LED driver
- The power supply that converts mains or battery power into the regulated current or voltage an LED needs, often with dimming. Driver quality largely decides flicker and lifespan.
- Constant current (CC) and constant voltage (CV)
- CC drivers regulate current, for example 700 mA, for high-power LEDs and COBs. CV supplies deliver a fixed 12 or 24 V to strips that limit their own current.
- PWM dimming
- Dimming by switching an LED fully on and off many times per second and varying the on-time (duty cycle). Low PWM frequencies can cause visible or on-camera flicker.
- Flicker
- Rapid, repetitive change in light output, often caused by poor drivers or low-frequency PWM. It is measured as percent flicker, Pst LM and SVM, and limited by IEEE 1789 guidance and EU ecodesign rules.
- SMD LED
- A surface-mount LED soldered flat onto a circuit board. Its four-digit name is its size in tenths of a millimetre: a 2835 measures 2.8 × 3.5 mm.
- COB (chip-on-board)
- Many bare LED dies bonded directly to one substrate under a single phosphor layer, forming one bright, uniform light-emitting surface for spotlights and downlights.
- CSP (chip-scale package)
- A package barely larger than the die, usually a flip-chip with phosphor applied directly, soldered to the board without a lead frame or bond wires.
- MCPCB
- A metal-core printed circuit board, usually aluminium, that spreads heat from high-power LEDs into a heat sink. The common 20 mm "star" board is an MCPCB.
- PPFD
- Photosynthetic photon flux density: the number of 400–700 nm photons landing on a surface, in µmol/m²/s. It is the grow-light equivalent of lux.
- IP rating
- The IEC 60529 code for protection against dust and water. IP65 resists water jets, IP67 survives temporary immersion and IP68 continuous immersion.
- Addressable LED
- An RGB or RGBW LED with a built-in controller chip, such as the WS2812B, so every LED in a chain can show its own color, set over a data line.
- OLED
- An LED built from thin organic layers instead of a crystalline semiconductor, used as self-emissive pixels in phones, watches and TVs.
- MicroLED
- A display in which every sub-pixel is a microscopic inorganic LED, typically under 50 µm, combining per-pixel contrast with high brightness and no burn-in.
Frequently asked questions
What is an LED?
An LED (light-emitting diode) is a semiconductor diode that emits light when current flows through it. Electrons and holes recombine at its p–n junction and release energy as photons, a process called electroluminescence. The color depends on the band gap; white LEDs add a phosphor to a blue chip.
Who invented the LED?
No single person. H. J. Round observed the effect in 1907 and Oleg Losev studied it in the 1920s. Biard and Pittman at Texas Instruments patented the first practical infrared LED (1961–62), and Nick Holonyak Jr. made the first visible red LED at GE in 1962. Akasaki, Amano and Nakamura won the 2014 Nobel Prize for the blue LED.
How long do LEDs last?
LED bulbs are typically rated 15,000–25,000 hours and LED fixtures 50,000–100,000 hours to L70, the point where output has fallen to 70%. Heat and driver quality matter most: a hot bulb in an enclosed fixture can fail within a few thousand hours. See LED lifespan.
Are LEDs more efficient than incandescent bulbs?
Yes, roughly 6–15 times. Incandescent bulbs give 10–17 lm/W and LEDs 80–230 lm/W, so an 8–9 W LED replaces a 60 W incandescent bulb and uses 75–90% less energy.
Do LEDs contain mercury?
No. Unlike CFLs and fluorescent tubes, LEDs contain no mercury. They do contain electronics and should be recycled as e-waste where possible.
Why do some LED bulbs flicker?
Usually because of an incompatible dimmer, a low-quality driver with poor ripple filtering, or low-frequency PWM dimming. Use LED-rated trailing-edge dimmers, check the bulb maker's compatibility list and choose products specified as flicker-free. See LED drivers and flicker.
What is the difference between SMD 2835 and SMD 5050?
A 2835 is a 2.8 × 3.5 mm single-chip LED on a large thermal pad, typically 0.2–1 W, and the most efficient choice for white light at 120–220 lm/W. A 5050 is 5 × 5 mm with three dies, ideal for RGB and addressable strips but less efficient for white. See the full comparison.
What does L70 mean for LED lifespan?
L70 is the number of operating hours until an LED's light output falls to 70% of its initial value, roughly when people start to notice dimming. It is projected from IES LM-80 test data using the TM-21 method. An L70 of 50,000 hours equals about 11 years of 12-hour days.
Why do LED lights glow faintly when turned off?
A faint glow, or "ghosting", comes from tiny leakage currents: an illuminated or smart switch, a dimmer that passes a trickle current, voltage induced in long cable runs, or a switch wired in the neutral instead of the live conductor. LEDs need so little current that a few milliamps make them glow. An LED-compatible switch or a bleeder (load) device fixes it.
What CRI is best for kitchen and bathroom lighting?
Choose CRI 90 or higher with an R9 value above 50, so food, skin and makeup look natural. 2700–3000 K suits most homes, and 3000–4000 K suits task lighting over worktops and mirrors. CRI 80 bulbs make reds look dull, which is most noticeable in kitchens and bathrooms.
Are UV-C LEDs safe and effective for disinfection?
They are effective when the dose is right: 255–280 nm UV-C damages microbial DNA and RNA, and enclosed water and air units with tested log-reduction data work well. They are not safe for direct exposure, because UV-C injures eyes and skin within seconds to minutes at close range. Many cheap gadgets emit too little UV-C to disinfect.
What is the difference between Mini-LED, MicroLED and OLED?
Mini-LED is an LCD backlight made of thousands of tiny LEDs grouped into local-dimming zones. OLED makes every sub-pixel from organic emitters, giving perfect blacks with some burn-in risk. MicroLED makes every sub-pixel from a microscopic inorganic LED: perfect blacks, very high brightness and no burn-in, but still very expensive. See the display comparison.
How do you calculate the right resistor for an LED?
Use R = (Vsupply − n × Vf) / If. For one 3.0 V white LED at 20 mA on a 5 V supply, R = (5 − 3.0) / 0.020 = 100 Ω. Round up to the next standard value and use a resistor rated at least twice its dissipation (I² × R). The resistor calculator does this automatically.
Why do LEDs need a driver instead of plugging straight into AC power?
An LED is a low-voltage DC device whose current rises exponentially with voltage. Mains power is 120 or 230 V AC and reverses polarity 100–120 times a second, so a driver must rectify it, step it down and regulate the current. Without one, the LED would conduct destructive current surges and fail almost instantly.
What color temperature should I choose?
Use 2700 K for living rooms and bedrooms, 3000 K for kitchens and bathrooms, 3500–4000 K for home offices, garages and workplaces, and 5000 K or more for workshops and color inspection. Keep one CCT per room, and consider dim-to-warm or tunable-white lamps for evenings. See the CCT guide.
What LED bulb replaces a 60 W incandescent?
An 800-lumen LED A19 bulb, usually 8–9 W. Match lumens rather than watts: about 450 lm replaces 40 W, 1,100 lm replaces 75 W and 1,600 lm replaces 100 W. Choose 2700 K to match the warm color of the old bulb.
Can LED bulbs be used in enclosed fixtures?
Only if they are rated for enclosed fixtures. LEDs shed heat by conduction, and a sealed globe traps it, raising the LED and driver temperature and cutting lifespan sharply. Look for "enclosed fixture rated" on the package, or use a fixture with integrated LEDs designed for the enclosure.
Is the blue light from LEDs bad for your eyes?
Normal white LED lighting is safe: general-service LED lamps typically fall in the exempt or low-risk groups of the IEC 62471 photobiological safety standard, like other lamps of the same color temperature. Staring into any bright point source is unwise. Bright, cool light late in the evening can delay sleep, so use warm, dim light at night.
Why do LED bulbs fail early?
Heat is the usual cause. Cheap bulbs overdrive their LEDs and use small heat sinks and low-temperature electrolytic capacitors, which dry out quickly, especially in enclosed or recessed fixtures. Power surges and incompatible dimmers also shorten life. Bulbs from reputable brands with longer warranties usually last far longer.
What is the difference between COB and SMD LEDs?
SMD LEDs are individual packages soldered across a board, spreading light over many points, and they are the most efficient option. COB LEDs bond many bare dies under one phosphor layer to form a single bright surface, ideal for spotlights with one crisp shadow and high CRI. CSP LEDs are smaller still, with no package at all. See COB vs SMD vs CSP.
Can I use LED bulbs with a dimmer switch?
Yes, if the bulb is marked dimmable and the dimmer is LED-compatible. Old incandescent dimmers often need a minimum load of 40 W or more and cause flicker, buzzing or dropout with LEDs. Trailing-edge (ELV) dimmers usually work best; set the low-end trim and check the bulb maker's compatibility list.
What do IP65, IP67 and IP68 mean on LED lights?
They are IEC 60529 ingress-protection codes. The first digit 6 means dust-tight. The second digit rates water: 5 resists water jets, 7 survives immersion to 1 m for 30 minutes, and 8 survives continuous immersion beyond 1 m under conditions the maker specifies. IP65 suits outdoor fixtures, and IP67–IP68 suits in-ground and underwater lights.
Do LED lights attract fewer bugs?
Generally, yes. Many night-flying insects are drawn most strongly to ultraviolet and blue light, which mercury-vapor and some fluorescent lamps emit and white LEDs emit little of. Warm-white (2700 K or lower) and amber LEDs attract the fewest insects, while cool-white LEDs still attract some.
How do white LEDs work?
Most white LEDs use a blue InGaN chip at about 450 nm coated with a yellow YAG:Ce phosphor. The phosphor converts part of the blue into yellow, and the mixture looks white; adding red phosphor gives warmer light and higher CRI. Some premium LEDs use a violet chip with red, green and blue phosphors instead.
About this encyclopedia
LEDlights.org is written to be the clearest, most complete free reference on LEDs. Articles cite primary papers, standards (CIE, IES, ANSI, IEC, IEEE, UN ECE) and government sources (U.S. DOE, IEA), listed at the end of each article. Photographs are credited where they appear; most are from Wikimedia Commons under free licences. Illustrations made with AI are labelled as AI-generated. Diagrams and calculators are original to this site. Key references used throughout:
- CIE S 017:2020, ILV: International Lighting Vocabulary, 2nd edition, International Commission on Illumination (CIE).
- IES LM-80-20, Measuring Luminous Flux and Color Maintenance of LED Packages, Arrays and Modules, and IES TM-21-21, Projecting Long-Term Luminous Flux Maintenance of LED Light Sources.
- ANSI C78.377-2017, Specifications for the Chromaticity of Solid-State Lighting Products.
- IEEE 1789-2015, Recommended Practices for Modulating Current in High-Brightness LEDs for Mitigating Health Risks to Viewers.
- U.S. Department of Energy (DOE), Solid-State Lighting Program, R&D Opportunities reports (2019–2022).
- E. F. Schubert, Light-Emitting Diodes, 3rd edition (2018).
- The Royal Swedish Academy of Sciences, Scientific Background on the Nobel Prize in Physics 2014: Efficient Blue Light-Emitting Diodes.