Lighting and displays: Science and limits
Why no white light can ever reach 683 lumens per watt
What this tab covers What physically happens in the conversion, step by step, the science behind it, and the hard limits that physics sets on how well it can ever work.Lamps and screens turn electricity into visible light. LEDs do it directly in a semiconductor; older lamps heat a filament or excite a gas. Lighting uses roughly 8-12% of world electricity, depending on whether industrial lighting is counted.
The conversion, step by step
Driver (power supply)Electrical (mains AC) → Electrical (low-voltage DC) · Driver, heat and optics together cost about 15% when LEDs are built into lamps and luminaires3LED chip emits blue light
Electrical (DC) → Light (blue photons) plus heat · White LED packages above 200 lm/W already convert over 60% of their electricity into light3Phosphor converts part of the blue to yellow and red
Light (blue) → Light (white mix) plus heat · Energy is lost because each longer-wavelength photon carries less energy than the blue one absorbed (Stokes loss)3Light reaches the eye
Light (all wavelengths) → Perceived brightness (lumens) · Typical LED sold today about 100 lm/W; best about 230 lm/W1
The science
Light output is measured in lumens, which weight each wavelength by how sensitive the human eye is to it; luminous efficacy is lumens per watt of electricity (lm/W). In an LED, electrons and holes meet in a thin layer of indium gallium nitride and release their energy as photons whose colour is set by the semiconductor's band gap. Most white LEDs are blue chips coated with a phosphor that re-emits some light at longer wavelengths. An incandescent or halogen lamp heats a tungsten filament until it glows, so most energy comes out as infrared heat. Fluorescent lamps pass a current through mercury vapour, which emits ultraviolet that a phosphor coating turns into visible light. Lasers force atoms to emit photons in step, giving a narrow, intense beam. LCD screens filter an LED backlight through liquid crystals and colour filters, discarding most of the light; OLED screens make light in each pixel from organic layers.
Limits set by science
| Limit | Value | Why it exists | Best today |
|---|---|---|---|
| Eye-sensitivity ceiling | 683 lm/W4 | By definition of the SI unit, 1 W of pure green light at 540 THz (about 555 nm, where the eye is most sensitive) gives 683 lumens. Any real white light must include red and blue, which the eye sees less well, so it scores lower. | Not reachable for white light |
| Lossless white light with good colour rendering | About 414 lm/W31 | The best a perfect, loss-free white source could do while still showing colours well. | Best LEDs about 230 lm/W, roughly 55% of this ceiling (calculated) |
| Practical ceiling for phosphor-converted LEDs | About 250-255 lm/W3110 | Phosphor conversion always wastes some energy (Stokes loss), so blue-plus-phosphor LEDs cannot approach 414 lm/W. | Premium and professional LEDs above 200 lm/W; DOE expects products to reach up to 250 lm/W soon |
| Ceiling for colour-mixed LEDs | About 325-330 lm/W3 | Mixing red, green, blue and amber LEDs avoids phosphor loss, but green and amber LEDs are still inefficient (colour-mixed packages about 110 lm/W). | About 110 lm/W |
| Lasers | Wall-plug efficiency up to about 40% for standard high-power fibre lasers and about 50% for IPG's high-efficiency range (company claims); IPG puts rival fibre lasers at 25-35%610 | Pump diodes convert electricity to light, and the doped fibre converts pump light to the laser wavelength, losing some energy at each step. Blue laser diodes passed 45% peak power conversion efficiency in 2019 (DOE). | About 50% (IPG YLS-ECO series, company claim) |
Sources disagree on lighting's share of electricity because of scope: the IEA counts about 2,200 TWh (8%) for buildings and outdoor lighting in 2024 and 2,500-3,500 TWh if industry is included; IEA 4E estimates about 3,600 TWh (12%) in 2025, down from about 19% in 2000 (sources 1 and 2).
Sources
- The next wave of LED lighting: smarter, circular and more efficient (commentary) International Energy Agency, 2026
- 2019 Lighting R&D Opportunities US Department of Energy, Solid-State Lighting Program, 2020
- The International System of Units (SI Brochure, 9th edition): definition of the candela BIPM, 2019
- High efficiency fiber lasers (YLS-ECO series) IPG Photonics, 2026
- 2022 Solid-State Lighting R&D Opportunities US Department of Energy, Solid-State Lighting Program, 2022