Home · Published 2026-08-12
Lighting Terms and Acronyms: A Practical Glossary
Plain-English definitions of lumens, lux, candela, Kelvin, CRI, efficacy, beam angle, IP ratings, drivers, and common lighting acronyms.
6 min read
ICBy Imogen ClarkeEditorial contributor
TL;DR
Lumens (lm): Total Light Output
Luminous flux is the total quantity of visible light emitted, expressed in lumens. Consumer packaging commonly uses lumens as the starting point for comparing output. It does not show where the light goes: two 800-lumen products can produce very different results when one has a narrow beam and the other a diffuse shade.
Lux (lx): Light Reaching a Surface
Illuminance is the density of luminous flux arriving at a surface and is expressed in lux, equivalent to lumens per square metre. A lux value is meaningful only with a measurement position, distance, orientation, surrounding conditions, and operating mode. A product's peak or average lux claim should therefore include its test geometry.
Candela (cd) and Beam Angle
Luminous intensity describes light output in a particular direction and is expressed in candela. Beam angle describes the angular spread of a directional beam according to a stated method. Together they help distinguish a concentrated spotlight from a wide flood, but photometric data provides a more complete picture than one beam-angle number.
Luminance (cd/m²): How Bright a Surface Appears
Luminance describes the light emitted, transmitted, or reflected in a direction from an apparent surface area. It is expressed in candelas per square metre and is relevant to the apparent brightness of screens, signs, windows, and luminous or illuminated surfaces. It is not interchangeable with illuminance in lux.
Watts (W), Lumens per Watt, and Power Factor
Watts measure electrical power. Luminous efficacy, expressed in lumens per watt, relates visible light output to power and can help compare efficiency when the measurement basis is consistent. Power factor is an AC electrical characteristic rather than a measure of light output; it can matter to circuit and system design but should not be confused with efficacy.
Kelvin (K), CCT, and Duv
Correlated colour temperature, abbreviated CCT, describes the colour appearance of near-white light and is expressed in kelvin. Lower common values look warmer and higher ones cooler. Duv describes position relative to the Planckian locus and can help explain why two sources with the same nominal CCT may look slightly greener or pinker, although it is often omitted from consumer specifications.
CRI, Ra, R9, and TM-30
The colour rendering index is a traditional colour-fidelity measure comparing a source with a reference. General CRI is commonly reported as Ra; R9 is a separate special index associated with a strong red sample and is not included in the average Ra value. TM-30 is a newer evaluation method that reports fidelity and gamut information. None of these values describes brightness or colour temperature.
LED, Driver, and Integrated LED
LED means light-emitting diode. A driver converts and regulates electrical power for an LED load. In an integrated LED luminaire, the light source forms part of the fitting rather than being a familiar replaceable bulb. Check whether the LED module or driver is serviceable, what warranty applies, and what happens when one component fails.
CCT, RGB, RGBW, and Tunable White
RGB combines red, green, and blue channels for coloured effects. RGBW adds a dedicated white channel. Tunable white varies white-light CCT, often using warm- and cool-white channels, while dim-to-warm changes toward warmer light as brightness falls. Product labels vary, so confirm the documented white range, colour capability, and controls.
IP Ratings
An IP code classifies protection provided by an enclosure against access, solid objects, and water under defined test conditions. The first numeral relates to solid-object protection and the second to water. A higher number is not a universal promise of suitability: installation position, seals, cable entries, maintenance, and the manufacturer's intended environment still matter.
Beam, Glare, Flicker, and Diffusion
A narrow beam concentrates light; a diffuser spreads and softens it, usually with some optical loss. Glare is excessive brightness or contrast that causes discomfort or makes seeing harder. Flicker is variation in light output over time and can arise from the driver, dimmer, supply, or their interaction. A product can score well on one characteristic and poorly on another.
Photometric Data, IES, and LDT Files
Photometric data records how a luminaire distributes light. IES and LDT are common file formats used by lighting software to model that distribution. Reliable design still depends on the correct product file, mounting, room geometry, surface reflectance, maintenance assumptions, and a competent interpretation of the results.
Useful Smart-Lighting Acronyms
BLE means Bluetooth Low Energy; Wi-Fi connects a device to an IP network; Zigbee and Thread are low-power mesh protocols; Matter is an application-layer interoperability standard that can operate over technologies including Thread and Wi-Fi. A logo does not guarantee every feature appears in every app, so check controller, bridge, border-router, and platform requirements.
How to Read a Lighting Specification
Begin with the exact model and intended environment. Compare useful output and distribution, CCT and colour fidelity, power and controls, dimming compatibility, ingress protection where relevant, dimensions, installation instructions, serviceability, warranty, and cited test standards. Missing context around a large headline number should count against the claim rather than be filled in by assumption.
Sources
- 1International Lighting Vocabulary
International Commission on Illumination
- 2LED Basics
U.S. Department of Energy
- 3Energy 101: Lumens
U.S. Department of Energy
- 4IP Ratings
International Electrotechnical Commission
- 5SLL Lighting Publications
CIBSE Society of Light and Lighting