Freelance engineering office for lighting design & planningColour temperature in kelvin
The colour of light is specified by the (correlated) colour temperature in kelvin. The higher the value, the cooler the light appears. Light sources are divided into three groups [1]:
| Designation | Colour temperature | Effect |
|---|---|---|
| Warm white (ww) | up to 3,300 K | cosy, comfortable |
| Neutral white (nw) | 3,300–5,300 K | matter-of-fact |
| Daylight white (dw) | above 5,300 K | cool, sober |
For comparison: daylight under an overcast sky is around 6,500 K [1].
Colour rendering: Ra or CRI
The colour rendering index Ra (CRI) describes how natural colours and skin appear in the light of a lamp. The best value is 100; the lower the value, the poorer the colour rendering [2]. Lamps often carry a three-digit code: the first digit stands for colour rendering, the second and third for colour temperature. Example: "930" means Ra 90–100 at 3,000 K, "840" Ra 80–89 at 4,000 K [2].
Glare and UGR
Direct glare occurs when overly bright luminous surfaces are in the field of view. For interiors it is assessed using the UGR method (Unified Glare Rating) of the International Commission on Illumination (CIE), which takes into account all luminaires of an installation as well as the brightness of walls and ceilings; the lower the value, the lower the glare [3]. The limits of the workplace standard EN 12464-1 form the series 16, 19, 22, 25 and 28 [4]. Reading, writing and screen work are among the tasks with a strict limit [4].
Important: a "UGR<19" on the data sheet only applies to a standard room. Whether the limit is met in your own room is only shown by the manufacturer's UGR table combined with room size and reflectances [3].
What the EU requires: ecodesign
Since 1 September 2021, the ecodesign requirements of Regulation (EU) 2019/2020 have applied to light sources [5]. Among other things, they require a colour rendering index of at least 80 – with exceptions, for example for outdoor and industrial applications –, colour consistency within a six-step MacAdam ellipse, flicker PstLM ≤ 1.0 and a stroboscopic effect SVM ≤ 0.4 at full load; standby power must not exceed 0.5 W [5]. These values are minimum standards – for living spaces it is worth exceeding them.
Buying checklist
- Colour temperature suited to the room (living spaces usually 2,700–3,000 K)
- Colour rendering Ra ≥ 90 for living spaces
- Dimmability and a compatible dimmer
- Glare control: recessed light source, honeycomb louvre or opal diffuser
- Compare luminous flux in lumens rather than watts
- Observe the protection rating (IP) for bathrooms and outdoors
- Low flicker: check the PstLM and SVM values in the data sheet
- Colour consistency: MacAdam steps (SDCM) – the smaller the value, the more uniform several luminaires look
Frequently asked questions
What does 2700 K on the packaging mean?
The lamp gives warm white light, similar to a classic incandescent bulb.
Is Ra 80 sufficient?
For corridors, basements and utility rooms, yes. For living spaces, kitchens and bathrooms we recommend Ra 90 or more.
What does SDCM or MacAdam step mean?
The value describes how far the light colour of individual lamps may deviate from one another. The smaller it is, the more uniform the light of several luminaires side by side looks.
Sources (in German)
- licht.de – Lichtlexikon: Lichtfarbe
- licht.de – Farbwiedergabe
- licht.de – Blendung und UGR
- ZVEI/licht.de – Positionspapier UGR-Verfahren
- EUR-Lex – Verordnung (EU) 2019/2020 (Ökodesign Lichtquellen)
Author: Paul Rostand Ndjeundoun, M.Sc. (Hons), Certified Lighting Expert (LiTG), Registered European Lighting Expert · As of November 2026. Standard values have been carefully researched but do not replace consulting the currently valid edition of the standard.