Fundamentals · 24 September 2026 · 2 min read
Colour temperature and CRI, explained
2700K or 4000K? CRI 80 or 90? Two numbers decide how warm a space feels and how true its materials look. Here is how to read them.
Correlated colour temperature (CCT)
CCT describes how warm or cool white light appears, in kelvin. Lower numbers are warmer: around 2700K reads as the warm glow of a home in the evening, 3000K as warm-neutral, and 4000K as the crisp neutral white common in offices and retail.
- 2700K–3000K — residential, hospitality, restaurants and lounges.
- 3000K–3500K — retail, galleries and mixed-use spaces.
- 4000K — workplaces, technical areas and spaces used mainly in daytime.
Consistency matters more than the exact number. Mixing colour temperatures in one view looks accidental; tunable-white and warm-dim luminaires let a single fitting shift across the day instead.
Colour rendering (CRI and R9)
The colour rendering index (CRI, or Ra) measures how faithfully a light source shows colours compared with a reference source, on a scale up to 100. Ra is the average of eight pastel test colours — so it can hide a weakness in saturated red, measured separately as R9. Reds matter for skin tones, food, timber and fabrics.
For hospitality, residential and retail we typically specify CRI 90 or higher with a strong R9. The newer IES TM-30 method goes further, reporting both fidelity (Rf) and saturation (Rg) for a fuller picture of how a source renders colour.
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