What colour temperature means
Correlated colour temperature, or CCT, describes whether white light looks warm or cool. It is measured in kelvin (K). Lower numbers look warmer, with more yellow and amber; higher numbers look cooler, with more blue.
CCT describes the appearance of the light only. It says nothing about how much light a luminaire delivers or how well it shows colours. Two luminaires with the same CCT can look quite different if their colour rendering differs.
- Warm white: about 2700 to 3000 K
- Neutral white: about 3500 to 4000 K
- Cool white: about 5000 K and above
Choosing a colour temperature
There is no single correct value. The right choice depends on the space, the people who use it and the surroundings. Common practice looks like this:
Outdoors, many communities now set an upper limit on colour temperature in their bylaws or lighting guidelines. DarkSky International, for example, recommends 3000 K or warmer for outdoor lighting. Check the local rules before you specify.
- 2700 to 3000 K: hotels, restaurants, homes and multi-residential buildings, and outdoor areas where the night sky and wildlife are a concern
- 3500 to 4000 K: offices, schools, retail floors, parking and most streets
- 5000 K and above: industrial plants, warehouses, inspection and detailed task areas, and sports venues where events are filmed or broadcast
Tunable white
Some luminaires let the colour temperature be changed after installation, either with a switch set once on site or continuously through a control system. Field-selectable CCT reduces the number of models a project has to stock. Continuously tunable white lets one space look warm in the evening and cooler during the working day.
Tunable products usually have slightly lower efficacy than a fixed-CCT version, because two sets of LEDs share the light output. Confirm the rated output and efficacy at the setting you plan to use.
Colour rendering index (CRI)
The colour rendering index, written CRI or Ra, measures how faithfully a light source shows the colours of objects compared with a reference source. It runs up to 100. A higher value means colours look more natural and are easier to tell apart.
Ra is the average of eight pastel test colours. It does not include R9, a saturated red. Red matters for skin tones, food, wood and many products, so specifiers often ask for a positive R9 value, or a minimum, as well as Ra. Some also use IES TM-30, a newer method that rates both colour fidelity and colour saturation.
- Ra 70: common for roadway, area and some industrial lighting
- Ra 80: the general standard for most indoor and outdoor work
- Ra 90 and above: retail, healthcare, museums, galleries and other colour-critical spaces
The trade-off with efficacy
CCT and CRI both affect how many lumens a luminaire produces per watt. Warmer colour temperatures and higher CRI values usually mean somewhat lower efficacy, because the phosphor has to convert more of the LED's blue light, and some energy is lost in that conversion. The difference varies by product, so always compare data at the exact CCT and CRI you intend to buy.
This is why a data sheet often quotes its highest efficacy at one combination, such as 4000 or 5000 K at CRI 70 or 80. If you choose 3000 K or CRI 90, check the lumen output again and adjust the design.
A quick checklist
Use these steps to keep the choice tied to the application, not to habit.
- Start with the application and any local rules on colour temperature
- Pick the CCT, then the minimum CRI, and add R9 where reds matter
- Keep CCT consistent within a space and check colour consistency between batches, measured in MacAdam steps (SDCM)
- Recalculate light levels with the actual output at the chosen CCT and CRI
- Ask for a photometric design that uses the right IES file for each option