Colour temperature and colour rendering: what do kelvin and Ra mean?

Kelvin: how warm or cool the light is

Colour temperature is measured in kelvin (K). The lower the number, the yellower and warmer the light; the higher, the whiter and bluer. Licht.de sorts it into three groups:

Within warm white there are further steps. A 2200 K lamp gives an amber light, such as the Lucide A60 filament lamp and the Ledvance 2200K filament lamp. At 2700 K you are at the colour of the old incandescent bulb, and 3000 K is a touch fresher.

Which light colour for which room

According to licht.de, homes mostly use warm white: the light feels calm and relaxing. Licht.de recommends warm white in the living room, at the dining table and at the bathroom mirror. Neutral and cool white light is activating, according to licht.de; offices, shops and public spaces mostly use neutral white and daylight white.

A few rules of thumb:

• Living room and bedroom: 2200 to 2700 K.
• Dining table: 2700 K; see our pendant lamps too.
• Kitchen and workspace: 3000 K, or up to 4000 K above the worktop if you want it fresher. Licht.de asks for at least Ra 80 in the kitchen.
• Bathroom mirror: warm white with Ra 90, as licht.de recommends. More in Bathroom lighting: which IP rating in which zone.

Keep to one colour temperature per room where you can. A 2700 K lamp next to a 4000 K one shows the difference straight away.

CRI or Ra: how true colours look

The colour rendering index is called CRI or Ra and runs up to 100. It says how natural colours look under the lamp compared with a reference light. Under the European ecodesign rules most lamps for homes and offices must reach at least Ra 80.

At Ra 90 or higher you mainly see the difference in skin tones, food, wood and fabrics. That is why makers put it in the name: Osram gives "Ra≥90" for the Osram PAR16 GU10 927, Philips "ExpertColor" for the MASTER LEDspot ExpertColor 940. Choose high colour rendering where colour counts: at the mirror, in the kitchen, by the wardrobe or above a painting.

The code on the pack: 827, 930, 940

Many LED bulbs carry a three-digit code. The first digit is the colour rendering, the last two are the colour temperature in hundreds of kelvin.

So the Osram Vintage 1906 Edison is a 927: warm white with Ra 90 or higher. The Osram E27 840 is neutral white with Ra 80 or higher.

Dim to warm and switchable white

An ordinary LED bulb keeps the same colour when you dim it. A dim-to-warm bulb gets warmer the further you dim it, as an incandescent bulb did. Philips calls this WarmGlow: the Philips WarmGlow GU10 goes from 2700 to 2200 K according to its product name, and the Philips mini-ball WarmGlow carries the code 922-927. Paulmann and Lucide put "Dim to warm" in the name, as on the Paulmann filament E27 and the Lucide GU10. Such a bulb needs a dimmer that suits it; the maker lists which dimmers work.

With switchable white you pick the colour yourself. Osram calls it CCT Select, as on the Osram Classic A100; Paulmann calls it White Switch, as on the Paulmann GU10. On some bulbs you switch with a small slider on the bulb, on others by turning the light switch off and on again quickly; the product description says which.

All bulbs are under LED bulbs and spotlights.