What kelvin means: colour temperature in plain words

The 2700K or 6500K on a lamp is not how hot it gets but a measure of its light colour. Here is where the kelvin scale comes from and how to read it.

People who see "3000K" on a lamp box often wonder whether the bulb heats up to three thousand degrees. It does not. An LED bulb stays cool enough to hold in your hand. The kelvin figure describes not the lamp's temperature but the colour of its light. To see why it is expressed as a temperature at all, it helps to picture a blacksmith's forge.

A scale that comes from heated metal

As a piece of iron heats in a forge, its colour changes: first it glows dark red, then orange, then yellow, and eventually whitish. Physicists describe this with an idealised model called a black body, whose glow has a definite colour at every temperature.

Colour temperature comes from here. If a light's colour resembles that of a black body heated to a certain temperature, that temperature is the light's colour temperature. It is measured in kelvins. The kelvin scale starts at absolute zero; a kelvin is the same size as a degree Celsius, but zero sits about 273 degrees lower.

The paradox: warm light, low number

In everyday language we call yellowish, orange light "warm" and bluish light "cool", because those colours remind us of fire and ice. Physics works the other way round: the hotter a black body, the bluer its light.

So the scale reads like this:

• low kelvin (roughly 1800–2700K): yellow-orange light that looks "warm";
• mid kelvin (roughly 3500–4500K): neutral white;
• high kelvin (roughly 5000–6500K and above): bluish light that looks "cool".

You only need to remember this reversal once: the higher the number, the cooler the light.

Compared with familiar light sources

To picture the scale, here is roughly where familiar sources sit:

• a candle flame, about 1800–2000K;
• an old incandescent bulb, about 2700K;
• a halogen lamp, about 2800–3200K;
• sunrise and sunset, somewhere between 2000 and 3500K depending on season and weather;
• direct midday sunlight, about 5000–5500K;
• an overcast sky, about 6500K;
• blue skylight in the shade on a clear day, higher still.

These are not exact constants, but they give the overall picture: the "cosy" light we are used to at home is close to candles and incandescent bulbs, while outdoor daylight sits higher up.

"Correlated" temperature for LEDs

An LED bulb is not a heated object; it makes light through a semiconductor chip and phosphor. Its light does not match a black body exactly, it only looks similar. That is why technical documents use the term correlated colour temperature (CCT): the temperature of the black body whose light looks closest to that of the LED.

This has a practical consequence. Two bulbs with the same kelvin rating can differ slightly, one a touch greenish and the other a touch pinkish. Kelvin tells you the overall tone of the light but not everything about its colour quality; for that you need further figures such as the colour rendering index (CRI).

The 827, 840 and 865 codes on packaging

Some lamps, especially tubes and panel-type sources, carry a three-digit code instead of a kelvin figure. The code combines two pieces of information. The first digit gives the tens of the colour rendering index: 8 means Ra 80 or higher, 9 means Ra 90 or higher. The next two digits give the colour temperature in hundreds: 27 for 2700K, 40 for 4000K, 65 for 6500K.

So "827" is warm white with Ra above 80, "840" is neutral white with the same colour rendering, and "930" is 3000K with Ra above 90. Knowing the code makes it much easier to find an exact match for an old lamp.

A simple way to choose

At home the most common range runs from 2700 to 6500K. As a general direction:

• warm white (2700–3000K) for relaxing and evening spaces such as bedrooms and living rooms;
• neutral white (about 4000K) for kitchens, bathrooms, hallways and work zones;
• cool white (5000–6500K) for storerooms, workshops, utility rooms and places that call for high concentration.

These are starting points, not rules. Personal taste, wall colours and the amount of daylight all influence the final decision.

On the Zaferoğlu Elektrik home page, the colour temperature simulator lets you move a slider and watch the same room change across kelvin values. It makes choosing between warm and cool light much easier before you buy.