Colour Temperature - What Does It Mean?

If you've never heard the terms 'colour temperature,' 'warm white,' or 'cool white' before, that's because these things weren't important considerations in the era of incandescent light bulbs. All light bulbs were more or less the same. With the development of various lighting technologies in recent decades, however, colour temperature now plays a big part in how a light bulb looks once it’s switched on.

While this gives consumers much more choice, it means that shopping for a ‘white’ light bulb is no longer as simple as it may seem.

What is colour temperature?

In short, colour temperature is a scale that measures how ‘warm’ (yellow) or ‘cool’ (blue) the light from a particular source is. It is measured in Kelvins (abbreviated to K), and the higher the number, the 'cooler' the light. The lower the 'K' number, the 'warmer' the light.

Imagine the situation: one of the three white light bulbs in your kitchen blows, so you visit your local hardware store to buy a replacement. You take the broken bulb with you and use it to find another one with the same cap and the same shape.

Job done, right?

Except, when you get back home and fit the new bulb, you find that although it is a ‘white’ bulb, the light it gives off is a completely different colour to the other – supposedly identical – bulbs installed in your kitchen. This is because its colour temperature is different.

It might not seem like a big deal, but colour temperature can have a big effect on the look and feel of a room, and mixing bulbs of different colour temperatures can look a bit odd. Fortunately, there are ways to find light bulbs with the right colour temperature.

Let the Kelvin scale guide you

Colour temperature is measured in ‘Kelvins,’ which is usually abbreviated to ‘K.’ It is measured on a numbered scale, where the higher the number, the ‘cooler,’ or bluer the light. The lower the number, the ‘warmer,’ or yellower the light.

For example, a wax candle emits a very warm white light of roughly 1000K, while at the other end of the scale, a blue sky would measure around 9000K.

Colour Temperature Scale

It should be easy to find the colour temperature of a new light bulb, as it should be stated clearly on the packaging. It's worth noting that the colour temperature of a bulb isn't the same as its brightness. This is measured on a different scale (in units called 'lumens,' see here for more information).

What if I don’t know the colour temperature of my current bulbs?

Although technically it’s measured on an endlessly varied scale, there are several standard colour temperatures that most light bulbs are manufactured to. This makes it much easier to find replacements. If the colour temperature is not printed on the bulb itself, take a look at the image above to get an idea of the different colour temperatures you can find.

2700K - Warm White

This is the colour temperature of traditional incandescent light bulbs and still one of the most common types used today. This is a nice, yellow glow, well suited to rooms meant for relaxing, such as living rooms and bedrooms. If a particular light in your home has a yellowish tinge to it, it is most likely a warm white light bulb.

3000K - Warm White/White

This is still a nice warm light, yet it’s not as yellow as a 2700K light bulb. Bulbs of this colour temperature are versatile, as they are still good for relaxed areas, but can be used in busier rooms, such as kitchens.

4000K - Cool White

Cool white light bulbs have none of the yellow glow of warm white bulbs, and may even seem to have a blue tinge to them. The light they emit is much crisper, and is well suited to more stimulating environments, like kitchens and workplaces. Many outdoor floodlights use cool white bulbs, as it provides a clear, bright light.

6500K - Cool Daylight

Daylight bulbs are so named because they replicate the natural light seen on an overcast day. If these are installed indoors, they would look very blue, and as a result, they don’t create a very cosy atmosphere. These are best avoided in bedrooms and living rooms, and are instead better for busier areas, workplaces and outdoor lighting.

It’s worth noting that light bulbs with a ‘daylight’ colour temperature are not the same as ‘full-spectrum daylight' bulbs. The latter produce light across the whole colour spectrum (rather than just mimicking the colour of daylight), and are used to treat Seasonal Affective Disorder (SAD).

Colour temperature can sometimes be hard to visualise without examples, so use our interactive switches below to see how bulbs with different colour temperatures illuminate a room.

Colour Temperature - Examples

Press the light switches to see how different colour temperatures might look in your home
Warm White
Cool White