If you’ve ever thought that some light bulbs make a room feel warmer than others, you aren’t imagining things.

Some bulbs give a rich amber glow, whereas others emit a crystal white glow. Placing the first bulb in your den will make it feel cozy and relaxed, whereas the second will create an ambience more akin to a doctor’s surgery than a comfy hang out.

Understanding these differences is critical to good lighting design, so how can you pick the right color for your lighting application?

In this beginner’s guide, we’ll explain how to pick the right bulb for any space by knowing how to use Correlated Color Temperature, or CCT.

What is CCT?

Correlated Color Temperature is a measure of the color appearance of a white LED. It’s measured in Kelvin and generally ranges from 2300K to 6000K. The higher the Kelvin, the colder and brighter the light.

For example, sunrise is about 3000K, whereas at noon the CCT is around 5500K or higher.

cct color temperaturen comparison

It’s worth noting that CCT does not give any information about the color rendering ability of the LED. You’ll want to look at the Color Rendering Index (CRI) for this instead. This is a measurement of how a light source renders the color of objects, compared to an ideal or natural light source.

Why is CCT important?

Do you ever feel unexplainably energetic at certain times of the day? Or perhaps you walk into a room and suddenly feel a drowsiness coming over you?

Surprisingly, this happens to a lot of us — thanks to our circadian rhythm.

The circadian rhythm is an internal, 24-hour clock which controls the sleep/wake cycle. It’s not as accurate an actual clock, however, and is open to influence from outside factors, such as light levels.

circadian rhythm lighting color temperature

A higher CCT light tricks our body that is in midday, suppressing melatonin, which makes us feel more awake. A low CCT light tricks our body that it is time to sleep, encouraging a relaxed and comfortable feeling.

So what’s this got to do with light bulbs?

By understanding how Correlated Color Temperature affects the circadian rhythm, we can pick the right Kelvin measurement to suit the space we are lighting. For example, you may want to apply an energizing Daylight Glow in an office space, but a subdued Warm White in your bedroom.

There are even studies to prove the role of CCT in interior design. In one case, published in the Journal of Circadian Rhythms, high correlated color temperature lighting was applied to one floor of a shift-based call center, whilst the other floor remained exposed to normal office lighting.

The result? Those who worked under the bright light were more productive, less tired and even showed signs of improved mental health and wellbeing!

Other experts have suggested that a range of CCT should be used in working spaces, to create the exact right atmosphere for a variety of tasks. For example:

  • Installing a high CCT light in brainstorming rooms could make workers invigorated and excited to share ideas.
  • Installing a low CCT light makes workers feel comfortable and trusting, ideal for HR meeting rooms and private offices.
  • Installing a middle CCT light, as close to daylight as possible, encourages a collaborative yet relaxed environment, ideal for general office space.
cct kelvin range productivity

And you can apply CCT know-how in your own home, too. If you want your living space to be relaxing, you’re better off using warmer lighting — in another study exploring CCT levels and heart rate, the majority of people found 3500K most relaxing.

How is CCT calculated?

The calculation of Correlated Color Temperature is a little complicated and there are various different approaches.

One practical approach is to use Judd's uniform chromaticity space (UCS), pictured below. This takes a chromaticity measure and gives CCT as a result.

cct kelvin graph chromaticity

Chances are, you’ll never have to measure CCT yourself. Simply review a bulb’s packaging and you should see its Kelvin measurement listed clearly, often shown on a scale from warm to cool.

Examples of CCTs for different applications

When choosing the Correlated Color Temperature for a specific application, ask what your goals are.

Do you want people to be alert or relaxed? Should the space feel cozy or clean? Does the space need to be multi-purpose with flexible task lighting or color tunable LEDs available?

Consider a supermarket lighting design project. The goal is to enhance the appearance of products and encourage the customer to buy:

  • In the vegetable section, use a warmer color (3000K) because it enhances the color of the produce. Combine with a high CRI light so customers can see the rich colors accurately.
  • In the frozen section, use a cooler color (4000-5000K) because the packaging does not need enhancement, but customers do need to be able to read product information clearly.
  • In the bakery and pastries section, consider a warm color to enhance the warm yellow colors of the bread.
  • In the warehouse, use high CCT lighting (5000K+) to increase alertness, raise visibility and minimize risk of accidents.

In a school lighting project, the goal is to encourage learning and collaborating:

  • For desk areas, use a cooler color (4000K) that will energize and allow good visibility. Try to minimize the impact of glare on whiteboards and screens.
  • For meeting rooms and staff offices, consider a warmer light (3000K) to make the space inviting, with lamps of 4000K for focused work.
  • For security and emergency lighting, consider high CCT lighting (5000K+) for maximum visibility.

For your own application, identify your goals and desired feel in the table below. Be sure to also consider how important accurate color rendering is for your application too — a CRI below 80 will wash out colors, whereas a high CRI will show all details.

CCT Commercial Applications Residential Applications Feel of Light
2300K (Amber Glow) High-end restaurants for accent or table lighting This is best suited for cozy places, like home bars and dens, ideally with dark-toned furniture. Rich and intimate, with an amber hue.
2700K (Warm White) Fashion retail, including mixed metal jewelry Ideal for comfortable cozy places with wooden furnishings. Bedrooms and living rooms. Comforting orange-yellow temperature.
3000K (Soft White Glow) This light is preferred by cafes, restaurants, lounges, bars and guest rooms in hotels. Also preferred for display of diamonds and silver. Good for general usage, in h ome offices and dining rooms. Inviting and pleasant, whilst still allowing good visibility.
4000K (Daylight Glow) Offices, schools and hotel lobbies and hallways. Perfect for designer spaces with minimal decor, kitchens, bathrooms and garages. Similar to natural daylight and gives an energetic feel.
5000K (Crystal White Glow) Ideal for outdoor security lighting and areas which need good visual rendering, like factories, workshops and hospitals. Outside or garage lighting. Crisp and illuminating, with a blue-white temperature.

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