If you are choosing lighting for a room, most people focus on light temperature (Kelvin) and brightness (Lumens). After all, what is light but color and brightness?
However, even lightbulbs with identical Kelvins can dramatically alter the feel of a room by changing how objects and colors in the room appear. For example, your beautiful new dark brown leather sofa might look black under a different type of lighting.
This happens because light sources are not created equally, and therefore generate light in different ways.
Take a look at daylight. You can see a broad distribution across the visible light spectrum where all colors are present. With all colors represented, natural daylight allows you to see the real colors of an object. More technically, we could say that colors are ‘rendered’ accurately.
For a fluorescent lamp, the distribution looks very different. There are spikes, which means those colors will be emphasized in objects illuminated by the fluorescent lamp. For this bulb, illuminated objects will appear bluer. This leads to less accurate color rendering.
With a high-performance LED, you can see a distribution similar to daylight, which will enable more accurate color rendering.
To measure the accuracy of this color rendering, we use something known as the Color Rendering Index (CRI).
What is CRI?
Color Rendering Index (CRI) measures how accurately a light source can reveal the colors of an object when compared with another light source, typically daylight. This is on a scale from 0 to 100.
For example, daylight has a full spectrum distribution so could score 100, whereas the particularly harsh fluorescent lamp above might score as low as 50.
Why is CRI important?
For most people, CRI has the largest impact when lighting your home. Lighting low CRI will make the room feel dull and uninviting, whereas lighting with a good CRI will bring your decorating choices to life, showing your furniture, paint colors and decor in their best light.
Expert interior designers invest considerable time in choosing the right lighting and the graphic below shows the dramatic impact this can have.
Have you ever bought an item in a clothing store then it looked different when you got home? This might be because the store had low CRI lighting, preventing you from seeing the true color of the item.
High CRI lighting is preferred in retail stores, as it enhances the appearance of products and creates a more welcoming space.
Every great photographer understands the power lighting has to transform a portrait. Chances are your favourite photograph (at least of a person!) has been taken in a high CRI environment. With high CRI values, skin tones are more natural with more depth, making people seem more healthy and vibrant.
During fine art restoration, high CRI lighting is essential.
Exposing art to daylight could damage the piece, so fine art restorers use an artificial high CRI light source. This enables the restorer to see any damage and ensure they are repaired with the correct color. Without high CRI lighting, the restorer could repair an area with an inaccurate color, which would be visible when the piece is displayed in an exhibition with different lighting.
During printing processes, excellent color reproduction is required to ensure quality. For example, when printing a poster, colors must be checked against comparison samples to ensure accurate printing. Without high CRI lighting, they cannot be compared accurately, leading to differences between the design and the finished printed product.
How is CRI calculated?
Although we all have lighting preferences, it is not possible to measure CRI visually, as the perception of light is an individual experience.
Instead, a series of calculations must be used. These calculations produce a score from 0 to 100. Bulbs with a score close to 100 will provide accurate color rendering, perfect for applications such as art restoration, museums or retail. At the lower end, the light will create an unnatural look with distinct color bias. With a score of 0, all colors will look the same.
To measure CRI, follow the process below:
1. Determine the color temperature (CCT) of the light source.
2. Using this color temperature, pick a fair comparison light source for which the CRI is known. For example, a light which accurately simulates daylight.
3. Shine both light sources on a series of color swatches called Test Color Samples (TCS) and measure the color reflection.
4. These numbers are averaged and entered into a complicated CRI formula. This calculates how well rendered the colors are on a scale from 0 to 100.
What is considered a high CRI?
For home and the office, a CRI of 80 is acceptable, helping to keep the space bright and welcoming and gives people a healthy look. A lower score will produce dull colors and skin tones will appear washed out - not a great look for your guests!
However, a CRI of over 90 is essential if color rendering is important to you. For example, in high-end retail, museums and galleries, a high CRI light displays colors accurately, showing products, art and people at their best.
If color rendering isn't a priority, you might want to prioritize other lighting factors, such as output and wattage. However, with most modern LEDs boasting CRIs in the high 90s and exceptional energy efficiency, it makes sense to choose high quality LEDs and to live life in full color.