What product facts do you usually look for on a lighting label? Brightness and lumen output, estimated annual energy costs, the colour temperature of the lamp (warm or cool) and of course the expected life of the bulb. Now it's time to add another feature to the list: the CRI of the bulb.
CRI, color rendering index, was first established by the International Commission on Illumination to evaluate how effectively light sources display color. Measured on a scale of 1 to 100, it is the only universally recognized color rendering metric. However, it’s vital to distinguish CRI from color temperature, which refers to how warm or cold a white light source is.
According to Rensselaer Polytechnic Institute, CRI is tested using 8 standard color swatches. The swatches are illuminated by a test light source and compared to a reference illuminant of the same correlated color temperature. CRI is calculated from the difference in chromaticities between the test light and the reference light.
The smaller the average difference in chromaticities, the higher the CRI. So a CRI of 100 means that the test light performed exactly the same as the reference illuminant.
Understanding the CRI value of a bulb is easy: the higher the CRI of a bulb, the more vibrant the colours will be. the lower the CRI, the darker the colours will be. bulbs with a CRI rating of 80 or higher are suitable for most applications, and bulbs with a CRI of 90 or higher are considered excellent. bulbs with a CRI of 80 or less are often considered to be inferior bulbs and cause colours to appear faded and difficult to distinguish.
Please note that the colourimetric index of a light is an independent measure of its colour temperature (or "CCT" (relevant colour temperature) in the case of fluorescent lamps and LEDs). Colour temperature is measured in degrees Kelvin (K) and ranges from 1,000 to 10,000.
Natural light is a combination of all the colours in the visible spectrum. The colour of sunlight itself is white, but the colour of an object in sunlight depends on the colour reflected by the object.
For example, an apple appears to be red because it absorbs all colours in the visible spectrum except red, which is reflected by it. When using artificial light sources, such as LED lights, we try to reproduce the colours of natural daylight in a way that makes the object look the same as it would look in natural light. In some cases the reproduced colours will look similar and at other times look completely different. the CRI measures the similarity of the reproduced colours compared to the colours reproduced in natural light.
As a result, the apples no longer have the bright red appearance they would have in natural light, a phenomenon that CRI has attempted to characterise by measuring the overall accuracy of the colour of various objects under light sources.
However, modern LEDs can push the CRI up to the 90. This means that if you choose a high CRI COB LED chip, you can get almost perfect natural colours.
Think of CRI as a qualifying indicator. If the appearance of colour is important to your design, look for 90 CRI or higher, not because it guarantees its perfection, but because it shows that the manufacturer has the ability to control the quality of its chips and cares about the quality of the light they produce. Just like MPG in a car, the actual colour rendering may vary depending on various conditions, but if a given luminaire cannot exceed 80 CRI, you have sacrificed colour rendering as a design feature. They may have traded the colour quality of the chipset for a lower cost or higher brightness. Not every application needs perfect CRI. remember also that it is the combination of CRI and colour temperature that really tells the story of how the human eye perceives light.