The color rendering index, also known as the CRI or Ra value, provides information about the quality of the color rendering of a light source. In this guide you will find a simple explanation of the complex subject of color rendering. You will also learn how to achieve natural-looking lighting with LED lamps.
Color rendering index definition
The color rendering index indicates how naturally the colors of objects or their surroundings appear with a particular light source. It is a comparative value with which the color impression of a light source can be determined. With the help of the color rendering index, the light quality of different illuminants can be compared. The following terms are also commonly used:
- Color Rendering Index (CRI)
- Rendering index (R)
- General rendering index (Ra)
All designations and their abbreviations refer to the color rendering index. For many lamps, the index is indicated as a CRI value or Ra value on the packaging or lamp.
The color rendering index has a value between 1 and 100 and has no unit. Natural light (sunlight) has a CRI of 100. Incandescent and halogen lamps also have the maximum color rendering index of 100. Fluorescent lamps, on the other hand, only have a CRI of 60 to 90.
We are used to see illuminated objects in their natural colors from the old light sources with filaments. With fluorescent tubes, the colors of the illuminated objects sometimes look slightly distorted. These look as if someone had put a grey veil over the colors, making them less luminous. The colors on the left in the picture look natural, on the right they look rather dull.
The effect becomes more visible when we go into an environment with red lighting with a red clothing, for example. Here, the red clothing no longer stands out from the red ambient light. If we go into the sunlight with the red clothing with Ra 100, the red color again is very noticeable.
CRI/Ra comparison of different light sources
The table gives an overview of the approximate CRI/Ra values to be expected for each illuminant.
|Light Source||Color Rendering Index (CRI/Ra)|
|Incandescent bulbs||up to 100|
|Halogen bulbs||95 to 100|
|LED bulbs||60 to 98|
|Energy-saving bulbs||80 to 89|
|Fluorescent bulbs||60 to 90|
|Mercury vapor lamp||45 to 50|
|Sodium vapor lamp||30 to 40|
Explanation in detail
This section is for those interested in the exact definition 🙂 The color rendering index is a photometric quantity used to determine the color impression of an illuminated object. It is a unit-less reference index. Sunlight serves as the reference. The index describes the quality of the composition of the white light of a light source.
This allows a statement to be made about the quality of the colors reproduced. The color rendering index can be used to compare the quality of the color rendering of different light sources.
How is the color rendering index determined?
The color rendering index is independent of the color temperature. For this reason, a suitable reference light source with identical color temperature is always used to determine the reference index depending on the light source. A black spotlight serves as the reference light source for all color temperatures up to 5000 K. All light sources above it are measured with a reference similar to daylight.
In order to determine the color rendering index for a light source, the following 14 test color samples were specified in CIE (1995).
|TCS01||Light greyish red|
|TCS02||Dark greyish yellow|
|TCS03||Strong yellow green|
|TCS04||Moderate yellowish green|
|TCS05||Light bluish green|
|TCS08||Light reddish purple|
|TCS13||Light yellowish pink|
|TCS14||Moderate olive green|
In the test procedure, the secondary spectra from the light source are measured and compared with the reference light source. The color rendering index is then calculated from the individual deviations (difference between artificial and natural light sources). For this purpose, the mean value is calculated from the values TCS01 to TCS08.
Color rendering index for LED lights
When switching to LED lighting, often only brightness and color temperature are taken into account. The color rendering index is not very tangible for many people and is therefore hardly noticed. The CRI value is another important parameter for good lighting quality, especially in the home.
What is the CRI value of LED lamps?
The first generations of LED light sources had a low color rendering index of 60 to 70. This often led to disappointment after the old incandescent lamp was replaced by a suitable retrofit model. The LED light was then often described as unnatural. Fortunately these times are over and the LED technology has developed quite a bit. LED lamps with CRI 95 are now available.
Also according to lighting regulatory guidelines a CRI of at least 80 is prescribed for the lighting in the living area. For outdoor and industrial applications, at least CRI 65 is required. This is why many lamps on the market now have a color rendering index of at least 80. In many cases, cheap no-name lamps do not even have a CRI value. Here you should expect a CRI of < 80.
Which CRI/Ra value to buy?
The color rendering of Ra 80 LED lamps is basically not bad. In direct comparison, however, the color rendering of a Ra 90 or even Ra 95 lamp appears much more natural. No general recommendation can be made here. Ultimately, it is a matter of taste whether the color rendering index really has to be as high as that of an incandescent lamp in order to perceive the light as pleasant.
In order to find the color rendering index that is pleasant for you, it is helpful to compare the lamps in the shop or order two or three lamps with different Ra values.
Color rendering vs efficiency
A small disadvantage of LED lamps with a high CRI value should not be concealed. As the CRI value increases, the efficiency of LED lamps usually decreases slightly. Manufacturers are trying to counteract this by mixing different light colors in the light source. This allows good color rendering to be achieved with high efficiency at the same time.
When buying LED lamps, especially those with high CRI values, you should pay attention to good efficiency.
You now know what the color rendering index or Ra/CRI value means. With this information you can compare the naturalness of the color rendering of different light sources. In living areas, LED lamps with a CRI between 80 and 90 are recommended. For particularly high demands on natural color rendering, there are also lamps with CRI 95.