Showroom of L.E.S.S. SA (LESS SA) of the Inspection Lighting Market with a Prisma, Chroma, Machine Vison, a Brightfield and a Mirage Table.

What does CRI mean and why is it important?

Selecting an optimal lighting system can be challenging as many factors need to be taken into account depending on your specific constraints. This article highlights the importance of the Colour Rendering Index (CRI) of a light source for quality illumination.

The use of high-quality lighting systems is of utmost importance to detect the tiniest irregularities, defects, scratches, or foreign matters on the inspected object, especially for visual inspection applications. When purchasing a lighting system, besides selecting the right colour of the light source (also known as the Correlated Colour Temperature, CCT), it is also important to understand how the illumination makes the colours of an object appear. This aspect, also known as colour rendering, may strongly affects the results of quality controls and analysis and must be considered in your purchase decision.

By emphasizing the importance of the so-called “Colour Rendering Index” (or CRI) and describing its significance in providing high-quality lighting systems, this article explains what it means, why it is important and helps you eliminate ambiguity related to this matter.

What is the Colour Rendering Index?

The Colour Rendering Index (CRI) of a light source provides a quantitative way to evaluate how well the colours of an object are revealed when illuminated by this particular source. In other words, how much this lighting system allows us to discriminate the different shades of an object.

As colour rending is related to the capability of our vision system, CRI evaluation is performed in comparison with the daylight of the sun for which our eyesight is optimized. For this reason, to have an ideal CRI, a light source must present a similar spectrum to the sunlight during the daytime, see Fig 1a. This means that the lamp spectrum must contain all colours contained inside the visible spectrum of the sunlight, from violet to red.

Artificial light sources always show discrepancies in the spectrum of the sunlight. An example is shown in Fig. 1. The sunlight spectrum is shown In Fig. 1a (on the left) and an LED spectrum is shown in Fig. 1b (on the right). Intuitively, the spectrum on the right-hand side is incomplete and may provide a misperception of some specific colours. The CRI indicates the level of difference that you should expect between an artificial light source and sunlight.

Fig. 1a: Reference daylight spectrum (D65 illuminant)

Fig. 1b: Typical spectrum of a white LED

How is the Color Rendering Index determined?

To calculate the CRI, eight CIE standard colour samples that span the visible spectrum are illuminated by the light source under evaluation and by the equivalent reference daylight illuminant, both having the same Correlated Colour Temperature (CCT). For both sources, the reflected light spectrum is measured, and the colour difference is evaluated. The latest is done by calculating the distance in the CIE colour space between the two measurements, giving the rendering indices Ri for the given standard colour sample using a dedicated formula. Practically, a Ri of 100 represents no colour rendering difference between daylight and the tested light source for the ith particular sample. On the contrary, a low value of Ri will show a big difference between the two reflections of the sample (source and reference).

What we refer to as the CRI, also denoted Ra, is the arithmetic mean of R1 to R8 and describes the colour rendition of these eight colour samples. For a more complete description, seven extra rendering indices of additional colours can be included (R9-R15). Fig. 2. Illustrated a typical Rth distribution of our lighting products. It shows a well-balanced Rth distribution and high colour rendering (Ra >90).

Fig. 2: CRI points of L.E.S.S. SA’s lamp

How to choose Color Rendering Index?

When deciding what kind of CRI will work best for your application, one has to make the difference between the Colour Rendering Index (CRI) and the Correlated Colour Temperature (CCT), which describes the appearance of white light. Warm white lights have low CCT values (< 4500°K) and cold white lights have high CCT (>5400°K). Be aware that the CCT and the CRI of a light source are independent factors. Your lighting system can exhibit a high CCT and low CRI and vice-versa. For example, an incandescent light source can exhibit a CCT as low as 2700K but a CRI above 95, and an LED-based system can exhibit a CCT above 6500K and exhibit a CRI lower than 80.

It is of common understanding that a CRI of 90 and higher is considered to have excellent quality in terms of colour reproduction, while a CRI below 80 is considered as good, and a CRI below 70 as limited quality in colour reproduction and may alter your colour visual perception as illustrated in Fig. 3.

Fig. 3. The colour of an Apple may appear natural or not depending on the level of CRI of the light source. 

Lighting Solutions at L.E.S.S. SA

All our inspection products exhibit a CRI above 90, with a well-balanced Rth distribution and the key benefit of good energy efficiency positioning us as utmost quality lighting systems.

From workplace lighting for watchmaking and machine vision for smaller samples to the more advanced microscopy instruments, L.E.S.S offers a wide range of products for visual inspection. The lighting systems have the same optical properties, including colour temperature, ultra-uniformity, and directionality, regardless of the lighting application. They are designed with an emphasis on users’ comfort and safety while offering highly precise and efficient lighting sources.

If you are interested in standardizing and improving all your manufacturing processes using a precise and consistent lighting system while ensuring user comfort and safety, feel free to contact us at L.E.S.S. for more details.

Article written by Dr. Yann Tissot, Dr. Jeremy Béguelin and Mathieu Charrière