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.

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.

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. 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. Jérémy Béguelin and Mathieu Charrière

Figure 3: Laser technology used to modify the shape of materials.

Table 1. Laser classification according to  the ICE 60825-1:2014

Table 2. Group of photobiological risk induced by lighting products according to the EN61471:2006

All our inspection lighting products are classified as Class 1 representing the safest laser product class, classified as “no risk” and demanding “no protective measures” according to the ICE 60825-1:2014 (green column in Table 1).

The importance of optimal lighting in visual inspection

Lighting is an important part of our everyday lives and can make all the difference in how we feel about a certain space or product. For example, the dim lighting of a restaurant is very different from the fluorescent white light in a hospital or ambient lighting used in living spaces.

Hand in hand with the development of industry, lighting technology has progressed tremendously over the course of the last few centuries. Only a couple hundred years ago, the first incandescent bulb was invented and today L.E.S.S. SA can produce high-quality light through the use of its property nano-active fibre technology and use it to study the world around us at a nanometre scale.

Visual inspection and the role of light

While many of us don’t think twice about the lighting we use in our offices and workspaces, lighting is of paramount importance for professions that routinely rely on visual inspection in their work. Visual inspection is a method for quality control and analysis that is used to detect any irregularities, defects, scratches, or foreign matter on the inspected object. During a visual inspection, a trained professional uses their naked eye to make judgments about the product quality. There are many industries, where visual inspection is practised regularly, such as watch and jewellery making, printed circuit board production, and pharmaceuticals. The lighting used during the visual inspection is of utmost importance, as it makes all the difference between noticing a subtle defect or inconsistency. For example, an unnoticed defect in a printed circuit board can lead to a device malfunction and even cause harm to the consumer. With the right colour temperature, light intensity and direction, the productivity and effectiveness of a visual inspection process can increase dramatically, resulting in higher quality and safer manufacturing processes.

Visual inspection for printed circuit board manufacturing

Printed circuit board (PCB) are an integral part of many electronic devices and their malfunction can have very serious consequences.

A visual inspection is the simplest and most effective way of PCBs inspection during the assembly and quality assessment. Inspecting the PCB helps to discover any defects or mishaps that might have occurred during assembly or manufacturing. Since PCBs have only gotten more advanced and complicated in their design, visual inspection after each phase of the assembly process is crucial. For the operator to be able to conduct the visual check successfully and in a consistent and reproducible manner, a high-quality, stable light source is essential. Our customers have reported a more than 25% increase in productivity after using our inspection lighting products. In addition, given the fact that visual inspection is done by a physical person, our lighting systems offer a comfortable and safe watch environment for the user.

Visual inspection in watchmaking

Watchmaking is another example of an industry that relies heavily on visual inspection. Each watch is made up of over a hundred complex yet miniature components.

Not only a visual inspection is employed during the actual watch assembly, but it also plays a crucial role during quality control. The surface of the watch needs to be visually inspected to make sure that it doesn’t have any scratches or damages. The face of the watch also needs to comply with the intricate design. An inspector also needs to be able to confirm that the watch is well-plated and if any gemstones are used, that they are firmly secured as well. In addition, the logo needs to be engraved or plated in the correct manner and in the right place. The scrupulous process of placing the tiniest watch details in their exact position and watch quality control can be made a lot easier and more productive if the right inspection tools are used.

Specifically designed for watchmaking, in close collaboration with the leaders of the watchmaking sector, L.E.S.S. has developed highly precise workplace lighting lamps – Prisma and Chroma. Placing a big emphasis on the comfort of the user, the lighting heads were designed to not dissipate heat. For the lamp’s easier and more precise position, it has been equipped with a top hinge, certified beyond 100 000 cycles. Prioritizing user’s needs, the lamps have also been designed on a modular approach, including accessories such as a tool tray or a magnifying glass that can be added on. The lamps can be directly fixed on a workbench, combined with the base, or interfaced with a mechanical arm, offering comfortable and ergonomic products that limit eye fatigue and neck and back pain during the operation.

Visual inspection in the pharmaceutical industry

For a pharmaceutical manufacturing industry, precision and quality control are vitally important, as they could have direct consequences on people’s lives and well-being.

Visual inspection remains to be the most trusted, fast, and precise quality control method for pharma. Every single drug container has to be examined for container defects and product defects. For example, vaccination ampoules, tablet blisters, plasma bags and insulin bottles are routinely checked for foreign particles, cosmetic defects and fit. A defect in the drug container can lead to improper storage and handling, which will impact the properties and safeness of the drug.

The drug formulations themselves have to be inspected for possible extraneous particulate matters (a process also known as parental drug detection), product precipitates, volume variations, and more. A defect in the drug formulation can have very serious implications for the health and well-being of consumers. Given how very subtle the difference between a compromised and an acceptable product can be, the colour temperature and lighting contrast need to allow for precise and accurate inspection at all times. In addition, visual inspection contributes to process knowledge and continuous process improvement.

The lighting system is a key element for successful visual inspection

In all industries where visual inspections are conducted routinely, the lighting system used is key to productivity, quality control, and assurance. The efficiency and precision of the lighting translate directly into the quality and safety of the product being manufactures, whether it’s a PCB, medicine, or a watch. Given that the visual check is done by a person, the human factor needs to be minimized. Therefore, the lighting parameters, such as colour temperature, directionality, and uniformity have to stay consistent throughout the visual check of the product and throughout the lifetime of the lighting device. At the same time, the lighting system needs to be safe and comfortable for the user, allowing inspection from different angles without putting a strain on the operator’s eyes.

Lighting solutions at L.E.S.S.

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 user’s 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.

Prisma

Das PRISMA ist der perfekte Ausdruck der Werte unserer Kunden: Er erreicht Spitzenleistungen und erfüllt die anspruchsvollsten Qualitätsstandards. Weit mehr als eine Innovation, die die Bedürfnisse von heute erfüllt, antizipiert er bereits die Erwartungen von morgen. Sein elegantes Design und seine außergewöhnlichen Leistungen werden Sie täglich begleiten.