Colour Rendering Index (CRI): A Guide to Accurate Lighting

Understanding Colour Rendering Index in Lighting

The Colour Rendering Index (CRI) is one of the most important measures in lighting design. It tells us how accurately a light source reveals the true colours of objects compared to natural light. Whether in retail, healthcare, photography, or everyday living, CRI directly impacts how we perceive spaces and objects. This guide explains what CRI is, how it is calculated, why it matters, and where it is most critical.

What is the Colour Rendering Index?

The Colour Rendering Index is a scale from 0 to 100 that indicates how well a light source shows the true colours of objects. A CRI value of 100 represents perfect colour rendering, equivalent to natural daylight or an incandescent lamp. Lower scores indicate less accurate colour reproduction, which can make objects appear washed out, dull, or even unnatural.

How CRI is Calculated

CRI is calculated by comparing how a set of standardised colour samples look under a test light source versus a reference light source, usually daylight or a blackbody radiator. The colour differences are measured, averaged, and then converted into a CRI score. The closer the colours appear to the reference, the higher the CRI rating.

Why CRI Matters in Lighting

Understanding CRI is essential wherever colour accuracy is important. A poor CRI light source may distort colours, while a high CRI source makes them appear vivid and natural. Here are some examples where CRI makes a significant difference:

Retail Environments

Shops and showrooms rely on accurate colour rendering to present products in their best light. A garment that looks dull under poor lighting may appear vibrant under a high-CRI source, improving customer confidence and sales.

Art Galleries and Museums

For curators and visitors, accurate colour is vital. High-CRI lighting ensures that paintings, sculptures, and artefacts are displayed as the artist intended, preserving authenticity and enhancing the viewing experience.

Photography and Filmmaking

Lighting quality directly affects image and video production. High-CRI lighting reduces colour correction in post-production and captures true-to-life visuals, saving time and delivering higher quality results.

Medical and Dental Applications

In healthcare, accurate colour rendering can make a critical difference. From diagnosing skin conditions to matching dental prosthetics, high-CRI lighting supports precision and ensures reliable results.

CRI Scale and How to Interpret It

Here is a general guide to CRI values and their quality:

• CRI below 50: Poor colour rendering – colours appear unnatural and distorted.
• CRI 50–79: Moderate colour rendering – suitable for areas where accuracy is less critical.
• CRI 80–89: Good colour rendering – ideal for most indoor environments.
• CRI 90–100: Excellent colour rendering – required for professional applications where precision matters.

CRI Value	Colour Rendering Quality
0–50	Poor – colours look distorted and unnatural
50–80	Moderate – suitable for general lighting
80–90	Good – accurate colour in most indoor spaces
90–100	Excellent – perfect for art, retail, healthcare

Limitations of CRI

Although CRI is widely used, it has limitations. The calculation is based on only eight test colours, which may not represent the full spectrum encountered in real life. In addition, CRI does not account for how different wavelengths interact with human vision. Alternative metrics, such as TM-30 or R9 (for strong reds), can provide a more complete picture of colour performance.

Conclusion

The Colour Rendering Index remains a key standard in lighting, helping designers, retailers, healthcare providers, and artists select light sources that deliver accurate and vibrant colours. While not perfect, CRI offers an accessible benchmark for evaluating performance. For applications where colour fidelity is crucial, choosing lighting with a CRI above 90 is essential.

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