Colour rendering index (CRI)

What is colour rendering index and why does it matter?

Colour rendering index, abbreviated as CRI or Ra, measures how accurately an artificial light source reproduces the colours of objects compared to natural daylight or an ideal reference light. It is rated on a scale of 0 to 100, where 100 represents perfect colour fidelity and 0 represents severe colour distortion. Every light source in your home, whether it is an LED, fluorescent, or tungsten bulb, has a CRI rating, and that rating directly affects how colours look under artificial light in the evening.

For architects and homeowners designing a space, CRI matters because it determines whether materials look right. A wooden floor that appears warm and rich under daylight can look dull or greenish under poor-CRI artificial light. Skin tones can look sallow. A carefully chosen paint colour can shift or flatten. These effects are subtle but pervasive, and they emerge only in the evening when you rely on artificial light. Most people do not consciously notice poor CRI until they spend time in a room lit with high-CRI sources and then return to lower-CRI lighting; the difference becomes obvious only in comparison.

Why does the average CRI hide the real problem?

The standard CRI metric (Ra) tests a light source against only eight standard colour samples: four pastel colours (light red, yellow, green, blue) and four saturated colours (red, yellow, green, blue). The final CRI score is the average performance across these eight samples. This averaging method creates a critical blind spot: a light source can score Ra 80 overall while performing very poorly on one specific colour, especially saturated red.

This is the key weakness of CRI as a single number. Imagine a LED that renders the pastel samples perfectly but severely desaturates reds. Its overall average CRI might still land at 75–80, passing a minimum threshold, but the red deficiency means skin, brick, wood, and food look wrong. You will not know this from the CRI number alone, which is why reading the individual colour test results (especially R9) is essential when selecting a light source for colour-critical spaces.

What is R9 and why does it matter for skin, wood, and food?

R9 is the CRI test result for saturated red, the ninth colour sample in the test sequence. It is the first number to fail in cheaper LED sources and the most visually noticeable because red appears everywhere in a home: in skin tones, wooden furniture and flooring, brick, terracotta, food, and many fabrics.

A high-quality light source with Ra 95 typically also has R9 above 90. A budget LED with Ra 80 might have R9 as low as 30 or 40, meaning reds are rendered as noticeably different from reality. Over an evening spent in a kitchen lit with such a source, the cumulative effect of seeing your food, your hands, and the wood cabinetry all slightly wrong can create a subliminal sense of wrongness about the space. This is why interior lighting plans that aim for colour accuracy should always specify both the CRI (Ra) and the individual R9 value, not the CRI alone.

How is colour rendering index different from colour temperature?

Colour temperature and CRI are often confused because both affect how light and colour look, but they measure completely different properties. Colour temperature, measured in Kelvin (K), describes the hue or character of the light itself: whether it leans warm (2700 K, like tungsten), neutral (4000 K, like daylight), or cool (6500 K, like bright noon sky). Colour temperature is a property of the light source, not of how it renders objects.

CRI, by contrast, measures how faithfully that light renders the colours of objects illuminated by it. You can have a warm 2700 K LED with excellent CRI (Ra 95) that renders reds beautifully, or a poor 2700 K LED with Ra 70 that makes the same red look muddy. The colour temperature sets the mood and warmth of the space; the CRI determines whether the objects in that space look right. A complete lighting specification therefore names both: "2700 K, Ra 90" means warm-toned light that also renders colours accurately.

What are TM-30 metrics and why is the industry moving toward them?

TM-30 is a newer colour rendering metric published by the Illuminating Engineering Society (IES) that addresses the limitations of the traditional CRI method. Instead of testing eight standard samples, TM-30 tests a light source against 99 colour samples, including real objects like skin, fabric, plants, and manufactured goods. The results are reported as two values: Rf (fidelity index, analogous to CRI but more comprehensive) and Rg (gamut index, which measures whether colours are shifted in saturation or hue).

TM-30 is more accurate than CRI because it avoids the averaging trap and because the 99-sample set better represents the colours actually found in homes and offices. However, TM-30 adoption is gradual. Datasheet manufacturers still list CRI as the primary metric, though high-end LED sources increasingly include TM-30 values. For now, specifying CRI remains standard practice, but reading individual colour samples (especially R9) and cross-checking against TM-30 values when available provides a more complete picture of how a light source will render colours in real use.

What colour rendering index should each room have?

CRI requirements vary by room function and by how critical colour accuracy is to the space. The table below provides typical recommendations for residential interiors:

The following table explains what typical CRI ranges mean in practice and how the deficiency affects the appearance of common materials:

Does high-CRI lighting cost more or use more power?

High-CRI LEDs do not consume more electricity than standard LEDs because the difference lies in the phosphor chemistry inside the LED chip, not in electrical efficiency. However, there are two genuine trade-offs to know about.

First, cost: high-CRI LED sources typically cost 15 to 30 percent more than equivalent standard LEDs at the same brightness and colour temperature. This premium reflects the more complex phosphor blend and tighter manufacturing tolerances required to achieve consistent high CRI.

Second, efficacy: some high-CRI LEDs exhibit a slight reduction in luminous efficacy (lumens per watt) compared to standard LEDs, typically a 5 to 10 percent loss. This means you may need to specify slightly more fixtures or higher wattage to achieve the same lux level in the room. In practice, this difference is often negligible for residential applications, but it is worth checking the datasheet if you are designing task lighting or ambient lighting where light levels must meet a specific target.

How do I read CRI on an LED product datasheet?

Every LED source worth specifying includes a datasheet that lists its photometric properties. Look for the photometric or electrical properties section; CRI is usually listed as a single number preceded by "Ra" or "CRI" (for example, Ra 90 or CRI 90). Some datasheets provide a range if the source is available in multiple versions.

Next, cross-check the colour temperature listed on the same sheet (typically 2700 K, 3000 K, 4000 K, or 5000 K) to ensure it matches your interior lighting plan specification. Finally, if the datasheet includes individual colour test results, look up R9 (saturated red) to verify it is above 70 for general use or above 90 if the space is colour-critical. Some premium datasheets also include TM-30 Rf and Rg values; these give a more complete picture but are not yet standard across all manufacturers.

When evaluating two similar LED sources, never choose based on CRI alone. Verify that both the CRI (Ra) and the key individual test result (R9) meet your space's requirements, and check whether the efficacy loss is meaningful for your target light level. This diligence ensures the artificial light in your home renders colours as faithfully as possible under evening light.