What CRI (Ra) is: how faithfully light shows colours

CRI (Ra) tells you how true to life colours look under a lamp. Here is how the figure is calculated and how to use it when choosing lamps.

You buy a white shirt in a shop and at home it looks faintly grey-green. Or fresh tomatoes look dull under a new kitchen bulb. In such cases the culprit is often not the colour temperature but the lamp's ability to render colour. That ability is expressed as CRI, or Ra.

What CRI measures

CRI (Colour Rendering Index) shows how accurately a light source displays the colours of objects compared with a reference light. The scale runs up to 100. A score of 100 means test colours look the same under the lamp as under the reference. As the number falls, some colours fade and others shift to a different hue.

At lower colour temperatures the reference is thermal radiation like that of an incandescent bulb; at higher ones it is a model of daylight. That is why incandescent and halogen lamps naturally score close to 100. It does not mean they are “ideal”, only that the comparison is made against them.

How Ra is calculated

The general index Ra is the average of results for eight moderately saturated test colours from a standard set: pastel reddish, yellowish, greenish, bluish and so on. A separate value is calculated for each (R1, R2 … R8), then averaged.

There is a weakness here. An average can hide problems. A lamp may render pastel colours well and saturated red poorly, and still achieve a high Ra. For that reason the extended set includes further test colours, the best known being R9 for saturated red. For serious colour work, people look not only at Ra but also at R9.

Do not confuse it with kelvin

CRI and colour temperature are different things. Kelvin tells you what tint the light itself has: warm, neutral or cool. CRI tells you how correctly other objects' colours appear under that light. Two 3000K lamps could be Ra 80 and Ra 95: both look equally warm, but under the second, skin, food and fabric look livelier and more natural.

Which value is enough where

As a general guide:

• below Ra 80: places where colour does not matter, such as streets, warehouses and plant rooms;
• Ra 80: the widely accepted minimum for most homes and offices, and the level the European indoor workplace standard EN 12464-1 sets for most tasks;
• Ra 90 and above: places where colour decisions are made, such as artists' and designers' desks, clothing and cosmetics shops, mirror and make-up lighting, photo and video work, museums and galleries.

It is not a case of “higher is always better”. A high-CRI lamp in a corridor or storeroom adds nothing. But in a few spots at home, such as the kitchen worktop, the mirror or a reading place, light with good colour rendering makes a noticeable difference to everyday life.

Where to find it on the box

CRI is usually printed as “Ra ≥ 80”, “CRI > 80” or “Ra 90”. Some manufacturers use a three-digit code such as “830” or “940”. The first digit gives the CRI band (8 for above 80, 9 for above 90), and the other two give the colour temperature in hundreds (30 for 3000K, 40 for 4000K). This coding is especially common on linear lamps and panels.

If the box does not state CRI at all, ask the seller, and for places where it matters, choose a lamp that states it clearly. When buying several lamps for one room, check that both the kelvin and the CRI match: lamps with different colour rendering side by side can make the same wall look two different shades.

A simple test at home

You can see the difference without a lab. Take a red apple, fresh herbs and a piece of white fabric. Compare them first by a window in daylight, then under the lamp you are testing. If the red looks brownish-grey, the green looks overly “neon” or grey, or the white fabric takes on a greenish or pinkish tint, the lamp's colour rendering is weak.

Zaferoğlu Elektrik manufactures various types of LED lamps under the Zəfər İşığı brand in Ganja. If a particular model's Ra value matters for your room, ask the company for it before buying.