A shirt that looks one colour in a shop and another at home may not have changed. Different light sources can alter how objects appear because they emit different distributions of light across the visible spectrum, which affects the colours reflected to our eyes.
The Colour Rendering Index, or CRI (also known by the Portuguese abbreviation IRC), is a measure of how faithfully a light source reproduces colours against a reference. The figure commonly listed in product specifications is Ra, an average based on eight moderately saturated colour samples, R1 through R8.
Ra can reach 100, but that does not mean a bulb is perfect in every aspect of lighting or that every colour will look identical under it. The reference used in the calculation depends on the light’s colour temperature. A CRI of 100 therefore does not mean a lamp produces sunlight or that its light is cool in appearance.
Colour temperature, expressed in kelvin (K), describes whether white light looks warm, neutral or cool. Around 2700 K is typically warm and yellowish, 4000 K more neutral, and 6500 K cooler and bluer. Colour temperature and colour rendering are separate properties: bulbs with the same temperature can still make objects look different, and a warmer bulb does not necessarily have a lower CRI.
One limitation of Ra is that it does not include R9, a sample representing saturated red. A lamp can therefore have a seemingly strong general CRI while rendering vivid reds less accurately. R9 may be worth checking when choosing lighting for food, fabrics, wood or faces.
As a practical guide, the US Department of Energy recommends CRI 80 as a minimum for indoor lighting and describes scores of 90 or above as indicating excellent colour fidelity. For shoppers who care about how food, clothing or décor looks, comparing products with Ra of at least 90 is one reasonable approach. These are selection guidelines, and lighting quality can involve trade-offs between colour fidelity, cost and luminous efficacy.
The CRI is not the only measure available. In 2025, the International Commission on Illumination recommended that the industry begin adopting the general colour fidelity index Rf, which uses 99 colour samples. During the transition, it recommends showing both Rf and Ra. The recommendation does not mean CRI has become useless or that product packaging has changed all at once.
Some products also list ANSI/IES TM-30 metrics. These include Rf for colour fidelity and Rg for average changes in the range of colours: values above 100 indicate an average increase, while values below 100 indicate an average reduction. An overall score can still conceal differences between colour groups, and fidelity is not the same as personal visual preference.
When comparing bulbs, check lumens for light output, watts for electrical power consumption, kelvin for the appearance of the white light, and CRI/Ra and R9 where available. Also check whether the bulb supports dimming and is compatible with the dimmer you plan to use.
A high CRI alone does not establish visual comfort, eye protection or flicker-free operation. It describes colour rendering, not every quality of a light source. Comparing the full specifications can help identify which bulb better shows the colours that matter to you, rather than simply which one produces more light.
Sources: Pplware.