When evaluating a light source in lighting design, we often look at its colour temperature (Kelvin) and colour rendering index (CRI).
Yet these two values do not fully explain how light reveals colour.
Two light sources with the same colour temperature and similar CRI values can produce different visual results on the same surface. For example, two sources rated at 3000 K and CRI 90 may render the same wood surface in noticeably different tones. The reason lies in their spectral power distributions.
Spectral power distribution describes how the energy emitted by a light source is distributed across different wavelengths. White light is not a single colour; it contains a range of wavelengths across the visible spectrum. From red to green, from blue to other colour components, this distribution varies from one light source to another. As a result, two sources may appear to have a similar white tone to the eye while containing different spectral components.
These differences directly influence how illuminated surfaces are perceived. Materials such as wood, textiles, skin tones and artworks, where colour character matters, can appear different under sources with identical Kelvin and CRI values. This is because the perceived colour of a surface depends not only on the properties of the material itself, but also on the spectral content of the light falling on it.
For this reason, the questions “What Kelvin?” and “Is the CRI 90 or higher?” are not enough on their own. The spectral distribution of light also needs to be considered. Kelvin describes colour temperature; CRI evaluates how accurately selected colours are rendered in comparison with a reference light source. Spectral distribution reveals which wavelengths are present in the light and how their energy is distributed.
The same white appearance does not guarantee the same colour rendering. The true character of light is understood not through a few numerical values alone, but through its spectrum as a whole.
