Spectral Power Distribution: Looking Beyond Colour Temperature

In the previous blog, we explained how the Planckian Locus and Δuv influence the appearance of white light. These parameters help us understand why two luminaires with the same correlated colour temperature can still look slightly different.

However, they describe only how we perceive light.
To understand what the light actually consists of, we need to look at Spectral Power Distribution, commonly referred to as SPD.

White light is not one colour

Visible light consists of wavelengths ranging from approximately 380 to 780 nanometres. Each wavelength corresponds to a different colour, from violet through blue, green and yellow to deep red. A white LED does not emit only one wavelength. Instead, it emits many wavelengths simultaneously.
The human brain combines these wavelengths into a single colour sensation that we recognise as white light.
This is an important distinction. The light itself consists of many individual wavelengths. White is how our eyes interprets their combined effect.

What is Spectral Power Distribution?

Every light source emits its own combination of wavelengths. Spectral Power Distribution describes how much optical power a light source emits at each wavelength across the visible spectrum.
It is normally presented as a graph. The horizontal axis shows the wavelength in nanometres, while the vertical axis shows the relative amount of optical power emitted at each wavelength. In this way, SPD reveals the complete spectral composition of the light.
It can be considered the spectral fingerprint of an LED. Two light sources may appear similar to the human eye while having very different spectral distributions.

CCT describes the result. SPD describes the composition.

This is where many misunderstandings arise.
Correlated Colour Temperature, or CCT, is not a measurement of the complete spectrum. It describes the overall colour appearance of a light source in relation to the Planckian Locus.
Many different combinations of wavelengths can produce a similar chromaticity and therefore the same reported CCT.
For example, two LEDs can both be specified as 3000K and both appear warm white. However, one may emit relatively more energy in the deep-red region, while another may contain more energy in the yellow and green regions.
Both light sources can have the same CCT, but they do not have the same spectrum.

Why does the spectrum matter?

Objects do not respond directly to colour temperature. They respond to the wavelengths that reach them.
A red object appears red because it reflects part of the red region of the visible spectrum. Green materials reflect primarily green wavelengths, while yellow and golden materials reflect a different part of the spectrum.
If the required wavelengths are weak or absent in the light source, the object cannot reflect them effectively. Its colour may therefore appear dull, distorted or less saturated, even when the luminaire has the expected colour temperature.
This is why Spectral Power Distribution has such a strong influence on colour rendering, material appearance and visual perception.

Why application-specific spectra exist

One universal spectrum cannot provide the optimum visual result for every application.
For this reason, LED manufacturers develop spectra for specific products, materials and environments.
A spectrum for fresh meat may place more emphasis on deep-red wavelengths. Bakery lighting can reinforce warm golden and brown tones. Jewellery lighting may be designed to improve brilliance, sparkle and material contrast. Museum lighting focuses on faithful colour reproduction across a broad range of colours.
These light sources may have a similar CCT, but their spectral compositions are intentionally different because they serve different visual purposes.

Understanding light as a whole

Professional lighting design should not be based on one specification alone.

• CCT describes how warm or cool the light appears.
• Δuv describes the subtle tint of the light relative to the Planckian Locus.
• SPD describes the wavelengths that form the light and their relative power.

Together, these parameters provide a more complete understanding of a light source than the Kelvin value alone.

At Light4U BV, we believe that understanding these principles helps lighting designers, architects and engineers make better decisions. The quality of light depends not only on how it appears, but also on the spectral composition that creates that appearance.