‘Human-centric’ lighting can enhance wellbeing

Dr Gareth Jones, Chairman of LED lighting testing and certification specialist, LUX-TSI, discusses evidence of the profound impact of daylight and artificial lighting on health and wellbeing, and explains how – with the right colour mix and intensity at different times of day – artificial light can ‘mimic, or even improve’, the light needed to maintain good circadian rhythm.

We ‘see’ light that passes into our eyes through a lens that is focused on our retina, which contains rods and cones that are receptive to light in wavelengths that are known as ‘visible’. Visible light is electro- magnetic radiation with wavelengths from 380-770 nm. Close to the visible light is the invisible ultraviolet (black light) and near infra-red (beyond red), and beyond that are radio waves, microwaves, X-rays, gamma rays, and so on.

Our vision has evolved to be highly adapted to typical natural light sources such as sunlight and firelight. White light consists of many wavelengths of light, as can be seen when it enters a prism or is split by raindrops in a rainbow. Although flames and the sun produce white light, their colours are very different. This is because the hotter an object gets, the more blue light is emitted (in a process called radiated emission). For example, heated objects (such as a poker in a fire) go from red to white hot as they heat up. Therefore, the colour of white light needed to be defined in order to measure and specify it. The way in which the colour of white light is defined is through the colour temperature or CCT, which is measured in Kelvin (where Kelvin (K) is 273 + Celsius (C); thus 25˚C = 298 K and 0˚C = 273 K). In simple terms, when an object is heated up to 3000 K (2727˚C), it will emit a similar colour (CCT) to that shown in Figure 1 (3000˚K), and when it is heated to 4200 K and 6400 K, the CCT will change, as seen in the other pictures.

The hotter the object, and the higher the CCT, the cooler the colour will look. For example, 3000 K is known as ‘warm white’, because it has a cosy, warm feel to it, like firelight, whereas 6400 K CCT has a much cooler appearance. It is important to understand this measure of the colour of white light in order to specify lighting and understand the impact that different levels of light have on our bodies.

During the day to night cycle, the sun rises at dawn, climbs higher in the sky, reaches its zenith, and then sinks back down towards dusk. When the sun is low in the sky, such as at dawn and dusk, scattering of the light by the air in the atmosphere causes more of the blue light


Light that passes into our eyes, through a lens that is focused on a retina that contains rods and cones that are receptive to light in wavelengths, is known as ‘visible’.

to be scattered towards the earth, resulting in less blue light being transmitted, and therefore a warmer reddish glow (also present at dusk for the same reason) with a CCT of 3000-3500 K.

At midday, when the sun is high in the sky, the full colour is seen; bright white light with a CCT of about 5500 K. When clouds are present this can increase to 7000 K. The sky appears blue because blue light is scattered more strongly in the atmosphere, and can therefore be seen all over the sky. The CCT of blue sky is about 10,000 K. Our bodies and minds have evolved to work well in and are used to a natural rhythm of increasing CCT and light levels from dawn until midday, followed by

reducing light levels and CCT in the evening. This day to night influence in the colour and intensity of the light has a profound effect on our health and wellbeing.

Health and wellbeing

Daylight affects our biological clocks, and helps us stay in tune with the time of day, which is known as the circadian rhythm. Our biological clock oscillates with an average periodicity of about 24.2 hours in the absence of any external time cues. Daily modulation of light patterns incident on the retina reset the biological clock and entrain its timing to match the local 24-hour light to dark pattern. Our bodies are attuned to certain behaviour at particular times of the

Figure 1: 3000 K is a ‘warm white’ colour, which has a cosy, warm feel (like firelight), while 6400 K CCT has a much cooler appearance.


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