‘Human-centric’ lighting can enhance wellbeing

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 electromagnetic 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.