My grandmother, colour perception and lighting specification

Traditionally in Cyprus, when somebody loses a very close family member, it is customary to wear black clothes as a symbol of mourning. Since losing her son at war, my grandmother always used to wear black or dark blue clothes. I remember many times having the conversation with her whether her blouse was blue or black and how it did not match her skirt which was definitely black. Back then I thought it was down to old age, not being able to recognise colours (which partly is correct), but if you have experienced something similar whilst being still ‘young and beautiful’ I can assure you that the reason for this is probably the light in the room, and not your eyes.

My grandparents in 1992.

My grandparents in 1992.

Colour is perceived by the human eye as the light that is reflected from an object. For example, a red apple absorbs certain amount of light and reflects the wavelengths of red light. The cones in our eyes receive the reflected light and then a signal is sent to the brain which then decides that the apple is red. This is quite straightforward and reminds us of science experiments at school and Isaac Newton, but does it affect our everyday in any way? In real life we very frequently change the light source and subsequently the initial ‘spectrum’ that reaches the object in the experiment. The light source can vary from a halogen to a compact fluorescent lamp, to an LED luminaire, to a high pressure sodium street light. This means that because not all light sources have the same light spectrum, the object will be reflecting a different total of wavelengths of light. Now imagine being a doctor looking at a patient’s skin, a chef at an expensive restaurant, a make-up artist or a shop owner who sells clothes that look different in the shop window from the changing room. Good colour perception is important in many sectors and modern LED technology has solutions. It is important to mention that usually the better the light source is in revealing colours, the more energy it will consume. At situations where accurate colour perception is not vital, for example a mechanical plant room, downgrading this aspect in the specification of the light source, is a good way to save on energy consumption. But how do we know what is the colour spectrum of a light source?

Different light spectra of different light sources. Daylight has a continuous light spectrum and it is a point of reference when talking about the colour of objects. (Image source:  Housecraft )

Different light spectra of different light sources. Daylight has a continuous light spectrum and it is a point of reference when talking about the colour of objects. (Image source: Housecraft)

Somebody who specifies lighting or simply wants to know more about how colours will look before installing a light in his house should be looking on the box for something called CRI- Colour Rendering Index which Wikipedia describes as ‘a quantitative measure of the ability of a light source to reveal the colours of various objects faithfully in comparison with an ideal or natural light source’. We can argue that colours look their best when seen under daylight, which has a full spectrum. An incandescent lamp or a candle also has a continuous spectrum and a ‘perfect’ CRI of 100. A good CRI for indoor spaces is above 80 while in places where we would need good colour rendering and have to use LED technology, we would be looking at something above 90. It is worth mentioning that the spectrum of a light source is independent from the ‘colour’ of the light. A warm white LED light can have better colour rendering from a cool white fluorescent light or worst from another cool white LED light.

Same colour of light can have difference colour rendering properties. (Image source:  Soraa )

Same colour of light can have difference colour rendering properties. (Image source: Soraa)