BLUE IS THE COLOUR?

With a background in theoretical lighting and ergonomics as well as lighting design, Neil Knowles was able to bring a wide range of expertise to bear for his ILP lecture on ‘how to be brilliant… at circadian lighting’ in the autumn

By Neil Knowles

Back in October I was asked to talk at a ‘how to be brilliant’ lecture organised by the ILP. ‘How to be brilliant’ – as ILP members may well be aware – is a series of lectures primarily for students and lighting designers at the start of their career, with a programme of events running throughout the year split between (currently) Scotland and London.

My talk was on ‘circadian lighting’. I don’t claim to be an expert, but my lighting experience, coupled with a background in theoretical physics (BSc) and ergonomics (MSc) means that I do have a deep understanding of the essentials.

This article is based on that informal evening talk, so imagine me giving it with a glass of wine in one hand! It is an overview of the basics – and therefore may be considered relatively ‘entry level’ – but is also an opportunity to remind ourselves, whatever our experience, about some of the fundamentals. I emphasise I don’t claim it is any more than that.

There were four parts to my presentation:

• Body clocks • Regulation and the third receptor • Cool white versus warm white – from black body curves to LED phosphors • Summary

BODY CLOCKS To start then: body clocks. Humans (and in fact most mammals) have an innate body clock. It runs on a 24.5- or 25-hour cycle, normally slightly longer than a day. It’s evinced by a number of measurable factors, such as core body temperature, which rises in the day and falls at night, and melatonin levels which rise in the evening and fall sharply in the morning (as figure 1 shows below). Note that these changes happen

before you are sleepy or tired; they are what make you tired. Without a correctly regulated body clock, all sorts of things go wrong, from performance at tasks to life expectancy and even the likelihood of bipolar disorder. Many of these things are documented problems of long-term shift work and many of the health issues associated with this kind of working relate to improper functioning of the internal body clock or its lack of synchronisation with daytime.

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Suprachiasmatic Nuclei (SCN)

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Figure 2. Studies on mice have shown how, if you knock out the part of the ‘master clock’ of the brain, the suprachiasmatic nucleus, you lose all track of what time of day it is or when you should be awake or sleeping

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Figure 1. The rise and fall of human melatonin levels during the day/night cycle. Both this slide and figure 2 above are taken from a talk, with thanks, by Professor Russell Foster, professor of circadian neuroscience at Oxford University

The core of the body clock is what is called the suprachiasmatic nucleus (SCN). This is a small part of the brain; it functions as a master clock.

Studies on mice have shown that if you knock out this SCN (say with genetic editing or a scalpel) the mice lose all track of what time of day it is and behave in erratic ways, essentially falling between asleep and awake at random (shown in figure 2 on the previous page).

If you isolate someone in an underground room with no means of telling the time – no TV, no radio, no internet (don’t do this with a millennial by the way) – they will carry on as normal, eating and sleeping in a roughly 24-hour pattern. But they will switch to their innate body clock period of around 24.5 hours, and therefore start getting up later and later. Something must be resetting our innate body clock every day, to bring it in line with reality. But what?

REGULATION AND THE THIRD RECEPTOR This brings me on to part two – regulation and the third receptor. Discovered recently, these are a type of light receptor present in the eyes. Everyone learns at school that there are two receptors: rods and cones. Rods are good in low light levels but are only monochromatic. Cones only work at higher light levels but allow colour vision.

The third receptor (so new it doesn’t have a proper name yet) does not allow you to ‘see’ and is not connected to the visual part of the brain.

These cells connect directly to the SCN. Experiments show the cells have a peak sensitivity of 470nm, which is blue, in fact exactly the blue of a bright blue sky. If you think about it, clearly it has evolved to be this. There is no point in our daylight-sensing cells working on purple, for example; they work on daylight. So you wake up in the morning, walk into the blue sky sunlit day and your SCN gets a massive wake-up hit, courtesy of your third receptor. It’s morning, wakey wakey! And, thus, your body clock is dragged back into line with reality, every day.

COOL VERSUS WARM WHITE But how much light? Highly controversial at present, and as yet unknown. Early studies suggested 1,200 lux. More recent ones suggest 75. I’ll be bold and say, ‘we’re not quite sure at present’. Exciting times. But what we are all agreed on is that walking to work in the morning sunlight is going to reset your body clock, dragging it back into tune with reality and not letting it drift.

So, 470nm light. But how do we make this then? Well, we could make LEDs with only this frequency of light emitted, specifically designed to wake us up and stimulate the

www.theilp.org.uk SCN. But nobody does, and if they did it would be blue, so not brilliant for warm and intimate bars, or where good colour rendering is needed.

Let’s therefore go back to basics. Take a lump of something; metal preferably, so it doesn’t melt or catch fire. Heat it up a bit. It gets hot and starts to radiate heat (actually infra-red radiation). Heat it more and it starts to glow, first a dull red colour, then yellow, then white-hot.

Keep going and it will glow blue hot. About this time, you’re probably thinking you’ve never seen anything glow blue hot. That’s because it’s around 6,000 degrees, and most metals have melted now, setting fire to anything nearby as they do. Anyway, you have, the sun is this hot on the surface; that’s why the sky is blue.

But where does the blue light come from? It comes from the sun. Rayleigh scattering is what splits the sunlight into two parts; blue for the sky, yellow coming straight through. So we end up with blue sky and yellow sun. How about we change to ‘the sun is this hot on the surface, that’s where all the blue light in the sky comes from originally’.

Describing mathematically the relationship between temperature and intensity of light and colour of light emitted was a major challenge to early 20th century physics. Classical theories could not do it; they broke down and predicted infinite amounts of ultra-violet light (in fact, the collapse of the Rayleigh-Jeans theory was known as the ‘ultra-violet catastrophe’).

It wasn’t solved until the physicist Max Planck arrived on the scene and suggested that perhaps light was ‘quantized’. This means it comes i n s p e c i f i c lumps, not a c o n t i n u o u s mass. This was revolutionary at the time; l i g h t w a s thought to be a w a v e , a n d therefore could come in any amount. The idea that it comes in lumps means it is in some ways like a particle not a wave. Quantum theory.

Planck’s law is not easy to derive (I covered it in the second year of my physics degree) but for those interested and with a good grasp of mathematics there is a description in this reference [1]. Planck’s law looks daunting but most of the terms are constants. The only two variables are the temperature, T, and v the frequency of light. If we pick and fix a temperature of, say, 3000K, you can draw a graph with light frequency on the bottom axis and amount of light up the side.

The graph below (figure 3) has three of these lines plotted, with temperatures fixed at 4500K, 6000K and 7500K. There are two things to notice about the differences between the three lines. Firstly, and most obviously, the peaks are much higher at higher temperatures. More energy is being emitted at every point; it is far brighter.

But the second is more subtle; look where the peaks are located. For the 4500K curve, it is peaking in the red part of the spectrum. In the 6000K curve, the peak has shifted left to be in the green part and at 7500K the peak is in the blue part.

If this was a lump of metal it would be glowing red hot (4500K), green hot (6000K) and blue hot (7500K). In lighting terms, these lumps of glowing metal are at different colour temperatures. In practice, what this means is we know blue light is the key for affecting the SCN, and different colour temperatures of light have different amounts of blue in them.

Don’t worry, we’re almost there. So, if we want to keep people alert and awake, we need to convince them it is daylight.

We need to produce light with lots of 470nm in it, so the SCN supresses melatonin production and your body stays awake. And what’s got lots of blue in it? t Figure 3. A illustration of Planck’s law 1 0.8 0.6 0.4 0.2 0

0 500 1,000 1,500 2,000 Wavelength [nm] 7,500k 6,000k 4,500k

High colour temperature light.

Actually, the manufacturer’s use of ‘colour temperature’ is an approximation. What labelling an LED tape ‘3000K’ means is that if you look at the emission graphs and compare them to an ideal (Planck’s law) graph; the one it is closest to is 3000K.

But LED does not generate light by heating stuff up, it does it from exciting electrons on a semiconductor substrate, causing them to emit photons to lose energy and then re-radiating these photons off a phosphor to blur the resultant colours.

So, what you actually get in this instance is a series of individual spectral lines from an LED; it is not a perfect Planck curve distribution but instead a vague approximation. The actual light emitted is dependent on each manufacturer, and the mix of phosphors used, and the manufacturing process. That’s why 2700K from two different manufacturers look different to each other.

SUMMARY So, in summary. Humans:

• Have innate body clocks • These are regulated by light • Only a specific frequency works • Lighting can be manufactured to emit this frequency

To finish, let’s look at a couple of examples of projects, both by Elektra Lighting. The first (the top image) is for Vodafone, with a cool light behind the stretch fabric ceiling throwing lots of blue light and making everyone alert and awake. By contrast, the image opposite is the Tivoli cinema, with warm lighting so everyone is nice and relaxed.

To end, I should emphasise this is a very new field, with lots of unknowns. For example, we don’t know how much light is needed to stop melatonin production. We don’t know how light affects the elderly or the young. Is it the same? Is there a difference across genders?

How many receptors are there in the eyes that do this and where are they located? We don’t know. So much to explore yet.

Until we do know this and more, all I can offer is to reiterate what has been said before: no mobile phones

before bedtime; their screens have lots of blue and therefore the light can keep you awake. But you knew that already. Hopefully now you know why.

Neil Knowles is director of Elektra Lighting

THE ILP’S 2020 ‘HOW TO BE BRILLIANT’ PROGRAMME – DATES FOR YOUR DIARY

The ILP’s ‘How to be brilliant’ programme of free, fun and friendly evening talks will be continuing during 2020, and dates for the London programme have now been agreed.

All the London lectures will be back at Body & Soul on Rosebery Avenue in Islington, north London, normally starting from 6pm (but do check online in advance).

The 2020 programme will kick off on 29 April, with further lectures taking place on 27 May, 24 June, 30 September, 28 October, and 25 November.

The speaker line-up was still being confirmed as Lighting Journal went to press but look out for updates online at www.theilp.org.uk/brilliant The ILP wishes to extend its thanks to Zumtobel, which has generously agreed to sponsor ‘How to be brilliant’ once again this year.

The intention is that ‘How to be brilliant’ lectures will also take place in Scotland through the year.

Again, more details on these will be available online and in the journal once the programme has been finalised.