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Light Levels

The new EN12464-1 standard for lighting in offices and workplaces calls for higher light levels – up to triple the previous standard set back in 2011. Which poses a dilemma: how to deliver those light levels and meet environmental targets- let alone the eye-watering electricity bill.

​Photo by bruce mars on Unsplash

We are on the brink of a new understanding of what a diet of light for healthy, happy people really means – and a brilliant new generation of lighting technologies – including affordable, reliable controls and ways of integrating daylight – the most sustainable light source of all.

In this newsletter – which is a bit longer than usual, I wanted to go into some of the detail – beginning with some basics that are probably familiar to you already. So if you enjoy this kind of thing, please read on. And if you feel your eyes glazing over, put your coat on and go for a walk – I’m planning something lighter for next week!


Many of the standard units were set over a century ago, when we knew a fraction of what we now know about how humans respond to light (this book / report is a fascinating detour (*) Sensitivity to light varies up to 50-fold between people, at different ages and different times of the day (*) , one in 12 men are colour blind (*), and the ageing eyes of a forty five year old only take in half the light levels of her ten year old son (*).


‘Brightness’- like loudness from a speaker, the number of photons or packets of light generated by the light source itself.

Image source.


How ‘bright’ a surface looks at a given distance from the light source. 1 lux is the brightness on a surface one foot away from one candle. The lux level falls away as you move away from the light source itself – and as you move away from the centre, or focus, of the beam.

The standard photopic ‘lux’ measurement that you see on the product specification sheet reflects how strongly the colour vision cells in the retina will respond to create a feeling of a bright light source. So the lux measurement is weighted towards the ‘green’ part of the spectrum – around 550nm.

Image source.

Traditionally, lux levels are measured on a horizontal surface – the desk or the floor. But the amount of light on other surfaces, including the walls and ceilings and the direction of your gaze – make the world of difference to how much light you’re actually getting – and your sense of brightness in a space. This paper explains how just looking towards the window can increase light exposure12-fold (*).

And this paper explains how, given control, office workers manage the overall light in a scene rather than the light levels on the desk with potential energy savings (*). This paper explains how those preferences can change through the course of the day.

Image source.

Another measure that focuses on the role of lighting in social interaction is ’cylindrical illuminance’. It describes how easy it is to see three-dimensional forms in space, particularly faces. This classic paper gives a great explanation and illustration (*). This blog explains the basics really well too.

Non-visual system –

Around 20 years ago, scientists in Seattle discovered a second ‘non-visual’ system that we now know drives powerful physiological responses including alertness and the body clock.

This system is triggered by a special group of cells in the top layer of the retina called the retinal ganglion cells (RGC’s). All the RGC’s send signals from the rods and cones back to the brain. But around 5% of those cells not only perform that ‘relay’ function, they also have a protein, melanopsin, that responds to light with a peak at around 460nm, or ‘sky blue’.

So they were called Intrinsically Photosensitive Retinal Ganglion Cells. This paper gives an excellent summary (*).

Photo by Josh Riemer on Unsplash

All bright light activates the IPRGC’s. But that response fades away. Only light in blue range triggers the long-lasting physiological response which, like a strong coffee, lasts for around two hours after the light is switched off. This paper gives an excellent explanation (*).

Image source.

This breakthrough led to the next generation of measurements. These are still under debate – and have yet to make it onto the packaging of many products.

The starting point is the CIE standard illuminant known as D65 that reflects the spectral power distribution of daylight at midday in Northern Europe (*).

In 2014, Rob Lucas and his team in Manchester proposed a measure that focuses on the amount of light in that peak melanopic response-triggering wavelength. This is known by various names including Equivalent Melanopic Lux or EML. This presentation is a great summary and offers some examples of ‘melanopic equivalence’ for different light sources (*).

Chris Cuttle and his team suggest a complementary approach – a switch from task to ambient illumination. In this paper, he points out that although straight downlighter may seem to be the most efficient solution, a more intelligent, human-centred approach can not only create more comfortable, stimulating places to work, but cut energy use too (*).

thanks for making it to the end!

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