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So, what do you see?

Image of the PEARL laboratory lit with blue light and smoke with some rigging in the foreground
PEARL under blue light with smoke

This week we have been talking a bit about lighting. A major infrastructure owner came to see us to find out if we could help them find out why their lighting standards are what they are, and whether they should be something different. Like many other infrastructure owners around the world, their view of lighting is set primarily by 'brightness' and is evaluated according to the

amount of light falling on some declared object - for example, the floor surface - from the light source. This is called 'Illuminance'. In the end, this means that if the light source is changed to be brighter or less bright the illuminance will change, so in fact the measure tells us about the power of the light source - which in effect is an input to the world we experience as people.

But there is a whole lot more to lighting. There is the absence of light - shadow - for example. Shadow is what gives us information about the world we are looking at: how you know that there is a step in the pathway is because the light reflected by the vertical part of the step is less than that reflected by the connected horizontal parts of the step. Light reflected from a surface is called 'Luminance'. Your brain has to choose whether this lower illuminance is due to the vertical part reflecting less light, or if there is a darker line on the horizontal surface and there is in fact no step at all. Making an error about this can cause a person to trip on a step they have not identified as such, or lead to an uncomfortable jarring sensation if they treat a horizontal mark as indicating a change in height that is not there. Making the lighting brighter (increasing the illuminance) would simply flood the area so that the difference in luminance from the vertical part of the step would be less easy to detect, thus the information about the surface is reduced.

There is a very good question about why we measure Illuminance when the measure that would be more informative might be Luminance. Mainly it is because Illuminance is much easier to measure, and is independent of the person who is looking at the scene, and depends only on the light source. Luminance, on the other hand depends on the height of the measuring device above the surface in question, the angle of view, the shape of the surface, and a number of characteristics of the material of the surface. Even if we do not consider the actual vision capabilities of a person, the light arriving at their eye is thus highly dependent on a number of different qualities of the surface in question - move the meter a small amount and the result could be very different. The question remains though: what are we actually interested in? If our question is "How bright is the light source in this environment?" then Illuminance will help us. If, however, the question is "How does the lighting in this environment work for people?", then we can only really discover this from Luminance. This requires a protocol for measuring Luminance that can be taken to represent the position of the human eye - so that we can have an idea of the light that would be entering the eye. This would include some light coming directly from the light source and also light reflected from other surfaces in the vicinity. What is clear though, is that just relying on Illuminance is particularly false.

There is also another issue though. Colour, or hue, is about what the light looks like. The eye has photoreceptors that are sensitive to three different parts of the visible spectrum, and these give a sense of colour. The triple sensor system that characterises human vision means that human eye is highly sensitive to colours. The computer or phone that you are using to read this article has three colour chips - red, green, and blue - and by blending the intensities of each of these, it is possible to create around 40 million colours. The human eye is sensitive to much more than this. The lighting system at PEARL is designed to match the colour range of the typical human eye, so we can experiment with how to manage colour in the lighting system to have the most beneficial effect for people - including those who find it difficult to distinguish between different colours, such as red or green (8% of men and around 0.5% of women have such a vision characteristic. From this we can start to determine what we should do about the colour of the lighting in our environment as well as how bright it is.

The possibility of being able to use colour in this way is only becoming feasible for the built environment as a result of the development of LED technology (see a previous post (3 July 2021) where I talked a bit about this). The mistake people are making at the moment is to consider that the colour of ordinary lighting - of a street, station, airport environment - is restricted to one instance that is dependent on the technology used to convert potential energy to light energy - so sodium produced an intense yellow light, mercury produces a purplish blue light, and so on. LEDs now mean that this is no longer the case, and we need to work on developing our approaches - and the associated standards/norms/guidances - to take this into account. The aim is to make the lit world better for people, other species and the planet as a whole, while reducing the over-use of lighting that obscures the beauty of the night sky for so many people. PEARL can help us to understand how to achieve this.

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