Is light a particle or a wave…or is it a field?

This was a question recently asked of Henrik Clausen - Director of the?Fagerhult?Lighting Academy, as well as Associate Professor at?Aalborg University?in Copenhagen. And here's how he answered...

Calculating the level of light falling on a desk, the wall or a workbench makes a lot of sense when that’s where a task is taking place. And, it used to be easy defining the task area.

Let’s imagine the traditional office as an example. There may be a picture on the wall, a book, a notepad and a phone on the desk. It was so simple to provide good lighting for this kind of set-up. But, a work environment doesn't look like this anymore. We look at screens now, and books and notepads are rarely used. It’s no longer as simple to provide good lighting.

Let me set some context. Consider a modern office: one room, one desk, one chair, one laptop. And in the old office: the notepad, a rolodex and the phone.

Let’s take a closer look at the ‘old office’...

The walls are white. We have a wooden desk, an upholstered desk chair and a 2 x 36 watt fluorescent lighting fixture with a lamella louvre, suspended from the ceiling over the desk. You get the idea.

The lighting standards require 500 lux on the task area, 300 lux on the immediate surrounding area and 100 lux on the rest of the horizontal surface. You enter the room, flip the switch and the space is filled with light. End of story…or maybe we should take a closer look, in slow motion.

The moment the copper parts touch each other inside the switch, the energy starts to flow towards the light fixture via the wires in the wall and ceiling. The electronic components come to life and send a high-frequency energy pulse to the fluorescent tube, and the glow wires in the tube start emitting electrons into the mercury gas-filled tube. Here they collide with mercury atoms and knock out the electrons. When an electron falls back into place, the mercury atom releases one (ultraviolet, invisible) photon – the smallest amount of light possible. This process happens all over the place inside the glass tube, so we have a huge number of photons moving towards the white light powder that covers the inside of the tube. Within the powder, the UV light transforms to white light. Not to visible light, as there is no such thing as visible light!

The light interacts with all of the surfaces inside the fixture. Some photons never make it out of the fitting, and end their short lives turning into heat. But others do exit, perhaps directly, perhaps after several interreflections. On the light’s journey towards the task area of the desk, some of it hits the walls and gets absorbed or reflected and, finally, some of the reflected light hits the task area of the desk. Overall, the entire office is illuminated.

Let’s take a closer look at the task area.

The illumination level of the task area, the colour of the light and the colour rendering capabilities of the light are all determined by the way the photons exited the fitting, the encounters they had on the journey described above, and the interactions they had with the plaster and paint on the walls before ultimately hitting the task area on the desk.

For me, the easiest way to imagine light on a journey is to think of the photons as balls in a pinball machine. They are being fired out, they bounce off all the objects they meet and, finally, they run out of energy, but new balls are fired.

For some people, it’s easier to imagine light as an electromagnetic wave of a certain wavelength, with white light containing all wavelengths. The wavelengths come in handy when talking about the colour of the light. Colour rendering – or actually Spectral Power Distribution – tells us how much light of each wavelength the white light from a light source contains, and is the basis for calculating all other illuminating engineering values.

If you ask if light is a particle or a wave, the correct scientific answer is: both.

That was in the good old days when it used to be simple. Now, let’s get back to the future!

In his 2008 book “Lighting by Design”, a brilliant teacher and lighting consultant – ?and a good friend – Christopher Cuttle introduced his take on the idea of light fields. I have read it many times, but I still find it difficult to explain, most likely because I’m not smart enough to understand the maths! But, I fully get the point, and I think that in the future this will be the basis for documenting advanced lighting designs.

We are still in the same office, but the tasks have changed over time, and the notepad, the rolodex and the phone are gone – replaced by a laptop computer. The light fixture is now equipped with light emitting diodes (LEDs) that are much more effective than fluorescent tubes and have a much longer lifespan. The junction within each LED emits blue light, and a light powder converts that to white light, much in the way described previously.

Imagine that you are standing in the doorway of the office and the person sitting at the desk turns to look at you. This likely happens several times every day, and we know less than we think about how the faces of the two people are illuminated…

Let’s think a little more about that.

With traditional calculation methods, we know how much light we have on surfaces, but not in the space between. Here we need the concept of the light field, because it describes what happens everywhere in a defined space, like a gravitational field, or a magnetic field. That's what we will need in the future.

Most seem to agree that the value of going to the office is for the interaction with colleagues, with light as the carrier of all visual communication. So, let’s continue with the aforementioned scenario…

The person in the doorway says “hi”, and their colleague at the desk turns around and says “hello there”. The light transfers the expressions on their faces and the rest of the visual communication between them. Is one person sad or embarrassed because of a work issue? Is another angry or disappointed? Has it been too long since the two last met? Are they having an office romance? Who knows…without light.

Once the conversation is over, the person at the desk turns back to their online meeting. And, if you agree that it can be hard to read people’s emotions in badly designed lighting, it’s almost impossible in an online meeting. I think we’ve all learned that lesson over the last couple of years.

Back to lighting…

When we look at someone, we use a perception system designed for the outdoor daylight we have existed within for millions of years. The qualities of the lighting determine how easy it is to read facial expressions – in bad lighting it’s hard to see if someone is telling the truth or lying.

You’ve probably had the experience of moving around in front of the bathroom mirror to see some detail of your face more clearly, or just to get a morning boost of self-confidence. It’s so hard to make lighting that makes people look the way they want to look.

Colour rendering and colour temperature are two highly important characteristics of lighting when it comes to ensuring good visual communication. A great deal can and does happen between the moment when the first photon was generated, and when the light illuminates your face. We know all of the data about a (new) light source, but we know much less about what happens on the way from that light source to your eye.

We need to find ways to document the light level, colour temperature and colour rendering at any given point in space, and the light field method can help us do that. But, the first step is to accept that we really need this deep level of knowledge, because all of this is pretty difficult to calculate, document, measure and compare, especially when we want this kind of information for every point in a 3D space.

I believe that the light field theory is our best shot at getting there. Some of the most important and interesting research in lighting is being undertaken in this area.

When I read: “Effects of inter-reflections on the correlated colour temperature and colour rendition of the light field” (C Yu et al, accepted 23 August 2022), I decided to share it with you, but in a tolerably gentle and accessible way. It’s for you to judge if I succeeded - and if it was worthwhile.

Learn more about the research here:

C Yu et al, paper: https://doi.org/10.1177/14771535221126902

Christopher Cuttle, Lighting by Design: https://www.routledge.com/Lighting-by-Design/Cuttle/p/book/9780750687683