Why are reflective jackets brighter at dusk?

12 March 2019

POLICE-FLUORESCENT

Police motorcyclist in hi-vis jacket

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Question

You sometimes see cyclists with super reflective jackets on. Why are hi-vis colours seemingly most vibrant at dusk, more than during the day or night?

Answer

Chris Smith received this question from listener Sam. It was over to chemist Ljiljana Fruk from Cambridge University to illuminate an answer... 

Ljiljana - It's an interesting question as well. The super reflective jackets are, of course, relying on fluorescent dyes which are embedded in these jackets so that when you are driving your bike during the daylight you can be noticed because fluorescence is the strongest in the UV light during the daylight from the sun. And then you have reflective materials which are basically reflecting the light that is coming from the car from your jacket.

So the question “why do you see these colours are little bit better in the dusk or dawn and not in the daylight” is basically also related not only to the colours which are in this jacket. Materials are also in the way of how we see, so the contrast is going to be much higher also when you are changing the daylight to dark. Because we also have in our eyes - this is really interesting - two types of the cells that are activated during the daylight and then there is the second type of the cell that is activated during the night.

Chris - These are the rods and cones. The rods are useful during the dark and low light conditions, the cones see colour and are much more active in the daytime?

Ljiljana - Exactly, exactly. There is this activation process that is also happening when you're changing the light at night. So it will be a combination of the materials that you are using so you will have fluorescent materials and reflecting materials, but you also have this contrast that you have around the surrounding lights as well as.

Chris - How do they get them so reflective though? They're just amazing some of these materials. And there's someone who came into our office the other day, she had a coat that looks just grey and as soon as you shone light on it - I noticed her cycling in the evening and I came along with my car behind her with the headlights - and there was no chance of missing her because she glowed like a beacon. It was bright white.

Ljiljana - Yeah, it's amazing how they've improved. And of course today we can have different kinds of materials that have structures which are nicely aligned so the light will basically reflect totally. I mean in nanotechnology it's very interesting because we are designing materials that can be so nicely structured that they not only change with the light that that is coming - so they can be super reflective or they can be super absorbing - but they can also change with the change of temperature or the humidity that you're have in your environment. So the material properties are getting much better and we have better ways of aligning them.

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