I read recently that fluorescent clothing was not effective at all at night. If this is the case, how effective is it during the day, and should I stop buying tops that are fluorescent to use on my daily commute?
There are several terms around this subject. A key thing here is reflectivity (or more specifically retroreflectivity), rather than fluorescence. Fluoresence is the property of a material where it absorbs light at one wavelength, e.g. ultraviolet sunlight, but then reflects back light from the visual spectrum. This is most useful in daylight conditions, using the sunlight to create more human visible light.
Everything (well, almost everything) is reflective to some degree, mostly diffuse reflection where light hits a surface and scatters, but this reflection goes in many directions and breaks apart into many wavelengths. Alternatively there is a property called retroreflection which reflects light back to its source, in contrast to mirror (or specular) reflection which just bounces off, as light does with a mirror, in a single opposing direction. Hence a person near the light source, e.g. a driver in a car, sees the retroreflective surface.
Thus in terms of visibility, you don't just need flourescent material at night because there isn't enough light in the usable spectrum, so a yellow jacket isn't going to be any more useful than a white t-shirt. So while they'll both be more visible than something black, in order to stand out in a driver's headlights, you need retroreflective material to actively react, rather than passively reflect.
The question asks how effective fluorescent clothes are during the day, and should he stop buying those tops for his daily commute.
Fluorescent clothing is very effective during the day, but not so much at night. From How Does Hi-Vis Clothing Work?:
The main thing to remember is:
“Fluorescent for daytime visibility, reflective for night”
Fluorescent for Day
Did you know...
Fluorescent colours help you to be seen in the daytime and near dusk.
- They work really well in dull or rainy weather and when daylight is fading. On a rainy day it's hard for drivers to see pedestrians and cyclists so it's a good idea to wear or carry something fluorescent.
- Fluorescent colours look really bright because of the way they absorb and emit different kinds of light.
- Many people don't realise it but... fluorescent colours don't show up in the dark or 'glow in the dark'.
- To be seen by drivers at night you need something reflective.
- To be seen day, dusk and night you need something that is fluorescent and reflective.
Different Fluorescent Colours
- Lots of colours can be fluorescent and used for high vis clothing and bags, not just yellow but red, pink, green and orange too. (These colours are listed on the EN1150 quality standard.)
- In industry, yellow and orange fluorescent clothing is often worn under health and safety laws (for quality standard EN471). You’ve probably seen police officers, postal workers, lorry drivers, builders and refuse collectors wearing these jackets.
- Lifeboat crews have orange lifejackets; this colour shows up best against blue and grey water. What colour were your armbands when you learnt to swim?
Horse-riders often wear pink as it is a colour not found often in nature and so shows up well against autumn leaves. The Safe Lanes scheme in Mayfield, Sussex has even encouraged motorbike riders to wear fluorescent pink so they can be seen! However...
The colour blue is not so effective for fluorescent clothing; because of its wavelength it is not good at emitting light and the human eye is not so good at seeing blue generally. So if you’ve got a blue school uniform it’s a good idea to also wear or carry something fluorescent to make yourself seen.
And it continues about why reflective material is good for the night, but not during the day.
Fluorescent/Reflective clothing only help you if there is a light pointing at you from about the same direction as the driver is coming from. The little light a fluorescent object will give out its self is not of any use as the car driver has the bright light from his/her headlights reflecting of the road just in front of his/her car.
Given that most people get knocked of bikes from the side, with car drivers pulling out, rather than from the back, it is very likely you will be outside of the car’s headlights beam at the crucial time you most wish to be seen.
So you need good lights!
However Fluorescent/Reflective clothing does help make you more visible and can make you show up a lot better in street lights. It may also help you win the legal case if you knocked of.