When cycling the dark winter commute, being seen is half the battle in staying safe.To be seen as the light fades, you need both Fluorescent and Reflective surfaces on your bike / clothing.Here is a short analysis on how these materials work to help when selecting clothing or accessories for increased visibility.Reflectance is the most easily understood of the two parameters.The reflector was invented by Mr. Arvi Lehti, a farmer in Finland. He invented it to make his livestock more visible at night.Inside a typical reflector there are two systems used. One uses spherical beads which is particularly good on flexible surfaces. Rigid structures can use prisms instead.Bicycle reflectors work on the prismatic principle. The diagram below shows how a simple 90° prism will reflect light back to the source for as long as the light hits at an angle greater than the "critical angle". Often reflectors will have their prisms arranged at different angles to give a wider field of view and this is the reason reflectors have to be orientated correctly.Reflective tapes and stripes contain spherical beads which reflect light in the same manner. This technology was developed in the 1930's to make cinema screens brighter and it has since been refined a number of times by companies like 3M. The beads are tiny (about 30µm in diameter) and they are embedded in a layer of adhesive on a metallised surface. The back face of the beads then act like tiny parabolic mirrors, sending the light back in the direction of the source.In sunlight reflectance is not so useful, reflecting light back to the sun will gain you nothing, but in the dark when other vehicle lights shine at a reflector it massively increases a rider's visibility. Studies in New Zealand have shown that a car driver is more likely to see a cyclist 3 seconds earlier in the dark, if they are wearing a reflective material.Fluorescence is a more subtle way of remaining visible as the light fades.Fluorescent materials take ultra violet light, light that is not normally visible to the human eye, and convert it into visible light.Visible light forms a small part of the electromagnetic spectrum as shown in the diagram below.Ultraviolet light is not visible to the human eye at 380nm, but it is still there and a fluorescent material can convert this light to a visible frequency, providing a bright glow when the light levels are low.More generally - any material will have some level of absorption, reflectance and sometimes transmission.Absorption is light absorbed by the objectReflectance is light reflected by the objectTransmission is light passing through the object and applies to materials such as glass.The reason a Banana is yellow is because it absorbs all the colours of the spectrum except the yellow wavelengths which are reflected, hence it appears yellow. This is true for any colour and is the science behind paint.However fluorescent materials are slightly different and the glowing appearance of a fluorescent jacket is caused by a conversion of energy; when the invisible ultraviolet falls on the fluorescent surface, it is absorbed and re-emitted as visible light radiation. This seemingly magic occurrence happens at an atomic level; the energy of the ultraviolet light causes the electrons in the jacket to jump into a higher energy orbit around the nucleus. When the electrons drop back into a lower orbit, they have to shed some of their energy and they do it in the form of a photon (visible light). Additionally the energy difference between the original electron orbit and the new orbit pattern will determine the colour of the fluorescence.So when cycling in the dark, ensure you not only have lights to see by, but also ensure you have a clean rear reflector and some fluorescent material on your bike.Some tyre walls have reflective tape which helps other road users see a bicycle "side-on" which can be important with all the other lights and reflectors pointing either forward or backwards.Take care out there.