Fluorescence is a property of a material where the ultraviolet rays are reflected back, which makes them appear to ‘glow’, increasing daytime visibility.
The effect is stronger in poor light conditions such as in fog or towards dusk. Various Fluorescent colours can be used such as yellow, orange, red, pink and green. Blue however is not so effective for emitting light and the human eye is not so good at seeing blue generally.
There are advantages to using yellow-green or orange for daytime use.
The florescent dyes absorb a wide range of wavelengths (including near UV and violet blue) and then re-emit them as light in the green/orange region, where the human eye has higher sensitivity.
Yellow and Orange fluorescent clothing are the two most popular colours used by industry today. Hi vis orange is the colour of choice for the railway industry, and works best against blue, grey or green backgrounds, whereas yellow is the current choice for roadworkers and general usage.
There has been much discussion about whether the hi-vis yellow/green or hi-vis orange provide more visibility. Although the eye is more sensitive to yellow/green, the contrast between orange and typical background colours is higher generally, and orange is becoming more widely accepted as the hi-vis choice by industry.