Fluorescent materials are substances that absorb ultraviolet (UV) light, which is not visible to the human eye. These materials then emit light at longer wavelengths that we can see. UV light is typically present only during the daytime.

The most striking effects of fluorescence are often observed at dawn and dusk. For personal protective equipment (PPE) and CE marking, the colors that enhance visibility are clearly defined. It is up to the user to choose the color that provides the best contrast to the surrounding environment based on the intended area of use. For example, orange may offer better contrast against a snow-covered background than yellow.