High-visibility (also hi-vis or hi-viz) clothing uses two specific material properties
Usually used as the major material in the fabrication of the garment.
This uses the property of a material where it absorbs light at one wavelength, but then reflects back light from the visual spectrum.
Reflectivity or more increasingly Retroreflectivity
Usually added to the clothing as bands of tape. The tape is added to give visual clues that garment is being worn buy a person by placing them on the body, arms and legs. The quantity and placement of these bands can affect how quickly the brain realises that this is a person, especially if they are not moving.
Generally, fluorescent materials work to increase daytime visibility (including dusk & dawn), whilst reflective materials are primarily used to increase night time visibility.