In order for people to recognize and avoid dangerous conditions, they must be see the hazard.
Adequate illumination is critical for people to detect and identify hazards in time to avoid them. We measure the illumination and reflectivity of objects to determine their relative contrast, and the resulting visibility.
Our eyes handle a huge range of light visibility, in part by using two types of receptors: rods, which work well under low light conditions, and cones that work best under bright conditions.
In low light conditions, such as the outer edges of a headlight or work light beam, brightness contrast is what matters most.
This is because the rods, which detect very low levels of light, only come in one model, whereas cones come in three models. Having three different types of cones allows us to differentiate colors, and having only one type of rods leaves us in a black and white world.
In the case of safety and forensics, establishing visibility is only the first step. The follow up question is always, “when was this object noticeable”.