Nighttime driving and mesopic vision in vehicle safety

A significant number of automobile accidents occur under low natural visibility conditions, especially at nighttime. This can be explained by some specific characteristics of nighttime driving, such as how human vision works within an intermediary visibility range – known as mesopic vision, which is a combination between scotopic vision (≤ 0.034 cd/m²) and photopic vision (≥ 3.4 cd/m²). In mesopic vision, the eye simultaneously uses cones and rods (photoreceptors in the retina), resulting in a unique interaction between sight and ambient lighting and, in practical terms, altering spectral sensitivity. These alterations improve visibility when ambient lighting has a high color temperature, in the range of 5000 K. Therefore, the spectral distribution of light from the headlamps plays a crucial role in vehicle safety.

It is important to note that the most widespread lighting systems in Brazil, equipped with incandescent bulbs, operate with a warmer color temperature, averaging 3000 K. Gas-discharge lamps and LEDs may operate at an average range of 5000 K. Initially, these differences do not pose a problem, since the incandescent bulb market also has special versions over 3000 K. The real problem is that there are inconsistencies in the country regarding the lawfulness of such lamps. This tends to limit the options available in the market to promote vehicle safety. Based on these factors, this paper correlates the technological diversity of vehicle light sources with nighttime driving and the underlying complexity of mesopic vision. The purpose is to clarify relevant questions in terms of vehicle safety in Brazil, possibly applying the conclusions hereunder to maintain the existing fleet and develop automotive forward lighting projects.