Black ice is dangerous because it is difficult to see. A thin, smooth ice layer is largely transparent at visible wavelengths, allowing the color and texture of the pavement underneath to remain visible. To a driver or a conventional camera, the road may therefore appear normal.
Visible-light images can contain indirect clues such as glare, reflections, or changes in road texture. These clues depend strongly on illumination, viewing angle, weather, and the pavement itself. However, they are not reliable.
Water molecules interact strongly with selected wavelengths in the near-infrared region. The exact absorption spectrum depends on whether the molecules are arranged as liquid water or as solid ice. That difference provides an optical signature that can be measured directly.
In the near infrared, both water and ice exhibit broad molecular absorption bands. Their spectra are similar, but the location, shape, and strength of individual features change when water freezes. Opaq uses these differences to discern water from ice.
The sensor head developed by opaq illuminates the road with several near-infrared lasers at carefully selected wavelengths. Instead of relying on the absolute reflected intensity, the system compares the signals from the different wavelengths. This differential measurement reduces the influence of road color, surface roughness, and changing distance while preserving the characteristic spectral differences between water and ice.
The prototype combines multiple lasers, collection optics, photodetectors, and signal-processing electronics in a vehicle-mounted sensor head to detect black ice in front of the vehicle.