Black ice is a tricky thing. You're sailing down the freeway because everything looks fine, when suddenly the next curve you maneuver sends you into a spin. Now, an infrared temperature sensor promises to alert drivers to this otherwise invisible road hazard, and help them decide when to slow down or pull off the road. Most temperature-sensing devices detect the ambient air temperature. But air and surface temperatures can vary by as much as 20 °F. Sprague Controls Inc., now part of Commercial Vehicle Systems Vision and Safety Group, has developed an infrared device that also senses the temperature of road surfaces and alerts drivers when it drops to 37 °F. Acting in real time, the RoadWatch device detects a 1 °F change in road temperature in less than 0.1 s. Because asphalt and concrete -- the materials typically used for road surfaces -- have good emissivity, infrared sensors can effectively measure the infrared energy (and thus the surface temperature) that they emit. The device focuses the energy through a refractive lens to better detect the radiation from a specific point on the road, minimizing skewed results from stray infrared radiation. The current version of the sensor uses a thermopile detector from Dexter Research Center Inc. of Dexter, Mich., but the Vision and Safety Group is designing a smaller detector with all the functions integrated into a single chip. With this, the company plans to switch to a smaller, more reliable silicon-based sensor from PerkinElmer Optoelectronics, said Gary Causley, general manager of the Canby plant. The new sensor also has fully integrated reference temperature, he said. RoadWatch has helped highway and municipal vehicles see where to apply salt or other road treatments for a couple of years. The company is now marketing the device to the heavy-truck industry and is in talks with automotive OEMs to try to get it into other vehicles, said James Sampson, director of marketing in Canton, N.C. In field tests, a major fleet put RoadWatch devices on 10 of its trucks for 60 days. Within that time, Sampson said, three of the 10 drivers saw accidents in front of them -- one of which was fatal -- involving trucks that had passed them when their temperature sensors were telling them to slow down.