Above-ground vehicle detectors using infrared light, for use with traffic signals, are considered. One variety, the passive detector, responds to heat energy emanating from the object that is to be detected. The detector incorporates ceramic pyroelectric sensor elements that are responsive in the 6 to 14 mu m region of the electromagnetic spectrum which corresponds to the wavelength of emissions from targets such as humans and motorised vehicles. The other, the active detector, is based on a twin transmitter and twin receiver enclosed in a single housing. The transmitter sections emit encoded streams of infrared light into two regions along the road. The receivers collect radiation from the same zones and signal processing is performed such that the presence of a vehicle in a zone can be detected. The design chosen has both regions includes towards oncoming vehicles. This reduces the effects of the variation in vehicle height on the speed measurement capability by ensuring that the same part of the vehicle cuts into both regions. For maximum precision in vehicle length assessment, the rays to the region nearest the detector are kept nearly vertical. The optics of each system are described and each system's advantages are given.
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