Driving becomes more difficult after sunset. Not only is visibility generally reduced, but an even more serious challenge on unlighted roads are the headlamps of approaching vehicles, which can dazzle a driver’s eyes. Thermal driving night vision systems can help drivers navigate in the dark, but to be of real help, they must be able to see both in the light and in the dark. They also must produce natural-looking images of the road, which is usually populated with animate and inanimate objects.
Nighttime driving conditions are enhanced by an NIR laser-based vision system. The naked eye is blinded by the headlights of an oncoming vehicle and can’t see the pedestrian (A). A thermal camera allows the driver to distinguish both the pedestrian and other vehicle but not the lane markings (B). The laser-based system, however, provides a clear view of the pedestrian, the vehicle and the markings (top).
Thermal driving night vision systems were introduced into the automotive market in the 2000 model year. The Cadillac DeVille that year used a thermal camera that was sensitive to the long-wave infrared part of the electromagnetic spectrum (the 8- to 14-μm band) along with a display that projected a virtual image of the road scene using the windshield.
Although this system excelled at detecting warm objects, such as pedestrians and animals, the image contrast was generally poorer for cool, inanimate objects, such as lane markings, signs and reflective road markings. Also, because automobile, windshields do not transmit long-wave infrared radiation, the camera had to be mounted on the outside of the vehicle, exposing it to the weather and, potentially, to damage from front-end collisions and road debris.
Lexus introduced the industry’s second thermal driving night vision system in 2002. Featured on the LX 470 sport utility vehicle, it had — instead of a thermal camera — an active thermal driving night vision system that used near-infrared illuminators and a CCD camera. Each illuminator consisted of an incandescent lamp filtered primarily to pass wavelengths longer than ~800 nm and designed to produce an angular distribution of light similar to that of a high-beam headlamp.
To improve the performance of vehicle-based active thermal driving night vision systems, researchers at Daimler-Benz AG in Ulm, Germany, developed an externally mounted diode laser as the near-infrared light source. Because the laser emits at essentially a single wavelength, a narrow bandpass filter can be used in the camera, which allows most of the laser light through while removing a large fraction of the near-infrared light from the headlamps of oncoming cars. This approach eliminates the image flaring that occurs in filtered-incandescent systems and allows the driver to have a clear view of the region near and behind oncoming traffic.
Placing the vision system on the exterior of a car would seem to be a natural design choice. However, from an engineering standpoint, it creates a nightmare because it is difficult to design an exterior lamp that is immune to water intrusion, which would wreak havoc with a laser and its power supply.
For this reason, engineers from Ford Motor Co. of Dearborn and Lear Corp. of Southfield, both in Michigan, designed and constructed a prototype system in which all components are positioned within the vehicle’s interior. Among the other advantages of this architecture are that it simplifies thermal management of the laser and its power supply; that the illuminator and camera apertures can be kept clear by placing them in the zone cleaned by the windshield wipers; that the number of common system components increases; and that the illuminator is protected from damage in front-end collisions.
The illuminator’s laser, which emits at ~810 nm, and its beam-forming optics, power supply and temperature management system are located in an overhead module. A lightguide collects and collimates the laser emission, and a holographic diffuser generates a beam pattern with horizontal and vertical spread angles (full width at the 1/e points) of 14° and 5.3°, respectively. A shroud that is sealed against the windshield prevents the laser light from being emitted into the passenger compartment. The illuminator meets the requirements of a Class 1 laser under all conditions.
This article comes from photonics edit released