1. Field of the Invention
The present invention relates to a distance measuring apparatus that measures a distance to a measuring target, a vehicle including the distance measuring apparatus and a method of calibration in the distance measuring apparatus.
2. Description of the Related Art
A stereoscopic camera is known which takes image data of a measuring target by using a plurality of imaging devices installed at a plurality of positions and measures a distance to or a position of the measuring target by using a difference in image positions (parallax) between the respective sets of image data of the measuring target taken by the plurality of imaging devices. Such stereoscopic cameras are used in safety apparatuses in ships, railway vehicles and so forth, and a Factory Automation (FA) field.
Such a stereoscopic camera is used, for example, in a system in which a stereoscopic camera is mounted in a vehicle and a driver's driving operation is assisted by providing information of the vehicle-to-vehicle distance, a presence of a pedestrian around the vehicle, if any, and so forth to the driver. In such a vehicle driving assistance system, improvements in the distance measuring range and the distance measuring precision are demanded.
Relationship between a parallax and a distance in a stereoscopic camera depends on various parameters concerning the stereoscopic camera (hereinafter, referred to as “stereoscopic camera parameters”). The stereoscopic camera parameters are determined by the respective image center positions of the plurality of cameras, the focal distances thereof, relative positions among the cameras and the attitudes.
When a distance is measured by using a stereoscopic camera, calibration is carried out in advance for the purpose of identifying the parameters of the respective cameras included in the stereoscopic camera. The precision in the calibration of the stereoscopic camera affects the precision of distance measurement in the stereoscopic camera.
It is difficult to directly measure the stereoscopic camera parameters. Therefore, the stereoscopic camera parameters are estimated, for example, by taking an image of a calibration target for which the three-dimensional coordinate values are known by the respective cameras and using the taken images for the estimation.
The calibration precision concerning a stereoscopic camera depends on the installation accuracy of a calibration target and the cameras.
Therefore, in order to improve the calibration precision concerning a stereoscopic camera, a technology is disclosed using a laser range finder separately installed behind the stereoscopic camera (for example, see Japanese Laid-Open Patent Application No. 2012-167944 (Patent Reference No. 1)). According to the technology of Patent Reference No. 1, the distance from the laser range finder to the stereoscopic camera and the distance to the calibration target are measured, respectively, thereby the distance from the stereoscopic camera to the calibration target is acquired, and thus, the calibration precision is improved.
Further, another technology is disclosed. In the technology, in order to improve the calibration precision of a stereoscopic camera, a laser light emitting unit is provided among a plurality of cameras included in the stereoscopic camera and a light emitting point due to plasma is formed in front. Then, the light emitting point is used as a reference point at a time of calibration (for example, see International Patent Publication No. WO2009/119229 (Patent Reference No. 2)).