Close Range Calibration of Long Focal Length Lenses in a Changing Environment

Abstract

ABSTRACT:University College London is currently developing a large-scale multi-camera system for dimensional control tasks in manufacturing, including part machining, assembly and tracking, as part of the Light Controlled Factory project funded by the UK Engineering and Physical Science Research Council. In parallel, as part of the EU LUMINAR project funded by the European Association of National Metrology Institutes, refraction models of the atmosphere in factory environments are being developed with the intent of modelling and eliminating the effects of temperature and other variations. The accuracy requirements for both projects are extremely demanding, so accordingly improvements in the modelling of both camera imaging and the measurement environment are essential. At the junction of these two projects lies close range camera calibration. The accurate and reliable calibration of cameras across a realistic range of atmospheric conditions in the factory environment is vital in order to eliminate systematic errors. This paper demonstrates the challenge of experimentally isolating environmental effects at the level of a few tens of microns. Longer lines of sight promote the use and calibration of a near perfect perspective projection from a Kern 75mm lens with maximum radial distortion of the order of 0.5m. Coordination of a reference target array, representing a manufactured part, is achieved to better than 0.1mm at a standoff of 8m. More widely, results contribute to better sensor understanding, improved mathematical modelling of factory environments and more reliable coordination of targets to 0.1mm and better over large volumes.

Temperature Compensation Method for Digital Cameras in 2D and 3D Measurement Applications

“…The temperature in the production hall can vary, and the temperature amplitude in a day–night cycle can also be very high. Taking into account the differences in environmental conditions, temperature is one of the most important factors that needs to be taken into account when considering the accuracy of measurements performed with any camera-based system [ 5 , 9 , 12 , 13 , 17 , 18 , 19 , 20 , 21 ]. The solution of this issue can be either software or hardware.…”

Research of the Camera Calibration Using Series of Images With Common Center of Projection

“…(Liang et al, 2014) propose a robust algorithm to calibrate the camera with a long focal length without using additional devices by adding a regularization term to computational process. (Robson et al, 2016) represents results of long focal length camera calibration in Violet and IR light. Figure 2.…”

Positional uncertainty in optical-based metrology systems in large volume manufacturing

“…The recently completed international collaboration on the project "Large volume unified metrology for industry and novel applications and research (LUMINAR)" [6] focused on the refractive index effects in an industrial setting. They found that in a typical aircraft industrial space the positional measurement uncertainty can be significant [7]. In addition, [8] found that a light beam will deviate from a straight line by 0.2 mm for every 15 m it travels horizontally when there is a vertical temperature gradient of approximately 2 K/m (and a vertical height of 6m).…”