IN-PROCESS PORTABLE PHOTOGRAMMETRY USING OPTICAL TARGETS FOR LARGE SCALE INDUSTRIAL METROLOGY

Abstract

Portable photogrammetry methods are being used more and more as a 3D technique for large scale industrial metrology applications. Optical targets are placed on an object and images are taken around it, where measuring traceability is provided by precise off-process pre-calibrated digital cameras and scale bars. According to the 2D target image coordinates, target 3D coordinates and camera views are jointly computed. Numerical approaches such as bundle adjustment have usually been adopted for computing the 3D scene, with high computation time, leading in practice to time consuming and user dependent iterative review and re-processing measuring procedures until an adequate set of images is taken, limiting the potential of photogrammetry for fast, easy-to-use, and precise measurements. In a previous work, a new efficient computing procedure was presented for solving the bundle adjustment problem in portable photogrammetry. In this paper, further steps are presented towards increasing the computational efficiency of the in-process approach in portable photogrammetry. Measurement of raw part surface geometry prior to its machining is adopted as the industrial case for evaluation, with large scale measuring volumes (100 m3). An increase of efficiency in the in-process computing capability is demonstrated on a consumer grade desktop PC, with a total in-process computing time ranging 1 second per image for all the images taken at all the computed industrial scenarios. Additionally, a method for the self-calibration of camera and lens distortion is integrated into the in-process approach, enabling the use of low cost non-specialized digital cameras for precise industrial metrology, ranging at the same performance reported for portable photogrammetry with precise off-process pre-calibrated cameras, with a relative precision of 1/10,000 (e.g. 0.1 mm error in 1 m). Keywords: metrology; precision; machine tool; image; photogrammetry; calibration