Geometric and radiometric analysis of a CCD-camera based photogrammetric close-range system

The advent of filmless imaging systems, especially of Charge-Coupled Devices (CCD), have created manifold opportunities and new applications which have led to significant changes in Photogrammetry. The exacting demands of Photogrammetry on the radiomet¬ ric and geometric characteristics of the imaging sensor and all other elements involved in the acquisition of imagery with solid-state sensors, require a detailed analysis of the fac¬ tors affecting the performance. This analysis can build on extensive knowledge developed over the last twenty years for the calibration of film based cameras. The modelling of systematic errors introduced by lenses is identical for both systems. Calibration and analysis techniques, such as the bun¬ dle adjustment with self-calibration, are important tools. Another source of a great deal of information are applications of solid-state sensors in astronomy. Exacting radiometric requirements have led to the development of specialized sensors, cameras, image acqui¬ sition systems, and calibration techniques. These techniques are currently only partially applicable to photogrammetric tasks though. They are tuned to be used with extremely specialized hardware (cameras, image acquisition systems, testing arrangements) which is typically not available for photogrammetric tasks. The task was thus to investigate the effects of cameras, signal transmission techniques, and frame grabbers as they are widely available on the market as off-the-shelf equipment. The individual components were in¬ vestigated first, then the combined radiometric and geometric performance of the system was analyzed, and finally the performance of a system for three-dimensional measure¬ ments was verified. This in turn required the installation of an image acquisition system with advanced capabilities to perform various comparative investigations, to set up a three-dimensional testfield, and to write a large amount of software for analysis and cali¬ bration purposes. The geometric regularity and the excellent radiometric characteristics of solid-state sen¬ sors make them ideal measurement devices. A number of investigations have shown that the regularity of the sensor element spacing is in the order of l/100th of the spacing. The uniformity of response from sensor element to sensor element is 1 % and better for many off-the-shelf cameras.The geometric regularity of the sensor would thus allow to mea¬ sure positions of targets imaged on such a sensor with a precision of 1/100 of the spac¬ ing and better. This is actually achieved and surpassed with special sensors in star tracking applications. The mechanical design and the electronics of CCD-cameras are usually not designed for photogrammetric purposes. The mechanical design is often not sufficiently stable, i.e. the assembly of lens, housing and sensor is not rigid. The camera electronics are designed for visual purposes (e.g. surveillance), thus potentially introduc¬ ing significant degradations due to a number of factors such a low-pass-filters, gamma correction, addition of video signals, to name but a few. The largest drawback with respect to the radiometric and geometric performance, but an advantage with respect to costs, is the use of standard analog video signals for the trans¬ mission of the imagery acquired by the sensor. These standards were developed in the 1950's for broadcasting and are not at all tuned for the requirements of precise measure¬ ments. The radiometric and geometric properties of these signals were analyzed, draw¬ backs discussed, methods for determination and compensation and/or elimination of deficiencies investigated and performed. Frame grabbers are another critical component of the image acquisition system. They must convert the analog transmitted imagery into matrices of numbers, the digital image.