Low cost monochrome camera systems based on CMOS sensors and C-mount lenses have been successfully applied to a wide variety of metrology tasks. For high accuracy work such cameras are typically equipped with ring lights to image retro-reflective targets as high contrast image features. Whilst algorithms for target image measurement and lens modelling are highly advanced, including separate RGB channel lens distortion correction, target image circularity compensation and a wide variety of detection and centroiding approaches, less effort has been directed towards optimising physical target image quality by considering optical performance in narrow wavelength bands. This paper describes an initial investigation to assess the effect of wavelength on camera calibration parameters for two different camera bodies and the same 'C-mount' wide angle lens. Results demonstrate the expected strong influence on principal distance, radial and tangential distortion, and also highlight possible trends in principal point, orthogonality and affinity parameters which are close to the parameter estimation noise level from the strong convergent self-calibrating image networks. Photogrammetric camera calibration is a heavily researched area building on a long history (Clarke and Fryer, 1998) . Commencing with a physical appreciation of geometric lens modelling (Brown, 1971) , through purpose-built lens design to minimise geometric image distortions (Mark, 1993) , to today's almost ubiquitous self-calibrating bundle adjustment, the success of established methods transcends many imaging disciplines. As discussed in Fraser (2012), one of the less appreciated aspects of the camera calibration process is the correction and modelling of geometric image discrepancies caused by variations in the wavelength of the imaging light. These discrepancies, broadly referred to as chromatic aberrations, are present to differing extents in all lens designs from basic 'C-mount' systems to the most advanced lenses for large format professional photography. The widespread use of digital camera systems equipped with either RGB mosaic (Bayer) filters, or separate filtered sensors, able to deliver three-channel colour images have prompted several camera calibration investigations. Reported work has concentrated on the modelling and correction of distortions in the three available colour channels and then recombining the images (e.g. Mallon & Whelan, 2007) or working through an extended RGB photogrammetric camera model (e.g. Luhmann et al, 2006; Cronk et al, 2006) to provide image coordinate corrections. In these cases the approaches were limited by the broad spectral imaging properties of the sensors with their three band outputs. The paper represents one starting point of a five year research project directed at enhancing the metrology capability of realtime photogrammetry with low-cost camera systems. This first paper seeks to improve our understanding of the optical capabilities of low cost sensors and lenses by imaging retro-target networks over a series of discrete wavelength ranges covering much of the spectral sensitivity of silicon. The aim is to investigate the performance of low-cost cameras for photogrammetry, not only in the visible portion of the spectrum but extending beyond into the near UV and IR regions where the illumination of targets by narrow-band light sources invisible to the human eye can have significant advantages. The reported method supports characterisation of the wavelength dependence of camera system for both chromatic aberration and photogrammetric parameters. Characterisation is achieved through multispectral imaging with 21 narrow-band filters applied to the light source whilst keeping the camera lens and unfiltered CMOS sensor arrangement constant. Each narrow band illuminant is used to capture a network of convergent images of a 3D retro-target test array in what is otherwise a conventional photogrammetric camera calibration process. A Balcar studio flash lamp was used as the light source, fitted with a softbox to diffuse the emitted light over a wide angle. Xenon arc flash has a broad continuous spectral power distribution, with significant power over the full imaging range of 400-950 nm. The spectral power distribution of this flash light was measured with an Ocean Optics HR2000 spectrometer at intervals of 0.5nm (Fig. 1) .
REF describe the effect of wavelength on camera calibration parameters in their research on low cost monochromatic camera systems to.
Multispectral calibration to enhance the metrology performance of C-mount camera systems
ISPRS - International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences
Computer Science Engineering