Fase 1 Planeación

During initial testing of the imaging system, before XEOM1 had even been de- ployed at a synchrotron beamline, it became apparent that some unusual fea- tures were present in some images. Specifically, many images were found to contain a series of bright ‘arcs’ (see Figure 4.10) which do not correspond to any physical features on the object or illumination source being imaged. The appear- ance of these arcs is particularly troubling as they are discernible in XEOL data images. Two characteristics indicate that they are not caused by some internal process in the sensor itself:

1. They are not present if there is no source of illumination, or the shutter is closed i.e. in dark frames.

2. Their position and sharpness is dependent on the illumination character- istics.

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Figure 4.10: Test images which demonstrate the artefacts observed in the FLI camera images.11These were produced by illuminating the optical system using a small filament lamp. Equivalent arc features are highlighted by the dotted lines. As the position of the lamp is changed, the artefacts shift so flat-field correction would not be sufficient to eliminate the problem.

The first option to be considered was the possibility of light being reflected or scattered by the internal walls of the optical column. The aperture stop (the limits of the first lens in the XEOM1 optical system) defines the cone of light rays which will pass directly through all the subsequent lenses and reach the sensor directly. However, despite the light absorbing properties of the black acetal plastic from which the column is constructed, it is still conceivable that light entering the system from outside this cone could still reach the sensor. The first test to be carried out in order to rule out this possibility was to insert a field stop into the optical column. Whereas the aperture stop determines the brightness of the image, a field stop determines the field of view of the system if the resulting image is smaller than the sensor size — if this is the case, the image is said to bevignetted. However, inserting a field stop with the same diameter as the primary ray bundle will still allow the full image to be formed, but any stray light that could be scattered into the sensor will be blocked i.e. it acts as a light baffle. This is most effective if placed between the final lens and the sensor, but when this test was carried out it was found that it had no impact on reducing the arcs. A further possibility was that of specular reflections from the

machined finish of the plastic surface. This was ruled out by a further test in which a tube of matte black flock paper was used to dull the inner surface of the final section of the optical column. Again, no suppression of the image artefacts was observed.

It was noted during the investigations into this problem that the surface of the CCD sensor in the FLI camera does not appear to be completely uniform. When viewed from an oblique angle, it can be noticed that the image produced by reflection of external light from the surface is distorted. The nature of the distortion suggests that the surface is not flat, as would be expected, but that the surface is ‘rippled’. Closer inspection of the ripples indicates that there is indeed a correlation between these and the image artefacts (illustrated in Figure 4.11). It is proposed that the surface of the sensor is slightly reflective (there is no anti- reflection coating) and the local curvature acts as a concave mirror which has a focal length approximately equal to twice the distance between the sensor and the inside face of the fused silica camera window. Consequently, the reflected light is focused back onto the CCD and the width of the image artefacts depends on the angular aperture of the source.

An additional unwanted feature which was apparent from fairly early on was a distribution of small dark circles across some images, also visible in Figure 4.10. As with the bright arcs, these are only apparent in situations where light is incident upon the sensor, and their positions also vary (although to a much lesser degree) depending on the illumination characteristics. These have been attributed to a build up of particulate matter on the surface of the sensor which appears to have increased in severity as the camera has continued to be in use. Considering that the front compartment of the camera is hermetically sealed, argon filled, and contains desiccant — these measures are intended to prevent the ingress of dust particles and water vapour (which would condense on the surface of the sensor when cooled) — this casts further doubt over the build quality of the camera.