Dalsa Piranha4 Series Manual

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Dalsa Piranha4 Series Manual Manual pdf 124 page image

124 Piranha4 2K and 4K Camera Color User's Manual03-032-20180-06 Teledyne DALSAAppendix D: The Sensor WindowCleaning and protecting against dust, oil, and scratchesThe sensor window is part of the optical path and should be handled like other optical components, withextreme care. Dust can obscure pixels, producing dark patches on the sensor response. Dust is mostvisible when the illumination is collimated. The dark patches shift position as the angle of illuminationchanges. Dust is normally not visible when the sensor is positioned at the exit port of an integratingsphere, where the illumination is diffuse. Dust can normally be removed by blowing the window surfaceusing an ionized air gun. Oil is usually introduced during handling. Touching the surface of the windowbarehanded will leave oily residues. Using rubber fingercots and rubber gloves can preventcontamination. However, the friction between rubber and the window may produce electrostatic chargethat may damage the sensor. To avoid ESD damage and to avoid introducing oily residues, avoidtouching the sensor. Scratches diffract incident illumination. When exposed to uniform illumination, asensor with a scratched window will normally have brighter pixels adjacent to darker pixels. The locationof these pixels will change with the angle of illumination.An important note on window blemishes:When flat field correction is performed, window cleanliness is paramount. The figure below shows anexample of what can happen if a blemish is present on the sensor window when flat field correction isperformed. The blemish will cast a shadow on the wafer. FFC will compensate for this shadow byincreasing the gain. Essentially FFC will create a white spot to compensate for the dark spot (shadow). Aslong as the angle of the incident light remains unchanged then FFC works well. However when the angleof incidence changes significantly (i.e. when a lens is added) then the shadow will shift and FFC willmakes things worse by not correcting the new shadow (dark spot) and overcorrecting where the shadowused to be (white spot). While the dark spot can be potentially cleaned, the white spot is an FFC artifactthat can only be corrected by another FFC calibration.