Piranha4 8K Color Camera User's Manual
75
Teledyne DALSA
03-032-20156-00
Appendix D: The Sensor Window
Cleaning and Protecting Against Dust, Oil, and Scratches
The sensor window is part of the optical path and should be handled like other optical components, with
extreme care. Dust can obscure pixels, producing dark patches on the sensor response. Dust is most
visible when the illumination is collimated. The dark patches shift position as the angle of illumination
changes. Dust is normally not visible when the sensor is positioned at the exit port of an integrating
sphere, where the illumination is diffuse. Dust can normally be removed by blowing the window surface
using an ionized air gun. Oil is usually introduced during handling. Touching the surface of the window
barehanded will leave oily residues. Using rubber fingercots and rubber gloves can prevent
contamination. However, the friction between rubber and the window may produce electrostatic charge
that may damage the sensor. To avoid ESD damage and to avoid introducing oily residues, avoid
touching the sensor. Scratches diffract incident illumination. When exposed to uniform illumination, a
sensor with a scratched window will normally have brighter pixels adjacent to darker pixels. The location
of 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 an
example of what can happen if a blemish is present on the sensor window when flat field correction is
performed. The blemish will cast a shadow on the wafer. FFC will compensate for this shadow by
increasing the gain. Essentially FFC will create a white spot to compensate for the dark spot (shadow). As
long as the angle of the incident light remains unchanged then FFC works well. However when the angle
of incidence changes significantly (i.e. when a lens is added) then the shadow will shift and FFC will
makes things worse by not correcting the new shadow (dark spot) and overcorrecting where the shadow
used to be (white spot). While the dark spot can be potentially cleaned, the white spot is an FFC artifact
that can only be corrected by another FFC calibration.
