Appendix D: The Sensor Window; Cleaning And Protecting Against Dust, Oil, And Scratches - Teledyne Piranha4 User Manual

Piranha4 RGB + NIR / Mono Camera User's Manual 107

Appendix D: The Sensor Window

Cleaning and Protecting Against Dust, Oil, and Scratches

The sensor w indow is part of the optical path and should be handled like other optical components, w ith
extreme care. Dust can obscure pixels, producing dark patches on the sensor response. Dust is most
visible w hen the illumination is collimated. The dark patches shift position as the angle of illumination
changes. Dust is normally not visible w hen the sensor is positioned at the exit port of an integrating
sphere, w here the illumination is diffuse. Dust can normally be removed by blow ing the w indow surface
using an ionized air gun. Oil is usually introduced during handling. Touching the surface of the w indow
barehanded w ill leave oily residues. Using rubber fingercots and rubber gloves can prevent
contamination. H ow ever, the friction betw een rubber and the w indow 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 w ith a scratched w indow w ill normally have brighter pixels adjacent to darker pixels. The location
of these pixels w ill change w ith the angle of illumination.
An important note on window blemishes:
When flat field correction is performed, w indow cleanliness is paramount. The figure below show s an
example of w hat can happen if a blemish is present on the sensor w indow w hen flat field correction is
performed. The blemish w ill cast a shadow on the w afer. FFC w ill compensate for this shadow by
increasing the gain. Essentially FFC w ill create a w hite spot to compensate for the dark spot (shadow ). A s
long as the angle of the incident light remains unchanged then FFC w orks w ell. H ow ever w hen the angle
of incidence changes significantly (i.e. w hen a lens is added) then the shadow w ill shift and FFC w ill
makes things w orse by not correcting the new shadow (dark spot) and overcorrecting w here the shadow
used to be (w hite spot). While the dark spot can be potentially cleaned, the w hite spot is an FFC artifact
that can only be corrected by another FFC calibration.
Teledyne DALSA 03-032-20217-01