Smear - bright line in the image
Smear (or smearing) can be defined as an undesirable vertical bright line that extends above and below a bright spot in an image.
Figure 1: Smearing above and below a bright spot
Basically, smearing is caused by electrons that move from an overexposed pixel to the shaded vertical shift register during the readout process. These electrons are caught by transferring registers that shift the charge from other pixels toward the readout line at the bottom of the CCD sensor. Therefore, the whole column shows this undesirable vertical artifact called smear or smearing.
Blank pixels are shifted in from the top of the light-sensitive area so that even a pixel that is "at the bottom" of this register becomes a bright pixel.Therefore, the vertical stripe above a bright spot is generated by shifting the previous image, while the smear stripe below is produced during the shift of the actual image.
Smear can be diminished by reducing the amount of light the pixels are exposed to. Moreover, decreasing the aperture size and concentrating the light to prevent it from reaching the interline transfer cells can help to reduce smear.
- Increase the shutter (integration) time.
- Turn the light source off before and after an exposure by trigger/shutter.
- Use a pulsed or flashed light source.
- Use an LCD/mechanical shutter
- Use an optical filter
Special case - dark smear:
If the image has several big bright spots with dark parts in between, smear can occur as a dark line that extends beyond and below a bright spot, which is interrupted by dark parts below and/or beyond. This dark smear is the inversion of smear explained above. Since the whole image is strongly exposed, smear can be seen everywhere except where the dark parts occur, which interrupt the shifting of overexposed pixels, so that dark smear can be seen.
Figure 2: Dark smear
Smear occurs in the shaded part of a CCD sensor and is sometimes confused with an artifact occurring in the light-sensitive part: a phenomenon called blooming. Especially on interline transfer CCDs, these artifacts often occur simultaneously.
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