Adaptive Optics for Vision Science

Chapter 10.2.7 - Science Camera

10.2.7  Science Camera

Ocular hazards limit the amount of light that can safely enter the eye and
ultimately strike the science camera. This places a premium on detector
quantum efficiency (QE) and fill factor. Both the Rochester and Indiana
cameras employ back-illuminated CCDs. Back illumination provides a QE in
excess of 75% at the desired wavelength, and the CCD pixel architecture
permits a fill factor of 100%. For typical retinal illumination levels,
photon noise dominates the detected retinal image even for moderate levels
of read noise (~50 electrons RMS). Because higher read rates generate higher
read noise, an appropriate balance of the two depends on the specific application
and available light budget. Dark noise of the CCD is largely unimportant,
owing to the short (~4 ms) exposure times. CCDs in current AO ophthalmoscopes
typically have 12 bits of dynamic range. For some retinal imaging
tasks, such as those that entail precision contrast measurements (e.g., classifying
cones), careful flat-fielding of the CCD is also important, as well as high
linearity of the detector. In the future, complementary metal–oxide–semiconductor
(CMOS) devices will be an attractive alternative to CCDs. CMOS
devices allow on-chip system integration, operate at a high speed, and are of
low cost and low power. At present they represent a maturing technology and
generally lag the CCD in nearly every other performance category, including
quantum efficiency and pixel fill factor.

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