| Comments |
"Rod photoreceptors detect and encode incident photons
exceptionally well, providing highly sensitive input
to the visual system at low light levels. They collect
sparse photons with high efficiency, as more than 25%
of the photons incident on the retina produce a rod response.
They maintain a low dark noise, equivalent to a
light producing one absorbed photon every 90 sec in a
human rod. And they generate reproducible responses
to each absorbed photon, allowing the number of photons
absorbed to be accurately determined from the rod
signals. In lights producing more than one absorbed
photon per rod every 10–20 sec, Poisson fluctuations in
the incident photons dominate the rod’s internal noise,
and the cell acts as a nearly perfect photon counter. The
performance of a mammalian rod is compared with that
of a cone photoreceptor, a photomultiplier tube (PMT)
and a charge-coupled device (CCD) in Table I. Rods
have lower dark noise and smaller pixel areas than the
PMT and CCD, but the PMT’s integration time is much
shorter. Cones have a lower quantum efficiency and
much larger dark noise than the rods, but a shorter integration
time." See note above table |