Sensor Wrap-Up - Nikon D300 Complete Manual

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and on my Web site: clean quickly and not in the presence of
high-powered light sources.
Another concern that comes up on Internet forums from time
to time is cosmic rays. A direct hit by a cosmic ray on the
right portion of a photosite could definitely damage it. But
cosmic rays are relatively rare here under the earth's
atmosphere. Even up in space we have a number of digital
cameras (NASA uses Nikon D2x bodies on the space shuttle),
and they don't seem to be worse for the wear due to cosmic
ray penetration. As they say, "take a chill pill" if things like
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this worry you. You're far more likely to drop your camera
than have it compromised in any way by excessive light
exposure or cosmic ray penetration.

Sensor Wrap-up

One final reminder about photosites: their light-catching
regions (photodiodes) don't fill the entire area the sensor array
occupies. This catches some digital newcomers by surprise, as
they imagine that the photosites are all jammed up against
one another and the entire sensor captures light. The
photosites are jammed together, but the light-sensing portions
of most sensors, including those in the D300, are significantly
smaller than the overall photosite size, partly in order to keep
light-produced electrons from migrating too easily to adjacent
photosites, partly to allow room for other signals on the chip
(power and data transfer, primarily).
I've also sidestepped one issue in this discussion of how a
sensor works: how the amount of light (an analog value)
becomes digital data. To make a very complicated story short,
the light photons captured by the individual photosites are
converted into electrons at each photosite. These electrons
are stored in a "well" on the photosite during the exposure.
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By "far more likely" I'm not talking about an order of magnitude or two. More like
orders of orders of magnitude. Computer memory is also subject to change by cosmic
ray. What most University computer science programs teach is that sometime in the
next century, one computer bit will be flipped randomly by a cosmic ray in a running
program and cause a problem. So I repeat: don't worry about these things.
Thom Hogan's Complete Guide to the Nikon D300 Page 81