Re: Dinosaurs

On Sat, 04 Feb 2006 11:02:29 -0800, "Roger N. Clark (change username
to rnclark)" <username@xxxxxxxxx> wrote:

Andrew Haley wrote:

Higher well capacity is also possible: there's no reason to suppose
the current capacity limits will be maintained forever.

In particular, there's no reason in principle why more layers couldn't
be fabricated beneath the sensor array to hold more charge. And if
you did that you could increase pixel density and therefore resolution
while maintaining SNR.

Andrew.

I agree that well capacities could be improved. But that is not
the ultimate limit. The ultimate limit is the number of
photons coming from the sun (or other light sources which
are all less intense than our sun). See my other post
this morning.

At f/8, a sunny bright day with a perfect optical system (100% transmission)
would deliver about 910,000 photons/second per square micron in the
focal plane in the green passband (the green response of your eye)
from an 18% gray card with the sun straight overhead and the lens
pointing straight down at the gray card (not a very interesting photo).
If you use ISO 100 speed and the sunny f/16 rule of 1/400 second,
you get only about 2300 photons/pixel per square micron (a 1 micron
pixel would have signal to noise ratio of 47. Current real cameras
lose light due to reflection and absorption in the optics and lower
quantum efficiency, and get only about 230 photons/pixel per square
micron for a signal to noise ratio of 15. For interesting photos
with shadows and the sun not straight overhead, the photon flux
drops, factors of 10 or more. Even so, yes, there is
room for improvement, but there are real and easily computable
upper limits due to the finite photon flux from the sun.
But obviously, with small pixels, one gets fewer photons, and
a larger pixel will collect more. It is up to the user
to decide how large a pixel they want (or can afford).

Don't optical coatings exist that absorb in the UV and re-radiate in
the visible? I don't know the exact wavelenghts they re-radiate in.
It might be possible to apply these to the windows of CCDs/CMOS
in order to "boost" incoming light.
-Rich


.

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