[Tig] new pixels
Thu Feb 14 09:16:28 GMT 2002
From: "Robert Lovejoy" <rlovejoy at home.com>
> The Times article fails to mention film's far greater latitude thus far
> any CCD device has achieved. I had little doubt they would get the
> resolution, but so far digital is far outperformed by film in handling
> highlights and low lights.
That's not quite right. CCD's are capable of dynamic range far exceeding
that of film. The trick is in the processing. While it is true that no
broadcast television camera (and I should say telecine) has been built to
exploit the broad dynamic range, that is, in no way, a reflection of the
current limits of CCD/CMOS imager technology. Back illuminated arrays, for
example, have vastly improved low light capabilities and less noise.
Remember that CCD's are "photon counters". Once the bucket is full, reset
it and keep countin` for greater upper end range!
There are various techniques used by CCD manufacturers to increase quantum
efficiency --a measure or how sensitive a pixel is to various light
wavelengths. Two that come to mind are: 1)Removal of anti-blooming
coating; 2) Back-side illumination.
The removal of the anti-blooming coating can double the quantum efficiency
at certain wavelengths (around red) with significant gains across the
spectrum. CCD pixels "wells" have a capacity measured in electrons and
blooming is the bleed of excess electrons when a pixel reaches saturation
(the well is full). Without an anti-blooming coating the technique used in
industrial/astronomical imaging for avoiding blooming is to take safe
short-duration exposures and layer them with software --a pixel begining to
reach saturation indicates the start of blooming.
Back side illumination is a technique that involves thinning a CCD sensor to
about 15 microns and mounting it front-side down on a substrate. The idea
is to expose the pixel wells directly to incoming light and vastly improve
quantum efficiency. The boost can be dramatic, going from 20% at red for an
anti-blooming equipped CCD imager to over 85% on an imager treated to both
the removal of the anti-blooming coating and the "back side illumination"
technique. Most of these, as far as I know, are used in Astronomy, these
guys truly count electrons when they photograph distant objects with very
long exposures, they even cool the imagers in order to reduce the Dark Count
(number of electrons per second that fill the wells when no light hits the
imager) from about 10 or more electrons per pixel per second at "normal"
temperatures to around 1 e-/pix/sec when the CCD is held at a temperature
of, say, -5 degrees Centigrade.
Neat stuff. We are getting there!
eCinema Systems, Inc.
ecinema at pacbell.net
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