🚀 go-pugleaf

sysrick@starbase.spd.louisville.edu (Rick McTeague) writes:

>In <1992Nov12.235528.2939@elroy.jpl.nasa.gov> jack@robotics (Jack Morrison) writes:

>>>The second CCD camera question deals with the CCD's response to IR.
>>>What is the response curve of a CCD camera?  How quickly does the CCD's
>>>sensitivity taper off as the frequency of the light drops below visible
>>>and into IR?   And how come I can see the IR on my monitor/view-finder?
>>>(try pointing your TV remote at the cam-corder).  My potential application
>>>is a pseudo-night-vision device.

>>It depends on the CCD used, and the optical filters in front of it that
>>make specific CCD elements sensitive to red, green, or blue. But as you've
>>observed, they do see some IR. Your monitor displays this signal as
>>visible image (TV remote IR LED's are actually pretty bright).

>CCDs are reasonably sensitive to near IR at room temperature. Get them
>cold (liquid nitrogen), and they're even more so; I have an astronomer
>friend who uses cooled CCDs as his IR image sensors.

CCD's get less sensitive to IR as you cool them, not more. The incoming
photon must combine with a 'packet' of heat energy (a phonon) to be
absorbed. The sum of the photon and phonon energies must exceed the
silicon band-gap to excite an electron. Lower temperatures mean fewer
high energy phonons, so the cutoff wavelength decreases. The reason
astronomers cool CCD's is to reduce the 'dark current' caused by those
(few) high energy phonons energetic enough to excite an electron by
themselves. This dark current limits exposure times to milleseconds at
room temperature, or centuries at -150 deg C or so. Usually the CCD's
are cooled as little as is necessary to give reasonably low noise within
the exposure time they require.
	If it weren't for this 'indirect' absorbtion process, CCD's
would only be sensitive to photons energetic enough to excite electrons
directly- the traditional 'LED red' color, and up.

(all the above applies to silicon CCDs only- as far as I know, the more
exotic materials aren't CCD's as such, but standard detector arrays
anyway).


>Rick McTeague
>Electrical Engineering Department, Speed Scientific School
>University of Louisville, Louisville, KY 40292
>(502) 588-7020

>Internet: sysrick@starbase.spd.louisville.edu


	Andrew Williams,
	Physics, Uni of West Australia.