Mark Roberts wrote:
> Someone on the rec.photo.digital newsgroup just posted asking if a
> digital camera can do infrared photography....
Hi Mark,
CCDs have spectral responsivity curves typical of other silicon devices,
meaning that their useful response extends out to 1000 nm or so.
CCD-equipped consumer camcorders extend somewhat beyond the red end of the
visible spectrum -- ever notice how your camcorder can "see" the flashing
LED in your TV remote (690 nm, IIRC), but your eyes of course can't? The
reason consumer camcorders do not respond terribly well out at 1000 nm is
not due to a limitation of the CCD, but rather to the optical filter mounted
in front of the CCD. This filter passes visible light quite well, but does
not pass very much infrared. I suspect digital cameras are equipped with
such filters as well. That makes sense, because the typical consumer wants
a photograph or a video to look like the view he/she sees with his own eyes.
If an off-the-shelf camcorder or digital camera responded instead like HIE
film, the typical consumer would probably not be too happy.
You ~can~ increase the IR sensitivity of such a CCD-equipped camera -- but
probably void the warranty -- by removing this filter. In fact, several
years ago when I lived in Champaign IL, I went to see an evening lecture at
the university given by a fellow involved in research on the Dead Sea
scrolls. His team used a consumer camcorder with the IR-blocking filter
removed to do near-IR imaging on recovered scraps of the scrolls. It's
amazing how much comes out in these near-IR images that is completely hidden
in visible-light images.
Good luck with your experiments!
Bill Peifer
Rochester, NY
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