Joe Gwinn wrote:
I think Moose meant by "oscillates between light and dark" the spatial
frequency, measured in cycles per millimeter on the film surface.
Of course that's what he means. He just sometimes thinks that using
alternate descriptions of jargony words and phrases may make it easier
for some people to hear the intended content. Jargon (scientific
vocabulary, inthis case) tends to increase the simplicity and accuracy
of communications between an audience with a size that decreases with
increases in the jargon density. I was trying to increase the audience
size.
Most digital sensors respond well to IR light out to about 1000 nanometers
(green light is about 500 nanometers, red light is about 600 nm wavelength),
unless the imager has a built-in filter. Response to the UV isn't usually all
that great, unless one has bought a special UV-enhanced CCD.
Boris wrote:
The broader the spectrum, the harder to control. All CCD's are sensitive to IR and UV,
but this is cut off by a filter in all professional cameras...except for the Dima*ge 7,
but note that this feature was "fixed" in the subsequent models at no cost.
Way back in my original response, I was careful to say I had no idea
about the IR and UV response of DC sensors, nor the effect of the glass
used in low (spacial frequency) pass filters. I understood the complaint
in the original post to confuse the spacial frequency low-pass filters,
which had been the subject of the thread to that point, with light wave
frequency filters. (Of course, in that case, low pass would pass IR and
exclude UV.) I was just trying to allay a possible misconception.
Boris, let me propose one practical and important reason why a
manufacturer might want to control light outside the visible spectrum
reaching the sensor. The problem already exists in cameras using film as
the sensor and is a result of the basic technology of color imaging. We
record color not as a contimuous process, but by sensing the relative
brightness of light seen through "filters", either literal or
figurative, that sample 3 (sometimes more, but generally 3) parts of the
visible spectrum. The whole color balance of the result depends on the
relative levels of the samples. When one of the sensors picks up light
outside the visible spectrum, there is no way to know how much of the
response of that sensor is due to the light from the visible subject and
how much from light invisible to our eyes. Thus, any imaging system
where color accuracy is desired would include both low and high pass
filters to limit the light reaching the sensors (CCD, COMS, silver
halide, etc.) to the visible spectrum. This is the same function as use
of dark red filters (light wave frequency low pass filters) to limit the
light received by IR film to the general range desired, since the film
is unfortunately also sensitive to light up in th visible spectrum.
Most (all that you and I use) photographic filters, unfortunately, do
not have sharp cut-off characteristics and conventional films (whether
by intent or by the nature of the beast), don't have so much sensitivity
beyund the visible spectrum that this is much of a problem except at
high altitudes, where controling UV becomes important. I don't know how
much this factor plays, but one of the wonderful things about DCs that
we tend not to focus on in our compulsion for resolution, is that they
generally have very high color accuracy, better than film. To the extent
that specialized applications like IR and UV imaging may have to be
sacrificed in cameras designed for the visible spectrum to improve color
accuracy, I consider that a good tradeoff for my photographic purposes.
So yes, they may be limiting your spectrum, but it's not a matter of
spending money on filtering just to 'get' you, there is a legitimate
photographic purpose.
Moose
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