From: Eugene.Leitl@lrz.uni-muenchen.de
Date: Fri Dec 01 2000 - 09:16:19 MST
Dave Sill writes:
> I don't get it. I don't see--excuse the pun--how selectively filtering the
> light reaching each eye can add information. The depth perception analogy
> doesn't work because in this case the filtering doesn't provide new
> information.
Imagine three bell-shaped, partly overlapping curves. Wavelength
vs. how much photons of this kind excites the receptor. The tails
decay rather slowly, so you should be able to see some NIR or UV,
given enough intensity (nevermind that enough intensity will damage
the eye, but at least for a while you can enjoy a radically expanded
experience ;) In fact some (imo rather foolish) people have purported
to see a very dim red when looking into an operating NIR laser diode
(don't do it at home, kids), at least for a while. I'm told an UV LED
(also not healthy to look into) also projects a sort of sickly
purplish glow (and making mineral-filler paper fluoresce), probably
combining the effects of emission and receptivity bell curves
tails. Additionally, some of the UV is being cut off by the cornea, so
some people with surgically removed cornea reported a wider
sensitivity to the blue region of the spectrum. (That sunrise is sooo
ultraviolet... wow, man). But that's irrelevant for our current
discussion, since we we will consider the eye as an integrated,
unhackable sensor.
Let's take just one of the bell-shaped curves for simplicity. There
are (rather expensive) colour filters based on interference effects
(think of it as hitech versions of butterfly's wings) out there, which
allow you to cut out a relatively narrow spectral window. If you put
such an interference filter which cuts out an area from within a given
photoreceptor's sensitivity/wavelength curve, you've changed the shape
of the total sensitivity/wavelenght curve, especially if it's
off-center (of the original sensitivity maximum). Of course, this
comes for a price: less light comes through in toto, and hence the
total sensitivity is degraded. You should be able to resolve more
colours when comparing a light source while with and without the
filter. In theory, you could use a filter function which smoothly
changes over time (moving a narrow window over your bell-shaped
curve), giving you a kind of time-multiplexed multispectral
vision. You would perceive that as an object smoothly changing hue,
highlighting some areas which you wouldn't be ordinarily able to
resolve. But the visual system is lousy at temporal comparisons, so
you try to put the other eye (or other part of the retina) to the
task. Another way to see more colours would be by misusing a
diffraction grating mirror as part of the optical system. You'd be
mapping photons usually converging on the same spot of the retina to
diverse nearby. Every object would acquire a multispectral halo in one
dimension. For instance, it is trivial to distinguish incandescents
and fluorescents using a common CD as a makeshift diffraction grating,
if you can't lay your hands on a real diffraction grating (which are
speciality items, and hence expensive. You might fish around in the
trash bin of your local research institute, and salvage one from an
old spectrophotometer, or something. Or order one from Edmund Optics,
if you're feeling particularly enterprising).
Of course, a true tetrachromate experience is much superior, since the
visual pathways have had time to adapt and remap themselves to utilize
to a colourspace with one extra dimension. The (minor) negative side
effect is that your colour resolution is even worse than before
(because the different cell types can't occupy the same place, notice
that this doesn't affect the cell type which is sensitive to the
entire VIS range, and which is responsible for high-lateral resolution
high-sensitivity -- this stuff is more important than color). And, of
course, you're likely to break the colour vision of your male
offspring. Yikes.
It would be cool if the retinal cell could modulate it's spectral
sensitivity rapidly, and the visual system could integrate the
information. But I guess we will have to wait for postbiology for
that.
Right now the best you can do is by using a multispectral imager with
a HMD/head-up, and (adaptively) map the colours into the common RGB
scheme, seeing the world in (changing, tunable) false colours. I
haven't heard of any gargoyles with such a hookup, apart from the
obvious (mapping NIR into VIS, using NIR filters and NIR light sources
(but the sun is pretty bright in NIR, too -- the NIR LED cluster lamp
plus CCD and/or MCP residual light amplifier is great for nightly
sneaking around, and doing all kinds of technology-assisted practical
jokes, etc.)).
Say, that's a rather large soapbox.
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