Re: Near IR Visual Observations

On Jun 3, 2:24 pm, Chris L Peterson <c...@xxxxxxxxxxxxxxxxxx> wrote:
On Sun, 03 Jun 2007 05:54:49 -0700, Martin Brown

<|||newspam...@xxxxxxxxxxxxxxxxxx> wrote:
It seems that the eye has some residual sensitivity to the
unattenuated near IR when the bulk of the visible light has been
filtered out. The visual oberservation of these "false" colour views
is quite interesting. I have tried it with a couple of other people
and they also report seeing red grass and orange trees after adjusting
to the dimness.

It's certainly true that the eye has some sensitivity to near IR. Most
people can see the IR LED in TV remote controls, for instance.

Indeed. But I have never seen colour in that. The light is
desaturated.

But in this case, it isn't at all obvious to me that you were seeing IR.
Even fully exposed, the film dyes are nowhere near dense enough to
attenuate daylight scenes below your photopic sensitivity threshold. I
think you were just seeing a color distorted world without a significant
IR contribution.

Although that is certainly possible the green of chlorophyll was so
massively attenuated that foliage and grass looked bright and red or
orange. Pretty much like you would expect to see in a monochrome near
IR image.

When you look through a true near IR pass filter, everything looks deep
red. You don't see any other color at all.

I have a set of near IR RGxxx filters and I agree. They only give deep
red or monochrome images. That is what makes the unexposed slide film
filter with some visible leakage so interesting - you get a rough and
ready false colour IR image in real time (after dark adaption). How
much is from the near IR is difficult to say, but subjectively it
looks very like the sort of distinctive luminance pattern that you get
with classic IR images. Conifers and deciduous trees stand out in much
the same way. Give it a try and see what you think. Sensia or Velvia
seem to work best.

Regards,
Martin Brown

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