Re: Why ARE the sparks blue / purple (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Mon, 29 Oct 2007 20:45:57 -0800 (PST)
From: Ian Macky <ian@xxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Why ARE the sparks blue / purple (fwd)

>> ...cameras are not sensitive to the same freqs as our eyes...
> (rough paraphrase)
>  Concur.
>  Also works the other way:
>    films, and sundry iameg sensors can be sensitive to
>    freqs our eys miss.  In Serious phtometeric work, film/whatever
>    is carefully calibrated:
>
>     Cameras may not lie, but they can be confused...

CCDs are only slightly sensitive to UV (cut-off about 350nm) but more
sensitive to near IR, which is why a low-pass filter is often included.
My E-10 produces an image with a 720nm cut-off filter, but not with a
900nm filter.  It *does* have an internal low-pass filter, drat.

The raw signal from a human eye is processed before the brain gets to
"see" it.  For example, I have red and green LED flashlights.  Shine them
both on a white surface at the same spot and lo!, I "see" yellow light.
But there is no yellow light, only a single wavelength of red and a single
wavelength of green, mixed.

Beware the senses, they are not are straight-forward as you might think.

The eye can be fooled easily.  What you "see" is not always what you get.

Another example of human sensor weirdness:

At a science hall in SF they have an exhibit with two alternating coils
of hollow copper pipe (say 1/4" ID), wound on about a 4" diameter form
(think of a double-threaded screw); one pipe runs hot water, the other
cold.  Because of the way they're wound, the pipes alternate, hot, cold,
hot, cold, etc.

Now, if you lay your hand on this thing you get a spinal reflex that
jerks it away because it fools the sensors into thinking it's VERY hot.
But it's not, you can manually override the reflex and hold you hand on
it and it's a weird feeling, truly, hot and cold, but it's not burning
hot and the reflex is in error.

--ian