Path: utzoo!utgpu!water!watmath!clyde!bellcore!faline!thumper!ulysses!andante!princeton!udel!gatech!purdue!decwrl!labrea!eos!jbm
From: jbm@eos.UUCP
Newsgroups: sci.electronics
Subject: Re: Seeing UV
Message-ID: <702@eos.UUCP>
Date: 12 May 88 22:51:20 GMT
References: <230@snjsn1.SJ.ATE.SLB.COM>
Distribution: na
Organization: NASA Ames Research Center, California
Lines: 62
Posted: Thu May 12 18:51:20 1988

From article <230@snjsn1.SJ.ATE.SLB.COM>, by greg@bilbo (Greg Wageman):
> In article <685@eos.UUCP> jbm@eos.UUCP (Jeffrey Mulligan) writes:
>>
>>Ultraviolet radiation causes the lens of the eye to flouresce.
>>This would produce a diffuse illumination of the entire retinal image,
>>but the effect would probably be more of a soft-focus effect,
>>(sharp edges with a fuzzy overlay) rather than out and out blurring.
> 
> The above may very well be true, but...

You bet your booties it's true!

I quote from page 116 Wyszecki & Stiles,  "Color Science", 2nd ed.,
John Wiley & Sons 1982:

"The flourescence of the human eye lens is caused by ultraviolet radiant
energy (lambda < 400 nm).  It produces a general veil of bluish light
over the retina.  The spectral composition of the flourescent light probably
agrees with the spectral distributions - all similar - determined for
the eye lenses of sheep, rabbit, and ox, by LeGrand (1948)."

> 
> ... But since "UV" is at the extreme end
> of the visible spectrum (actually what you are seeing is the extreme
> deep violet visible portion), its refraction will be far enough from
> the correct focal point that it will indeed be out of focus.
> 

The lens absorbs UV, unlike IR which can be seen by simply jacking
up the power to compensate for the lower sensitivity of the visual
pigments.  Continuing the above quote:

"The retina is also flourescent to ultraviolet and possibly
to short-wavelength visible light, but in the normal eye, absorption
in the eye lens removes most of the exciting radiant energy
before it can reach the retina.  Aphakic observers (eye lens removed)
are able to see stimuli of wavelengths much deeper in the ultraviolet
than are normals, the color seen in the range of 360 to 310 nm
being blue, not violet (Gaydon, 1938).  This change of color may
be the visual response to a mixture of bluish-green flourescent radiant
flux with the original ultraviolet stimulus; however, this is
controversial."

Greg's point about chromatic aberration is certainly valid in the
visible range, but it is interesting that this is not usually
noticeable.  There is a nice demonstration of this at the Exploratorium
in San Francisco:  They have an array of pinholes illuminated from
behind with white light.  Most people see a bunch of white spots.
When the same spots are observed through a purple filter (which
transmits only red and violet), you see a red dot surrounded by
a bluish cloud.  (I myself see a cross made up of a red and a blue
member due to my astigmatism.)  The point is that the blue component
of the white light is just as blurred when it looks white, but
that the brain compensates for the chromatic fringes present at the edges of
white objects.


-- 

	Jeff Mulligan (jbm@ames-aurora.arpa)
	NASA/Ames Research Ctr., Mail Stop 239-3, Moffet Field CA, 94035
	(415) 694-5150