Re: Red Eye, Reprise

["Donald E. Simanek" <dsimanek@eagle.lhup.edu>]

On Thu, 9 Jan 1997, David Thiessen wrote:

> NOW HERE IS WHY I ASKED:  I have a domino mosaic picture of Einstein on the
> wall up at the front of the room illuminated with fluorescent lights.  A
> student took a flash photograph using a simple camera with outdoor color
> film in it.

You misled us. We thought you wanted to know about the human eye case. 
Nothing posted so far indicates there's any relation between your the
effect seen in your dominos and the red-eye effect of the human eye. Of
course, we haven't ruled out a connection either. 

More information is in order. In the photos is the red on the black
portions of the dominos, or on the white dots? (Use a magnifier.) I'd
guess it's on the black.

Many materials scatter light with a directional dependence on color.
Artists make use of this in choosing pigments for paintings. You can have
a material which behaves like a mirror for one color while scattering
other colors. The reason could have to do with size of the pigment
particles, and due to diffraction. This hypothesis is testable in your
case.

Try photographing mosaics with other types of dominos. Some are plastic,
some older ones are Bakelite, some are composition stone. Try
photographing other flat black surfaces.

Do your dominos have a smooth surface, or a textured surface?

> When I asked the other science teachers here at Deerfield what causes Red
> Eye, they gave the same "reflect off the red retina" answer. When I handed
> them the photograph of the Einstein mosaic, they break out laughing.

I still haven't gotten to a good library, or a knowledgable authority. But
all the color pictures I've seen of human retinas show them to be
reddish-orange, over the whole area (not just the blood vessels). The
mosaic of the ends of the receptor cells could conceivably have
diffractive scattering which could give color differentiation.

> You can guess why.  The domino Einstein has Red Eye (and nose bridge).  The
> camera was aimed directly at the eyes.  Had it been aimed at the moutasche,
> I'm sure it would have reflected red.

So, why did they laugh? Did they suppose the two examples had anything to
do with each other? 

> I await your explanation of why the black dominoes which relected the flash
> directly back to the film appeared red.  Is it the combination of a little
> bit of white light from the dots plus the "black light :-) " from the black
> paint added up to red?  I seem to remember seeing a demo of a rotating
> black and white disc create the sensation of red in my eyes.  What is going
> on here?

The rotating disk produces a sensation of several colors, rusty red, dusky
green, and a sooty blue. This is due to the timing of the firing of the
receptors in the eye. There's no reason to suppose it has anything to do
with your example of the domino portrait. One can speculate what it might
have to do with red-eye, however, for we may have a short term fading
effect of pigments in the retina from the intensity of the light from the 
flash. Still, I think that hypothesis is reaching a bit.

> Why my impatience?  I plan to pass this picture around at the joint meeting
> of CAPO (Chicago Area Physics Organizations) next Wednesday (1/15) at
> Elmhurst College and I would like your input before I go.  If you live in
> the Chicago area, you might like to attend.  It is the annual joint meeting
> of the four physics teacher sharing groups and we always have a good time.
> Phood and Phellowship at about 5:30 p.m. and sharing at 7:00 p.m.  The
> Elmhurst College physics building is at the SE corner of the campus.

The University of Chicago has an excellent program in medical physics, and
surely one of its faculty members could give you an authoritiative answer
to the human red-eye question.

	-- Donald

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 
Dr. Donald E. Simanek                            Office: 717-893-2079
Prof. of Physics                    Internet: dsimanek@eagle.lhup.edu
Lock Haven University, Lock Haven, PA. 17745          CIS: 73147,2166 
Home page: http://www.lhup.edu/~dsimanek            FAX: 717-893-2047
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