NavList:

On Aug 4, 2006, at 5:38 PM, Ken Muldrew wrote:

>
> On 4 Aug 2006 at 20:50, George Huxtable wrote:
>
>> The only way that I can envisage heat-damage behaving in a different
>> way from light-damage is that one might expect heat to diffuse away,
>> more effectively, from a tiny spot into the surrounding area, than 
>> it would
>> if a larger area was being heated. That wasn't an argument 
>> deployed by Ken,
>> but it might have some validity, depending on the time-scales 
>> involved.
>
> Heat removal from the eye will mostly be through the vasculature. 
> There
> are a couple of venules removing blood from the retina but they are
> intertwined with the arterioles and so form something of a heat 
> exchanger.
> Thus while photochemical damage is caused by atomic level 
> transitions (or
> ionization?), heat injury does not depend on the energy density as 
> much as
> the total energy being delivered. It's possible that much of the 
> energy
> that is dumped on the retina can go into heating. I'm not sure if 
> anyone
> has a good understanding of heat transfer within the eye although the
> principle worry of leaking microwave ovens is cooking the humours 
> of the
> eye so perhaps there has been work done on it.
>
> I don't know if a sextant scope or binoculars can provide enough 
> heat to
> cause damage or not, but I would be surprised if an astronomical scope
> aimed at the sun didn't cause retinal burning.
>
> Ken Muldrew.
>

I expect free radicals generated by ejecting electrons from molecular 
orbit would be the main causes of damage in the photoreceptors, at 
least initially.  This is so in plants.  Then there is the question 
of removing the heat after the free radicals have been pacified 
(sorry, what else does one do with radicals?).

IR radiation could cook eye humours equally as well as lower 
frequency microwave radiation.  A simple Google search turned up the 
following

http://www.lsbu.ac.uk/water/vibrat.html

This would affect the cornea and lens also.  I wouldn't expect them 
to have any vasculature for heat exchange, and for that to be handled 
by conduction, and convection in the humours.

However, nearly instantaneous damage would almost assuredly involve 
the photoreceptors.  I have no clue how long "nearly instantaneous" is!


Regarding the optics, wouldn't the diameter of the objective be the 
main parameter involved in potential damage, since one is 
concentrating the energy from that cross section into a smaller 
one?   Isn't magnification secondary here?

Fred

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