From: cam@castle.ed.ac.uk (Chris Malcolm)
Newsgroups: rec.bicycles.tech
Subject: Re: Contact lenses and blood vessels in the eye!?
Date: 17 Nov 92 02:55:43 GMT
Organization: Edinburgh University

In article <1992Nov16.183553.13270@kestrel.edu> king@reasoning.com (Dick King) writes:
>In article <28173@castle.ed.ac.uk> cam@castle.ed.ac.uk (Chris Malcolm) writes:
>>In article <1992Nov13.182426.911@kestrel.edu> king@reasoning.com (Dick King) writes:

>>>In the rain, i was faced with the choice of doffing my glasses and having
>>>20/400 vision for the whole ride home, or keeping them on and having my vision
>>>deteriorate from 20/20 to perhaps 20/800 or worse on the way home as the
>>>glasses covered with water.

>>Contact lenses are not the only solution here.  [many unworkable "solutions"
>>deleted for brevity]

>Mr. Malcolm either has never cycled enough to perspire and this fog up the
>inside of a helmet visor, or doesn't need corrective optics.

I've been cycling on and off in all weathers, frequently in rain and
mist, and frequently perspiring, for 35 years.  When not cycling I'm
motorcycling, where the problems are worse, since the dress required
to withstand 80mph freezing rain provokes instant heavy sweating when
stationary. And when not on two wheels I'm a spectacled pedestrian in
one of the world's dampest cities, where it rains all winter, and
summer is marked by the increased incidence of sea-fogged days.  I'm a
visor and spec fogging expert.

Without specs I can focus no further than 3.5 inches in front of my
eyeball. Mr King clearly doesn't understand the detailed physics of
fogging. On a bike visor fogging is rather like getting wet -- simply
wearing a bunch of waterproofs would rapidly convince anyone that it
was impossible to stay dry on a bike in heavy rain. Similarly just
sticking on a visor and riding off into freezing rain will very likely
result in gross fogging. Intelligence is required.

>None of these "solutions" works.  RAIN-X on my glasses was an improvement over
>bare glass, but only barely.  The droplets can hide a pretty big pothole
>or car before they fall off, and they tend to leave behind a trail of even
>tinier, more numerous, and more annoying droplets.

That's the trouble with rain on the specs, whether or not you apply
RAIN-X: the droplets subtend much too much of the visual field, and
can't easily be moved fast enough to be "seen through" by the brain's
scene construction mechanisms, which are designed not to notice small
transient imperfections.  Hence the need to have a rain-catching
surface further out -- such as a visor.  At a typical visor distance,
and an amount of head shaking that hardly needs to be practised
consciously unless the road is very smooth, the RAIN-X droplet size
can be easily "seen through". In fact, as it happens, a typical visor
distance is just about the optimum for seeing through these droplets:
just as spectacle lenses are too close, car windscreens are too far.

Fogging of a visor is not simply due to perspiring. It is necessary
for the perspiration to create an air-draught of high humidity, and
for the visor to be sufficiently colder than this hot wet air to
precipitate the moisture. It is usually not difficult to avoid these
conditions, despite heavy perspiration: one has to arrange that one's
clothing vents are not so disposed as to channel the sweaty air up to
the visor, and that the configuration of the visor invites dilution of
such sweaty air as does pass its way by a sufficient admixture of
fresh air. One must also avoid breathing on the visor.  This requires
attention to the shape and fitting of the visor -- some are very good,
some very bad. Most are too deep.  Once these principles are grasped
it is not difficult to avoid visor fogging on a bicycle, even when
sweating in rain at a few degrees C.  The visor also protects the
spectacles from the wind and rain, thus permitting body heat to keep
them above the dew point.  The only conditions in which fogging is
hard to avoid is when the air itself is at or close to the dew point,
such as mist, fog, or smir.

There are also treatments available which can greatly reduce the
receptivity of visor materials to fogging, but they are difficult to
use and not well understood, so I didn't mention them. Fortunately
there should soon be commercially available visors made of inherently
unfoggable material.
-- 
Chris Malcolm    cam@uk.ac.ed.aifh          +44 (0)31 650 3085
Department of Artificial Intelligence,    Edinburgh University
5 Forrest Hill, Edinburgh, EH1 2QL, UK                DoD #205