Re: For newbies going to viewing sites

"Chris L Peterson" <clp@xxxxxxxxxxxxxxxxxx> wrote in message
news:bqkr72h4c83eaua7john48qidriupqs7rh@xxxxxxxxxx

On Wed, 31 May 2006 17:28:07 GMT, "John Nichols"
<bejay@xxxxxxxxxxxxxxxx> wrote:

I wear photogray lenses, which get quite dark in sunlight. I imagine
this
is just as good as regular dark sunglasses?

The recommendation for military pilots is that sunglasses absorb 85% to
prevent reduced dark adaptation that night. Not sure what the absorption
is for the photogray lenses.

There are many flavors of photograys, but the ones I've had never gave me
the sensation of full protection. I later performed some experiments with UV
lamps and found out that the most common photogray tint doesn't absorb
longwave UV.

Photograys seem to just reduce the intensity on the visible uniformly, but
if they are uncoated and made of glass the let a lot of UV pass through. In
particular the mercury 365nm triplet partially passes through. This can be
easily demonstrated by putting a glass photogray lens against a cotton
shirt. If you illuminate it with a blacklight lamp, the cotton behind the
lens will still fluoresce. This is an indication that the 365nm line
responsible for the UV fluorescence goes through the glass.

I am not sure how UV protection affects nightvision though, but it sure
sounds a good idea to have all UV blocked during daytime.

Chris L Peterson
--
Ioannis

.