>So whats thermal lenses and whats the advantages???

I've been playing several years and can get along just fine with
standard single-pane lenses, but unfortunately, I'm in the minority.

Most people that play paintball find their lens fogging up now and
again, usually on days of high humidity, (or light rain) and when
they've been running a lot and then stop to rest.  I've seen people
just have slight problems, (just a tad foggy) all the way to complete
white-outs. (you can't see their face inside the mask!)  If it gets
bad like that, you're OUT.  You can't be taking your mask off to wipe
away the fog, (UNSAFE!) and sometimes you just have to plop down
on the ground and wait for the game to end so you can pull off the mask
and see where you're going.  If there's a ref on the field though,
they'll lead you back to camp usually.

There are several ways to avoid fogging.  You can spend about $15
extra on a "thermal lens", which is a dual-pane setup sort of like
on your house's windows.  Fogging is caused by too much of a temperature
difference between the sides of the lens, (plus some humidity) and if
you have that "air space" between the lenses, it cuts the temp difference
in half for each pane. (plus there is no humidity in the sealed area
between the panes, so they can't fog internally)  The inner pane is
usually also coated with a thin layer of clear rubber, which helps
prevent fogging, but is very fragile and scratches very easily.

Second is to go buy some no-fog.  It's a liquid you squirt on the
inside of a standard lens.  It's a quick-dry crystalline compound of
some sort, and basically glazes the inside of the lens with a coating
that doesn't like water, which usually prevents fogging.  You usually
have to re-apply this every other game or so, and the layer it puts
on the inside is not 100% clear, so you lose just a little bit of your
vision, but it DOES work.  I keep some on-hand for the uncoated
lenses I bring with me to loan-out. (MY mask has a dual-pane, and you
NEVER put this stuff on a coated lens!)

Third is a fan.  They're these little CPU fans mounted to the top of
the mask.  The idea here is to keep the air circulating through the
inside of the mask, keeping the temperature inside the same as
outside, thus preventing the fog.  I haven't seen one lately, but I
remember them being very quiet, and running on a single 9v battery.
One battery lasts several days.  They are very effective, but don't
like loose hair!   *ouch*

The other problem is if you wear glasses.  I've loaned out my spare
goggles, treated with no-fog, to a guy wearing glasses that has a
VERY high metabolism - he's the type that leaves his bedroom window
open at night IN THE WINTER because that's his kind of sleeping
temperature. (the human torch?)  Anyway, his glasses fogged in nothing
flat.  No good to see thru your goggles if you can't see through
your glasses!  I suppose you could use no-fog on them, but that stuff
leaves behind that nasty crystalline film, and you'd have to use
alcohol to remove it, (which most plastic/glass lenses are OK with)
but it's still an added hassel.  In the case of glasses and big
fogging problems, a fan may be the best solution.  Otherwise, a
thermal lens is the best way to go.  No-fog is the cheap way out.