At 06:06 20 February 2011, 2G wrote:
On Feb 13, 8:29=A0pm, brianDG303 wrote:
I've posted this before and hope I'm not boring everyone.

Using an excellent and expensive Crawford UV meter and testing a
whol=
e
line-up of gliders one day there was a distinct pattern of older
gliders passing UV and newer gliders not. =A0Eric, one of these
days
I'll be out at EPH the same time as you are and we can do some
research on the line-up out there, but we won't get wavelength
charts

Now we're talking evidence instead of anecdote! Can the meter
quantify
the difference in transmission?

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us"
to
email me)



“Plexiglas sheet absorbs the shorter wavelengths of
ultraviolet (UV) energy, but transmits most of the
longer wavelengths (those near the visible region),
Figure 5.The UV transmittance, unlike the visible
and infrared transmittance, of colorless Plexiglas sheet
varies from one formulation to another.Therefore,
Plexiglas MC sheet does transmit slightly more of the
longer UV wavelengths than does Plexiglas G sheet.
Even after prolonged outdoor or artificial light
exposure, the spectrophotometric characteristics of
Plexiglas sheet in the UV and visible ranges do not
change significantly. Most of the drop in UV transmittance
of Plexiglas sheet takes place in the first two
years after exposure to sunlight. No measurable change”

“Three Plexiglas sheet formulations that can filter
UV light are available. Plexiglas UF-4 sheet absorbs
most ultraviolet radiation. Plexiglas UF-3 sheet
absorbs virtually all ultraviolet radiation and some
of the visible light in the violet wavelength region.
Plexiglas UF-5 sheet, a new, special formulation in
the MC grade for framing applications, filters the
maximum of harmful UV radiation.”


From reading this material we are led to believe that most acrylic
Plexiglas does not transmit most UV light and does not degrade much itself
when exposed to sunlight.

Why then the high rate of UV transmission in older canopies? Was there a
different formula for the acrylic that was used to make canopies in the
70s and 80s? If so, when did the formula change?

Have these older canopies degraded over time due to exposure or were they
always unable to filter UV?

R5