Werehatrack <
[email protected]> wrote:
> On 16 Aug 2004 09:09:43 -0700, [email protected] (wle) wrote:
> >i tried to measure the SPF with my camera.
>
> Won't work; the reasons take a bit of explaining. With an incident
> light meter, you could get a better estimate...but it would still be
> wrong, as the visible spectrum dominates in that instance.
>
> >i discovered they cut the [visible] light by
> > about 1 f stop, which as i understand SPF would be about SPF 1.
In the name of science, I did this experiment, outside in the sun
with an incident light meter pointing up. A white cotton t-shirt
reduced the reading by 1 f-stop. A black cotton t-shirt reduced
the reading by 4.5 f-stops. Both were typical Hanes-type shirts.
However, it would be grossly incorrect to assume that the black
t-shirt is that much (11 times) better at protecting you from the sun.
It's obvious that most of the light hitting the meter is scattered
or reradiated from the white t-shirt. It is actually pretty difficult
to make materials that transmit well in the UV. Most materials,
other than some metals and mirror coatings, don't reflect well in
the UV either; they like to absorb UV. (UV-transmitting optics have
to be made from special glass. This is one reason why even cheap
sunglasses protect your eyes from UV light.) I am sure that the
light meter, measuring visible light, isn't telling us much that is
useful about the SPF value of a t-shirt.
My opinion (that the experiment is meaningless) is driven by experience:
t-shirts and jerseys do very well at protecting from sunburn, at least
as well as sunscreen. If I wear something that doesn't quite cover up
to the tan line, I get a vicious sunburn but only up to the edge of the
clothing. This doesn't appear to correlate with color or material
of the clothing.
> That's not correct. The average white T-shirt has an SPF of 5 to 9.
>
> See http://www.physsportsmed.com/issues/1997/05may/potera.htm
>
> Additionally, some T-shirts are treated with UV-active agents to
> brighten them; these block slightly more of the UV as a result. Some
> laundry detergents leave a small amount of such brighteners behind as
> well.
>
> >[the published data allegedly refers to UV light.]
> >
> >so then i started thinking about my arms.
> >they are tan below the t shirt line, much whiter above.
> >yet i wear sun screen every day, SPF 30+.
>
> Yup. Funny how that works, eh?
>
> >yet - here is the question -
> >if the t shirt really has such a low SPF, why does it appear to do much
> >more screening than the sun screen?
>
> It screens *everything*. UVA and UVB cause sunburn and skin damage,
> but tanning can occur without them in many individuals. Shade, which
> is what a shirt provides, supresses all melanin production triggers.
>
> >the t shirts are clearly screening more than the sunscreen.
> >but how?
>
> By being moderately opaque.
[reformatted]
> >since i ride about 2 hours a day, all year, that;s all i wear in the summer.
> >[don;t want to have 7-10 bike jerseys at $xx a pop.]
> You're not alone, in case you haven't noticed.
BTW, if this is a real issue you can buy cotton-poly blend t-shirts. Or
get some soccer jerseys at the thrift store. Both will get less soaked
with sweat. Anyway, bike jerseys can be washed in the shower and dry
in a day, so you don't need 7-10 of them.
Actually, it's because they're $5 for 7 shirts at wal-mart
