Re: Why black plastic cameras heat up like ovens in the sun

On 7 Aug 2008 10:01:14 GMT, Chris Malcolm <cam@xxxxxxxxxxxxxxxxx> wrote:
: RichA <rander3127@xxxxxxxxx> wrote:
: > On Aug 6, 12:32?am, Cal I Fornicate <priv...@xxxxxxxx> wrote:
: >> RichA wrote:
: >> > They often hit temps 20 degs or more higher than metals.
: >> > IR absorbtion.
: >>
: >> I love this guy. His comments aren't just unbelievable, they are
: >> ludicrous and it is really obvious that he doesn't give any practical
: >> thought to what he says.
: >>
: >> Ok genius, why is it that my black plastic enclosed security camera that
: >> is bolted above the front door, in full sunlight, gets hot, but never to
: >> hot to touch, while the metal alarm bell bolted to the wall next to it
: >> get so hot that it can raise blisters if handled before being allowed to
: >> cool?
: >>
: >> Why is my black plastic lens hood always cooler than my all metal lens
: >> when both have spent time in hot sun?
:
: > Because you live in an alternate universe. Only certain types of
: > PAINT applied to metals will allow them to absorb IR.
:
: I seem to have lost track of which universe we're supposed to be
: discussing. Which universe is it in which metals don't absord IR
: unless painted with certain kinds of paint? I suspect I've never
: visited that one.

No, he's right about that. Painting a metal object black does cause it to
absorb more light and therefore heat up more. Of course it's the paint (the
less reflective the better) that does the actual conversion to heat, but that
heat is quickly transferred to the metal because of the latter's high
conductivity. Now I suppose it may be the case that not all paint that's black
to visible light is black to IR. But visible light carries energy too
(sunlight carries quite a bit of it), and that energy will heat the object if
it isn't reflected away. That's even true of UV, BTW. If you use a paint that
doesn't reflect UV (don't put any sunscreen in it!), the UV will heat the
object too.

Bob
.

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