"Ifff you go out in the sun today..." Better make sure your camera/lens is metal!

On Sat, 17 Jul 2010 07:33:17 -0700 (PDT), RichA wrote:
: Horrifying how black plastic attracts heat. If you shoot an IR image
: (or even use a bright flash) you can see why. Black plastic either
: painted or not absorbs IR hugely whereas the metal reflects a lot of
: it (it appears purple in photos).
:
: What is the downshot of a hotter camera body? Reduced lifespan and
: more noise in the images.
:
: http://www.pbase.com/andersonrm/meta...eat_absorption

Rich, did you sleep through high school physics? An object isn't black because
it absorbs IR; it's black because it absorbs all *other* wavelengths and
*radiates* in the IR band. That's why if you lay different-colored cloth
squares on snow, the black square sinks into the snow fastest and the white
square sinks hardly at all. Canada gets plenty of snow in the winter, so they
must have showed you that in the seventh grade. Did you play hooky that day?

It's summer in our half of the world, and North America is "enjoying" a heat
wave just now. Take a sheet of polycarbonate plastic and a sheet of
black-painted steel and lay them side by side in the sun at 9:00 AM. Wait
until 1:00 (EDT), when the sun is about at its highest and the two sheets have
gotten as hot as they're likely to get. Place one hand on each sheet and leave
them there for as long as you can stand. Which hand do you think will start to
smolder first? Right, the one touching the metal. That's because metal absorbs
heat, and gives it back, faster than plastic. So if you want to protect a
group of sensitive electronic objects from the heat, surround them with black
plastic, *not* with black metal.

We all know that metal has its place and that it's better than plastic for
some applications. There are many reasons for this, one being that metal tends
to fail more gracefully under certain kinds of mechanical stress. But your
constant carping about plastic, even in cases where its advantages are easily
demonstrable, has made you a laughingstock.

Bob

Robert Coe wrote:
On Sat, 17 Jul 2010 07:33:17 -0700 (PDT), RichA wrote:
: Horrifying how black plastic attracts heat. If you shoot an IR image
: (or even use a bright flash) you can see why. Black plastic either
: painted or not absorbs IR hugely whereas the metal reflects a lot of
: it (it appears purple in photos).
:
: What is the downshot of a hotter camera body? Reduced lifespan and
: more noise in the images.
:
: http://www.pbase.com/andersonrm/meta...eat_absorption

Rich, did you sleep through high school physics? An object isn't black because
it absorbs IR; it's black because it absorbs all *other* wavelengths and
*radiates* in the IR band. That's why if you lay different-colored cloth
squares on snow, the black square sinks into the snow fastest and the white
square sinks hardly at all. Canada gets plenty of snow in the winter, so they
must have showed you that in the seventh grade. Did you play hooky that day?

It's summer in our half of the world, and North America is "enjoying" a heat
wave just now. Take a sheet of polycarbonate plastic and a sheet of
black-painted steel and lay them side by side in the sun at 9:00 AM. Wait
until 1:00 (EDT), when the sun is about at its highest and the two sheets have
gotten as hot as they're likely to get. Place one hand on each sheet and leave
them there for as long as you can stand. Which hand do you think will start to
smolder first? Right, the one touching the metal. That's because metal absorbs
heat, and gives it back, faster than plastic. So if you want to protect a
group of sensitive electronic objects from the heat, surround them with black
plastic, *not* with black metal.

We all know that metal has its place and that it's better than plastic for
some applications. There are many reasons for this, one being that metal tends
to fail more gracefully under certain kinds of mechanical stress. But your
constant carping about plastic, even in cases where its advantages are easily
demonstrable, has made you a laughingstock.

I think- but am not about to test- that I'd prefer my hand on black
plastic in the sun for several hours vs. uncoated steel @1/4 inch or
more....big physics question. Also, black doesn't 'attract heat'.
Absorbs, yeah, but colors do not attract or repel the path of the sun's
rays. At least not measurably.

Anyway, time to brush with the stainless steel toothbrush...Ouch!


--
john mcwilliams

To be cool in the sun, you want an object to reflect in the visible
and near IR (out to say 2 or 3 microns) and be perfectly absorbing
(and hence, radiating) in the farther IR on out into the microwave.

The usual way to do this is to use a paint which is very white,
ideally with barium sulfate as the colorant, though the more
common anatase will do OK too.

To FEEL cooler if you touch it, in addition you want a lower
thermal conductivity ... metal is bad for this, plastic good.

To cool down faster if put in th shade, you want a lower
total heat capacity and higher themal conductivity.

Doug McDonald

Doug McDonald wrote:
To be cool in the sun, you want an object to reflect in the visible
and near IR (out to say 2 or 3 microns) and be perfectly absorbing
(and hence, radiating) in the farther IR on out into the microwave.

The usual way to do this is to use a paint which is very white,
ideally with barium sulfate as the colorant, though the more
common anatase will do OK too.

Hence the famous Canon white long lenses. Outnumbered the black lenses
at both the World Cup Footie matches and the US Open golf. Not so easy
to see with the Brit open.

To FEEL cooler if you touch it, in addition you want a lower
thermal conductivity ... metal is bad for this, plastic good.

To cool down faster if put in th shade, you want a lower
total heat capacity and higher themal conductivity.

The btu's retained by a slab of metal is important, too. A piece of
plastic with an external of 130Ëš F would quickly drop with a hand placed
on it; not so for metal, much more retained heat. Ow!

--
john mcwilliams

"Doug McDonald" wrote
...

To be cool in the sun, you want an object to reflect in the visible
and near IR (out to say 2 or 3 microns) and be perfectly absorbing
(and hence, radiating) in the farther IR on out into the microwave.

The usual way to do this is to use a paint which is very white,
ideally with barium sulfate as the colorant, though the more
common anatase will do OK too.

To FEEL cooler if you touch it, in addition you want a lower
thermal conductivity ... metal is bad for this, plastic good.

To cool down faster if put in th shade, you want a lower
total heat capacity and higher themal conductivity.

Doug McDonald


I was under the impression that titanium was the white colorant.

Robert Coe wrote in
:

On Sat, 17 Jul 2010 07:33:17 -0700 (PDT), RichA
wrote:
: Horrifying how black plastic attracts heat. If you shoot an IR image
: (or even use a bright flash) you can see why. Black plastic either
: painted or not absorbs IR hugely whereas the metal reflects a lot of
: it (it appears purple in photos).
:
: What is the downshot of a hotter camera body? Reduced lifespan and
: more noise in the images.
:
: http://www.pbase.com/andersonrm/meta...eat_absorption

Rich, did you sleep through high school physics? An object isn't black
because it absorbs IR; it's black because it absorbs all *other*
wavelengths and *radiates* in the IR band.

But the plastic and rubber absorb it MORE than anodized black metal, which
you can see if you fire a camera sensitive to IR at them. The metal will
appear purple, it will even be several shade lighter, not black, while the
plastic will still appear black because it is absorbing everything.

"Robert Coe" wrote in message
...

Rich, did you sleep through high school physics? An object isn't black
because
it absorbs IR; it's black because it absorbs all *other* wavelengths and
*radiates* in the IR band. That's why if you lay different-colored cloth
squares on snow, the black square sinks into the snow fastest and the
white
square sinks hardly at all. Canada gets plenty of snow in the winter, so
they
must have showed you that in the seventh grade. Did you play hooky that
day?



Uhm! Black is the absence of color.




--
Peter

On Sat, 17 Jul 2010 19:58:02 -0400, "Peter"
wrote:
: "Robert Coe" wrote in message
: ...
:
: Rich, did you sleep through high school physics? An object isn't black
: because it absorbs IR; it's black because it absorbs all *other* wavelengths
: and *radiates* in the IR band. That's why if you lay different-colored cloth
: squares on snow, the black square sinks into the snow fastest and the
: white square sinks hardly at all. Canada gets plenty of snow in the
: winter, so they must have showed you that in the seventh grade. Did you
: play hooky that day?
:
: Uhm! Black is the absence of color.

Which is another way of saying that a black object absorbs all visible
wavelengths (and reflects none). Which is what I said. Your point is ???

Bob

"Robert Coe" wrote in message
...
On Sat, 17 Jul 2010 19:58:02 -0400, "Peter"
wrote:
: "Robert Coe" wrote in message
: ...
:
: Rich, did you sleep through high school physics? An object isn't black
: because it absorbs IR; it's black because it absorbs all *other*
wavelengths
: and *radiates* in the IR band. That's why if you lay different-colored
cloth
: squares on snow, the black square sinks into the snow fastest and the
: white square sinks hardly at all. Canada gets plenty of snow in the
: winter, so they must have showed you that in the seventh grade. Did
you
: play hooky that day?
:
: Uhm! Black is the absence of color.

Which is another way of saying that a black object absorbs all visible
wavelengths (and reflects none). Which is what I said. Your point is ???


Since when does black radiate in any band?


--
Peter

On 7/17/2010 8:19 PM, Peter wrote:
"Robert Coe" wrote in message ...
On Sat, 17 Jul 2010 19:58:02 -0400, "Peter"
wrote:
: "Robert Coe" wrote in message
: ...
:
: Rich, did you sleep through high school physics? An object isn't black
: because it absorbs IR; it's black because it absorbs all *other* wavelengths
: and *radiates* in the IR band. That's why if you lay different-colored cloth
: squares on snow, the black square sinks into the snow fastest and the
: white square sinks hardly at all. Canada gets plenty of snow in the
: winter, so they must have showed you that in the seventh grade. Did you
: play hooky that day?
:
: Uhm! Black is the absence of color.

Which is another way of saying that a black object absorbs all visible
wavelengths (and reflects none). Which is what I said. Your point is ???


Since when does black radiate in any band?



Since the Big Bang.

Anything radiates if it is hot.

However, a "red hot" piece of silver or aluminum will not radiate
as much as a piece of tungsten at the same temperature, in the visible
region, because they are "whiter", that is, reflect more.

At room temperature the same thing applies, only they radiate
around 10 microns in the IR. This is really true and is why,
for example, the old-fashioned glass Thermos bottles are
silvered on the inside.

The thermodynamic proof of this is one of the classic examples of
physics, and is the obvious (to a physicist) fact that led Hawking
to his Nobel Prize: Black Holes are black, therefore the MUST
radiate.

Doug McDonald