"Ifff you go out in the sun today..." Better make sure your camera/lens is metal!
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From: Robert Coe on 17 Jul 2010 13:47 On Sat, 17 Jul 2010 07:33:17 -0700 (PDT), RichA <rander3127(a)gmail.com> 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/metal_versus_plastic_heat_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
From: charles on 17 Jul 2010 16:11 On Sat, 17 Jul 2010 07:33:17 -0700 (PDT), RichA <rander3127(a)gmail.com> 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/metal_versus_plastic_heat_absorption Some temperatures I measured with a non-contact IR thermometer. Canon kit lens ( the old bad one) 128-140 depending on which part of the lens was measured. vivitar metal lens from late 1970'2 117.6 Lenses were left in the So Cal sun for about an hour, warm day, about 86 now Dark piece of iron, 117.5 Light colored iron (galvanized?) 108.5 White automobile 112.4 Dark automobile 147.9 Iron and autos have been in the sun all day, presumably at equilibration. All temps in Fahrenheit, we don't have Celsius in California.
From: G Paleologopoulos on 17 Jul 2010 16:49 "Doug McDonald" <mcdonald(a)scs.uiuc.edu.remove.invalid> wrote news:i1ssjb$oud$1(a)news.acm.uiuc.edu... > > 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.
From: Rich on 17 Jul 2010 17:28 Robert Coe <bob(a)1776.COM> wrote in news:98p346hl4t9752r9774j6db5ioq0b1brjf(a)4ax.com: > On Sat, 17 Jul 2010 07:33:17 -0700 (PDT), RichA <rander3127(a)gmail.com> > 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/metal_versus_plastic_heat_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.
From: Peter on 17 Jul 2010 19:58
"Robert Coe" <bob(a)1776.COM> wrote in message news:98p346hl4t9752r9774j6db5ioq0b1brjf(a)4ax.com... > 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 |