"Ifff you go out in the sun today..." Better make sure your camera/lens is metal!
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From: RichA on 17 Jul 2010 10:33 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
From: John McWilliams on 17 Jul 2010 14:06 Robert Coe wrote: > 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. 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
From: Doug McDonald on 17 Jul 2010 14:29 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
From: John McWilliams on 17 Jul 2010 15:12 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 130Ë F would quickly drop with a hand placed on it; not so for metal, much more retained heat. Ow! -- john mcwilliams
From: John McWilliams on 17 Jul 2010 17:38
charles wrote: > 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. Well, he just posted in another thread, which i just saw, and I am in CA, therefor, we occasionally have C in CA..... Are there any conclusions you wish to draw from your tests? -- john mcwilliams |