Temperature change in a metal due to heat from a distant animal?

[ChromeBit]

In bolometers, incoming radiation warms a strip of material. this material will have a large thermal coefficient of resistance, leading to a (small) resistance change in the material. the inventor, Samuel Pierpoint Langley apparently used this to detect a cow from 1/4 of a mile away using a bolometer with a platinum element.

I was curious as to how much of an increase in temperature the material would have, as surely most of the radiation would be absorbed by the air before it reached the element?

 

[haruspex]

In bolometers, incoming radiation warms a strip of material. this material will have a large thermal coefficient of resistance, leading to a (small) resistance change in the material. the inventor, Samuel Pierpoint Langley apparently used this to detect a cow from 1/4 of a mile away using a bolometer with a platinum element.

I was curious as to how much of an increase in temperature the material would have, as surely most of the radiation would be absorbed by the air before it reached the element?

Atmospheric absorption depends a lot on circumstance and the exact wavelength in question.
According to Fig 18-14 at http://www.cnofs.org/Handbook_of_Geophysics_1985/Chptr18.pdf, in a rural atmosphere at 50% RH, for a wavelength around 10μm the attenuation is only 1% per km.
Even at 99% RH, 18-15 has it at 10% per km.

 

[plasmon_shmasmon]

Just to be clear, it's not the "heat" from the animal, it's the light.

 

[haruspex]

Just to be clear, it's not the "heat" from the animal, it's the light.

@ChromeBit wrote "radiation", so I do not understand what point you are making. There is no reason why the visible light from the cow would have more intensity than that from the background, so we are are surely discussing infrared radiation.

 

[berkeman]

Samuel Pierpoint Langley apparently used this to detect a cow from 1/4 of a mile away using a bolometer with a platinum element.

Interesting. Do you have a link to a reference you can share? I'd be interested in reading about how he did this. Was it a cold winter night so the thermal background was very low? Did he use a parabolic mirror reflector to concentrate the IR radiation from the direction of the cow onto the detector?

 

[haruspex]

Interesting. Do you have a link to a reference you can share? I'd be interested in reading about how he did this. Was it a cold winter night so the thermal background was very low? Did he use a parabolic mirror reflector to concentrate the IR radiation from the direction of the cow onto the detector?

... and had he chased the cow across a field first?

 

[plasmon_shmasmon]

@ChromeBit wrote "radiation", so I do not understand what point you are making. There is no reason why the visible light from the cow would have more intensity than that from the background, so we are are surely discussing infrared radiation.

My bad, all. I posted before reading (or thinking) carefully.

So, I assume focusing through a refracting telescope would not have worked well? Is glass opaque in that wavelength range?

 

[berkeman]

My bad, all. I posted before reading (or thinking) carefully.

So, I assume focusing through a refracting telescope would not have worked well? Is glass opaque in that wavelength range?

Think "hot car" effect. Why does the interior of a car heat up in the sunshine?

 

[haruspex]

Think "hot car" effect. Why does the interior of a car heat up in the sunshine?

Careful with that... real greenhouses work more from inhibiting convection than from differential transmission of light frequencies by glass.

 

[berkeman]

Careful with that... real greenhouses work more from inhibiting convection than from differential transmission of light frequencies by glass.

True, but the OP's question is about glass. This is not a great reference, but is the first hit from a quick Google search:

https://climaterx.wordpress.com/2013/04/11/can-infrared-light-pass-through-glass/

Anyway, I'd use a parabolic first-surface mirror (FSM) if I wanted to concentrate IR onto a target. I'd probably have to save up my allowance for a couple of months to buy it though...

 

[haruspex]

the OP's question is about glass

Not the OP - that was ChromeBit.
Anyway, my point is that the hot car effect is also probably more to do with inhibiting convection than with the glass blocking IR.
There are many types of glass available, varying in transparency across the IR range.

 

[Tom.G]

Anyway, my point is that the hot car effect is also probably more to do with inhibiting convection than with the glass blocking IR.

Considering that a blackbody at 75F peaks around 10 microns, and common glasses are down 70% in transmittance beyond 4micron or so, I expect them to be a pretty good high-pass filter for radiant thermal energy.

An example would be the effectiveness of a Sun Shade placed in the car window. Living in Southern California I can attest to their effectiveness!

 

[haruspex]

Considering that a blackbody at 75F peaks 10 microns, and common glasses are down 70% in transmittance beyond 4micron or so, I expect them to be a pretty could high-pass filter for radiant thermal energy.

sure, but:
a) for relative importance of blocking convection need to compare:
- car with windscreens that do not block IR (e.g. Prof Wood's quartz sheets) with
- car with normal glass windscreen but convection active (side windows open a little, say)
b) the glass gets hot and reradiates in both directions, so 50% of what was blocked still escapes.

 

[russ_watters]

In bolometers, incoming radiation warms a strip of material. this material will have a large thermal coefficient of resistance, leading to a (small) resistance change in the material. the inventor, Samuel Pierpoint Langley apparently used this to detect a cow from 1/4 of a mile away using a bolometer with a platinum element.

I was curious as to how much of an increase in temperature the material would have, as surely most of the radiation would be absorbed by the air before it reached the element?

Did you google this? Or...the wikipedia article on the subject has the cow/invention example in it and then appears to me to answer your main question in the very next sentence. So I'm not sure why you didn't already find what you are asking for, particularly if that was your source. Can you clarify if you were more interested in the absorption of the air or sensitivity of the material and what you've already read?