Ironing Out the Differences

[bvf]

When I was a young lad I spent some of my scarce pocket money on Kodak Infrared film (B&W, not the Ektachrome stuff - I never got to that level). In reading around the subject I came across a technical publication (may actually have been an Eastman one) which included a photograph of a clothes iron photographed in IR and glowing brightly. I tried to emulate this, but did not succeed.

 

So I thought I'd try again with my current gear, and got some success. The problem is that the exposures are long (16 minutes, f/1.8, ISO 800) and I couldn't get a dark enough environment - even though I had the iron in what my wife called a coffin. So there was some reflected light pollution. Another problem was that the iron has a safety cut-out to turn itself off if it hasn't been moved for a while, and as I had masked the indicator light I wasn't sure whether the iron was actually on throughout the course of the exposure.

 

But here are the results - camera was a full-spectrum Sony A6000, lens a Canon EF 50mm f/1.8 ...

 

1. Visual Reference Image.

 

 

2. IR image, using a Midwest Optical LP1000 filter. The iron was turned on.

 

 

3. Same as (2), but with the iron turned off.

 

 

4. Difference between images 2 and 3, to bring out the effect of the hot iron.

 

 

5. Image 4 with post-processing:

 

[Stefano]

Interesting. Wonder why they got bright images with film. Maybe irons back then ran hotter?

[microbat52]

I would have thought you need to get to the 400-500C to see emitted NIR?

[Stefano]

I would have thought you need to get to the 400-500C to see emitted NIR?

The lower limit is about 300°C: https://www.ultravioletphotography.com/content/index.php/topic/2928-drones/page__view__findpost__p__25475

[Andy Perrin]

You don’t need to go all the way to the Wein’s law temperature to see some light. Even if you aren’t right at the peak emission you can get enough to be detectable in a photo.

[ulf]

The lower limit is about 300°C: https://www.ultravio...dpost__p__25475

When taking those soldering iron images I did not push the limit as Bernhard has done above.

They were not taken in a pitch black room with high ISO and very long exposure times.

If I remember correctly my images were taken hand held.

[bvf]

Interesting. Wonder why they got bright images with film. Maybe irons back then ran hotter?

 

Well, one thing occurs to me. If you look at my imge you will see the bright spots. Here we are looking through the steam holes to another plate, which is presumably a lot hotter than the surface used for ironing. The old photo I saw may have been an old-fashioned dry iron without a steam machanism, and so perhaps the ironing surface was indeed hotter than on a modern iron - more like the hidden plate in my image.

 

But on the other hand ... according to https://www.digitaltruth.com/products/product_tests/infrared_film_010.php Kodak High Speed Infrared film could reach only to about 950nm, whereas with a full-spectrum camera you can record up to about 1100nm. So the film would have needed a higher temperature to record an image.

 

I wish I could find that old photo again, but it doesn't seem to be on the interweb.

[Andy Perrin]

I think you are just not accounting for the fact that the surface emissivity matters a LOT for thermography. A steel surface is different emissivity from a cast iron surface which is different from aluminum. Water has an emissivity of 0.95 while aluminum is 0.1. In the holes is probably water. Bernard, if you want you could try a stove burner. Those will have a higher emissivity than polished steel.

[Stefano]

Does water have high emissivity in the near infrared? Emissivity is not constant for all wavelengths.

 

Also the holes may have a high emissivity since they are a close approximation to a blackbody.

 

 

[bvf]

Water is irrelevant - the iron was dry.

 

But you are right that the materials in the ironing surface (polished metal, steel or some other) are almost certainly different from what was behind the holes (quite possibly cast iron). If my scientific memory serves me correctly, shiny metal surfaces don't radiate very well.

[Stefano]

The darker a material is, the better it radiated blackbody radiation. A small hole is often used to approximate a true blackbody, you can treat it as if it was a black piece of some material, although it is literally a hole. Shiny metals (such as tinfoil and spoons) appear very similar in LWIR as they do in visible light.

 

Example:

[Andy Perrin]

The darker a material is, the better it radiated blackbody radiation.

 

Stefano, that’s incorrect. Darker materials in visible light have nothing to do with emissivity at other wavelengths that are significantly different. Really stark example is that the emissivity of fresh snow is almost 1 in LWIR while it is white in visible light. As we know, it shifts a lot as you go visible-> infrared.

 

Emissivity = 1-reflectivity-transmissivity = absorbtivity. We know very well that water is dark in near infrared so it has high absorption and the reflectivity at the surface is a few percent. Conclusion is that water has high emissivity in near infrared.

[Stefano]

Yes, you are right. Emissivity is not constant also because absorption is not constant. I think it has to do with the amount of energy transitions electrons can do, or something like that. If a material has a lot of those possible states it can absorb a lot of different photons (making it dark) but also emit them.

 

Another nice example is that water is dark (or glowing) in LWIR, but liquid nitrogen, although being much colder, is almost transparent, and appears similar in LWIR to water in visible light.

[SteveE]

A long time ago I read in Popular Science or the like that bolting a stack of old style uncoated double edged razor blades together had been found to approximate a black body radiator. Somehow this was due to the fact that the cross section of the blade where it narrows down from the full thickness to the cutting edge was curved, thus causing photons to bounce back and forth many times in the grooves formed between the blades.

[Andy Perrin]

SteveE, that’s like the cavity example Stefano mentioned upthread. It makes sense to me — there is a distinction between surfaces with very high emissivity (which are often called black bodies) and cavities, which produce black body radiation by trapping photons in the cavity. Different mechanisms with same end results. Your example sounds like it’s in-between!

[SteveE]

Now that the second hamster has woken up, I seem to be able to recall that the curved edge causes incident photons to reflect deeper into the groove with each bounce, so i guess it would function mostly as a cavity.

[microbat52]

You don’t need to go all the way to the Wein’s law temperature to see some light. Even if you aren’t right at the peak emission you can get enough to be detectable in a photo.

 

I thought what Bernard was trying to reproduce was "glowing brightly" though?

[dabateman]

Now that the second hamster has woken up, I seem to be able to recall that the curved edge causes incident photons to reflect deeper into the groove with each bounce, so i guess it would function mostly as a cavity.

 

Your lucky, I think my hamster lost the wheel and just fell asleep.

 

Bernard,

Just set your oven to really hot. I am in USA so can't remember the C equivalent of 450F, but that might do. Open the door, stand back and snap some images of the bottom heater coil. If you have gas, then try the top coil used for browning.

 

[Stefano]

The coils are much hotter than the air inside the oven. I have a small one (the portable kind) and the resistive elements glow red-hot. The Draper point is around 525°C, so those elements are probably at 550-575°C. The oven itself may reach 220-240, not more.

[Andy Perrin]

 

 

I thought what Bernard was trying to reproduce was "glowing brightly" though?

Glowing brightly in the photo is just a question of exposure settings, not how bright the light is in absolute terms. It just has to be detectable above the background noise.

 

At 400C, the peak would be at 4000nm or so but the light can be detected in near infrared for sure. That’s partly because our cameras are quite good.

 

They do make MWIR cameras that work in that range. InSb sensors.

[bvf]

Bernard,

Just set your oven to really hot. I am in USA so can't remember the C equivalent of 450F, but that might do. Open the door, stand back and snap some images of the bottom heater coil. If you have gas, then try the top coil used for browning.

 

In our oven (fan-assisted electric) the heating elements are hidden. In the past I have photographed the grill and of course got a bright element in IR (but you could see it with the naked eye as well, of course). The point about trying an iron was (a) to re-capture my youth, and (b) see how low in temperature I could record something - assuming that an iron is at a lower temperature than a grill.