At what temperature objects start to glow?

[raxo2222]

Lets say we have 1x1 km black radiator in planet shadow.

It warms uniformly at 1 K/s starting at 200 K, when it would be visible?

that its edges would have different color.

Edited December 29, 2017 by raxo2222

[tomf]

Wikipedia says 798K

https://en.wikipedia.org/wiki/Draper_point

Edit:

Which is weirdly specific because as Zeiss says below it is going to depend on the quality of your eyes and the darkness of your surroundings.

Edited December 28, 2017 by tomf

[Zeiss Ikon]

I'm used to thinking in Fahrenheit (I blame bad upbringing), on a real world experience basis.  In a blacksmith's forge, the iron or steel (not a perfect black body, but a reasonable real-material approximation) begins to give off a barely detectable glow (in a lit smithy -- much dimmer than outdoors, but bright enough to read small print easily) at around 850 F (for verification, molten aluminum can be cool enough to have no glow, but easily gets hot enough to glow, even to cherry red for some casting operations).

That converts to: (850-32)*5/9 = 454 C, plus 273 = 727 K.  I'd call that entirely consistent with @tomf's answer from Wikipedia (especially given there's some variation in what you can see as a "glow" depending on the lighting and the condition of your eyes).

[KG3]

10 hours ago, Zeiss Ikon said:

  In a blacksmith's forge, the iron or steel (not a perfect black body, but a reasonable real-material approximation) begins to give off a barely detectable glow (in a lit smithy -- much dimmer than outdoors, but bright enough to read small print easily) at around 850 F 

I am a blacksmith.  Rule of thumb around a forge is hot steel looks allot like cold steel.  A horseshoe at a gray heat is still hot enough to take the skin off of you fingers.  Also, I can't read small print easily at any temperature.

[Chakat Firepaw]

12 hours ago, tomf said:

Wikipedia says 798K

https://en.wikipedia.org/wiki/Draper_point

Edit:

Which is weirdly specific because as Zeiss says below it is going to depend on the quality of your eyes and the darkness of your surroundings.

The temperature is specific because that's when a blackbody begins to radiate in wavelengths considered "red" rather than "infra-red".  The exact dividing point between the two bands is a bit arbitrary because the longest wavelength that can be seen varies between people, (some can see a little bit into the infra-red, some can't see a little bit of the red part of the visible spectrum).

[magnemoe]

17 minutes ago, Chakat Firepaw said:

The temperature is specific because that's when a blackbody begins to radiate in wavelengths considered "red" rather than "infra-red".  The exact dividing point between the two bands is a bit arbitrary because the longest wavelength that can be seen varies between people, (some can see a little bit into the infra-red, some can't see a little bit of the red part of the visible spectrum).

Its also an issue with light level, been in cabins with no electricity but it has an wood stove, the stove looks weird if all light is off as some parts start to glow very weak. 
Same in army tents with an wood stove, here the stove is just sheet metal so it start glowing far easier, if cold the stove should glow well enough for you to dress in, if you you complained to the guy manning it

[Chakat Firepaw]

1 minute ago, magnemoe said:

Its also an issue with light level, been in cabins with no electricity but it has an wood stove, the stove looks weird if all light is off as some parts start to glow very weak. 
Same in army tents with an wood stove, here the stove is just sheet metal so it start glowing far easier, if cold the stove should glow well enough for you to dress in, if you you complained to the guy manning it

I was talking about why that temperature was specific:  It's about when the light emitted starts including the visible band, not when there are enough photons to see.

[Zeiss Ikon]

1 hour ago, Chakat Firepaw said:

I was talking about why that temperature was specific:  It's about when the light emitted starts including the visible band, not when there are enough photons to see.

The emission bell curve includes some visible light even a hundred degrees or more colder -- but unless the surroundings are very dark, you won't be able to see it (even if you're one of those who can see deeper into infrared than most).  Even if they are, until the light emitted within the wavelength sensitivity range of your eye's rods and red cones is above the sensitivity threshold for your particular eyes, you won't see anything.

Hence why there's some variability in the temperature at which you can see an object start to glow.  The Draper Point was established as the point at which nearly everyone can see a faint glow in a room light enough to work in.

BTW, @KG3, I agree, steel that looks cold can still be around 800 F -- which is hot enough to melt any sort of tin-lead solder, even pure lead. Not quite enough for aluminum -- I just looked it up and pure aluminum melts at 1221 F.  Yet, I have seen aluminum poured and it looked silvery, with no perceptible red (I presume it was an alloy with a lower melting point, or the bright fluorescent room light covered the glow).

When i learned to weld, we were taught this, and taught to put any heated pieces (welded, cut, or hot bent, for instance) that hadn't been quenched on the steel work bench, draw a circle around them with a soapstone, and letter "HOT" beside the circle, to avoid surprises.

[KG3]

1 hour ago, Zeiss Ikon said:

 

BTW, @KG3, I agree, steel that looks cold can still be around 800 F -- which is hot enough to melt any sort of tin-lead solder, even pure lead. Not quite enough for aluminum -- I just looked it up and pure aluminum melts at 1221 F.  Yet, I have seen aluminum poured and it looked silvery, with no perceptible red (I presume it was an alloy with a lower melting point, or the bright fluorescent room light covered the glow).

 

I've tried making horseshoes out of aluminum bar stock.  Tricky stuff.  A propane gas forge runs about 1800 F.  You put the bar stock in and count to about 15 then pull it out and check to see if you can work it.  It's so easy to over heat the stuff.  It goes from workable to a puddle in seconds.  I've never seen any color (or glow) on it.  Maybe in a very dark room? 

[Zeiss Ikon]

10 hours ago, KG3 said:

I've tried making horseshoes out of aluminum bar stock.  Tricky stuff.  A propane gas forge runs about 1800 F.  You put the bar stock in and count to about 15 then pull it out and check to see if you can work it.  It's so easy to over heat the stuff.  It goes from workable to a puddle in seconds.  I've never seen any color (or glow) on it.  Maybe in a very dark room? 

Well, again, alloying matters.  Pure lead melts at 700+ F, but the alloy I use for bullet casting (lead, antimony, and a little tin -- both antimony and tin melt higher than lead) melts around 600 F.  Most aluminum used for things other than electrical wire  contains some  copper, and often a little silicon.  Copper melts at the verge of orange glow, pure aluminum (at 1221 F) would be a nice cherry red -- but the alloy might well melt below 900 F, which would be cool enough you might not see the glow in a well lit work area, certainly not against the yellow glow of the forge interior.

Just looked it up -- 6061 has a solidus (lower end of the partial melt range) of 582 C, still above the Draper Point, but close enough to it that lighting matters very much.  Especially if you're used to heating iron or steel to yellow to give a little more working time, you might not notice the very faint glow at that temp -- and that's where it transitions to "puddle," even though the puddle would be less than fully fluid.  And that 6061 melts about 80 C lower than pure aluminum, even though it's 96% or so aluminum and contains less than 1% of any single alloying element (biggest is magnesium at up to 1.25%, then copper no more than 0.4%, then zinc, not to exceed 0.25%).  I found references to other aluminum alloys that have liquidus (fully melted) below 500 C, which is decidedly below the Draper Point.

Edit: just occurred to me there's another factor in "glow" temperature -- emissivity.  A higher emissivity will glow at a lower temperature, and generally more "silvery" a metal is, the lower its emissivity (this is why gold -- with a very low emissivity in IR -- is used as insulation against radiation cooling for stuff going to space, like the LEM descent stage).  A perfect black body has the highest possible emissivity, a perfect reflector (and an aluminum coating is close-ish, hence its use on telescope mirrors) will have the lowest possible.

Edited December 29, 2017 by Zeiss Ikon

[PB666]

On 12/28/2017 at 4:44 AM, raxo2222 said:

Lets say we have 1x1 km black radiator in planet shadow.

It warms uniformly at 1 K/s starting at 200 K, when it would be visible?

that its edges would have different color.

Every object in the universe glows (even a black hole emits hawking radiation). The question you are asking is when to the 'red' cones and rods detect the glow, look up an anatomy angle.

Here is something you may not know.  https://en.wikipedia.org/wiki/Matter_wave, Fortunately most of the particles in the universe do not act as singularities. Thus you can ignore this hypothesis in the glow equation

But one object that left a remnant glow is the Universe

The CMB has a thermal black body spectrum at a temperature of 2.725 which means we all glow at a temperature of at least 2.725 unless there has been the application of energy to remove that glow (as in insulation and laser refridgeration systems).

[KG3]

23 minutes ago, Zeiss Ikon said:

 

Just looked it up -- 6061 has a solidus (lower end of the partial melt range) of 582 C, still above the Draper Point, but close enough to it that lighting matters very much.  Especially if you're used to heating iron or steel to yellow to give a little more working time, you might not notice the very faint glow at that temp -- and that's where it transitions to "puddle," even though the puddle would be less than fully fluid. 

This is true, the aluminum that I've had the misfortune to over heat kind of just turned into a mealy mush and lost it's structural integrity when cooled.  I believe all of the aluminum horseshoes I've come across are cast not forged.  It seems like all the different manufacturers have their own secret recipe as far as the alloy goes.  We tend to work these shoes cold because heating them seems to ruin whatever tempering they do at the factory and because it's not too hard to just smack them into shape cold with a hammer. 

[Zeiss Ikon]

7 minutes ago, KG3 said:

This is true, the aluminum that I've had the misfortune to over heat kind of just turned into a mealy mush and lost it's structural integrity when cooled.  I believe all of the aluminum horseshoes I've come across are cast not forged.  It seems like all the different manufacturers have their own secret recipe as far as the alloy goes.  We tend to work these shoes cold because heating them seems to ruin whatever tempering they do at the factory and because it's not too hard to just smack them into shape cold with a hammer. 

I suspect most horseshoes are a ZA alloy.  These are zinc-aluminum-copper die casting alloys with solidus well below the Draper Point, and they don't take well to being partially remelted (hence why much/most die cast metal is just thrown away when it fails -- can't braze or solder it, can't weld it).  These are also sometimes call Zamack.  The alloys are similar in strength to cast iron, but lighter, and a bit less brittle.  You can do limited cold working on thick enough pieces of ZA (especially the versions with higher aluminum content), and they're cheap to produce (both zinc and aluminum are cheapish, and the low temperature and ability to die cast cut production costs).  You get a shoe that costs less than a premade steel one, weighs a little less, and wears about as well.  Also won't spark on rocks or pavement, a good thing if you ride in tinder-dry conditions.

[PB666]

8 minutes ago, KG3 said:

This is true, the aluminum that I've had the misfortune to over heat kind of just turned into a mealy mush and lost it's structural integrity when cooled.  I believe all of the aluminum horseshoes I've come across are cast not forged.  It seems like all the different manufacturers have their own secret recipe as far as the alloy goes.  We tend to work these shoes cold because heating them seems to ruin whatever tempering they do at the factory and because it's not too hard to just smack them into shape cold with a hammer. 

That's because thin sheets of aluminum oxidize before they melt. I have had the same thing happen. If you a thin foil hot enough it will literally burn in the fire and leave nothing but a grey/black residue.

I have worked a fair amount of aluminum, you can't really melt it, you have to get it hot and turn it firing the long edge more than the short edge.

The other thing about aluminum is it likes to curl, im not sure what the physics is but it like to curl at a perpendicular to the axis that you are working so its good to have a jig that holds it strait.

15 hours ago, Chakat Firepaw said:

The temperature is specific because that's when a blackbody begins to radiate in wavelengths considered "red" rather than "infra-red".  The exact dividing point between the two bands is a bit arbitrary because the longest wavelength that can be seen varies between people, (some can see a little bit into the infra-red, some can't see a little bit of the red part of the visible spectrum).

if its infrared you cannot see it, if its red you can. Its not physics of the universe, its the physics of the eye. To a blind person everything that produces heat is infrared.

[kerbiloid]

Somebody should mention.
https://en.wikipedia.org/wiki/Incandescence

Spoiler

Spoiler

 

 

Edited December 29, 2017 by kerbiloid

[PB666]

can the OP or moderator please fix the typo in the title.

[Zeiss Ikon]

10 hours ago, PB666 said:

if its infrared you cannot see it, if its red you can. Its not physics of the universe, its the physics of the eye. To a blind person everything that produces heat is infrared.

There's an accepted definition of the demarcation between infrared and visible deep red -- a specific wavelength of light (700 nm) that's considered "visible" and anything longer is "infrared."

It's actually pretty easy to demonstrate, however that most people can see a little into the defined infrared.  It's pretty easy to make an IR-pass filter that looks black to the eye in a lit room (several layers each of specific colors of filter gel sheet will do it).  Install those in a pair of goggles with a good seal to your face, however, and wear them for a while, and you'll discover that you can actually see through them, especially in bright incandescent light or direct sunlight.  Human eye sensitivity extends down to around 750 nm in most individuals, 780 or even a little lower for some.  The sensitivity is very low in this region, however, so you need help to be able to see this light without it being covered by other light more to your eye's liking.

A pair of these goggles will block out the "visible" light that would normally overwhelm this (rather low) sensitivity, and allow your eyes to adjust to the dimness (like standing in a darkroom for a few minutes, or waiting under a starry sky until you can see to walk by starlight alone).  You'll then see only by light that's normally considered invisible -- and you'll see things that surprise you.  Black plastics (some of them) become transparent, but will show black marker that would be invisible black-on-black by "visible" light.  If it's summer, you'll see the Wood effect (foliage appearing bright against a black sky) originally discovered with infrared sensitive photographic film.  Some kinds of clothing become translucent, too.

All this to say, the border between IR and visible is pretty fuzzy.  We normally consider near IR to be invisible, because if there's any significant amount of other light visible, it'll overwhelm even strong IR.  In fact, however the ambient lighting has a strong effect on how deep into the red/IR spectrum you can see.  Some IR LEDs are actually visible if your eyes are sufficiently dark adapted.  Try it with your TV remote some time (some of them use a 750 nm LED, which is within the range of most humans if their eyes are fully dark adapted).

 

[PB666]

And there are probably genetic variants in humans that can see lower in the IR than everyone else. I don't think that I have ever been in a completely dark room, but I do know that our eyes can see what is not there

like showing three different colors on a screen and then switching to light we see the opposite colors in those positions. The problem with seeing infrared is that our bodies emit in the infrared part of the spectrum
37'C = 310K and using Weins law is 9343 nm which is between 5000000nm and 700 nm (actually closer to deep red than to MW radiation). And if our rods could see it, since we are continually exposed to a low level of light our brains would just subtract it out. Rembmer that this lambda is the center of the blackbody profile, its not the shortest wavelength that we produce, that at 1/10th the amplitude you have wavelengths of 2700 at 1/100th you are approaching that limit. 

where lambda peak is the wavelength. This is Wein's law. http://hyperphysics.phy-astr.gsu.edu/hbase/wien.html

To state this in another way our bodies potentially could detect IR hv but there many reasons that it would not and should not.

1. As the threshold lamba of the cones approached our bodies IR shortest wavelength threshold the signal from the body would increase, eventually is would saturate all the receptors and red from other sources would not be visible. We would walk around all the time as if we were blinded by an internal sun. We have to recall that our eyes only let in some of the light that hits them and that the aperture limits the amount of light crossing the retina to about a 10th the flux through the aperature per unit area. The infrared source of the body lies against the rods, it would be like staring into a flashlight. Eventually, in childhood, I think the body would simple mask all red receptors.

2. The light that comes from the back of the retina could not be focused, which means that the brains ability to interpret structure or patterns would not be focused. This could cause problems for the development of a childs visual cortex.

3. The amount of energy in an hv photon of lower frequency does not as readily do the type of chemistry required to create excited states that upon relaxation can interrupt the neurological process.

[KG3]

11 minutes ago, PB666 said:

.

1. As the threshold lamba of the cones approached our bodies IR shortest wavelength threshold the signal from the body would increase, eventually is would saturate all the receptors and red from other sources would not be visible. We would walk around all the time as if we were blinded by an internal sun. We have to recall that our eyes only let in some of the light that hits them and that the aperture limits the amount of light crossing the retina to about a 10th the flux through the aperature per unit area. The infrared source of the body lies against the rods, it would be like staring into a flashlight. Eventually, in childhood, I think the body would simple mask all red receptors.

2. The light that comes from the back of the retina could not be focused, which means that the brains ability to interpret structure or patterns would not be focused. This could cause problems for the development of a childs visual cortex.

3. The amount of energy in an hv photon of lower frequency does not as readily do the type of chemistry required to create excited states that upon relaxation can interrupt the neurological process.

 

I never thought of that.  Pit Vipers can see IR.  They can't see IR with their eyes, they use a completely different organ.

https://en.wikipedia.org/wiki/Loreal_pit

The loreal pit is the deep depression, or fossa, in the loreal area on either side of the head in crotaline snakes (pitvipers). It is located behind the nostril and in front of the eye, but below the line that runs between the centers of each. It is the external opening to an extremely sensitive infrared detecting organ. The loreal pit is bordered by lacunal scales.The loreal pit is thermal regulating system. Pitvipers maintain their temperature of body through loreal pit.

 

Good thing they are cold blooded.  They need to be colder than their prey or they would blind themselves. 

[PB666]

1 minute ago, KG3 said:

 

I never thought of that.  Pit Vipers can see IR.  They can't see IR with their eyes, they use a completely different organ.

https://en.wikipedia.org/wiki/Loreal_pit

The loreal pit is the deep depression, or fossa, in the loreal area on either side of the head in crotaline snakes (pitvipers). It is located behind the nostril and in front of the eye, but below the line that runs between the centers of each. It is the external opening to an extremely sensitive infrared detecting organ. The loreal pit is bordered by lacunal scales.The loreal pit is thermal regulating system. Pitvipers maintain their temperature of body through loreal pit.

 

Good thing they are cold blooded.  They need to be colder than their prey or they would blind themselves. 

Pit vipers are so beautiful, its a shame that have such nasty tempers. We have to remember that pit vipers are largely pokiotherms so their temperature is lower than ours, its advantageous if you are a mammal killer.
The other side of that is in order to regulate temperature they have to keep the back of the membrame cooler than the rest of the body in order to 'see' radiation sources. Pit vipers have a membrane buffered from the body by air. 

I wonder if the reason they strike is that the animals heat on the membrane irritates them, hit the sucker and see if he heats me further. That would explain why they are so ill tempered.

[Zeiss Ikon]

On 12/29/2017 at 9:04 PM, PB666 said:

A our bodies emit in the infrared part of the spectrum
37'C = 310K and using Weins law is 9343 nm which is between 5000000nm and 700 nm (actually closer to deep red than to MW radiation).

Infrared emission at 310K contains so little of the wavelengths approaching 700 nm that it's below the sensitivity threshold of the eye's receptors -- just like a body at 500K (temperature of a slow oven, near enough) emits so little visible light we consider it "invisible" (even at that temperature, the IR-pass goggles and long adaptation probably wouldn't show you anything).

Remember that the human eye's sensitivity to IR is both limited to the shortest end of that spectrum (for most individuals, shorter than 750 nm), near the visible deep red, and of very low sensitivity (it takes rather a lot of radiation below 700 nm to register with even a fully dark-adapted eye, hence why the goggles work best outdoors on a sunny day).  Even the IR sensitivity of common silicon video sensors (with the IR block filter usually used with those devices) doesn't extend far enough to pick up thermal IR from a human body; you need specially designed sensors to go below about 1000 nm.

Edited December 31, 2017 by Zeiss Ikon
typo

[PB666]

2 hours ago, Zeiss Ikon said:

Infrared emission at 310K contains so little of the wavelengths approaching 700 nm that it's below the sensitivity threshold of the eye's receptors -- just like a body at 500K (temperature of a slow oven, near enough) emits so little visible light we consider it "invisible" (even at that temperature, the IR-pass goggles and long adaptation probably wouldn't show you anything).

Remember that the human eye's sensitivity to IR is both limited to the shortest end of that spectrum (for most individuals, shorter than 750 nm), near the visible deep red, and of very low sensitivity (it takes rather a lot of radiation below 700 nm to register with even a fully dark-adapted eye, hence why the goggles work best outdoors on a sunny day).  Even the IR sensitivity of common silicon video sensors (with the IR block filter usually used with those devices) doesn't extend far enough to pick up thermal IR from a human body; you need specially designed sensors to go below about 1000 nm.

yep, but I wonder how much of that insensitivity is inherited and how much is down throttling of red response. Most  smaller animals don't see red at all.

[Zeiss Ikon]

6 hours ago, PB666 said:

yep, but I wonder how much of that insensitivity is inherited and how much is down throttling of red response. Most  smaller animals don't see red at all.

As I understand it, all mammals see red, they just can't distinguish it from green (unless they're rodents or primates), like a human with dichromatism (aka color blindness).  I'm not sure what having only yellow-blue cones in your retina would do to change the wavelength limit of your vision, though -- what I recall is that all kinds of cone cells (red-green and yellow-blue, plus the third "alternate red-green" that about one person in 100,000 has) have similar wavelength limits, but they have different sensitivity peaks (the red-green cones peak around 600 nm, as I recall, while the blue-yellow peak close to 500 nm -- and the summation of their curves places the actual sensitivity peak of human vision very close to 540 nm, a little to the green side of the yellow sodium emission line).

[mikegarrison]

The receptors in the eyes do not have a perfect squarewave filter. Their response to frequencies higher and lower than the peak frequency tails off rather than cutting off entirely. And then the brain interprets "color" by comparing the signal from the different kind of receptors and judging how far into the tail each one is.

[PB666]

8 hours ago, Zeiss Ikon said:

As I understand it, all mammals see red, they just can't distinguish it from green (unless they're rodents or primates), like a human with dichromatism (aka color blindness).  I'm not sure what having only yellow-blue cones in your retina would do to change the wavelength limit of your vision, though -- what I recall is that all kinds of cone cells (red-green and yellow-blue, plus the third "alternate red-green" that about one person in 100,000 has) have similar wavelength limits, but they have different sensitivity peaks (the red-green cones peak around 600 nm, as I recall, while the blue-yellow peak close to 500 nm -- and the summation of their curves places the actual sensitivity peak of human vision very close to 540 nm, a little to the green side of the yellow sodium emission line).

We really get into off-topic discussion. Although I must admit the OP baited the argument with a poorly phrased question. I always thought many mammals can't see red, particularly nocturnal animals and that is why we use red-lights in displays at zoos. People who are color blind are also poor at seeing red, they see green better than they see red. They taught us this during grad school because they wanted people to avoid putting red in presentations. For example if you have a choice between a black white presentation and white red presentation you choose black white. Red is unavoidable such as in rhodamine is used as a probe, and some people false color their images (but that is discouraged by the Journals).