Infrared Incandescence: How Hot Does It Need To Be?

[ricardo7890]

From what i understand thermal cameras like flir can see "heat" because objects at room temperature release Ir light like hot metal would visible light. But most cameras like commercial security cameras can see IR but don't work like thermal cameras because they aren't sensitive to the right range of the IR spectrum.
But if an object was hot enough would it give off IR light that a normal ir sensitive camera could see it, but not produce visible light, and if that is the case how hot would say a piece of iron have to be to give of this wavelength of IR but not visible light.

 

[NascentOxygen]

But if an object was hot enough would it give off IR light that a normal ir sensitive camera could see it, but not produce visible light, and if that is the case how hot would say a piece of iron have to be to give of this wavelength of IR but not visible light.

Hi ricardo7890. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif

You can experiment with IR at home, using your electric stove and digital camera. In the dark of night, set one of the stove's top elements to medium heat and as it warms up watch it on the monitor of your digital camera. Does it show up on the screen as green-grey well before it's visible to the unaided eye?

At night, while monitoring the screen of your digital camera, you can illuminate things using your tv remote control as an IR flashlight.

If you want to read up on the topic, see radiation from hot bodies and Wien's Law. http://www.egglescliffe.org.uk/physics/astronomy/blackbody/bbody.html

 

[Drakkith]

From what i understand thermal cameras like flir can see "heat" because objects at room temperature release Ir light like hot metal would visible light. But most cameras like commercial security cameras can see IR but don't work like thermal cameras because they aren't sensitive to the right range of the IR spectrum.

Yes, the security cameras have to illuminate their field of view with special LED's that emit the required frequency range of the camera I believe.

But if an object was hot enough would it give off IR light that a normal ir sensitive camera could see it, but not produce visible light, and if that is the case how hot would say a piece of iron have to be to give of this wavelength of IR but not visible light.

Yes and no. The effect you are referring to is known as "thermal" or "black-body" radiation. http://en.wikipedia.org/wiki/Black-body_radiation
Any object above absolute zero in temperature will emit a broad spectrum of radiation. This emission is random, but follows a relationship based on the temperature of the object. Simply put, hotter objects emit more radiation at higher frequencies (higher frequencies are more energetic) than colder objects. A normal incandescent light bulb that you screw into your lamp or whatever uses a filament that is heated to about 3,000 kelvin. At this temperature objects emit a range of wavelengths that includes the visible and infrared, with the amount of radiation peaking in the infrared range. That's right, a normal light bulb puts out more power in the INFRARED range of the spectrum than in the visible range! This means a large amount of power is wasted, which is the reason for the push to change to compact fluorescent lamps (CFL's).

Here is an interactive graph you can use to see the output of an object at a certain temperature. It's designed for use with stars, but the same rules apply to any object. Drag the bar to whatever temperature gives you the right range and you can see for yourself.
http://www.pas.rochester.edu/~afrank/A105/mha/Blackbody_Nav.swf

That one only goes down to 2500 k, so here's one where you can enter whatever temperature you want.
http://homepages.ius.edu/kforinas/physlets/thermo/blackbody.html

In any case, the radiation emitted at higher frequencies drops off, BUT IS NEVER ZERO! So every so often the wall of your room may emit a red or blue photon, but the frequency at which this happens is so incredibly low you'd never be able to see it. It's like...really REALLY low...