Temperature

[Hanshotfirst.kc]

I've read a lot of posts about LD temp, storage and operating temp, etc., but, no info on how hot the laser beam/dot actually is.

By the way, if the "beam" is the laser travelling from LD to surface, what is the impact point called?

So, how hot does the point of impact get? (IP) It's obviously, in the case of balloon popping, hot enough to melt latex, but how hot IS that?

Paper burns at around 440 deg, f, but, smoulders around...what temp?

Has anyone either a) shot the IP with a laser IR thermometer or b) sharpied the end of a digital thermometer (meat type, not "I'm sick" type) and measured the temperature?

 

[Justin]

We get asked this kind of question a lot, and it's actually an invalid question.. allow me to explain:

The point at which the laser hits an object will get hot because the energy in the laser light is being absorbed by the material in the target. "Temperature" is a measure of the average kinetic energy of a substance, so when the laser light is absorbed by a match or balloon it causes the molecules of the match to vibrate (random kinetic energy) and this is called "heat".

Light has no mass, and therefore doesn't really have any kinetic energy. Light does not have a "temperature" because it is not matter, it is energy. Light can deliver energy to a substance and will generate heat, but the amount of heat generated will depend on the absorbtion spectrum of the material, its thermal conductivity and so forth. The laser beam itself has no intrinsic temperature. Shooting the laser at a thermometer will only tell you how much energy is being absorbed by the thermometer head, it won't tell you anything about the laser.

 

[Hanshotfirst.kc]

"Shooting the laser at a thermometer will only tell you how much energy is being absorbed by the thermometer head, it won't tell you anything about the laser."

Well if you don't know, just say so. Kidding, of course. I know the laser isn't "hot". I don't think I implied that in my initial post, but, if I did that's not what I meant.

I didn't think that if you looked at the beam in an FLIR camera that you'd see anything like a huge heat signature slicing through the room. (you might if it was heating and interacting with dust, but, that's still not the beam heat.) A laser isn't like a blow-torch, it's not a place where combustion is occuring. Got that. In order to have combustion, you need fuel, heat, oxygen, and a chemical reaction. Laser is just an energy delivery method. Apparently, when a laser hits an absorptive surface, the energy transference is enough to raise the temperature to the point of combustion, or at least smoldering in some cases. The energy transfer is enough to heat a balloon to popping, light a match, burn a retina...I understand that the beam is delivering energy to the point of impact, and that the beam itself has no heat. I phrased the question wrong.

What I meant to say was, at the impact point, what is the highest temperature recorded on an object? The laser will cause the temperature of the object to spike, and this will vary according to laser mW, correct? So, with a 125mW laser, if I sharpied the back of my hand and held it there, would my skin temp, due to imparted energy from the laser, rise to 150? 212? 300? It could not climb past the maximum energy contained in the beam, but, with a 125mW laser, what is that upper limit? With a 250?

We measure and interpret that energy transference as heat. So, how much of a temperature spike can a laser cause in a point of impact?

 

[marianne]

all i can tell you is that it would sting like crazy

 

[a_pyro_is]

"Shooting the laser at a thermometer will only tell you how much energy is being absorbed by the thermometer head, it won't tell you anything about the laser."

Well if you don't know, just say so. Kidding, of course. I know the laser isn't "hot". I don't think I implied that in my initial post, but, if I did that's not what I meant.

I didn't think that if you looked at the beam in an FLIR camera that you'd see anything like a huge heat signature slicing through the room. (you might if it was heating and interacting with dust, but, that's still not the beam heat.) A laser isn't like a blow-torch, it's not a place where combustion is occuring. Got that. In order to have combustion, you need fuel, heat, oxygen, and a chemical reaction. Laser is just an energy delivery method. Apparently, when a laser hits an absorptive surface, the energy transference is enough to raise the temperature to the point of combustion, or at least smoldering in some cases. The energy transfer is enough to heat a balloon to popping, light a match, burn a retina...I understand that the beam is delivering energy to the point of impact, and that the beam itself has no heat. I phrased the question wrong.

What I meant to say was, at the impact point, what is the highest temperature recorded on an object? The laser will cause the temperature of the object to spike, and this will vary according to laser mW, correct? So, with a 125mW laser, if I sharpied the back of my hand and held it there, would my skin temp, due to imparted energy from the laser, rise to 150? 212? 300? It could not climb past the maximum energy contained in the beam, but, with a 125mW laser, what is that upper limit? With a 250?

We measure and interpret that energy transference as heat. So, how much of a temperature spike can a laser cause in a point of impact?

As was said earlier, it would depend on how much of the light was absorbed, and on how fast the thermal energy could leave the sharpied spot on your hand.

 

[pseudonomen137]

What I meant to say was, at the impact point, what is the highest temperature recorded on an object?  The laser will cause the temperature of the object to spike, and this will vary according to laser mW, correct?  

Yes, but it would also very greatly by the wavelength of the laser, ambient temperature, humidity, wind current, material of the object, shape of the object, absorbance of the object, etc etc etc.

For instance if you said "I have a 1W laser shining at a piece of a aluminum, how hot would it get maximum?". Well, if this is polished aluminum, it may only absorb a few 10s of mWs of energy. If its anodized deep black, it may absorb nearly the entire watt. Right there the question is shot. After that though, there's the wavelength of the laser, how well the aluminum heatsinks that incoming energy, etc. Basically, a cascade of other variables, which can all make of break the answer.

So yeah, the question does have its sound basis in theory, but in practice its next to impossible to have a perfect enough setup to offer a worthwhile, repeatable answer. Sorry :-/

 

[Hanshotfirst.kc]

Thanks, Pseudo.
OK, now I understand it fully.
Just because I sharpie the end of a meat thermometer and laze it, doesn't mean that it will show a constant heat every time. And, depending on what chaotic elements are in place, it can vary from time to time, place to place.

It CAN be measured as a "neatness" factor, but, it really has no overall bearing on laser power, and it will change depending on the emissivity of the object (emissivity is a measure of reflection for energy).