[Eternal Student]

Hi.

You (Hamdani) seem to be looking for precise and absolute answers but I think the situation is generally left quite vague and much as suggested by @Bored chemist .

How much spectra can be added to a narrow laser spectrum until it starts being a thermal radiation?

        You could artificially re-create a black body spectrum and some might call this "thermal radiation".   However, some people will reserve that name to radiation that was produced as a consequence of a body being at some well defined temperature and radiating naturally.   You can re-create the same spectrum but it doesn't always acquire the name "thermal radiation".   Sometimes the term "thermal radiation" describes the origin or cause of the radiation instead of the properties or spectrum that it has.
       This is much like the term "gamma ray" being kept separate from the term "x-ray".   You can have an X-ray of such high frequency that it is identical to a gamma ray but if it didn't originate from the nucleus of an atom then you just don't call it a gamma ray.

Here's the description from Wikipedia, just to illustrate the situation.   In their definition, not all thermal radiation has a black body spectrum and not not all black body spectrums were produced in the process of thermal radiation.   However, you should be aware that others will use the term "thermal radiation" and "black body radiation" much more interchangeably than this.

Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter....
If a radiation object meets the physical characteristics of a black body in thermodynamic equilibrium, the radiation is called blackbody radiation.

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

Best Wishes. 

[hamdani yusuf (OP)]

And  what is left isn't strictly "thermal", but it's close enough that we can ignore the difference.

How much can we remove until the difference can no longer be ignored?

If you have more blue light than red, the colour temperature can be very odd.

It's actually quite common for hot enough objects.

https://en.wikipedia.org/wiki/Wien%27s_displacement_law#Discovery
The law is named for Wilhelm Wien, who derived it in 1893 based on a thermodynamic argument.[4] Wien considered adiabatic expansion of a cavity containing waves of light in thermal equilibrium. He showed that, under slow expansion or contraction, the energy of light reflecting off the walls changes in exactly the same way as the frequency. A general principle of thermodynamics is that a thermal equilibrium state, when expanded very slowly, stays in thermal equilibrium.

Wien himself deduced this law theoretically in 1893, following Boltzmann’s thermodynamic reasoning. It had previously been observed, at least semi-quantitatively, by an American astronomer, Langley. This upward shift in νmax with T is familiar to everyone—when an iron is heated in a fire, the first visible radiation (at around 900 K) is deep red, the lowest frequency visible light. Further increase in T causes the color to change to orange then yellow, and finally blue at very high temperatures (10,000 K or more) for which the peak in radiation intensity has moved beyond the visible into the ultraviolet.[5]

[hamdani yusuf (OP)]

Sometimes the term "thermal radiation" describes the origin or cause of the radiation instead of the properties or spectrum that it has.

Or maybe it's based on the effect instead.

If a thermal radiation is filtered by a linear polarizer, is it still considered thermal radiation?

[Bored chemist]

How much can we remove until the difference can no longer be ignored?

It depends what you are doing.

It's actually quite common for hot enough objects.

Not many things are that hot.
The colour temperature of a blue LED is extremely high- which is absurd given that the emitter is near room temperature.
So, as I said...

If you have more blue light than red, the colour temperature can be very odd.

[Bored chemist]

If a thermal radiation is filtered by a linear polarizer, is it still considered thermal radiation?

It depends what you are doing.

[hamdani yusuf (OP)]

You (Hamdani) seem to be looking for precise and absolute answers but I think the situation is generally left quite vague and much as suggested by @Bored chemist .

I'm fine with non-binary concepts and fuzzy variables. As long as it can be stated clearly what kind of changes can be made to shift the membership of "thermality" of a radiation.
Here's an example.

https://en.wikipedia.org/wiki/Fuzzy_logic#Fuzzification

[hamdani yusuf (OP)]

How much can we remove until the difference can no longer be ignored?

It depends what you are doing.

Let's say I'm passing it through light filters.

It's actually quite common for hot enough objects.

Not many things are that hot.

It depends on how you count them.

[hamdani yusuf (OP)]

The colour temperature of a blue LED is extremely high- which is absurd given that the emitter is near room temperature.

https://en.wikipedia.org/wiki/Color_temperature
The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of a color comparable to that of the light source. Color temperature is a characteristic of visible light that has important applications in lighting, photography, videography, publishing, manufacturing, astrophysics, horticulture, and other fields. In practice, color temperature is meaningful only for light sources that do in fact correspond somewhat closely to the color of some black body, i.e., light in a range going from red to orange to yellow to white to blueish white; it does not make sense to speak of the color temperature of, e.g., a green or a purple light. Color temperature is conventionally expressed in kelvins, using the symbol K, a unit of measure for absolute temperature.

Color temperatures over 5000 K are called "cool colors" (bluish), while lower color temperatures (2700–3000 K) are called "warm colors" (yellowish). "Warm" in this context is an analogy to radiated heat flux of traditional incandescent lighting rather than temperature. The spectral peak of warm-colored light is closer to infrared, and most natural warm-colored light sources emit significant infrared radiation. The fact that "warm" lighting in this sense actually has a "cooler" color temperature often leads to confusion.[1]

[Bored chemist]

Let's say I'm passing it through light filters.

Why?

[Bored chemist]

The colour temperature of a blue LED is extremely high- which is absurd given that the emitter is near room temperature.

https://en.wikipedia.org/wiki/Color_temperature
The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of a color comparable to that of the light source. Color temperature is a characteristic of visible light that has important applications in lighting, photography, videography, publishing, manufacturing, astrophysics, horticulture, and other fields. In practice, color temperature is meaningful only for light sources that do in fact correspond somewhat closely to the color of some black body, i.e., light in a range going from red to orange to yellow to white to blueish white; it does not make sense to speak of the color temperature of, e.g., a green or a purple light. Color temperature is conventionally expressed in kelvins, using the symbol K, a unit of measure for absolute temperature.

Color temperatures over 5000 K are called "cool colors" (bluish), while lower color temperatures (2700–3000 K) are called "warm colors" (yellowish). "Warm" in this context is an analogy to radiated heat flux of traditional incandescent lighting rather than temperature. The spectral peak of warm-colored light is closer to infrared, and most natural warm-colored light sources emit significant infrared radiation. The fact that "warm" lighting in this sense actually has a "cooler" color temperature often leads to confusion.[1]

Thanks for posting the bit of wiki that says what I already said.

[Bored chemist]

I'm fine with non-binary concepts and fuzzy variables. As long as it can be stated clearly what kind of changes can be made to shift the membership of "thermality" of a radiation.

Then you need to "clearly state" what you are talking about.

How many questions will you ask before you realise that all the answers are "It depends" because you are asking vague questions?

[hamdani yusuf (OP)]

Why?

Because

It depends what you are doing.

[hamdani yusuf (OP)]

Then you need to "clearly state" what you are talking about.

Which part hasn't been clear yet? I even put an illustration above.

[hamdani yusuf (OP)]

How many questions will you ask before you realise that all the answers are "It depends" because you are asking vague questions?

It depends on what, exactly?

[Origin]

It depends on what, exactly?

I am really curious, are you any clearer on what temperature is after these 10 pages or is temperature still as much a mystery as when you started this thread?

[alancalverd]

How much spectra can be removed from a continuous spectrum until it stops being a thermal radiation?

Any or none. "Thermal" radiation merely denotes the source of the radiation, not the observed spectrum.

[Bored chemist]

How many questions will you ask before you realise that all the answers are "It depends" because you are asking vague questions?

It depends on what, exactly?

Lots of things, for example

Let's say I'm passing it through light filters.

Why?

[hamdani yusuf (OP)]

How much spectra can be removed from a continuous spectrum until it stops being a thermal radiation?

Any or none. "Thermal" radiation merely denotes the source of the radiation, not the observed spectrum.

What are the requirements for a radiation source to be called thermal?

[hamdani yusuf (OP)]

It depends on what, exactly?

I am really curious, are you any clearer on what temperature is after these 10 pages or is temperature still as much a mystery as when you started this thread?

So you think you already understand it?
Explain yourself.

[Bored chemist]

What are the requirements for a radiation source to be called thermal?

Quote from: Bored chemist on Yesterday at 13:12:17
It depends what you are doing.