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How much spectra can be added to a narrow laser spectrum until it starts being a thermal radiation?
And what is left isn't strictly "thermal", but it's close enough that we can ignore the difference.
If you have more blue light than red, the colour temperature can be very odd.
https://en.wikipedia.org/wiki/Wien%27s_displacement_law#DiscoveryThe 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]
Sometimes the term "thermal radiation" describes the origin or cause of the radiation instead of the properties or spectrum that it has.
How much can we remove until the difference can no longer be ignored?
It's actually quite common for hot enough objects.
If a thermal radiation is filtered by a linear polarizer, is it still considered thermal radiation?
It depends what you are doing.
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 .
Quote from: hamdani yusuf on 11/04/2022 13:02:44How much can we remove until the difference can no longer be ignored?It depends what you are doing.
Quote from: hamdani yusuf on 11/04/2022 13:02:44It'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.
https://en.wikipedia.org/wiki/Color_temperatureThe 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]
Let's say I'm passing it through light filters.
Quote from: Bored chemist on 11/04/2022 13:12:17The colour temperature of a blue LED is extremely high- which is absurd given that the emitter is near room temperature.Quotehttps://en.wikipedia.org/wiki/Color_temperatureThe 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]
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.
Why?
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?
It depends on what, exactly?
How much spectra can be removed from a continuous spectrum until it stops being a thermal radiation?
Quote from: Bored chemist on 11/04/2022 13:59:13How 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?
Quote from: hamdani yusuf on 11/04/2022 13:24:34Let's say I'm passing it through light filters.Why?
Quote from: hamdani yusuf on 11/04/2022 11:41:49How 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.
Quote from: hamdani yusuf on 11/04/2022 16:00:58It 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?
What are the requirements for a radiation source to be called thermal?
Quote from: Bored chemist on Yesterday at 13:12:17It depends what you are doing.