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Emissivity. Which one is the right one?
09-25-2016, 08:44 AM
Post: #1
Emissivity. Which one is the right one?
Hello Forum,

I have a basic question about emissivity. Before taking a thermographic measurement, we need to se the emissivity parameter on the device. How do we know, a priori, which emissivity we should use if we don't know what material we are looking at? Also, emissivity depends on many factors (temperature, wavelength, surface texture, etc.). It sounds like we can only and always get imperfect temperature measurements....

b) Why do most thermal cameras focus only on the 9-14 micron spectral range? I know there are atmospheric windows there. Any other reason? Maybe, at the most ordinary temperatures, between 0 C and 2000C, the peak wavelength in the emission spectrum is found within that range, 9-20 micron?

thanks for any comment,
techyon
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09-26-2016, 08:22 AM
Post: #2
RE: Emissivity. Which one is the right one?
Most cameras are LWIR as they use microbolometer detectors, LWIR is also a good spectral match for objects at lower temperatures.

The emissivity of an object can easily be measured, this measured value will be correct to use again in the same context.
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09-28-2016, 09:09 AM
Post: #3
RE: Emissivity. Which one is the right one?
Thank you Bobberry.

I guess "...LWIR is a good spectral match or objects at lower temperatures...." is the key statement that explains why these thermal cameras narrowly focus on that region of the infrared spectrum.

As far as the emissivity, can you point out the techniques that are used to determine the emissivity of a certain material? Is the emissivity a surface parameter only? Could the internal temperature of the object be different from its surface temperature?
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09-28-2016, 10:06 AM (This post was last modified: 09-28-2016 10:09 AM by Bobberry.)
Post: #4
RE: Emissivity. Which one is the right one?
I guess "...LWIR is a good spectral match or objects at lower temperatures...." is the key statement that explains why these thermal cameras narrowly focus on that region of the infrared spectrum." There are thermal cameras available that operate in other parts of the spectrum. Most cameras use microbolometers as it is a cheaper detector to produce, and for most people do, it makes no difference if they use a LWIR of MWIR camera. There are some applications where the spectral region chosen is a very important factor, but for most things it makes no difference.


[/font] "Could the internal temperature of the object be different from its surface temperature?" Yes "Is the emissivity a surface parameter only?" No other factors also matter.


[font=Helvetica Neue]
"As far as the emissivity, can you point out the techniques that are used to determine the emissivity of a certain material?" See ISO 18434-1
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09-29-2016, 08:04 AM
Post: #5
RE: Emissivity. Which one is the right one?
Could the internal temperature of the object be different from its surface temperature?

Not quite sure what you mean by this question. Throw a steak on the grill and the outside can be burned and the inside still frozen.
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