temp from light lumiance

[saurabhshelar]

Hi,



Is it possible to calculate the temperature from light lumiance?

There are some hard tasks in this like temperature may b low even though light lumiance is more (due to ac) or in some cases temperature may be more than light lumiance.

Is there a generalized and perfect way to find temp from light lumiance?

 

(I only have light lumiance and no more device. I want to calculate temp from light lumiance only)

Edited May 26, 2013 by saurabhshelar

[ajb]

If you have a black body, or in reality a near black body, then you can use Wien's displacement law or Stefan-Boltzmann law. The second one sounds closer to what you are looking for.

 

Stefan-Boltzmann law says that the power emitted per unit area of the surface of a black body is proportional to the fourth power of its absolute temperature

 

[math]j^{*} = \sigma T^{4}[/math],

 

where [math]\sigma = 5.67 \times 10^{-8} \: \:\: W m^{-2} K^{-4}[/math].

[Enthalpy]

There are technological limits.

 

The body must be hot enough to radiate light: light strong enough that the detector measures this instead of ambiant light (including light produced by the surroundings' temperature), light at a wavelength that allows measurement.

 

300K produces light in the far infra-red, around 10µm, and needs special detectors. Hotter objects are easier: a 3000K filament in a light bulb is nearly white and very brilliant. Colder objects are measured at THz or rather GHz frequencies, but require extremely sensitive receivers that are isolated from ambiant noise: it's done in radioastronomy, but damned hard.

 

Also, the object's emissivity (how easily it radiates as compared to the perfectly emitting black body) must be known, and it depends on wavelength and temperature, so the apparatus uses to work on one material and context only, for instance steel exiting a furnace.