I did a light absorption test using UV-Vis spectrophotometry on three types of materials. From the test results, I was surprised because it was found that a material with a higher absorbance had a lower temperature generation. What causes this phenomenon? Based on the phenomenon, is it probable for the light radiation energy to be converted to other than heat energy? If so, what type of energy is generated?
Dear Alfan Sarifudin ,
you should know, that the reflectivity R*) of a sample is not only a function of the refractive index 'n', but also a function of the absorption properties of the sample [k = extinction coeffcient; a= absorption coefficient; k = a * lambda/(4pi)]
For a beam geometry close to normal incidence you have for the reflectivity R:
R = [(n-1)2 + k2]/[(n+1)2 + k2]
So for increasing absorbance the extinction coeffcient 'k' (k = a*lambda/(4pi)) will also increase and you will have an increase of R.
So your missing fraction of light energy is not changed into heat, but is reflected from the sample surface as light energy...
*) See for example :
https://eng.libretexts.org/Bookshelves/Materials_Science/Supplemental_Modules_(Materials_Science)/Optical_Properties/Metallic_Reflection
or any other physics/optics book
Best regards
G.M.
Dear Dr Gerhard Martens
I have also measured the reflectance and transmittance of the material with opposite results. The higher the absorbance, the lower the reflectance and transmittance. This result is still following the theory that you convey. However, with higher absorbance also lower reflectance and transmittance, why is the material temperature lower?
Dear Alfan Sarifudin ,
thanks for feedback...
There can be no loss of energy other than by reflection and transmission. All of the light energy, which is stuck in the sample will be converted into heat.
So, in addition to the change of temperature, you have to consider the heat capacity of the material as well as the amount of material .
So please multiply the change of temperature with the 'heat capacity' and the 'mass' of your sample (= amount of energy picked up by your sample *)) and compare your results...
*) at constant/fixed measurement times for each sample...
and pay attention onto 'thermal isolation' of your sample(s).
The temperature rise is defined both by absorbance and heat removal capability. Can it be that temperature rise is smaller because heat is more effectivlty removed e.g. via higher heat conductivity or higher infrared emission coeffisient ?
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