Transmissivity

November 17, 2009

A little story bit about heat and light.

 

Transmittance is defined as the fraction of incident light that passes through a sample. The fraction that is absorbed is as mentioned earlier, the absoptance. By this deffenition we realize that trancmittance is dependant on the sample geometry, material and surface finish. We also must realize that transmittance is specific to specific wavelength.

Outline of the experiment

Meassuring transmissivity of an object is relatively straight forward. Our meassuring device will be this thermal imaging camera that senses infra red light. The setup is shown here:

 

  • Since we want to find the reduction in radiation that reaches the measuring device, we first use a reference measurement. ( It is important to note here, that we adjust the emmisivity of our reference object to 1, and note the temperature )
  • Then the samples of interest are placed in-between the reference and the measuring device. This will affect the amount of radiation reaching the device.
  • We manually adjust the input emissivity on the deviceโ€™ computer terminal, until we arrive at our known reference temperature. The value of emissivity is noted. The difference between the new value and the reference is value is the transmissibility of our sample.

Data

The computed data are listed in the table. It is observed that the dark plastic material has a higher transmissibility than the clear. Initially this seems counter intuitive, as we with our eyes can see through the clear bag, but not the dark. But when we consider that visible light and infra red are merely traveling at different wavelengths, we can conclude that the two materials impede radiation at different bandwidths. The camera used, detects only certain a band of electromagnetic waves traveling at higher frequencies, in the infra red spectrum. Our eyes, detects waves moving in a lower band of frequencies, and the result is clear, as the plastic bag turns opaque in the cameras view.

Transmissivity measurements and values

Experimental-grade PE plastic garbage bag. Placed in the cameras FOV during one of the measurements.

 

The image below is from a different experiment, where we investigated the cameras capability with rounded surfaces. We are looking at a round aluminium block, standing upm It is clear that depending on the angle between the camera and the surface, the meaaurements will vary. The object seems to be warmer near the top and bottom, and also in the middle. The top and bottom are because here we have a larger density of molecules pr. viewed surface area. ( A bit like the reverse of the sun shining on earth, giving rise to temperature differences. )

 

The part in the middle that seems warmer is a reflection from the camera, and corrosponds to the camera being heated to 35 degrees. The very bright parts below are reflections.

Here we have a picture of the “cavity effect”. The cavity effect describes the circumstances at wich the radiation enters a caity and is reflected, multiple times. This results in multiples of radiation summing together, and exiting together. This will make the cavity appear warmer. Shown in the picture below, is a horisontal row of 4 holes woth the only varing parametre being the depth. From left to right: D: 5, 10, 15, 20 mm

Cavity effect

So now you know, a little bit about non-contact heat meassurement ๐Ÿ™‚ And i didnt even tell you about emmissivity and reflectance ๐Ÿ˜›

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