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Thread: Emissivity Questions

  1. #1

    Emissivity Questions

    After speaking to my managers about the course I have attended, I need some help answering my boss’s queries, was hoping you can help me out with it:

    1) We did some tests (for fun), inspecting different coloured PCV tapes on a hot surface(metal cup with hot water) using our thermal cameras. All PCV tapes display the same colour on the imager.
    Question: why does the Zebra has its black stripes displayed on the imager even though it is the same material(skin) throughout?

    2) Why does the “E” increase with temperature on a clean metal?
    Last edited by CustomerQuery; 08-06-2013 at 05:47 PM.

  2. #2
    With regard to your first question, your observation with different colour tapes is correct. The PVC tapes of different colour will emit at the same rate, and consequently show the same temperature (colour) on the thermal imager screen. Colour does not affect emittance unless we are dealing with a change in material qualities. For example, a metallic paint which contains metal flake or metal particles will have a lower emittance than a non metallic paint. Now with regards to your Zebra scenario, or any coloured object sitting in sunlight, we are now dealing with an object that is absorbing a whole array of energy (ie many other wavelengths of energy) and not just infrared. With sunlight dark coloured objects (ie black) will absorb more solar energy than light colour objects (white).

    Now I have never imaged a Zebra before, and I cannot comment on whether there are other physiological properties that would influence the radiance of either black/white fur. What I can tell you is that all things being equal, a black coloured surface will absorb more solar radiation than a white one, and therefore will be hotter when exposed to sunlight. It's emittance in the infrared spectrum will be the same.

    With regard to your second question about E increasing with temperature for clean metals. This is not a simple answer and I would call on the physicists here to contribute. There are dozens of white papers published on this subject matter which can be found on the internet and are usually pages and pages in length. To offer a simplified statement would not do justice to the complexity of the issue... however as a starting point you need to be aware of the following:

    Emissivity is a surface property that is highly dependent on material temperature, wavelength, direction, surface morphology, contamination, aging and oxidation.

    Now specific to your question of emittance vs temperatue for clean metals, in general the E value will increase with temperature. Emittance is a result of atomic and molecular vibration, and as an object becomes hotter the frequency of the vibrations are increased. The frequency is related to wavelength (which usually becomes shorter with higher temp). With real bodies (sometimes referred to as non grey bodies), emissivity will vary with wavelength and the emissivity of highly reflective materials is generally higher at shorter wavelengths that at longer wavelengths. As it's been explained to me this has something to do with shorter wavelengths being able to fit into the microscopic "nooks and crannies" of the materials surface, rather than being reflected.

    Another issue that must be taken into account is that metals at high temperatures react easily with oxygen in the atmosphere, forming metal oxides. These oxides can have a different emissivity and/or characteristics from that of the base metal.
    Brenton Ward
    Level III Infraspection Institute Certified Thermographer
    Level II Airborne Ultrasound

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