Message Board Thread - "Does Curvature Affect Emissivity"

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Does Curvature Affect Emissivity Vishnu 10/11/2005
Hi,
I am working with polymer fibers on the order of 100 microns to 2 mm. There is a not a lot of literature on whether curvature of the object affects the emissivity.

It has been shown that emissivity increases with the thickness of films. So does this mean it would increase with diameter for a cylinderical object. Also if indeed emissivity decreases with fiber diameters, then how do I account for spatial resolution.

Can I accurately measure the temperature of objets as small as 70-80 microns? If not, can I quantify the error in any way ?

Thanks a bunch !!
 
Re:Does Curvature Affect Emissivity JKEngineer 10/12/2005
You could certainly set up a test situation to answer these questions. Take a set of fibers at known diameters along with a thick slab of the same material in a known environment. Set up the camera with the known information and see how you read the temperatures for the materials with them set to various known temperatures. Don't forget that spot size and focus range, which will be camera and optics dependent, will have a major effect on your ability to do this at all.

Jack
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Re:Does Curvature Affect Emissivity Doctir bob 10/12/2005
Assuming the fibers are opaque in the waveband of your IR camera you know the emissivity and reflected apparent temperature, your two major concerns are spatial resolution and viewing angle (curvature). You need about 3X3 camera IFOV's to have good measurement resolution.

When the viewing angle to the surface normal is greater than about 50 degrees, emissivity for dielectrics begins to drop. For a working distance, d, much greater than cylinder diameter, R, the measurable arc length is about 1.75R. The attached shows the math for this.
 
Re:Does Curvature Affect Emissivity electricpete 10/18/2005
Other issues to consider are the IFOV and MFOV of your instrument. Typically MFOV ~ 3 * IFOV. Typically these would represent target sizes for detection and measurement of a perpendiular surface. It sounds reasonable to adjust them for the geometric aspects described by Dr Bob. One more aspect is that the 50 degree number of course will depend on the material. I believe in general high emissivity materials emit over a wider angle while low emissivity materials emit over a narrower angle (close to perpendicular)
 
Re:Does Curvature Affect Emissivity electricpete 10/18/2005
I'm sorry. I didn't see that Dr. Bob was specifically addressing dielectrics. I withdraw my question of whether 50 degrees applies.
 


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