Measurement with IR Thermometer
Date: Fall 2013
If you do an experiment using different colors of paper and an IR thermometer, when you measure the "temperature" of the papers outside, what is the correct term for what is actually being measured?? Irradiance, thermal flux, radiant emittance, reflectance, radiosity or is it a combination?
I assume you have some sort of optical IR radiation detector. These are usually based on the equation for “black body” radiation. This assumption assumes the body is “black”. I do not mean the color, but that the target is radiating the thermal energy which is calculated to be the “temperature”. Often this IR radiation is independent of the optical color of the target. The instrument is measuring the IR radiation, which may or may not, correspond to its visual “color”.
The terms you ask about: “Irradiance, thermal flux, radiant emittance, reflectance, radiosity” would have to be computed from the various definitions of these terms – this gets very technical and involved.
The issue to keep in mind is that the IR detector is measuring the radiation to which the instrument is sensitive. This may or may not have any corresponding meaning in terms of the other measures of measurements unless the configuration of the object and detector are arranged to focus on one of the variables.
It is true that some people may call something "this" while others may call it "that" while they are both talking about absolutely the same thing. So an absolute answer to your question is not possible because words are simply local conventions applied to things.
The simple answer to "what is the correct term for that which an IR thermometer measures" is "temperature." :) Referring to the referenced Wikipedia article, other terms that can be used to describe the measurement are "thermal radiation" and "IR emissivity".
All IR thermometers are reflective spectrophotometers measuring in the IR. They measure Dr, the density of reflection/emission. The instrument does not care if the radiance is emitted or reflective, all it does is measure (think in terms of quanta). Dr(pronounced Dee-are) is the correct term.
Point of fact is that Dr measurements for taking human temperatures must be "massaged" to fit the first and second standard deviation. The laser is a reflective control. Splines in discontiguous mathematics are required for a predictive fit. Only large differences are considered.
The Kodak E700 - 800 series of clinical analyzers uses Dr for the predictions of human blood analytes in hospitals.
Hoping this helps, Peter E. Hughes, Ph.D., Milford, NH
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