We’ve been trying to preach to the inexperienced about Spectral Emissivity vs (just plain) emissivity.
The former is the subject used in Thermal Infrared Radiation Thermometry (Pyrometry, to some) and users of Thermal Infrared Imagers (Thermographic or Thermography Cameras) while the latter is the domain of radiation heat transfer considerations (except of course when spectral issues, like windows and atmospheres get in the way of the radiation transfer).
See the Emissivity Trail Pages at About Temperature Sensors if you’d like a brief rant or two.
But popular ignorance of details not withstanding, it is still a bit of a shock to see the term ‘Emissivity” a prominent feature on both instrumentation and engineering websites, Here’s another one with some sample text (no numbers here) from the Engineering Toolbox website. (Note: we corrected their misspelling of “emissivity” – as mentioned in our semi – rant pages on About Temperature Sensors, the word seems to be misspelled as often as it the term and the values are misunderstood and misused!)
The radiation heat transfer emissivity coefficient of some common materials as aluminum, brass, glass and many more
The emissivity coefficient – ? – indicates the radiation of heat from a ‘grey body’ according the Stefan-Boltzmann Law, compared with the radiation of heat from a ideal ‘black body’ with the emissivity coefficient ? = 1.
The emissivity coefficient – ? – for some common materials can be found in the table below. Note that the emissivity coefficients for some products varies with the temperature. As a guideline the emisivities below are based on temperature 300 K.
Surface Material
After Note: We have tried over the past ten years or so, with very limited success, to point out to organizations that should know better, including at least one each manufacturer of “Infrared Thermometers” and One Prominent Maker of Blackbody calibration furnaces, that they need to mend their errant ways and get with the one true religion of Spectral Emissivity.
Heck, the Church of the Flying Spaghetti Monster got a better response and Rodney Dangerfield gets more respect.
There are a few bright lights at the end of the emissivity “black hole”, the new facilities at several national Metrology Laboratories, such as the one at NIST dealing with buy percocet online target=”_blank”>Infrared Optical Properties of Materials and the “Modern emissivity measuring facility for industry-orientated calibrations developed at PTB“.
Hope springs eternal!
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Hi
I am trying to find out what material to use as a transmitter of infrared radiation, with the limitations that the surface shall never be allowed to be hotter than 88 degrees Celsius, that the surface at such temperature shall not cause damage to humans if touched and that it will have a maximum thickness of 4 mm.
When I read all of what you write and refer to I understand that it is complex but do I understand fully, no I do not and can I answer my question, no
Can you help me?
Best Regards
Lars Enström
Not quite certain what you mean as a transmitter of Infrared Radiation. Do you seek a specific portion of the Infrared, or the whole thing (It’s relatively wide portion of the EM spectrum)?
If you are seeking a window material that will meet your other conditions, the most complete technical resource on the web, one that I admire and think is perhaps the best so far, is on the website RefractiveIndex.INFO . While this is nominally a site with a physics slant, it includes a transmittance calculator to enable one to consider a very wide variety of materials available. You’ll have to check the other properties separately.
If you just want a selection of commercially available infrared windows, one can do a Google search and wade trough all the commercial offerings or, if that is not quite what you seek, check the available technical data from some of the optics makers around the world, like http://www.crystran.co.uk
Personally, if you are considering something really inexpensive, look at some of the plastic materials available; most will fit your other specs.
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