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).
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.
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 Infrared Optical Properties of Materials and the “Modern emissivity measuring facility for industry-orientated calibrations developed at PTB“.
Hope springs eternal!