Let's talk about emissivity, reflectance, and transmittance of infrared energy. This gets very technical so bear with me...
To understand emissivity, reflectance, and transmittance, one must also understand the principal "Conservation of Energy"
Infrared radiation obeys many of the laws which apply to visible light. When infrared energy strikes an object it may be (1) partially reflected; (2) partially absorbed; (3) partially transmitted.
Conservation of Energy means that "The total energy displacement in an object is equal to the amount of energy striking the object.
The mathematical or scientific formula for conservation of energy (for those who want to know) is:Reflected energy + Emitted energy + Transmitted energy = 1 which equals Total Energy
Now for some definitions...Transmittance:
Is the ability of a material to transmit infrared energy through
Good transmitters are used as lenses for thermal imaging cameras and systems. The 2 commonly used materials for such lenses are germanium and silicon.Reflectance:
Is the ability of material to reflect
Good reflectors appear like their surrounding temperature. Their apparent temperature is often quite different than their true temperature.
Examples of good reflectors include shiny metals, glazed ceramics, glass, mirrors, and high polished surfaces. Good reflectors are poor emitters of infrared and have low emissivity ratings.Emissivity:
Is a common measure of a material's thermal energy absorption and radiation capability. This measure is the material's emissivity rating
Emissivity scales range from 0.0 (zero) to 1.0 (one) depending on the type of material. A material that absorbs the total
amount of IR energy which strikes it, has an emissivity of 1.0 and is also known as a "black body". (Black bodies are used to calibrate thermal imagers and equipment)
Emissivity and reflectance are opposites when transmission is not a factor.
Objects or materials that are good absorbers are also good emitters of IR energy. Good emitters radiate IR energy well and appear like their true temperature.
Examples of good emitters include wood, brick, soil, water, and many painted surfaces.
Emissivity ratings are important when measuring true temperatures of object and materials. Thermal imaging when applied to residential and commercial applications are typically not concerned with true or accurate temperature measurement (in most cases). We are more concerned with relevant temperatures as compared to surroundings. Because there are so many different materials used in home and commercial construction, true temperatures must be averaged as relevant temperatures. We know that good emitters (good absorbers) include many of the materials used in residential and commercial construction. Good emitters typically have an emissivity of 0.85 - 0.98. Residential and commercial thermographers will average the ratings and typically calibrate their imagers at ~0.94
When one needs to become more accurate with temperature measurement, the thermographer will adjust the imager closer to the material's actual emissivity rating. Emissivity tables for different materials can be found throughout the internet. A fairly good one relevant to our applications can be found at:http://www.raytek.com/Raytek/en-r0/IREducation/EmissivityNonMetals.htm
The following images are nearly identical except for the emissivity settings of the camera... notice the temperature measurement differences are quite substantial. Emissivity and reflectance can play havoc with true temperature measurement.