Infrared Temperature Challenges - Viewing IR Through a Window
In the next series of articles we’ll be discussing some of the common challenges of using infrared temperature measurement devices. In this article, we’ll focus specifically on the challenges of infrared temperature through a window. In other words, attempting to measure the temperature of an object that’s sitting behind some transparent material like crystal or glass.
Applications that might require an object to be measured through a window are much more common than you might expect. One of the most common applications is measuring the temperature of an object inside of a furnace. Furnaces are extremely common in a number of manufacturing processes such as those used to heat metals, plastics, glass and ceramics. Some furnaces are designed with a window, similar to your kitchen oven, to allow for the heating process to be monitored.
For the application that we described above, it makes a lot of sense to use an infrared thermocouple or pyrometer, due to it’s contactless nature, but as the title of this article suggests, there are some additional concerns that anyone attempting this should be aware of.
Transmission
The transmission of IR through a window can be defined as the proportion of infared radiation from an object that is able to pass through a window and ultimately reach your IR measurement device. If a window allows for 100% transmission, you would have a perfectly accurate measurement. As you may have guessed, 100% transmission isn’t possible when viewing IR through a window. Additionally, IR windows are typically spectral in nature so transmission varies depending on the wavelength of your device.
The IR energy that’s not transmitted is lost by being either reflected or absorbed. The very simple RAT equation describes the total IR energy read by your measurement device.
R (reflection) + A (absorbtion) + T (transmission) = 1
Transmission only comes into play when an IR window is present and while IR windows do absorb and reflect some IR energy, transmission is the main factor to account for when attempting to measure IR energy through a window.
How can I use IR for my application?
The main consideration when attempting to use IR temperature measurement for your application is the material of the window. Some windows are designed specifically with IR measurement in mind, others will completely prevent you from getting an accurate reading. Materials like Saphire or Chalcogenide glass for example have high transmission, but are not generally not ideal for furnace applications due to their temperature limits. Below are some examples of materials and the wavelengths that IR can be transmitted at through these materials:
|
Material |
μm (top range) |
μm (bottom range) |
|
Sapphire (Al2O3) |
5 |
0.165 |
|
Quartz (SiO2) |
6.5 |
0.27 |
|
Cleartran (ZnSe) |
2.38 |
0.0833 |
|
Calcium Fluoride (CaF2) |
7.7 |
0.11 |
|
Magnesium Fluoride (MgF2) |
7.5 |
0.125 |
|
Silver Bromide (AgBr) |
2.2 |
0.0286 |
|
Silver Chloride (AgCl) |
1 |
0.036 |
|
Barium Fluoride (BaF2) |
6.7 |
0.074 |
|
Cadmuim Telluride (CdTe) |
1 |
0.04 |
|
Caesium Iodide (CsI) |
4 |
0.02 |
|
Gallium Arsenide (GaAs) |
0.7 |
0.065 |
|
Germanium (Ge) |
0.55 |
0.06 |
|
Irtran-2 |
1.7 |
0.072 |
|
Potassium Bromide (KBr) |
4 |
0.04 |
|
KRS-5 |
2 |
0.025 |
|
Lithium Fluoride (LiF) |
5 |
0.17 |
|
Sodium Chloride (NaCl) |
4 |
0.0625 |
|
Silicon (Si Dependant on Doping) |
8.33 |
0.066 |
|
Zinc Selenide (ZnSe) |
2 |
0.0454 |
For more information see: https://www2.lbl.gov/mmartin/bl1.4/IRwindows.html
* If you’re thinking about using IR measurement devices for your application we advise that you contact the window manufacturer to get the material of the window.
To further complicate the situation, even within the ranges provided above, materials differ in their transmittance at each of those wavelengths. You can see this behavior in the chart below.
For more information see: https://www.optics.arizona.edu/sites/optics.arizona.edu/files/msreport-christina-kucerak_.pdf
Which IR Measurement Instruments should I use?
As you can see in the charts above, most IR windows have a low transmission range between 0 - 8 μm. A majority of the IR thermocouples on the market have a spectral range around 5 - 20 μm, so you’ll most likely need to find one that’s designed to have a lower spectral range. Lucky for you, IOThrifty has a number of products that can serve that need:
PyroUSB PC Configurable Infrared Temperature Sensors with 4-20 mA Output (some versions)
- IR-CA-PUA2-151-LT: 2.0 - 2.6 μm
- IR-CA-PUA2-751-HT: 2.0 - 2.6 μm
IRt/c.10A - Infrared Thermocouple with 10:1 Field of Veiw for Metals and Non-Metals
- Lo E Version (metal): 0.1 - 5 μm
FibreMini Fiber Optic Pyrometer for Harsh Applications: 2.0 - 2.6 μm
As always, if you have any questions or require any assistance please reach out to IOThrifty’s team of experts.