In some applications it can be desirable or necessary to mount an infrared temperature sensor behind a window to protect it from particularly harsh environmental conditions. This is often the case when monitoring a process inside a furnace, oven or pressure vessel. In these cases it is important to know what type of window to use as this can have a huge impact on the accuracy of the measurements the sensor makes.
If an application requires a window the first thing to consider is that the infrared sensor used will need to have adjustable emissivity; as, for example, offered by the the Convir IL series from Calex Electronics. This feature will be needed to compensate for the transmission losses in the window. The size of these losses will vary greatly with the window material used and its thickness, but there will always be some level of loss.
To calculate the emissivity value which should be used, the transmission of the window should be multiplied by the emissivity value of the target to be measured. Therefore;
Emissivity value to be used = Emissivity of target X Transmission of window
The next, and possibly most crucial aspect to consider is what material to use for the window. This will depend on the wavelength at which the infrared sensor operates, and the temperature and pressure the window itself will need to withstand. The table below shows the wavelength, transmission and maximum temperature at which some common window materials can be used.
Window Material Transmission Range Transmission Maximum Temperature
Zinc Selenide (ZnSe) 4 to 14µm Around 72% 250ºC
Germanium (Ge) 2 to 14µm Around 46% 70ºC
Calcium Flouride (CaF2) 0.2 to 7µm Around 94% 1200ºC
Sapphire (Al203) 0.2 to 4.5µm Around 85% 2000ºC
Quartz Crystal (SiO2) 0.4 to 3µm Around 92% 490ºC
The environment in which the window will be placed will also impact on the selection of window material. For example, Zinc Selenide and Calcium Flouride are both water soluble so should not be used in areas where steam or water vapour will be present for extended periods of time.
Once the window material has been selected the next consideration must be the dimensions of the window. The diameter used will depend on the field of view of the infrared sensor that will look through it. The visible diameter of the window must be larger than the measurement spot of the infrared sensor. Therefore the amount of the window obscured by mounting must be added to the required visible diameter to ensure there are no obstructions to the sensor's field of view.
The thinner the window the less the transmission losses will be. However, if the window will be under pressure, for example as part of a vacuum chamber, then the pressure it will need to withstand will have to be taken into consideration when specifying it's thickness. For a circular window the thickness required to avoid deformation can be calculated using the equation below;
T = K x [(p x D2) / S ]½
For an unclamped window K = 1.06
For a clamped window K = 0.866
T is the minimum thickness of the window
D is the unsupported diameter
S is the apparent elastic limit
p is the pressure differential
To provide the maximum possible transmission, windows should be kept clean and free of scratches. A lint-free cloth or air puffer should be used to remove loose particles from the surface.
All of these windows are available from Calex and despite their exotic natures are relatively inexpensive, especially in comparison to the cost of repairing the lens of an infrared temperature sensor.