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Image of transmission correction through polymer mesh IR WindowsTransmission Correction through Polymer Mesh IR Windows – Mission Impossible?

Posted on: 27th, January 2014

When looking at the Crystal IR Windows transmission diagrams, we have been able to assume that there is an uninterrupted path to our target. The use of polymer mesh IR windows supported by a metal, means that this is not always the case.

Due to the complex thermal radiations signals and amounts we have to show this diagram in two parts due to the combination of optic materials; the Polymer and the Mesh.

Above the dotted line we see the signals associated with the polymer portion of the 2 part optic.

  • The orange arrow represents the energy reflected by target.
  • The blue arrow represents the energy reflected by the window.
  • The green arrow represents the energy emitted by target.
  • And the red arrow represents the energy emitted by the window.

Figure 1: Polymer Mesh IR Window

Now let’s take a look at the signal properties associated with the steel mesh portion of this optic (below the dotted line). It is clear to see that there are only 2 signals seen by the thermal imager. This is because there is NO TRANSMISSION allowed by the steel mesh, so the reflected by target and emitted by target do not reach the camera. This leaves the reflected by mesh signal and the emitted by mesh signal.

With such a number of signals, it creates inconsistencies due to the polymer/mesh combination, focus, target and window temperature, making it nearly impossible to calibrate over the standard PDM temperature range.

After looking at the two optics;

  • Crystal (uncoated),
  • Crystal (HydroGARD coated)
  • Polymer/Mesh combination

It is possible to summarize the differences in transmission characteristics between Polymer/mesh optics and CorDEX IW series crystal optics:

  • Polymer/mesh optics mean you are dealing with multiple complex signals seen by the thermal imager.
  • Measurements can be severely affected by focus and angle due to the not transmissive mesh.
  • Repeatable transmission correction across the PDM range is not possible. Thus, the polymer/mesh optic IS suitable for non-measurement based thermography only.
  • With CorDEX IR windows we can achieve simple, repeatable signals to the thermal imager.
  • There is minimal surface reflection from the CorDEX IW Series Crystal optic.
  • The Crystal optic does not affect target focus.
  • Quantitative (measurement based) thermography is now possible with the transmission correction algorithm available on-line, and on the IW Series Productivity App for iPhone.
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