A common application that is well suited to IR filters is that of measuring thin film plastics. Since the process of making thin plastic film itself is highly temperature critical, it is imperative to evaluate both the temperature and the uniformity of the plastic as it exits the extruder or web process. The product is typically moving at high speeds which precludes the use of contact temperature methods.
Most plastic films have spectral characteristics similar to polyethylene (depicted in the figure below) which is transmissive in both the short wave and long wave IR regions. Measuring thin film plastics can be challenging since without using a filter, you see “through” the plastic and measure the objects behind the plastic, rather than the plastic itself.
Spectral transmission of Polyethylene film with spectral response of plastics filter