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
Analysis of the IR spectral characteristics of most plastics shows that a significant absorption band exists at 3.4µm. This absorption band is a region where the plastic will appear opaque to the IR camera, allowing the camera to measure the temperature of the plastic and not the background. The width of this absorption band will vary with the thickness of the plastic being measured. The thicker the plastic, the deeper the absorption band will be. Utilizing a filter which allows only the energy from a region around 3.4µm is ideal for this type of plastic film.