Optical Filters in FLIR Science-Grade Infrared Cameras

From FLIR Systems

Figure 1. Propane leak
Figure 1. Propane leak imaged with FLIR GF320 hydrocarbon gas camera

This comprehensive technical article looks at the roles that Infrared bandpass filters can be used for in Infrared cameras. For instance they can be helpful to make a material opaque, or see through a material, or to visualise some gases.

The article discusses traditional uses of cold and warm bandpass filters in Infrared cameras and includes a case study where the advantages and shortfalls of using these filters with a Science Grade Infrared camera are shown.

By Austin Richards

Introduction:

It is often desirable to restrict the spectral range of an infrared camera to a particular band to observe gas, plastic or other materials. These materials may be highly transparent to most wavelengths of infrared radiation, yet will absorb strongly in certain narrow ranges of wavelength.

The most common embodiment of this concept are the optical gas imaging cameras, built by FLIR Systems and others.

These cameras use a narrow waveband cold filter to make the camera see in a very particular spectral range. The camera will not detect radiation outside the narrow waveband, making these cameras fairly insensitive to scene radiance.

They are really suitable only for specific imaging applications, though they can be calibrated to measure scene temperatures.

Figure 1 shows an image of a large propane leak taken with a hydrocarbon gas imaging camera. The camera’s cold filter transmits in the 3.2-3.4 micron waveband only, where hydrocarbon gases show significant absorption due to a C-H bond stretch mode resonance.

Read the extensive and detailed article in PDF format with this link (5.6MB):
FLIR-Article2018Q