In the coming weeks with will be posting about the unique design considerations and material requirements for infrared optical systems. In this post we will focus on the definitions we use as optical engineers.
Infrared radiation includes electromagnetic waves with 0.75-1000 µm wavelength band. This spectral region is divided on five intervals:
Near-infrared radiation (NIR) - 0.75-1.4 µm wavelength band;
Short-wavelength infrared radiation (SWIR) - 1.4-3 µm wavelength band;
Mid-wavelength infrared radiation - 3-8 µm (MWIR) wavelength band;
Long-wavelength infrared radiation - 8-15 (LWIR) µm wavelength band;
Far-infrared radiation (FIR) - 15-1000 µm wavelength band.
IR-radiation has a number of features which make it useful it for solving of the set of specific problems. The main feature is that objects with temperature above absolute zero emits IR-radiation. Intensity of radiation emitted by the object and spectrum of this radiation (spectral concentration of a radiance) depends on temperature of this object. Thermal imaging optics is optical devices which can collect IR-radiation emitted by objects (MWIR and LWIR usually) and analyzes it. One of the most widely-spread such devices is thermal imager.
Thermal imaging systems demonstrate distribution of the temperature of the imaged object. In case of monochromatic thermal image, areas of depicted body or scene which have higher temperature are brighter and cold areas are darker. In case of colored thermal image hot areas are red and cold areas are blue.
VIS camera works with radiation reflected from object
IR camera works with radiation emitted from object
Thermal imagers are used in many areas of the science and technology - space astrophysicist, heating engineering, weather forecasting, military science and other. The main tasks which are solved with help of thermal imagers are:
Detection of the heated object on the thermal background - for example searching of the people in distress from air in a sea or other difficult to reach places;
Exploring of the thermal distribution - for example can be used for exploring of the weather from the space.
In the next post we will review in more detail the optical materials appropriate for thermal and IR imaging.