What are Thermal Camera Lenses
This article is a basic introduction to Thermal Camera Lenses, which refers to the lens components used for infrared thermal imaging.
First of all, before understanding the thermal camera lenses, we must figure out what are thermal imager and thermal camera, and how these two work. Object with a surface temperature higher than absolute zero (-273°C) emits infrared magnetic radiations (that is, the radiations with wavelengths beyond 700nm, but above 1000 microns) outward. Unlike a visible camera which produces images by capturing visible light, an infrared imager is a device that forms the image by detecting the temperature of the object. Because the thermal imaging system detects heat rather than light, it can be used 24/7, moreover, it is a piece of passive equipment, and there is no extra radiation, so it does not expose the user's location.
The infrared spectrum can be divided into five categories according to different wavelength ranges: Near Infrared (NIR), wavelength range 0.75 microns-1.4 microns; Short Wave Infrared (SWIR), wavelength range 1.4 microns-3 microns; Mid Wave Infrared (MWIR), wavelength range 3 microns-5 microns; Long Wave Infrared (LWIR), wavelength range 8 microns-12 microns; and far-infrared, which ranges from 15 microns-1000 microns. Among them, The MWR and LWIR spectral regions are the two major wavelength ranges for thermal imaging. MWIR and LWIR radiation could penetrate the atmosphere and produce images detecting the temperature difference of the objects, helping us to scout the environments better in fogged weather or at night.
Figure 1. This picture exhibits Shalom EO's Thermal Imaging Lenses as an example
To be more specific about the working principle, a thermal imager or camera uses optics to focus the infrared radiation from the objects in the scene onto an infrared detector, and then the digital data from each detector element are converted into a standard video format that can be viewed on a standard video monitor or recorded on a videotape. The detectors inside the thermal imagers are divided into two categories: Cooled Photon (Quantum) Detectors and Uncooled Thermal Detectors. Detectors of thermal imagers exist to receive the thermal radiations entering via the thermal camera lenses and to produce visual representations of the temperature differences. Therefore the self-heating of the detector will increase the thermal noise and result in a loss of sensitivities. A cooled Photon detector produces a direct electrical effect after absorbing infrared radiation, while a thermal detector produces a temperature change after absorbing the infrared waves, and generating an electrical effect. The material properties of the detectors affect how temperature changes induce the electrical effect. In comparative terms, cooled photon detectors are more sensitive. Cooled photon detectors are often integrated with a cooler to maintain the detector at -130 degrees Fahrenheit or less and maintain high sensitivities. Cooled Quantum Detectors are often incorporated into MWIR thermal imagers. An uncooled thermal detector, on the other hand, does not contain a temperature control gadget and might suffer from loss of sensitivities but will be much cheaper than a cooled detector and perform well enough at room temperature. For the most part, uncooled infrared FPA detectors/sensors are designed to match with Long Wave Infrared (LWIR) thermal imaging camera lenses operating at 8-12 micro.
When designing and manufacturing infrared camera lenses, materials different from that of visible lenses are chosen. The material should be transparent to the infrared wavelength range of interest. For example, silicon might not be as a good choice as germanium when producing LWIR Thermal Camera Lenses since silicon is opaque to wavelengths above 7000nm. Apart from materials, it is important to weigh and balance other factors such as field of view, weight, dimension, and the focusing mechanisms of the infrared camera lenses as well.
Figure 2. A Single Infrared Germanium Lens from Shalom EO
A set of finished thermal camera lens modules constitute more than one single lens. A group of single IR lens components of various materials (e.g. Materials with complementing coefficient of thermal expansions are often combined into one lens module to realize passive optical athermalization) forms, and functions (e.g biconvex to collect and converge thermal radiations while balancing out spherical aberrations, plano-concave lenses to diverge collimated radiations or collimate converging radiations, achromatic doublets to eliminate chromatic aberrations, etc) is selected and engineered with deliberations to cater to the intentions of the lens assemblies.
Apart from the materials, there are also other critical parameters that define the functions and capabilities of infrared lens assemblies, including Sensor Size and Resolution, Depth of Field (DOF), Focal Length, Field of View (FOV), f-number, Transmission of Materials, Image Distortion, Modulation Transfer Function (MTF), Spherical Aberration and Coma. Read more in our technical article Lens Selection Tutorial to learn more about the definition of these glossaries.