Infrared Thermal Cameras: Technology Overview and Buyers’ Guide

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Please check out our Infrared Thermal Cameras section for more information or to find manufacturers that sell these products.

Thermographic imaging has many industrial applications, including monitoring equipment and testing the thermal efficiency of buildings. Recently, infrared thermal cameras have found application in the medical field, as demonstrated by their use in airports during the 2008 swine flu pandemic.¹ The wide range of manufacturers (Table 1), models, and uses can make IR thermal camera selection challenging. This article aims to help in the selection process by providing background on the technology, clearly defining specifications, and discussing key considerations necessary for matching the right infrared thermal camera with the end user’s needs.

Thermographic imaging

Thermal imaging systems perform two primary functions: imaging and temperature measurement. This is accomplished by measuring the electromagnetic (EM) radiation that every object emits, given its temperature exceeds absolute zero (–273 °C). The amount of EM radiation emitted and its wavelength distribution over the EM spectrum will depend primarily on the temperature and emissivity of the object. Planck’s equation describes this relationship. Simply put, IR thermal imaging converts images produced by longer wavelength radiation to a visual wavelength we can see.

Figure 1: IR thermal camera components.

Basic components of an infrared thermal camera

Figure 1 outlines the generalized optical system used by IR thermal cameras to form an image based on an EM radiation emitting body. One or more detector elements convert EM radiation into electronic signals proportional to the radiation detected. The electronic processing element then converts the detector(s) output into a video signal. The video signal is then displayed as a visual image on the instrument display along with any accompanying data.

Lens system

The lenses of an IR thermal camera focus incoming IR radiation onto the detector element(s). Common materials used for lenses include germanium (Ge), silicon (Si), and zinc selenide (ZnSe). Lenses are designed with different focal lengths depending on the thermal camera’s intended use.

Basic detection system

The detector, or detector array, largely determines the potential level of thermal and spatial resolution. There are generally two types of IR detectors: thermal detectors and photon detectors. The former relies on IR radiation physically heating the detector element triggering a secondary physical mechanism proportional to the radiation collected on the element. Photon, or quantum, detectors measure incident radiation (photons) interaction with the material of the detector at a molecular level. Typically, this interaction involves an electron absorbing a photon, moving from one quantum energy level to another.

Signal processing

The electronic output from the detector elements is processed to produce a thermal image and/or temperature measurement. Embedded in the detector output is the radiation signal from the body of interest plus the background radiation from other objects. The signal processing element functions to remove the ambient or background radiation signal component and convert the electrical output into a standard video format.

Display

The image generated by the signal processing element will be viewed on a separate external display (i.e., computer monitor) or a small direct-view display located on the IR camera itself. Direct-view displays are typically liquid crystal displays (LCDs), and can be in color or monochromatic. Generally, color displays are color coded to depict temperature differences throughout the field of view of the imager.

Environmental challenges

The atmosphere along the pathlength between the thermal imaging equipment and the object of interest plays an important part in the quality of the thermal measurement. Molecules such as water vapor, carbon dioxide, oxygen, and carbon monoxide present in the atmosphere absorb radiation in the wavelength range of interest. For example, if the instrument operates within a shorter wavelength (3–5 μm), it is more susceptible to errors in humid air environments. Long wavelength instruments (8–12 μm) can typically provide a more robust measurement, but may not give the precision needed when looking at small changes in temperature. The measured radiation will be the sum of the target EM emission, reflected sunlight, the reflection from other radiating bodies, and the atmosphere along the pathlength between the body of interest and the thermal camera. Table 2 lists important environmental measurements to consider and collect when precise thermographic data are required.

Key purchacing considerations

Before purchasing an IR thermal camera, it is important to consider the intended application and the corresponding system requirements. Table 3 outlines several factors to consider when selecting an IR thermal camera.

Defining requirements

1. Resolution: The resolution of an IR thermal camera is generally reported as the number of pixels in the horizontal and vertical axis (e.g., 320 × 240). Low-resolution cameras will have ≤160 × 120 or 19,600 pixels, medium-resolution cameras have around 320 × 240 or 76,800 pixels, and high-resolution cameras around 640 × 480 or 307,200 pixels. In determining the necessary resolution, users should consider the size of the intended target, the distance between the camera and body of interest, and the need to zoom in while maintaining a quality image.
2. Accuracy: For IR thermal cameras, accuracy is the agreement between the result reported by the IR thermal camera and the true temperature of the targeted body. This may be reported as a percentage or a temperature tolerance (i.e., ± 2% or 2 °C).
3. Temperature range: Equally as important as matching the maximum and minimum temperature measurable by the thermal imager to the test body, understanding the increments into which this temperature can be divided is critical to accurate temperature measurement.
4. Field of view: The field of view (FOV) is defined as the area visible to the thermal imager at a given distance. Generally, FOV is described in horizontal and vertical degrees (i.e., 22° × 15°). Instantaneous field of view (IFOV), or spatial resolution, is the smallest detail within the FOV that can be measured at a set distance. IFOV is typically measured in small fractions of angular degrees called milliradians (mRad).
5. Result display: Cameras with dual imaging capability, those able to display both thermal and visible wavelength images, are useful in many applications. Because the visual wavelength image is typically of higher resolution and relatively wide field of view, superimposing the thermal image on the visible image allows the user to easily identify the areas of interest.

Validation

Once the system requirements are well understood, including environmental specifications (i.e., operating and storage temperature range and humidity), an IR thermal camera and the necessary components should be identified. To assure the end user of a product’s conformance with specifications, a validation should be performed prior to placing the IR thermal camera in use. Typically, the accuracy of a validation device (i.e., thermocouple or resistance temperature detector) is at least four times greater than that of the equipment being validated.

Conclusion

Depending on the application, certain regulatory issues may drive the selection of devices used in specific environments. Other purchasing considerations include vendor reputation, availability of accessories, training, rapid service, and customer support.

Reference

1. Bitar, D.; Goubar, A. et al. International travels and fever screening during epidemics: a literature review on the effectiveness and potential use of non-contact infrared thermometers. Euro Surveill. 2009, 14, 1–5.

About the Author

T. Keith Brock, BS, is a Contributing Writer; e-mail: [email protected].

Please check out our Infrared Thermal Cameras section for more information or to find manufacturers that sell these products.