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Mar 13, 2020 | 10:55 am 2410 0

How to choose a camera for infrared imaging

How to choose a camera for infrared imaging

Before choosing an infrared camera, there are several factors users must understand, specifically the different types of IR detectors, cameras and specifications and how these can affect system performance. There are allot of factors to consider as far as temperature variance, shutter speed, and ultimately cost and value of the thermal imaging camera.

Accurate measurements

It is relatively easy to understand the relative temperature variances in a color-mapped thermal image device, as depending upon the color palette settings warmer colors tend to be hotter temperatures (red, orange, yellow) whereas the cooler colors tend to be colder temperatures (green, blue, purple). This system provides a consolidated view of cold to hot in an easy to read visual interface. If accurate temperature measurements are required though, several potential error sources must be considered: emissivity of the surface, the viewing angle of the camera, surface reflections, and the transmission characteristics of the target, atmospheric conditions, spatial resolution limitations and motion blur. If one or more of these considerations are overlooked, temperature accuracy will suffer significantly.

Reflections are another source of error. Hot objects in the vicinity of the target being measured may reflect from the target. Since the total temperature includes reflected energy, any reflections can mask emitted energy and degrade measurement accuracy.

Depending on their physical characteristics, some targets may have transmission issues. Thermal cameras can see through some types of materials, particularly some plastics. Thus, it is important to ensure that a lot of energy is not being transmitted through the object being observed, otherwise the temperature behind the target may be detected rather than the surface temperature.

Camera considerations

Good quality IR cameras have a temperature accuracy of about +/-2o Celsius, or 2% of full scale, whichever is larger. Unfortunately, it does not matter whether the camera costs $5,000 or $100,000, the accuracy is not much better than +/-2 °C. If more accurate measurements are required, the use of thermocouples may be a better option.

There is a reason why there is a large price difference between calibrated thermography cameras and un-calibrated cameras. High end, factory-calibrated IR cameras require that precise temperature calibration is performed in a laboratory. Such cameras also need to compensate for the temperature drift of the detector, which requires temperature sensor and compensation electronics inside the camera. These features are typically not included in lower-cost, non-calibrated cameras.

On most IR cameras, there is an iris-type shutter that is used to compensate for temperature drift. Periodically, clicking noises may be heard from a thermal camera indicating that the shutter has been shut in front of the detector, and the camera is recalibrating to compensate for thermal drift. This automatic calibration typically takes a few seconds, executing during startup or if the camera detects an internal temperature drift. This temperature recalibration needs to be taken into consideration for automated imaging applications because the camera is "blind" during recalibration and the continuous image data stream will be interrupted. This is commonly referred to as NUC and can be manually done on several devices to ensure the correct calibration of the device.

NETD Values

Noise-equivalent temperature difference (NETD) defines the detector's ability to distinguish between very small differences in thermal radiation in the image. A typical NETD of an uncooled microbolometer is somewhere below 50milli-Kelvin (mK). Cooled cameras that are photon-based, or have photon-based detectors provide better than 20mK. Thus, if a high-degree of measurement sensitivity is required, a cooled camera may be needed.

In cameras with multiple measurement ranges, the NETD depends on the selected measurement range. The smaller the temperature range, the better the NETD. Matching the NETD of the camera to the application is important and this is particularly true if lower temperatures need to be measured or the application requires greater temperature precision.

Application considerations

When choosing a camera, it is important to know the field of view, accounting for moving targets, as well as seeing through obstructive enclosures.

If moving targets need to be imaged, it is important to understand how fast the target moves through the field of view of the camera or, more specifically, the distance (in pixels) that the target moves during the integration time. This can occur for motion blur in tracking moving targets. It is recommended to use a higher pixel resolution sensor and core to provide a clearer image so there is no thermal smearing.

Lenses for thermal image cameras are made from exotic and expensive materials, such as Germanium, Zinc Selenide, or Sapphire that transmit specific IR wavebands and are matched to the detector of the camera. IR lenses are also curved aperture lenses, increasing the cost of manufacturing these high quality and high end optical lenses.

Protective enclosures are also necessary when installing cameras in hostile environments. Since IR cameras cannot look through standard glass windows, camera enclosures require exotic windows of Germanium, Zinc Selenide or Sapphire. The window material must be matched to the specific camera and the environment to allow heat transmission through the protective enclosure.

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