Thermal Imaging

To evaluate the thermal performance of a component or assembly, many designers use a thermocouple attached to a specific device. And for many applications this approach is quite adequate. However, increasingly gaining broad acceptance, is the use of thermal imaging for real-time thermal analysis.

Thermography is a non-contact method which provides a thermal map of the unit under test, using 65,000 or more individual temperature points. Thermo-graphy evaluates all of the system components involved, not just those attached to thermocouples and thermal images are produced in real-time showing dynamic thermal events which might be missed by slower responding thermocouples.

PALM-SIZED CAMERAS

Today’s modern thermal imaging systems are fast and flexible and can be configured in a matter of seconds to measure changes made in a design. Advancement in IR (infra-red) technology and in particular the development of focal plane array systems has spawned the release of high performance IR cameras that offer significantly improved capabilities for evaluation applications. Inframetrics’ ThermaCAM demonstrates this, it has the look and feel of a camcorder and the analytical power of workstation-based IR systems.

The original ThermaCAM range included a Sterling electric microcooler which eliminated the need for liquid nitrogen cooling. This Inframetrics microcooler is a NASA, space-qualified cryogenic device and extremely energy-efficient.

Size, weight and power consumption have been dramatically reduced due to the microcooler and the incorporation of low-power components. The resulting cordless camera has a power consumption of less than 10W and weighs less than 2kgs. The camera can operate continuously from standard camcorder batteries for more than two hours. Interchangeable lenses add to versatility ranging from the inspection of complete electronic assemblies to microscopic analysis of VLSI chip hybrids.

So with all these benefits, why develop an uncooled, microbolometer-based system?

UNCOOLED ADVANTAGES

Uncooled technology allows the camera to work in a much wider dynamic range,14-bit. This means that for the first time temperature ranges between 40 C and 1500 C can be addressed. Uncooled technology also allows operation in the long wavelength, 8 m to 12 m range which is less sensitive to solar reflections than shortwave infrared.

One of the disadvantages of a cooled camera is the time it takes to be ready for operation. This may not be an issue but, where instant operation at switch-on is an advantage, uncooled cameras offer this instantaneous response.

Microbolometers have been developed and demonstrated in commercial and military imaging systems. For radiometric applications, they offer the advantages of high sensitivity, solid state uncooled performance, broad dynamic range and longwave spectral sensitivity. However, not all microbolometers are what they seem.

Most uncooled cameras currently available do not have dedicated radiometric microbolometer detector packages. Most use an imaging module developed for military applications where temperature measurement is not important. To make them suitable for use in an industrial environment, these imaging modules are equipped with an additional warm shield which is not temperature stabilised. Detector shielding is needed to bounce photons back from the detector and prevent stray radiation falling onto the detector. But, while outside the camera temperature will vary with ambient temperature typically 100C to 500C , the temperature inside the camera will vary as radiation is emitted from the warm shield and falls onto the detector. A change of just 1/300th of a degree in detector temperature will result in a 10C change in temperature readout.

To adjust for the temperature drift NUC (non-uniformity corrections)are made, this can occur as often as once a minute. but temperature readout can drift significantly between two non-uniformity corrections. As a non-temperature controlled radiation shield varies in temperature, this energy must be subtracted from the overall detector signal, which also reduces the overall dynamic range and results in reduced camera temperature ranges.

Developed over a three-year period with the aid of an $8.9 million dollar US Government military technology commercial-isation programme, Inframetrics’ second generation uncooled FPA detector technology includes the first microbolometer detector designed specifically for precision temperature measurement in an uncooled IR imaging system.

This package controls parasitic radiation from reaching the sensor. It is a small, robust, reliable and relatively low-cost vacuum package with an integral TEC (thermoelectric cooler). The system incorporates an internal temperature controlled radiation shield which is thermally coupled to the TEC. This shield is crucial to the control of background radiation and minimising offsets due to ambient temperature drift.

Inside surfaces of the radiation shield are covered with a high emissivity coating to minimise background reflections from reaching the sensor. Both the sensor and the radiation shield are mounted to a common diffuser circuit board which acts as a thermal interface to the TEC and an electrical interface to the sensor. This interface allows the radiation shield temperature to be controlled by the TEC servo loop over the system operating range.

Benefits include: significant reduction of system drift and improvement in measurement accuracy, elimination of the need for frequent non-uniformity corrections, the provision of wider dynamic ranges and larger temperature spans and the provision of more stable temperature measurements.

Now this technology is available to UK industry in the form of the new ThermaCAM Ultra X95 series of cameras. In addition to the performance benefits ensured by the uncooled detector, the UltraX95 includes several features that improve ease of use. These include direct access operation keys which are backlit for easy use in low-light environments, auto Hot Spot finder and a Delta Mode that calculates temperature rise automatically. ThermaCAM Ultra offers wide temperature ranges; -40 C to 90 C; -20 C to 200 C; 20 C to 500 C and 450 C to 1500 C.

In common with all models in the ThermaCAM range, Ultra operates from standard Sony camcorder batteries, offering up to 30% longer running times than other systems. A choice of display options is available for the system, including a colour LCD viewfinder or 3in colour LCD screen. New image documentation options include a bar code reader or a digital voice recorder. Data is stored digitally, allowing up to 500 images to be kept on a single removable memory card.

The digital memory allows the devise to be used with standard PCs and laptops so digitised data can be analysed in detail using purpose-designed software. In total, five different image processing programmes and two application-specific modules are now available. All programmes are compatible with Windows 3.1 and Windows 95/98 environments.

TherMonitor 95 is for multiple image analysis and comparison, ThermaGRAM 95 for real-time analysis of rapidly occurring events or time based studies, for pixel-by-pixel emittance correction there is CirPASS and Positrak gives high resolution for inspecting large PCBs and lets you mosaic several images.