What’s hot? – thermal tracing applications

Materials World magazine
2 Jul 2009

Thermal tracing equipment used for military applications is successfully being transferred to applications such as wildlife welfare and clothing. Chris Lavers, Senior Lecturer in Sensors and Stealth at Plymouth University at Britannia Royal Naval College, Dartmouth, UK, reports

Military devised technology allowing one to see in darkness or through heavy dust and smoke is now widely available for civilian use. Thermal imagery provides an effective ‘swords into ploughshares’ technology for applications including wildlife assessment and management. It allows the thermal properties of fur and feathers to be examined or used as templates for thermal material design. Also climate change and a rising human population are increasing pressures on wildlife, so high-tech monitoring could provide critical information to preserve biodiversity.

Working with wildlife

The imaging approach – developed from comparative visible, near infrared (NIR) and thermal observation comparisons – does not stress or harm animals under observation or endanger zoo staff. Unlike night sights, the technology can also be used in the day and gives calibrated temperature measurements that help veterinary staff.

However, heat cameras cannot see through glass, and water absorbs heat, so the method cannot be used to directly view marine species. Removal of marine organisms from water for short periods may be complemented with NIR imagery in shallow water (littoral) environments. Marine mammals, such as otters, may be monitored at the water/air interface. Passive thermal imaging is less obviously suited for reptiles, which have little internal heat.

At Paignton Zoological Park, and the Buckfast Butterfly and Otter Sanctuary, both in Devon, UK, heat cameras demonstrate how endangered wildlife species, that are vulnerable to man’s activities and climate change, may be researched in captivity without stress.

Evaluation of material parameters in existing biodiversity has the potential to create templates in future biomimetic designs to boost biodiversity. Zoological parks provide excellent centres to observe animals from different habitats, with techniques applicable to management and evaluation in the wild. This work extends non-radiometric comparative visible, NIR and heat wildlife photography to focus on radiometric calibrated data.

Testing the temperature

All animals emit heat, dependent on temperature and skin emissivity (e). Radiant heat distribution is proportional to T4 in absolute temperature (T) and skin emissivity, consequently, small temperature changes create large heat variations which are especially detectable at night when there are less confusing heat reflections. Human skin has e=0.98, so only two per cent of heat is reflected at the skin/air boundary. High emissivity means animals absorb and radiate most of the incident energy. As many animals radiate heat well, skin temperature distribution can be monitored remotely. Using a cylindrical model approach, typical animal output power levels are modelled in the table (top, right).

Peak emission wavelength λpeak, is related to T:λpeak=2,900/T measured in micrometres. For a rhino at 27ºC, this is near 10µm. Animals near outdoor ‘ambient temperature’ have peak emission in the far infrared, but, for most wildlife, radiated power levels are unknown. A baseline of ‘healthy’ animal parameters provides diagnostic data for comparison to ‘sick’ animals. Traditional ‘contra-lateral’ (left vs. right hand) methods for non-radiometric cameras are used qualitatively and successfully by vets, but temperature data are also quantitatively useful. Because infrared cannot be seen by eye, detected heat is converted to visible light, and colours are chosen to increase contrast for specific details, which might otherwise be missed.

Thermal health

Radiated intensity levels vary considerably between species, with the bull African elephant radiating over 13kW. Elephants cannot sweat and rely upon efficient heat transfer via their ears, which permit large area evaporation and appear cooler than other parts. A hot trunk is seen alongside tusks at near ambient background temperature. The visible tusk is largely dentine (ivory) with an outer enamel layer.

Veterinary work has observed many heat related issues, for example, a pregnant beagle will radiate more abdominal heat than a non-pregnant control, and may be up to 6ºC warmer. Radiometric cameras allow accurate evaluation of a pregnant animal’s surface temperature at an early stage of pregnancy (24 days) through to birth.

Day-time thermal analysis can reveal confusing complexities. Zebra images (see image right, middle) obtained outside under strong sunlight show that dark pigments absorb visible and heat radiation, while white pigments radiate heat directly and heat from absorbed visible light, as well as reflecting thermal radiation. Dark visible regions appear brighter in thermal bands than visible white stripes, which contribute little to daytime heating.

Strong patterning creates turbulent thermal mixing around animals, while in the visible, temperature induced refractive index changes make zebra silhouettes shimmer, creating a ‘shifting mirage’ rather than a clear static outline. Zebra retain residual stripe patterns because they experience day heating, without strong heating stripe patterns are absent in the heat bands. A similar problem occurs with naval camouflage, which must not heat up via visible to heat conversion mechanisms. Natural concealment will, no doubt, inspire future generations of camouflage design. Black stripes have high emissivity and low visible reflectance – so black stripes absorb visible radiation and absorb/reradiate heat efficiently, giving a surprising ‘negative’ of a visible image.

Butterfly wings

Recent biological research may offer templates for future sensor systems. The Morpho butterfly’s blue iridescence results from light scattering from multilayer diffraction elements in its periodic wing photonic structures. Such features are found among many insects.

Future man-made photonic and metamaterial periodic arrays will reflect some wavelengths and absorb others. Pigments responsible for visible absorption often absorb in the NIR, but iridescent feathers and butterfly wings result from physical photonics structures rather than pigmentation. Little is known about the thermal qualities of these delicate layered structures.

Visible light absorbed in a photonics structure can be reradiated as heat. Although thermography does not harm fragile butterfly wings, passive ambient infrared imagery does not reveal significant detail. This is because there is limited material with neither a large heat specific capacity nor significant blood supply, and so wings are poorly contrasted against the ambient background. Instead, a non-destructive test (thermographic signal reconstruction) from UK-based TWI for inspecting space shuttle wing leading edges was used to reveal sub-surface flaws and irregularities.

As a sample cools, its surface temperature is affected by internal structural flaws that obstruct heat flow, revealing previously invisible detail. The zebra butterfly wing generates temporal changes. This temporal technique may be valuable in shallow depth analysis of thin biological specimens, but cannot show physiological or circulatory features. At thermal wavelengths, surface interference effects in the visible blue no longer function, but visible to heat transfer can result in the complementary thermal patterns of the Peacock.

Natural photonics structures create visible interference, but are ineffective at heat wavelengths. The only interaction is via absorption. Peacock’s black pigment below photonic structures will absorb nearly all the incident visible light, so weak visible interference from surface structures is seen. At high elevations some butterflies abandon iridescence for light-absorbing brown to survive colder temperatures. Heat radiates into wings from the butterfly’s body and vice-versa. At altitude, iridescence has no obvious benefit at heat wavelengths. Butterfly photonic structures create modest visible blue interference flashes but absorb heat poorly, whereas brown wings have greater light absorption and weak thermal absorption, so light is absorbed – generating heat.

Clothing applications

Man-made photonics structures with constructive interference in the heat bands may be incorporated into spacesuits or desert garments to mitigate heat loss effects. Researchers are looking at using biological templates to design man-made absorbing materials with improved light and thermal absorption, for example, by investigating gold/palladium deposition onto Morpho wings.

Such structures can be designed to have low light absorption and low light to heat transfer. Depending on the precise thermal interference structure on the underpinning natural absorbing biochrome layer, little or much heat may be reflected from a designed surface. Artificial photonics structures may also have applications in the design of thermal wavelength filters.

The most promising imagery has been revealed by looking at the thermal properties of live butterflies. Hatched butterflies channel hemolymph into their wing structures, initially in a non-uniform manner before developing characteristic dragonfly circulation loops in the upper wing surfaces. These circulation loops capture heat and transfer it efficiently to the main body. Once the body is sufficiently warm, some degree of regulation is observed.

Similarly, a butterfly does not want to lose heat to its wings in cold weather. This is vital for large butterflies and moths that have a greater area over which to lose heat, as seen in the homogeneous thermal pattern of the Attacus Atlas moth found in southwest Asia.

The thermal behaviour of many animals, if replicated, may provide thermally efficient fabrics and materials, could, as well as highly structured electro-optical sensor components. Low emissivity paints or coatings developed from these principles for building structures may reduce the 42% of non-transport energy used in the UK to heat buildings.

Further information: Chris Lavers