Originally developed for surveillance and military operations, thermal cameras are now widely used for scientific research. Advanced IR cameras enable scientists to observe and study all types of environments, including the oceans, land, and sky (space). Different cameras can excel at different things, so researchers need to understand the tradeoffs between different types of thermal camera modules and the impact those differences will have on imaging performance. Specifically, users need to have a clear understanding of key camera specifications including resolution, sensitivity, and spectral range.
Thermal cameras capture infrared (IR) energy to create images through digital or analog video outputs, with the details defined by differences in temperature. A thermal camera is made up of a lens, a thermal sensor, processing electronics, and mechanical housing. The lens focuses infrared energy onto the sensor. The sensor can come in a variety of pixel configurations from 80 × 60 to 1280 × 1024 pixels or more. These resolutions are low in comparison to visible light imagers because thermal detectors need to sense energy that has much larger wavelengths than visible light, requiring each sensor element to be significantly larger. As a result, a thermal camera usually has much lower resolution (fewer pixels) than visible sensors of the same mechanical size.
Infrared cameras come in three basic types: short wavelength, mid-wavelength, and long wavelength. A mid-wavelength infrared camera was recently used to study butterfly wings. Mid-wavelength cameras typically detect infrared wavelengths in the spectral range of 2 to 5 microns, and they deliver higher resolution with accurate readings.
One important specification that is often overlooked at the expense of resolution, is thermal sensitivity, the specification that defines the smallest temperature difference a camera can detect. A thermal camera’s sensitivity will have a direct impact on the image clarity and sharpness that the camera can produce. Thermal devices measure sensitivity in milliKelvins (mK). The lower the number, the more sensitive the detector. Thermal sensitivity, also referred to as Noise Equivalent Temperature Difference (NETD), describes the smallest temperature difference observed when using a thermal device. In effect, the lower the NETD value, the better the sensor will be at detecting small temperature differences. Integrators and developers should look for manufacturers that can provide NETD performance at the industry-standard 30° C. The table below can be used to generally rate the sensitivity of a thermal detector.
Increased sensitivity makes thermal imagers more effective at seeing smaller temperature differences, which is especially important in scenes with low thermal contrast and when operating in challenging environmental conditions like fog, smoke, and dust. Selecting a more entry-level, essentially lower-cost thermal camera, that features “acceptable” to “satisfactory” thermal sensitivity results in a product that offers low contrast scenes resulting in poorer image quality, reduced detection range, and limited situational awareness compared to cameras with greater sensitivity. Devices with better thermal sensitivity are suitable for a wide variety of uses from search and rescue to research and science.
It turns out that butterflies are just as striking in thermal as they are in the visible light spectrum. According to a study published in Nature by researchers from Columbia Engineering and Harvard University, it’s possible to examine the thermodynamic properties of butterfly wings and the importance of radiative cooling in keeping these delicate structures fluttering. Nanfang Yu, associate professor of applied physics at Columbia, describes how thermal imaging played an important role in the study.
“It’s the most non-invasive way to measure temperature,” Yu explains. In the study, the team identified the complex living structures in butterfly wings that expertly aid in thermoregulation. With a thermal camera such as the FLIR T865, “you begin to essentially see the skeleton of the butterfly,” said Yu. “It’s almost like an x-ray — you are seeing the framework, the wing veins, the membrane… the whole cross-section of the wing material.” In thermal, the bright colors and patterns of a butterfly wing all disappear, and what you see instead is the underlying structure of the wing itself.
Past studies of butterfly wings have been limited by using equipment like thermocouples to measure temperature. Even the smallest probes are large compared to the thickness of a butterfly wing, and the act of measuring can affect the local temperature. Additional inaccuracies may occur because the measurements are only point-by-point. With thermal, “you can measure and map the entire temperature distribution,” said Yu. His team has been able to view and measure the difference in temperature between the wing veins, membrane, and other structures like scent pads. They found that the areas of butterfly wings that contain live cells (wing veins) have higher thermal emissivity than the “lifeless” regions of the wing (the membrane).
“This imaging technique enables us to examine physical adaptations that decouple the wing’s visible appearance from its thermodynamic properties,” Yu said in an article from Columbia Engineering. “We discovered that diverse scale nanostructures and non-uniform cuticle thicknesses create a heterogeneous distribution of radiative cooling — heat dissipation through thermal radiation — that selectively reduces the temperature of living structures such as wing veins and scent pads.”
Measuring the temperature of butterfly wings with thermal imaging isn’t without obstacles. “The challenge here is that, in the case of the butterfly wing, the thermal camera gives you a temperature reading, but you cannot trust the temperature reading,” said Yu. “The butterfly wing is semi-transparent in the infrared, so when you are looking at a butterfly wing in a thermal camera, you’re not just receiving the thermal radiation of the wing itself, you’re also receiving the thermal radiation generated by the background behind the wing.” A similar phenomenon can be observed with a thin sheet of plastic, like a plastic grocery bag, which just like a butterfly wing is opaque in the visible light spectrum but transparent in the infrared.
In addition to mapping out the thermal distribution of butterfly wings, the researchers also conducted behavioral studies that they observed in thermal. Using a small light as a heat source, they demonstrated that butterflies use their wings to sense the direction and intensity of sunlight. At the “trigger” temperature of approximately 40° C, all the species they studied turned within a few seconds to avoid the light and keep their wings from overheating.
This isn’t Yu’s first time using a thermal camera to study insects. “When I was joining Columbia in 2013, the FLIR camera was one of the first pieces of equipment I bought while I was setting up my lab,” said Yu. Though his research is mainly focused on nanophotonics, Yu is particularly interested in the intersection between biology, photonics, and physics. His research friends in the field of biology “often they probe me with questions regarding the life history of animals they are studying… I’m quite interested in helping them solve these mysteries from a physics and photonics point of view.”
In an earlier collaboration with a nanobiologist colleague, Yu studied Saharan silver ants, which forage during the heat of the day in one of the hottest terrestrial environments on earth. In this study, published in Science in 2015, the researchers also used a FLIR scientific camera to monitor the body temperatures of the ants. They wondered how such small insects could survive such harsh conditions. “The interesting thing here is understanding how small and light insects — tiny ants or the thin wings of butterflies —manage thermodynamically, because they are, by default, very bad at it,” explains Yu. Due to their small thermal capacity, small animals like insects can heat up to extreme temperatures within a few seconds.
The silver ants deal with extreme heat using the very fine hairs that cover their bodies. These hairs serve two functions: backscattering light in the visible and infrared wavelength to reduce the amount of absorption from solar energy, and enhancing thermal emissivity, so when the body of the ant is heated it can better distribute the heat in the form of thermal radiation.
“We wanted to find out how small animals were hardwired to survive extreme heat,” Yu says. His latest study continues exploring the question of how small insects manage to keep cool. Butterfly wings are covered with mechanical sensors to detect overheating, and their wing scales contain nanostructures that help facilitate radiative cooling. Besides the biological interest of these findings, Yu thinks they could serve as inspiration for the design of heat-resistant nanostructures and heat-sensing aircraft.
Yu and his colleague Naomi E. Pierce, Hessel Professor of Biology, plan to continue their research on butterfly wings. Pierce is the Curator of Lepidoptera at the Museum of Comparative Zoology at Harvard and has access to a large collection of butterflies and moths. They are currently conducting an extensive scan of the collection using a thermal camera and hope to gain an understanding of the factors that contribute to the design of a butterfly wing. Yu compares the work to “deciphering a complex book” because of the many diverse elements that have played a part in the evolution of the butterfly wing. Clearly, this is one book that’s worth reading closely to see what other discoveries we might uncover.
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