Cooled Versus Uncooled Thermal Cameras
Cooled or Uncooled?
Thermal imaging cameras equipped with a cooled detector offer distinct advantages in comparison to their counterparts with an uncooled detector, albeit at a higher cost. In the realm of contemporary thermal imaging technology, a cooled thermal imaging camera incorporates an imaging sensor intricately linked to a cryocooler. This cryocooler effectively reduces the sensor's temperature to cryogenic levels, a crucial step to mitigate thermally-induced noise below the signal originating from the observed scene.
Cryocoolers, though highly precise mechanical systems, exhibit wear and helium gas leakage over time. Typically, they necessitate rebuilding after 10,000-13,000 hours of operation.
The question arises: under what circumstances should cooled thermal imaging cameras be favored for research and development (R&D) applications?
The answer is contingent upon the specific application at hand.
Cooled thermal imaging cameras prove invaluable when discerning minute temperature variations is imperative, when optimal image quality is paramount, in scenarios requiring high-speed capabilities, for temperature profiling or measuring exceedingly small targets, for visualizing thermal phenomena within precise electromagnetic spectrum ranges, or when synchronization with other measuring instruments is essential. In these instances, a cooled thermal imaging camera stands as the instrument of choice.
These infrared images depict a comparison of tire capture results while rotating at a speed of 20mph. The image on the left was acquired using a cooled thermal imaging camera. At first glance, it may seem as though the tire is stationary, but this effect is the outcome of the cooled camera's exceptionally rapid capture rate, which has effectively frozen the tire's motion. In contrast, the uncooled camera's capture rate is insufficiently swift to capture the rotating tire, resulting in the wheel spokes appearing transparent.
The thermal images displayed above illustrate the maximum achievable close-up magnification when using both cooled and uncooled camera systems. On the left, you'll see an image captured with a 4× close-up lens paired with a 13μm pitch cooled camera, resulting in an impressive 3.5μm spot size. On the right, an image was taken with a 1× close-up lens in conjunction with a 25μm pitch uncooled sensor, yielding a larger 25μm spot size.
Cooled cameras generally exhibit superior magnification capabilities compared to uncooled counterparts, primarily due to their ability to sense shorter infrared wavelengths. Thanks to the heightened sensitivity of cooled cameras, it's possible to use lenses with a greater number of optical elements or thicker elements without compromising the signal-to-noise ratio. This, in turn, enables more effective magnification performance.
Appreciating the enhanced sensitivity of cooled thermal cameras can sometimes be a challenge. How can one truly grasp the advantages of a 50mK sensitivity uncooled thermal camera when compared to a 20mK sensitivity cooled thermal camera? To shed light on this benefit, we conducted a brief sensitivity experiment. In this comparison, we pressed our hand against a wall for a brief moment, creating a thermal handprint. The initial two images capture the handprint immediately after removal, while the subsequent set of images reveals the thermal handprint's residual signature after two minutes. As you can discern, the cooled camera continues to detect most of the thermal features of the handprint, whereas the uncooled camera only retains a partial trace. Evidently, the cooled camera excels in discerning finer temperature distinctions and maintaining them over longer durations than the uncooled counterpart. This translates to the cooled camera offering superior target detail and the ability to detect even the subtlest thermal irregularities.
One of the significant advantages offered by cooled thermal cameras lies in their ease of implementing spectral filtering, enabling the revelation of details and measurements that would otherwise remain beyond the reach of uncooled thermal cameras. In the showcased initial example, we employ a spectral filter, which can either be positioned within a filter holder behind the lens or integrated into the dewar detector assembly, to capture images through a flame. The objective was to assess and characterize the combustion of coal particles within the flame. By applying a "see-through flame" spectral infrared filter, we tailored the cooled camera to a specific spectral waveband in which the flame becomes transparent. Consequently, we successfully imaged the combustion of coal particles. The first video, without the flame filter, exclusively displays the flame itself, while the second, with the flame filter, provides a clear view of the coal particle combustion process.
The second example uses a Nitrous Oxide filter that filters to where Nitrous Oxide is absorptive to the IR and therefore we can “see it” with the cooled thermal camera. The application was to design a better Nitrous Oxide mask and scavaging system; so the first video is imaging the older mask design and the second video is imaging the new mask design. As you can see, the older mask design is leaking a lot of Nitrous gas into the room, which is concerning for many reasons. The new mask design has minimal leakage and appears to be a better solution.