Product research and development on internal combustion engines, brake rotors, tires, and high-speed airbags are just a few of the areas that truly benefit from high-speed, high-sensitivity thermal characterization testing. Unfortunately, traditional forms of contact temperature measurement such as thermocouples are not practical to mount on moving objects, and non-contact forms of temperature measurement such as spot guns — and even current infrared (IR) cameras — are simply not fast enough to stop motion on these high-speed targets in order to take accurate temperature measurements.
Without the appropriate tools for adequate thermal measurement and testing, automotive design engineers can lose time and efficiency, and risk missing defects that lead to dangerous products and expensive recalls. For example, US automakers recently recalled millions of cars, SUVs, and trucks due to faulty airbags with problems ranging from micro-cracks in passenger activation systems, to defective inflators. These flawed systems are not only dangerous to drivers, but also harmful to the bottom line for manufacturers who face lawsuits, fines, and loss of public confidence.
Next-generation IR camera technologies may offer engineers a solution. These cameras incorporate 640 × 512- pixel high-resolution detectors that can capture images at a rate of 1000 frames per second. Additionally, newer detector materials, such as strained layer superlattice (SLS), offer wide temperature ranges with a combination of great uniformity and quantum efficiency beyond that of earlier MCT and QWIP detector materials. These new technologies, plus the ability to synchronize and trigger remotely, give engineers and technicians the tools they need to address the difficulties of high-speed automotive testing.
The High-Speed Challenge
Measuring temperature on objects that are moving fast is challenging. Traditional forms of temperature measurement such as thermocouples are not practical for systems in motion. Non-contact forms of temperature measurement such as spot pyrometers lack the fast response rates necessary to take accurate readings on fast-moving objects or to thermally characterize a high-speed target accurately. Infrared cameras with uncooled microbolometer detectors are also unable to measure temperature accurately at extremely high speeds. These cameras have long exposure times that cause blurring in the thermal image.
In order to visualize and take accurate temperature readings on extremely fast moving targets, you need a cooled thermal camera with a short exposure time and fast frame rate. Let’s explore both detector types to better understand the benefits and drawbacks of each as they relate to high-speed thermal measurement.
Thermal vs. Quantum Detectors
The difference between thermal and quantum detectors comes down to how the sensor translates infrared radiation into data. Thermal detectors such as uncooled microbolometers react to incident radiant energy. Infrared radiation heats the pixels and creates a change in temperature that is reflected in a change in resistance. The benefits of uncooled microbolometer cameras include durability, portability, and low price. However, the drawbacks include slow frame rates — around 60 frames per second — and slow response times (time constant). Because of this, uncooled microbolometers can’t produce a crisp, stop-motion image of a fast-moving object. Instead, the slow frame rate and response time lead to blurring in the image and ultimately inaccurate temperature readings. Slow frame rates also prevent these cameras from accurately characterizing objects that heat up quickly.
In comparison, quantum detectors made of indium antimonide (InSb), indium gallium arsenide (InGaAs), or SLS are photovoltaic. The detectors’ crystalline structures absorb photons that elevate their electrons to a higher energy state; this changes the conductivity of the material. Cooling these detectors makes them very sensitive to infrared radiation, with some able to detect temperature differences of less than 18 mK or .018 °C. Quantum detectors also react quickly to temperature changes, with a time constant on the microsecond time scale, rather than multiple milliseconds. This combination of short exposure times and high frame rates makes quantum detectors ideal for stopping motion on high-speed targets for accurate temperature measurement, as well as proper characterization of how thermal temperatures rise over time on fast-heating targets. These cameras are generally more expensive and typically larger than uncooled microbolometer cameras — factors some research teams may need to take into consideration.
Fast Frame Rates are Not Enough
As mentioned briefly before, the ability to record hundreds or thousands of frames per second is only part of what is required to stop motion. Another element of the equation is integration time, or how long the camera collects data for each of those frames.
Integration time is analogous to shutter speed in a digital camera. If the shutter stays open too long, any motion in the image it captures will appear blurred. In the same way, IR cameras with long integration times will record blurred motion. A bouncing ball, for example, will look like a comet, with a trail of motion behind it.
The number of analog-to-digital converters, or channels, a camera has, plus the ability to process pixels at high speed, are also important. High-speed IR cameras typically have a minimum of 16 channels and have processing speeds — or pixel clock rates — of at least 200 MP/ sec. Most low-performance cameras have four channels and run at pixel clock rates below 50 MP/sec.
The temperature of your target can have an impact on integration speed and, ultimately, the digital count. The camera converts digital counts into radiance values used for the temperature readings on your target. Hotter targets emit more radiant infrared energy, thus more photons, while colder targets emit fewer photons. The challenge becomes how to accurately measure temperature on colder targets at fast frame rates, because fast frame rates require shorter integration times.