THERMAL IMAGING & VISION SYSTEMS FEATURE
Stop-motion image of inflating airbag
ADVANTAGES OF LWIR SLS THERMAL CAMERAS T
hermal infrared cameras have reimagined how we perform thermal
measurements for research and science testing. In recent years, we have seen significant readout and camera electronic advances that push the limits of resolution, speed and sensitivity. This allows us to solve many of the most difficult thermal testing challenges, such as high speed thermal measurement on air bags, failure analysis on micron-scale electronics, and optical gas imaging on visibly translucent gases. However, it was not until the recent introduction of Type II Strained Layer Superlattice (SLS) that we saw significant advances in thermal imaging. This new detector material brings thermal camera performance in line with their read-out integrated circuit (ROIC) and camera electronic counterparts. The integration of SLS into commercially available thermal cameras offers a new longwave IR (LWIR) solution with significant improvements in speed, temperature range, uniformity, and stability that costs less than analogous detector materials.
SPEED IMPROVEMENTS While SLS works in both the longwave and midwave infrared bands, you will see the biggest performance benefits when it is filtered to the LWIR band exclusively. In fact, one of the key benefits of SLS is its short integration times, or snapshot speeds, compared with other infrared camera materials. Tables 1 and 2 demonstrate the difference between the LWIR SLS and the MWIR indium antimonide (InSb) performance metrics. Looking just at the first temperature range in row one, we see that SLS offers 12.6 times faster snapshot speeds than that same range for the MWIR InSb detector camera. Faster snapshot speeds allow you to
Table 1 (Top): LWIR SLS Camera Performance Metrics; Table 2 (Bottom): MWIR InSb Camera Performance Metrics
stop motion on high speed targets in order to get accurate temperature measurements. If the integration time is too slow, blurring in the resulting image could impact temperature readings. Similarly, faster snapshot speeds allow for faster frame rates. Quite often, the long integration time requirements of InSb and other detector materials cause the camera to operate at a frame rate that is slower than the detector maximum. For example, say you have a camera that can image at 640 x 512 at 1,000 frames per second, but it operates in a bandpass that requires an integration time of 1.2ms. The camera would not be able to achieve its full frame rate potential due to the longer integration time constraint. This can cause problems when imaging targets that heat up quickly. Slower sampling
can cause you to inaccurately characterise the thermal transient of your part, perhaps missing a critical temperature spike in the boot up cycle on an electronics board.
WIDER TEMPERATURE BANDS A second benefit of LWIR SLS thermal cameras is their wider temperature bands. In Table 1, we see that the LWIR SLS camera has a starting temperature range from -20°C to 150°C with one integration time. To achieve the same temperature band with MWIR InSb, you would need to cycle through (superframe) three integration times, each representing a different temperature range. Cycling through three temperature ranges in order to superframe them into one complete -20°C to 150°C temperature range results in only one superframe image per three frames captured from the camera. This means three times more work when calibrating the camera as well as a one-third reduction in overall frame rate. Looking at Tables 1 and 2 again, we see there is another point to consider: LWIR SLS cameras allow you to measure higher temperature ranges before needing an ND filter. The SLS camera evaluated allowed for measurements up to 650°C before needing an ND filter, whereas a MWIR InSb camera only measures up to 350°C before an ND filter is required. This is partly a function of the SLS operating in the LWIR band versus the InSb operating in the MWIR waveband. To illustrate this, let us look at the
graph in Figure 1, which shows the spectral emissive power of a 30°C ideal blackbody. The area under the curve represents the power within that waveband, which is much larger for the LWIR band than the MWIR band. When
INSTRUMENTATION | NOVEMBER 2017
FLIR Systems’ Ruud Heijsmann discusses how the integration of SLS into thermal cameras is demonstrating significant improvements in speed, temperature range, uniformity and stability
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