Evaluation of three thermal imagers for skin temperature measurement using a blackbody source and a spatial resolution test object

BACKGROUND. We evaluated three modern focal plane array uncooled microbolometer thermal imagers to assess their suitability for measuring skin temperature in a clinical setting. The imagers were the FLIR A40M, A320, and the portable E30. METHOD. Test equipment comprised of the Land P80P blackbody source and a heated spatial resolution test object. For each camera we tested drift in reading after switch-on and with varying ambient temperature, agreement between the device and the blackbody across the typical skin temperature range, linearity, uniformity of reading across the image field, and spatial resolution. RESULTS. When measuring a fixed blackbody cavity temperature, all three cameras gave stable temperature readings to within ±0.2°C within 45 minutes of switch-on. Agreement between the blackbody and imager was different for each camera: the bias was 0.83°C for the A40M, -0.08°C for the A320, and 0.09ºC for the E30. Plots for linearity showed r2>0.99 for each camera. When the blackbody was placed in the four quadrants of the field of view, the maximum difference in temperature recorded between quadrants by any of the cameras was only 0.2°C. When ambient temperature was varied across an 8°C range, no camera drifted in its reading of the blackbody temperature by greater than 0.4°C. The A40M and A320 detectors (320 x 240 pixels) showed a better spatial resolution than the E30 detector (160 x 120 pixels). CONCLUSIONS. All three cameras showed a radiometric performance that would be suitable for most skin thermal imaging applications. The offset bias of the A40M device highlighted the importance of regular quality assurance: this offset could be easily corrected once detected. Of particular note was the performance of the low-cost E30 device: despite its limited functionality and low spatial resolution, it performed comparably to the more expensive devices in the blackbody tests.