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A substrate-free microelectromechanical system (MEMS) focal plane array (FPA) has been successfully developed. Since there is no silicon substrate beneath the FPA's pixels, the infrared radiation (IR) loss has been significantly reduced. However, at ambient pressure, it is difficult to obtain a clear visible image due to the sensitivity loss caused by the thermal conductance of air. To improve the packaged FPA's performance, its thermal sensitivity and response time were investigated under varying air pressure. The high-contrast IR thermal images of a human hand were captured at room temperature and under an air pressure of 0.01 Pa, and the noise-equivalent temperature difference of the FPA was also estimated to be about 373 mK compared with the 1.4 K under a pressure of 10 Pa. What is important is that the FPA's response time was increased from 50.1 to 336.6 ms by the decrease in air pressure. In addition, we present an approach to package the MEMS FPA at a low-pressure vacuum with two optical windows, which is an effective way to increase FPA's sensitivity.