The low-frequency noise is a ubiquitous phenomenon and the spectral power density of this fluctuation process is inversely proportional to the frequency of the signal. We have measured the 1/f noise of a 640×512 pixel quantum well infrared photodetector (QWIP) focal plane array (FPA) with 6.2μm peak wavelength. Our experimental observations show that this QWIP FPA’s 1/f noise corner frequency is about 0.1mHz. With this kind of low frequency stability, QWIPs could unveil a new class of infrared applications that have never been imagined before. Furthermore, we present the results from a similar 1/f noise measurement of bulk InAsSb absorber (lattice matched to GaSb substrate) nBn detector array with 4.0μm cutoff wavelength.