Although long-wavelength infrared imaging technology is crucial in applications such as terrestrial remote sensing and astronomy, it faces a fundamental challenge from the overwhelming thermal background radiation. This background photon flux often pushes conventional detectors to the limits of their background-limited performance (BLIP). The main limiting factor here is not the intrinsic noise of the detector, but the shot noise of the background itself. In this paper, a key classification is demonstrated to distinguish between two superficially similar but fundamentally different detection architectures (difference detector and differential detector). According to the application and implementation of the detector, the background photocurrent of the conventional difference detector sets a background-determined threshold for the detectable signal difference, while the differential detector is a device that directly measures the differences of the target physical quantities at the physical perception level. Only the weak difference signals are integrated, resulting in extensive cumulative sampling to improve the signal-to-noise ratio to an unprecedented level. In particular, the differential detection technology path based on the quantum well infrared photodetector (QWIP) is introduced. QWIP provides an ideal physical basis for realizing high-performance long-wavelength infrared differential detectors with its extremely low dark current, precise electrical controllability and intrinsic spectral selectivity, and has made significant progress in experiments. Finally, Fisher information theory and Cramer-Rao bound are used to provide rigorous theoretical support for differential detectors.