This review offers new perspectives on the subject and highlights an area in need of further research. It includes an analysis of current scientific literature mainly covering the last decade and examines the trends in the development of electronic, acoustic and optical-fiber humidity sensors over this period. The major findings indicate that a new generation of sensor technology based on optical fibers is emerging. The current trends suggest that electronic humidity sensors could soon be replaced by sensors that are based on photonic structures. Recent scientific advances are expected to allow dedicated systems to avoid the relatively high price of interrogation modules that is currently a major disadvantage of fiber-based sensors.

We have analyzed the various noise processes occurring in photon infrared detectors operating at room temperature and show that background-limited performance for photon detectors is potentially achievable at room temperature over the 3–13 μm band even in restricted fields of view. We discuss practical embodiments in which this might be realized and give quantitative estimates of the material properties required to achieve this performance. A critical feature to achieve the highest performance is that devices within an array should not be radiatively coupled to each other.

Using heterostructures of InSb/In1−xAlxSb, grown by molecular beam epitaxy, diodes have been fabricated which emit at room temperature with a peak wavelength in the range 5.5–5.8 μm and an internal efficiency of 0.2%.

Long wavelength diodes in Cd_xHg_1−xTe show large deviations from ideality in their reverse characteristics. The excess currents are attributed in many published papers on band to band tunneling at high reverse bias and to trap assisted tunneling at low reverse bias. Measurements of photocurrent multiplication, current–voltage characteristics, and noise have been made on long wavelength loophole diodes to determine the breakdown mechanism. This has produced strong evidence that the reverse characteristics of good quality diodes of this type are limited by impact ionization. At higher biases, there is evidence of an additional breakdown mechanism, probably tunneling.