The physical principle of the distributed fiber optic fire detector (lateral photovoltaic effect) was discovered by accident, and as with other discoveries in scientific research, such accident was the inevitable result of much analysis. The lateral photovoltaic effect was discovered and reported during research on the metal-semiconductor oxide (Cu-Cu2O) structure: when a beam of light shone on the oxide (Cu2O) surface, a current output was measured at the metal electrode on the edge of the oxide. Further research showed that the output photocurrent decreased exponentially with the distance between the incident light point and the electrode. Due to a lack of special attention to this phenomenon, it was not until 1957 that Wallmark studied the PN junction formed by N-type Ge and heavily doped P-type In+ semiconductor again. The distributed fiber optic fire detector illuminated a ring-shaped Ge-In semiconductor P+N with light, and the lateral potential difference between two points on the N-type layer was discovered again, with the potential difference approximately proportional to the displacement of the light point.

For this phenomenon, the linear fiber optic heat detector explained the physical mechanism of the lateral photovoltaic effect using the carrier transport theory in semiconductors. On this basis, test samples were prepared: electrodes on the junction surface output the lateral photocurrent excited by the incident light — usually called the point-electrode type or Wallmark-type electrode. Experimental tests were carried out on the response sensitivity of the lateral photovoltaic effect and its linear correlation with position. This research produced the experimental device of the PSD (position sensitive detector), made a comprehensive test and in-depth analysis of its basic characteristics, and laid the foundation for the research and preparation of PSDs. For the international research status of the PSD with lateral photovoltaic effect: an inhomogeneously illuminated semiconductor PN junction can produce the lateral photovoltaic effect, and its detailed physical explanation was completed by Lucovsky in 1960. On the basis of Wallmark's research, Lucovsky made a detailed and in-depth study of the lateral photovoltaic effect of the PN junction under inhomogeneous illumination. Using solid-state electronics theory combined with the current continuity equation and the diffusion equation, he established the mathematical relationship of the lateral potential difference distribution on the PN junction surface, described the distribution characteristics of the PN junction potential difference, derived the steady-state distribution function of the lateral potential difference considering the junction width and junction capacitance through rigorous theoretical reasoning, and analyzed the solution of the second-order differential equation with Green's functions, obtaining the dependence of the diffusion current on time.