Thermal Detectors: Principles and Applications

This lecture discusses various thermal detectors, focusing on their principles and applications. It begins with the historical context of infrared radiation discovery by Sir William Herschel, who utilized a gas thermometer to measure temperature changes due to infrared exposure. The lecture then introduces the Golay cell, a type of photoacoustic detector, explaining its operational mechanism where infrared light heats gas, causing membrane deflection. The instructor highlights the challenges in miniaturizing Golay cells for camera applications, noting that while miniaturization is possible, it adversely affects sensitivity due to increased thermal conductivity. The lecture further elaborates on the general schematic of thermopiles, bolometers, and pyroelectric detectors, emphasizing the importance of thermal isolation for effective operation. The concept of thermocouples is introduced, detailing how they measure temperature differences using the Seebeck effect. The discussion includes the behavior of N and P semiconductors in thermocouples, illustrating how electric fields and voltage differences arise from temperature variations, ultimately leading to the development of thermopiles for imaging applications.