This chapter provides a selective account of important solar and infrared radiation observations and instrumentation developments. The ability to measure solar and infrared radiation has evolved through centuries of scientific discovery and engineering applications. Historically, measurements have played a critical role in the development of accepted physical laws. The measurement of direct solar radiation, the amount of “beam” radiation from the direction of the sun, is essential for understanding the total solar irradiance output from the Sun. Pouillet’s pyrheliometer was the first to be called a pyrheliometer and was designed as a calorimeter for measuring solar energy. Electrical leads from the strips and the thermojunctions are connected to the instrument controls. The Callendar pyranometer enjoyed considerable popularity during the first two decades of the twentieth century. Seeking improved measurement accuracy, Charles Greeley Abbot developed his pyrheliometer based on the principles of calorimetry. The bimetallic receiver, mechanical linkage, and recording drum were housed in a weather-resistant metal case.