This lecture covers the concepts of radiometry and photometry, focusing on the measurement of electromagnetic radiation and light within specific wavelength ranges. It discusses the history of photomedicine, including the anatomy of the human eye, age-related macular degeneration, and photodynamic therapy with Visudyne®. The instructor explains the principles of radiometry, such as radiant energy, irradiance, radiant power, and radiant flux, as well as photometry terms like luminous energy, illuminance, luminous flux, and luminance. Various units and conversion factors in radiometry and photometry are also explored.