Radiometry is a method used to measure electromagnetic radiation. It measures the entire spectrum of light wavelengths, visible and non-visible (visible, ultraviolet and infrared light). Photometry is the term used when considering the range of visible light wavelengths exclusively. The main difference between the two is that radiometry measures how light properties behaves, spreads, and interacts with physical material in space. Where photometry is concerned with the interaction of light with the human eye.