Actinometers are instruments used to measure the amount of radiant energy that is absorbed by an object. There are several different synonyms for actinometers, including pyranometers, radiometers, and photometers. Pyranometers are specifically used to measure the amount of total solar irradiance that is received at a particular location, while radiometers are designed to measure the amount of radiation that is emitted by a source. Photometers are instruments that are used to measure light intensity, and they are often used in photography and other imaging applications. Regardless of the specific type of instrument being used, actinometers are an essential tool for measuring the amount of radiation that is absorbed by various objects and materials.