Actinometer is an early photographic device used to measure the intensity of light and calculate the correct exposure for film or photographic paper. Typically consisting of light-sensitive materials such as silver salts or chemical emulsions, actinometers helped photographers determine how long to expose their materials to achieve proper image density. While largely replaced by modern light meters and digital exposure tools, actinometers were essential in the early days of photography for achieving consistent results and understanding the behavior of light on photosensitive surfaces. Knowledge of actinometry remains relevant in historical photographic processes and experimental photography.