I saw the photographer in the studio holding a sensor to test the aperture size of the camera before taking pictures. What is the scientific name of this sensor?

An exposure meter is an instrument used to measure the intensity of light. The exposure meter is used to determine the appropriate exposure time in photography. When the film sensitivity and shutter speed are known, the exposure meter gives the aperture value to obtain the best exposure. Exposure meters are also used for lighting to control the brightness of lighting, or as hand-held instruments to ensure that the light intensity meets certain requirements.