In the tidbits of some star videos, there is always a staff member holding a tool, running to his head from time to time and watching it over and over again. What is this?

The tool taken by the staff is called an exposure meter. Exposure meter is an instrument to measure the surface brightness of the object being photographed or the luminous intensity of the luminous body. In the process of photography, the exposure meter can accurately provide the illumination or brightness of the subject through various known conditions and instantaneous objective conditions, and provide the photographer with the combined parameters of aperture and shutter used in photography. Exposure meter is an indispensable tool in professional photography.