What is that thing you look at next to the model's face with a small instrument in professional photography?

That exposure meter is divided into incident exposure meter, incident light meter and reflected light meter. The exposure meter in the camera we use is a reflective exposure meter, also known as behind-the-mirror metering.

However, the exposure instrument in the camera has a defect, that is, the metering area is too large, which leads to inaccurate metering. Even metering, metering area will account for more than 3% of the picture, which will lead to the failure to correctly expose the scene you want to expose. When shooting portraits, the luminosity of faces is very strict. Usually, the exposure meter behind the mirror can't meet the photometric accuracy. So the photographer chose to hold the event exposure instrument. This kind of exposure meter is simple to use, accurate in degree and widely used in artistic photography. This is very useful for the correct exposure of the work.