Since film exposure is controlled by the camera's shutter and aperture settings , it is essential to know what those settings should be in order to take a picture that is properly exposed. A photographer must measure the amount of light to know how much of it should be allowed to strike the film. With this knowledge, the photographer can then accurately select shutter and aperture settings so that no more and no less than the proper amount of light reaches the film. These terms all amount to the same thing. There is no difference in their meaning, and any one of these terms may be used throughout this site and in photography books and manuals to refer to the taking of a light meter measurement.
All exposure meters have light-sensitive cells.
They convert light energy into electrical energy, which can be measured more easily. There are distinct advantages to both types, and most professional photographers will have at least one of each type of meter. A third type, a multipurpose meter, is a combination of the two, and can be set to read either reflected or ambient light. Nearly every camera sold today has a built-in exposure meter. Exposure meters that are built into cameras are reflected light meters, which means they measure the light reflected from a subject, its luminance.