There are various types of meters available to make photography easier and more accurate. The most common meters are light meters, color meters and flash meters. Most cameras have built-in exposure meters, but some people prefer separate meters. Built-in exposure meters basically read the light reflected. However this light measurement is sometimes inadequate as built-in meters can fail to compensate for high or low contrast images. For accurate photography you need proper exposure control. Built in meters sometimes read the situation inaccurately and result in underexposed photographs.
Handheld meters enable more control and accuracy. The incident meter, which measures the amount of light falling on the subject, is suitable for studio photography where accurate illumination balance and color reproduction are necessary. If your subject is far away, reflected light type meters that measure light reflected from all areas of a scene to the camera are better.
Older meters did simple jobs such as measuring ambient light. Newer meters are more advanced and can read short pulses of light from electronic flash units. Meters work when an electrically charged coiled-galvanometer movement reacts to light reaching a photocell. This reading is measured in arbitrary light value units or EVs (Exposure Value). It is then transferred to a calculator dial which gives you the correct combination of aperture and shutter speed. In flash-reading meters, the shutter speed is a fixed value. Other meters often have the galvanometer movement and calculator dial in one function.