A light meter is a device that measures light.

There are two types of light meters:

• Incident light meter: Measures light falling on the meter itself. Those are characterized by a dome-like shape which is used to average incoming light.
• Reflected light meter: Measures light being reflected from an object towards the light meter.

All digital cameras have a reflected light meter built-in. Actually, they have several, up 1200 of them! If you put your camera to Spot metering mode, then it measures light from one area (between 2%-10% of the frame, configurable on some higher-end Nikon DSLRs) which is what hand-held reflected light meters do too.

This is how a digital camera measures exposure. Hence, there is little use for a stand-alone reflected light meter for digital photography. However, the read-value is highly dependent on the subject, since dark/less-reflective subjects reflect less light than light/more-reflective ones.

An incident must be placed very close to your subject, so that it receives the same light. Once you do that, the reading is good for that ambient light, no matter what color or how reflective is your subject. These meters are typically used in studio environments where light is setup for portraits or product photography. You only need to meter when the lighting changes.

Since these devices are separate from your camera, you have extra work to do to use them. First, you usually need to input the ISO and sometimes aperture or shutter-speed into the meter. When the reading is taken, the meter gives you the missing parameter, usually shutter-speed or aperture. You put these values into your camera in Manual exposure mode.