The Incident Meter

The incident meter is something of the opposite of the reflective meter. It measures the light falling on the subject, rather than that being reflected from the subject. These are the most common type of handheld meter.

The incident meter uses a diffusing dome over its sensor, which is pointed at the camera from the subject. The ASA (film speed) is set, then the meter is activated. Depending on whether the meter is analog or digital, you either dial in the given exposure value, or with digital, the meter will display an exposure setting.

Gossen Luna-Pro S
Gossen Luna-Pro S. From the top, the diffusion dome, the Exposure Value scale, time and ƒ/stop scales, ASA/ISO setting. The meter is activated by the toggle switch on the right side.
Sekonic L-398A. The great thing about this fully analog meter is that it does not require batteries. Similar in operation to the Luna-Pro, the meter is activated by pressing the button in the center. The scale is in footcandles. It is still being manufactured, and is fairly inexpensive.
Sekonic L478DR
Sekonic L478DR. This electronic meter is fairly typical of contemporary meters, although the touchpad interface is somewhat unique. It is a multifunction meter, able to be used in incident (ambient) mode (as shown), as a strobe meter, or as a reflective meter, with an optional attachment. In incident mode, it can be set to either aperture-priority or shutter-priority.