An 'incident' meter measures the amount of light falling on the scene. A reflectance meter measures the amount of light being 'reflected' from a surface in the scene.
Meters are 'calibrated' to give an exposure indication which yield a certain 'density' in Film film. While the method of determining the ISO for a digital camera is different, the general goal is to have the esposure result in known levels.
One can determine one's own 'exposure index', by using a grey card, a calibrated step wedge or similar to find an 'personal' ISO value.
For reflectance meters:
In practice, one has to take in to account the approximate reflectiveness of objects in the scene. Black 'paper', 'dark' wood, etc, may be 5-10% reflective, or less depending... caucasian skin tone may be 30% reflective... a blank white sheet of paper 80-90%, etc.
So if one assumes that a meter is calibrated to an 18% grey card... then if one takes reflective readings of those items listed above, one would be off by one or two stops.
Hence one needs to 'adjust', usually mentally, for such surfaces.
Since the incident meter reads only how much light is falling on the surface, the exposure the meter indicates, has to be interpreted in the sense of 'if there were an 18% grey card in the scene". However, one does not have to adjust based on the reflectivities of the objects...
A 'spot meter' is a form of reflectance meter which has a narrow area of sensing the light, and usually allows the photographer to obtain readings at a distance, and over a 'small' area, as most non-spot reflectance meters read a 30 degree angle, which can include other objects, which have different reflectivities. For example a black cardboard square sitting beside a white cardboard square. The wide angle of the ordinary reflectance meter would yield an average of 'black reflectance + white reflectants'... whereas a spot meter would yield two exposure readings for each of the squares.
Sure you can move close to the subject, but if one is too close, then one's own shadow may figure into the reading and skew the results... or if one is taking reading on a person's face... one can not realistically get 'close enough', or at an angle that matches the camera's view... and so a spot-meter would allow one to take such a reading off a small area.
Edited by jeclark2006, 25 June 2014 - 01:30 PM.