1. An array of electric charges, either stationary or moving, in which the density of charge is proportional at each point to the light values of corresponding points in an optical image to be reproduced.
2. An electric charge measured from an arbitrary reference line which is used in finding the electric field set up by fixed electric charges in the area of a conductor.

The electric conductor, with its distribution of induced surface charges, is replaced by one or more of these fictitious charges.