Method and apparatus for implementing discharge and develop, REaD IOI electrostatic printing machines that determine image area charge potentials of without using potentials within interdocument zones. The method includes charging a photoreceptor's image area to a charge potential, interrogating the image data to be used to produce a latent image on that charged image area to identify a white section, exposing the charged image area according to the image data to form a latent image, determining the potential of the white section of the latent image, and equating the potential of the white section to the charge potential. The printing machine includes a photoreceptor having a charge retentive surface of a sufficient length to hold a plurality of image areas; a charging station charging one image area to a potential that is to be determined; an image data source producing a digital representation of a latent image that is to be produced; an exposure station exposing the image area to produce a latent image; a data interrogator for identifying white sections; and an electrostatic voltmeter for measuring the potential of the identified white area.