an optical probe beam traverses both crystals and probes their refractive indexes a polarizer analyzer optics arrangement may be used to detect passage of charged particles through the gap between the two crystals the arrangement of two crystals may be chosen such that the dependence of the magnitude of the electro optic signal is minimized with the beam position the detected optical phase shift either inteferometric or polarization rotation is proportional to the amount of charged particle beam current and can be deployed to cancel noise in a primary beam this detection method is limited by the laser photon beam shot noise which can be made smaller than the charged particle beam shot noise if the number of photons per detection event is greater than the number of charged particles per detection event therefore this could provide for a noise cancellation method for the electron beam shot noise and allow for throughput increase less electron per pixel required since primary beam current is now measured according to another aspect of the present disclosure an electro optic apparatus may be configured to operate as a beam position monitor bpm signals of electro optic rotation occurring in the eo materials on either side of a vacuum gap through which the charged particle beam passes are read separately in each material and subtracted the resulting signal is linearly dependent to the charged particle beam 39 s position relative to the two crystals hence the processed signal corresponds to the position of the beam centroid the sum of the signals on the other hand would also provide a beam current reading by using pairs of crystals oriented along the x and the y directions the position of the center of the charged particle beam can be read.