PURPOSE: To extract a data continuously by an arrangement wherein the two-dimensional image of voltage distribution on the surface of a panel irradiates the surface with an input beam of optical energy through an electrooptical modulation means.

CONSTITUTION: A modulation means 30 is directed to capture at least a part of an input beam 24 delivered from the first face 32 thereof to the means 30 and impinging on the second face 34 of the means 30 contiguous to the region 40 on the surface of 14 of a PUT 16. An exciting voltage source 15 is coupled with a conductor on the PUT 16 as a variable voltage source for the position 12 of the PUT 16. More specifically, the bias function and the exciting function are distributed between the power supplies 37, 15 while being separated. The voltage on the surface 14 or at the position 12 in the vicinity thereof interacts with the means 30 and the optical transmission of the beam 24 is varied through matching with the position 12. It is observed as a variation of spatial dependency in the parallel matching of an output beam 42 with respect to the beam 24 and the variation is proportional to the voltage at the position 12 on the surface 14.