The invention consists of a variable transmission field mesh which has the unique capability of isolating the uniform, accelerating electrostatic field in the photocathode region from the focus field in the drift tube while both regions are effectively isolated from the control signal. This isolation controls the electronic image intensity after the image has been generated by the photocathode, but before the image enters appreciably in the drift-tube region, without seriously degrading characteristics of the image tube such as sensitivity, resolution, and image distortion. The invention also contemplates intensity control of the output signal from the tube by actually sensing the intensity of the electron image as well as the transmisivity of the storage grid and feeding the resulting signal back to the variable transmission field mesh. A preferable construction is with a double-decked sandwich field mesh consisting of three conducting, aligned meshes separated by two insulating or vacuum layers. The middle mesh is the control mesh while the one facing the photocathode controls the field at the photocathode and the mesh facing the drift-tube maintains the drift-tube electrostatic field at essentially zero. The potential on the control mesh may vary from a value considerably below the potentials of the other two meshes for low transmission to considerably above for high transmission. Essentially, the invention relates to an electronic iris for controlling intensity of the signal in an electronic correlation tube through signal enhancement or intensity control utilizing a control mesh in association with the storage grid. Essentially, the invention relates to an electronic iris for controlling intensity of the signal in an electronic correlation tube through signal enhancement or intensity control utilizing a control mesh in association with the storage grid. Heretofore, it has been well known to utilize electronic image storage tubes for the purpose of correlation of similar optical images, such as shown in prior art U.S. Pat. Nos. 3,290,546 and 3,239,766. However, in these prior art systems there is no control for the particular intensity or light conditions of the optical input images. Therefore, for example, when the optical input image is an actual present image, the light intensity can vary considerably depending upon the atmospheric conditions and the actual location of the detector. Some type of iris control or shutter control is necessary, be it mechanical or electrical, to appropriately control the light intensity so that an image of constant average intensity is achieved. The general object of the invention is to meet the needs of the art by providing an electronic iris and intensity enhancement or reduction control in a correlation tube by controlling the signal in proportion to the intensity of the optical input signal, either before storage on the storage mesh, or after the storage mesh has performed its storage function, so as to provide an image of average intensity in both the write and read modes of operation. The aforesaid object of the invention and other objects which will become apparent as the description proceeds are achieved by providing in a correlation tube the combination of a closed tube drawn to a vacuum, a photocathode at one end of the tube, and an electrical anode at the other end thereof, means to direct and focus electrons emitted from the cathOde down the tube towards the anode, a storage mesh mounted in the tube between the cathode and anode adapted to collect electrons and represent an optical image depicted by the electrons as an electrical image, and means to control the storage capability of the storage mesh in proportion to the intensity of the light impinging on the photocathode.