Image pickup tube with screen and field grids

Claim 1

An image pickup tube comprising a photoelectric cathode (8) emitting electrons as a function of impinging radiation in accordance with the image; a porous storage target (12) of the secondary emission induced conductivity type; means scanning reading electrons over said target; means to accelerate and focus the electrons including: a fine mesh field grid (19) having a transparency of at least 60 percent and closely spaced from said target (12), said field grid being connected to a source of potential of such magnitude relative to the spacing of the grid that the electrical field between grid (19) and target (12) is substantially uniform and a screen grid (FIG. 3:37) having a transparency of more than 80 percent located between the target (12) and said first grid (19), said screen grid being located close to said target, said field grid (19) being formed of wires of from 5 to 8 microns diameter, having a mesh spacing of from 25 to 30 mesh per millimeter and located from 5 to 10 millimeters from the target; said screen grid being located from the target by a distance of from 0.2 to 1 millimeter, and formed of wire of from 5 to 8 microns diameter, having a mesh spacing of from 12 to 20 mesh per millimeter.