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Both the photocathode and the image plane of such an electrode configuration are curved concave as seen from the anode aperture.

The field-flattening property of the biconcave fiber coupler can be utilized to alleviate the peripheral resolution losses resulting with a flat phosphor-screen or coupling member.

For the same reason, the output fiber plate is planoconcave, its exposed flat side permitting contact photography if a permanent record is desired.

As it will be shown later, the field-flattening properties of the interstage and output fiber coupler comprise indeed the main advantage of such a design.