The disclosure relates to image intensifier tubes of the proximity focusing type, wherein it especially concerns the positioning of a primary screen with respect to a slab of microchannels. An image intensifier tube comprises a sealed chamber containing a primary screen and a slab of microchannels. The slab of microchannels is fixed to the body of the chamber. According to one characteristic, the primary screen is fixed to the slab, from which it is kept at a distance by means of at least one insulating shim. The result thereof is greater precision and greater uniformity of the spacing between the primary screen and the slab of microchannels.