1. An apparatus for superimposing an overlay image on a portion of a field of view of a user looking through said apparatus at a scene external to said apparatus, said apparatus comprising: a display screen for generating said overlay image; an optical system for creating a virtual image of said overlay image which appears to said user as covering a portion of said field of view, said virtual image being in focus when said field of view is seen by said user as being in focus; and a transmission screen having a two-dimensional array of pixels that may be set to a first state in which said pixels are transparent and a second state in which said pixels are opaque, said transmission screen being located such that a pixel set to said opaque state blocks the user from viewing a portion of said field of view of said external scene, allowing the user to view only said virtual image in that portion of said field of view, said transmission screen being located at a distance from said user such that said transmission screen is out of focus when said virtual image is seen as being in focus by said user.