A vectograph is a type of stereoscopic print or transparency viewed by using the polarized 3D glasses most commonly associated with projected 3D motion pictures. Credit for the concept of the vectograph is due to Joseph Mahler, cousin of composer and conductor Gustav Mahler. He immigrated to the US from Czechoslovakia in 1938 and was hired by the Polaroid Corporation, where he worked with its founder Edwin Land, to develop his idea into a practical process. An elementary vectograph is a polarizing filter sheet that encodes a photographic image as areas that polarize light more or less strongly, corresponding to the darker and lighter areas of the image. When the sheet is viewed by itself in ordinary light, a pale image becomes visible. When viewed through a uniformly polarizing filter sheet oriented in the same plane of polarization, the image is almost completely invisible. If either sheet is rotated 90 degrees about its axis, so that their planes of polarization are at right angles, the image becomes boldly visible. If two such images, made to polarize in opposite directions and each encoding one of the images of a stereoscopic pair, are superimposed and viewed through glasses containing appropriately oriented polarizing filters, each eye sees only one of the images and a single three-dimensional image is perceived by people with normal stereoscopic vision. Vectographs in their native form are transparencies, to be viewed by transmitted light or projected onto a suitable non-depolarizing screen, but by limiting the density of the images and backing the vectograph with a non-depolarizing aluminum-based paint, a print for viewing by reflected light can be produced. Vectographic prints and transparencies can serve many of the same purposes as their anaglyph equivalents, but with the visual advantage that they do not require the use of viewing filters which are of disturbingly different colors for each eye. However, while anaglyph images can be produced by virtually any photographic or mechanical printing process capable of producing a two-colored image, vectographic images require specialized materials and printing technologies, limiting their practical application. During World War II, stereoscopic aerial reconnaissance photographs were printed in the form of vectographs, both as reflective prints for use by troops in the field and as transparencies to be projected onto a screen for group viewing and discussion. Instructional use was made of projected vectograph slides illustrating three-dimensional coordinate systems. Although originally a monochrome "black-and-white" process only,