PHOTOGRAPHIC STEREOGRAMS ese are analogous to the rst type of signal transmission: the photographic images are dissected into strips, which are interlaced and separated directionally by a retroreecting lenticular screen positioned in register over the print, so that each line is visible only from immediately in front and a change of viewpoint changes the viewed image to the new viewpoint. is type of “barrier” stereogram appeared in a primitive form early in the history of photography and was originally given the somewhat pompous name parallax panoramagram. In a modern sophisticated form, such stereograms have undergone a revival and are widely used in portraiture; they also appear regularly on advertising tokens and calendars, and even on large hoardings, but they are outside the scope of this book and are not discussed further.