"Open the doors," says Jean Carlu, eminent poster artist, to Manager Sloan of the National and Civilian Defense Exposition at New York's Grand Central Palace. In the background is part of the 15 x 30 foot poster Carlu designed for the Office of Emergency Management (OEM). The panel shown represents the Arsenal of Democracy while another of equal size is entitled The Four Freedoms. These panels, designed according to a technique in photomontage used only once before, will be seen in a number of cities from coast to coast