Design, fabrication and characterization of transparent retro-reflective screen.

A transparent retro-reflective screen, which can be used as head-up-display (HUD) or a see-through screen for head mounted projection displays (HMPD) is proposed. The high optical gain of screen enables the use of low power projectors to produce very bright content. The screen assembly is based on retro-reflective microspheres, patterned on an optically clear substrate using steel stencil as a shadow mask. The incident light is retro-reflected as a narrow angular cone to create an eyebox for the viewer. The optical gain and transparency of screen is varied by changing the fill factor of the mask. The optical design and fabrication of the screen is presented. The retro-reflective and transmission characteristics of screen are evaluated. The impact of fill factor on screen luminance and transparency is studied. The screen provides high luminance (up to 280cd/m2 with 50% transparency) from about 40cm to >3m when used with a low power (15 lumen) mobile projector. Unlike regular diffusers, luminance remains nearly constant with projection distance. Furthermore, the screen offers prominent see-through capability with small degradation in modulation transfer function for transmitted light. For a particular camera and imaging configuration, MTF10 (10% cutoff) for 50% transparent screen is reduced from 37 cyc/deg to 30 cyc/deg when screen is inserted at an intermediate distance.