Uniform scattering patterns from grating-diffuser cascades for display applications

A cascade of a thick grating and a thin diffuser is shown to scatter radiation efficiently and uniformly over a wide angle. Cascading the grating with a diffuser causes the single-beam power spectrum of the diffuser to be replicated at each diffraction angle of the grating. The grating period is chosen so that the first diffraction order falls near the one-half point of the power-spectrum peak of the diffuser. The relative strengths of the diffraction orders are optimized to obtain uniformity of the resulting intensity distribution in the plane of the diffraction orders. The intensity distribution in the perpendicular plane is governed solely by the diffuser. Such a cylindrical system is considered on the basis of the requirements for projection TV's of a large horizontal span (100 degrees) and a narrower vertical span (approximately 15 degrees). Broadband illumination is studied by consideration of three simultaneously illuminating wavelengths. Experimental results are given for a cascade of a grating formed in photoresist and an etched-glass diffuser.