Various embodiments involving structures and methods for illumination can be employed, for example, in projectors, head-mounted displays, helmet-mounted displays, heads-up displays, back projection TVs, flat panel displays as well as other optical systems. Certain embodiments may include prism elements for illuminating, for example, a spatial light modulator. Light may be coupled to the prism in some cases using fiber optics or lightpipes. The optical system may also include a diffuser having scatter features arranged to scatter light appropriately to produce a desired luminance profile. Ellipsoid- and parapoloid-shaped combiners may be employed in embodiments of displays such as for example head mounted displays, helmet mounted displays, and heads-up display. Other embodiments are possible as well.