An optical engine for combining or splitting color light beams, compatible with modern high-speed projection systems, is described. The invention utilizes a unique pentaprism core element, partially bounded by five faces, two each on either side of a plane of bilateral symmetry and one opposite, bisected by the plane of bilateral symmetry. Two of the faces of the pentaprism are coated with dichroic films. Functioning as a light combiner, the pentaprism uses these dichroic surfaces and its geometry to interact with incoming tri-color light beams which, after first passing through appropriately modulated light valves, are selectively transmitted and reflected such that all beams exit, axially aligned, from one side of the pentaprism. Functioning as a light splitter, the pentaprism uses the dichroic surfaces and its geometry to split a white light beam into its component primary colors and pass these colors out of the pentaprism. Turning mirrors are used to direct the tri-color beams into or out of the pentaprism region at the necessary angles for proper operation. The turning mirrors are configured to increase compactness of the optical engine while maintaining nearly equal path lengths for all of the tri-color beams.