When considering several projection system designs, one of the important characteristics is the total luminous flux the system will deliver to the projection screen. This is the main factor controlling perceived brightness to the viewer for a given screen size and gain. This luminous flux depends on a number of factors, such as lamp lumen output, reflector collection efficiency, projector architecture, light valve aperture ratio and the transmission/reflection efficiency of each optical component in the system. Many of these factors are amenable to accurate estimation prior to construction of a prototype projector. However, quantitative estimates of lamp/reflector light collection efficiency are difficult to make accurately. This paper will give an analytical approach based on etendue (optical extent) that can accurately predict lamp/reflector collection efficiency prior to the construction of a prototype projector. Collection efficiencies and projector lumen throughputs for several lamp/reflector/projector combinations will be estimated with this technique.