Abstract: A filter IR spectral analyzer has a highly efficient compact imaging optical collector having a first concave reflector of ellipsoidal design or shape, having a light source at one of its foci and a second concave reflector of spherical design which has its focus at the center of the light source. The light emitted from the source that strikes the ellipsoidal primary mirror converges to a second focal point located in an exit aperture in the center of the second reflector. The light that does not strike the ellipsoidal primary mirror reflects off the secondary mirror and returns to the source for reabsorption and re-emission. Compactness is achieved for multi radius versions by the use of a segmental spherical secondary mirror with each facet being radiused with its origin at the source so that all act together as though they were one large spherical mirror. Four possible configurations are described.