We discuss the design of optical concentrators based on dielectric and hollow compound parabolic concentrators (CPCs), for use in free-space infrared communication receivers. In order to acheive a high signal-to-noise ratio in a direct-detection receiver, it is desirable to use an optical bandpass filter that passes the signal but attenuates ambient radiation. Placement of a planar bandpass filter at the entrance aperture of a CPC results in a receiver having a narrow passband and high gain, but a narrow acceptance angle. The addition of a second, angle-transforming CPC at the entrance aperture allows the receiver to achieve simultaneously a narrow passband and an acceptance angle approaching 90 deg. We have employed a Monte Carlo ray tracing method to calculate the optical gains of several optical concentrator designs. We find that the optical gains of single and double CPCs are, respectively, about 94% and 93% those of ideal optical concentrators, while addition of planar bandpass filters decreases these gains to about 88% and 86%, respectively. We compare the performance and size of CPC-based concentrators with those based on dielectric hemispheres fitted with hemispherical bandpass filters.