This article presents a passive technique to measure the THz emission intensity of a sample at several emission angles with vertical and horizontal polarization relative to the plane of the sample surface. The optical system employs a polarization rotator and a movable ellipsoidal mirror that enables the detector to remain stable. The detector is a heterodyne receiver at 280 GHz and 490 GHz. The RMS difference between simulated and measured brightness temperature was within about 2%. The system is highly sensitive and accurate to within 5 K from 40 to 70degrees of measured emission angles in the vertical and horizontal planes.