An ideal diffuse reflectometer can be defined as a reflectometer with a throughput which is independent of the angle of reflected radiation, as measured at the sample. For integrating spheres, effects related to the detector's field of view (FOV), the beam port, and internal baffles can result in a throughput which is nonisotropic. This paper analyzes these three sources of nonideal behavior and suggests three sphere designs using nonimaging concentrators which minimize FOV related errors. A technique for measuring the error due to the beam port is also discussed as well as ways of minimizing perturbations caused by baffles.