A lot of confusion exists within the optical community about the definition of Bidirectional Scatter Distribution Function (BSDF), how it should be measured, and how it should be specified. This paper defines BSDF, and then explores the practical factors that limit the measurable upper and lower values of BSDF. The upper value, which is limited by detector aperture size, can take on values exceeding 107 sr-1 in practical cases. The lower limit, sometimes called the electronic noise equivalent BSDF, is determined by the incident power and the detector solid angle as well as the system electronic noise, and needs to be specified with all three values. For systems that are noise limited by the detector, the minimum BSDF can be expressed in terms of the detector parameters (responsivity, noise, active area, and detectivity). The upper and lower limit for several systems has been explored and these results are presented.