Nonreciprocal light transmission through a photorefractive medium is a consequence of the asymmetric self-diffraction at the photoinduced grating produced by two antiparallel beams. We report for the first time one way viewing experiment with KNbO3 as the nonlinear, photorefractive medium. By illuminating the one way device with two antiparallel beams of equal intensity P = 10 mW (I = 19 W/cm4) the light transmission factors were 113% and 59% for the two beams, i.e. the transmissivity in the first case is larger than unity despite of an absorption constant of α = 1.5 cm-lx The grating build-up time was 1.7 seconds at this power level and 0.1 second at 270 W/ce. The transmission asymmetry caused by the non-linear interaction between a beam and its own reflective from the back face of the crystal was investigated for several wavelengths X. We measured 57% and 27% for two antiparallel beams at X = 514.5 nm with an optimized crystal cut with light propagation direction 37' to the b-axis and perpendicular to the a-axis.