A review of papers and patents related to sensor based variable rate technology (sVRT) is presented. The rationale for an sVRT system using optical sensing to vary top-dress nitrogen fertilization of winter wheat is presented. The current OSU sensor is presented with some of the calibration results for the 1996 crop season. Reasonable correlations for use of radiance based on NDVI (Perry et al. 1984) for detecting vegetative nitrogen mass are presented.