We report 100% conversion of infrared photons into visible photons through three wave interactions in a nonlinear medium, present the first experimental evidence of overconversion, and confirm the classical theory of up‐conversion in the high‐conversion‐efficiency region. A laser pump light feedback technique is described that promises to make the process practical with modestly powerful pump lasers and less than perfect nonlinear crystals. The nonlinear medium used was a LiIO3crystal that cannot be 90° phase matched. The ``walk off'' that resulted helped make possible the attainment of 100% conversion efficiency.