Dr. Hansen has spent a large part of his career inventing and developing research and diagnostic tools for the analysis of peripheral blood cells. Most notably in 1976, he invented the first optical cell counter using a laser light scattering principle, which allowed for platelet counting for the first time to be accurate below 10,000/μl with substantial benefit to patients receiving chemotherapy. This light scattering principle is used today in all optical hematology analyzers worldwide. In 1980 Dr. Hansen invented a flow cytometry method that identified and gated lymphocytes in whole blood and detected cell surface immunofluorescence on these lymphocytes. The method for the first time eliminated the need to isolate mononuclear cells and became the first objective test for immunosuppression in humans. This gating method remains in use for all flow cytometry immunophenotyping today. Dr. Hansen is an inventor on more than 30 patents, 18 of which concern measurements of peripheral blood cells and were all commercialized.