Electric signals are generated due to changes in electric resistance of the fluid when cells suspended in an electri cally conducting medium pass through a light beam in the sensing area. Based on the fact that blood cells have lower electric conductivity than saline solution has, Wallace Coulter designed a cell counter that has dominated the American market for >30 years. The principle he used to measure cell volume is now called the Coulter Principle. This principle was also used in some older models of flow cytometers, such as the fluorescence-activated cell-sorting(FACS) analyzer(Becton Dickinson) which contains a mercury-cadmium arc lamp. The advantages of using mer- cury and xenon arc lamps are their low cost, ease of opera- tion, and broad range of spectral output. The disadvantages are nonuniform radiance, low brightness, and instability of the arc. Because the disadvantages outweigh the advantages, mercury arc lamps are no longer used in the new models of flow cytometers