Axis scales for histograms and 2D plots have traditionally been displayed on arbitrary scales: either a channel number or a logarithmic intensity scale. For some applications this is not adequate and the investigator needs to know the absolute number of antigen molecules or binding sites on the surface of the cell. Several companies have specially designed beads of different intensities where the precise numbers of dye molecules are known. By correlating the intensity level in terms of channel number to the number of dye molecules it is possible to specify your results in terms of molecules of antigen, antibody or whatever you wish.
This type of calibration has traditionally been very labor intensive, and involved using your flow cytometry software to obtain a set of statistics and plugging those statistics into a spreadsheet to obtain a calibration curve. Results were obtained as standard channel values from the flow cytometry software and then transformed using the spreadsheet into number of binding sites. FCS Express automates the entire process for you. Once you have set up a channel calibration, all plots will automatically display the calibrated axes and statistics will be displayed as calibrated values.