1. A method of setting the fluorescence sensitivity of an optical scanning system, the method including the steps of: A. producing an excitation signal with a predetermined power level; B. scanning a first scan line of a predetermined calibration area of a sample using the excitation signal; C. determining if signals corresponding to N pixels in the first scan line are saturated; D. if the signals are saturated reducing the excitation signal power by a predetermined factor; E. repeating steps B-D until the signals associated with fewer than N pixels in the first scan line are saturated; F. scanning a next scan line of the calibration area; G. determining if signals corresponding to N adjacent pixels in the next scan line are saturated; H. if the signals are saturated reducing the excitation signal power by the predetermined factor; I. repeating steps G-H until the signals associated with fewer than N pixels in the next scan line are saturated; and J. repeating steps F-I until a last scan line in the calibration area is scanned.