A method of determining from measurements of beam properties, characteristics of an electron source that use field emission in an electron beam system, comprising: varying an extractor voltage applied to the field emission electron source and measuring the electron current at different extraction voltages; determining a relationship between angular intensity I' and the extraction voltage; calculating from the relationship a field factor β corresponding to a ratio of the applied electric field and the extraction voltage, the calculation including an expression of the form AK X, where A and X are functions of the source geometry and are independent of the emitter tip condition and K = I ′ / J where J is the emission current density, and determining from the field factor at least one additional derived characteristic of the source, including at least one of: determining an intrinsic emission energy spread from the field factor, the extraction voltage, the work function, and the source temperature; determining an intrinsic virtual source size d v (int) from the beam geometry, source temperature, and extraction voltage; determining a corrected emission energy spread from the intrinsic emission energy spread and the angular intensity; determining a corrected virtual source size d v from the emission current density; and determining a beam brightness from the angular intensity, the corrected virtual source size, and the extraction voltage or from the temperature and emission current density. varying an extractor voltage applied to the field emission electron source and measuring the electron current at different extraction voltages; determining a relationship between angular intensity I' and the extraction voltage; calculating from the relationship a field factor β corresponding to a ratio of the applied electric field and the extraction voltage, the calculation including an expression of the form AK X, where A and X are functions of the source geometry and are independent of the emitter tip condition and K = I ′ / J where J is the emission current density, and determining from the field factor at least one additional derived characteristic of the source, including at least one of: determining an intrinsic emission energy spread from the field factor, the extraction voltage, the work function, and the source temperature; determining an intrinsic virtual source size d v (int) from the beam geometry, source temperature, and extraction voltage; determining a corrected emission energy spread from the intrinsic emission energy spread and the angular intensity; determining a corrected virtual source size d v from the emission current density; and determining a beam brightness from the angular intensity, the corrected virtual source size, and the extraction voltage or from the temperature and emission current density. determining an intrinsic emission energy spread from the field factor, the extraction voltage, the work function, and the source temperature; determining an intrinsic virtual source size d v (int) from the beam geometry, source temperature, and extraction voltage; determining a corrected emission energy spread from the intrinsic emission energy spread and the angular intensity; determining a corrected virtual source size d v from the emission current density; and determining a beam brightness from the angular intensity, the corrected virtual source size, and the extraction voltage or from the temperature and emission current density.