A method and apparatus for measuring bandwidth of light emitted from a laser which may comprise: first and second wavelength sensitive optical bandwidth detectors providing, respectively, an output representative of a first parameter indicative of the bandwidth of the emitted light as measured respectively by the first and second bandwidth detectors, and an actual bandwidth calculation apparatus adapted to utilize these two outputs as part of a multivariable linear equation employing predetermined calibration variables specific to either the first or the second bandwidth detector, to calculate a first actual bandwidth parameter or a second actual bandwidth parameter. The first actual bandwidth parameter may be a spectrum full width at some percent of the maximum ("FWXM"), and the second actual bandwidth parameter may be a portion containing some percentage of the energy ("EX"). The first and second bandwidth detectors may an etalon and the outputs may be representative of a fringe width of a fringe of an optical output of the respective etalon at FWXM. The precomputed calibration variables may be derived from respective three dimensional plots representing, respectively, detector outputs in relation to a calibrating input light with known values of the first and second actual bandwidth parameters.