To measure the insertion loss of a tunable optical bandpass filter—whose passband shape rarely matches the test laser spectrum—you must use a broadband source and a reference trace; first, connect the broadband fiber-coupled source directly to the OSA and sweep once over the filter’s specified wavelength range, saving this as Trace A; next, route the broadband source into the tunable filter and connect the filter’s output to the OSA as Trace B; then set the OSA to compute Trace C = Trace B − Trace A and perform an automatic sweep across the wavelength range while tuning the filter’s micrometer to shift the passband peak; at each wavelength of interest, use the OSA’s peak-search function on Trace C to read the insertion loss.