The instrumental width represents the fundamental resolution limit of a spectrometer, interferometer, or other optical measuring device. Even if an ideal, infinitely narrow spectral line enters the system, diffraction, slit width, and internal aberrations will broaden the measured signal into a finite profile. Engineers must deconvolve this baseline broadening from raw data to determine the true linewidth of a laser source.
We measured a 2 GHz spread, but the instrumental width of that old Fabry-Perot is at least 1.5 GHz, so the actual laser linewidth is much narrower.
Always measure a known ultra-narrow reference source to calibrate the instrumental width before trusting your spectrometer's output on an unknown beam.
Related terms
Linewidth
The spectral width of a single-frequency laser's output, usually given in Hz, kHz or MHz at half maximum.
Free spectral range
The frequency spacing between neighboring resonances of a laser cavity, etalon or interferometer.
Finesse
The sharpness of an optical cavity's resonances: free spectral range divided by the resonance linewidth.