The preferred embodiment of the invention uses known atomic or molecular absorptions as absolute wavelength standards for calibrating wavelength measurement instruments used in tunable lasers. Examples of atomic and molecular absorptions are carbon and molecular oxygen that have absorptions including 193.0905 nm and 193.2728 nm, respectively, for use with a tunable Argon Fluoride excimer laser at approximately 193 nm. A wavelength measuring device (e.g., a wavemeter) is equipped with a gas cell containing the absorption gas. During a calibration procedure, the wavelength measured by the wavemeter is compared to the atomic or molecular absorption. The wavemeter's calibration constants are then adjusted accordingly to match the wavemeter's output to the atomic or molecular absorption wavelength. Such calibration procedures, therefore, calibrate the wavemeter to absolute standards and correct for any drift in the wavemeter that may occur between calibrations. Some gases, such as molecular oxygen, have multiple molecular absorptions within the tunable range of the laser. The use of multiple absorptions during calibration procedure enhances the precision of the procedure due to the proximity of an absorption line to the final wavelength of interest. After calibration, the laser is tuned to the final wavelength of interest using the calibrated wavemeter.