Accuracy and Traceability

How can the Optical Reference Lab standard mirrors be accurate to better than 0.25%?

Metallic mirror reflectance standards produced by the Optical Reference Laboratory are traceable to NIST calibration values between 250 and 2500 nm.  Those values for an aluminum mirror at 8 degrees AOI are shown in the graph below.  The error bars represent the uncertainties estimated by NIST for different spectral ranges, given in the adjacent table.  The blue spectral curve was generated by a smoothed cubic fit through the NIST data points.
 
  NIST calibration spectrum graph, Optical Reference Laboratory
 
NIST Uncertainty table, Optical Reference Laboratory
 
NIST traceability is limited to their measured data points and the accompanying uncertainties.  However, recent comparisons of the NIST calibrated mirror to several dielectric low loss laser mirrors indicated that in practice, the calibration values are somewhat more accurate than the estimated uncertainties would predict.  The adjusted spectrum shown below in red was constructed using the data from these comparisons, with related accuracy estimates shown in the table.  This adjusted spectrum, still well within the NIST uncertainty limits, is used by the Optical Reference Laboratory in producing secondary calibrated mirrors. 

Adjusted calibration spectrum graph, Optical Reference Laboratory
 
Adjusted Errors table, Optical Reference Laboratory
 
It must be stressed that the NIST method for estimating uncertainty is fundamentally different than the way errors are estimated as the low loss mirrors are used, making comparisons difficult.  In practice, achieving superior accuracy requires that instrument noise and sampling reproducibility are strictly controlled.  For more information on how the measurements with low loss mirrors are performed and interpreted, please consult the applications note on this topic in the Downloads section.