Significance Comment

Summary

An important paper describing the non-lambertian nature of reflectance standards - some additional discussion of the implications of the work would have been welcome.

Novelty

While the study of non-lambertian behaviour of reference materials is not new, the approach taken by these authors is new and comprehensive. In particular the use of PCA to decompose the spectral variations as a function of geometry is novel for these materials, while being in keeping with other recent publications of the same authors. It has allowed them to present a vast data set in a simple and clear manner, allowing the reader to grasp the central differences between the standards chosen. The descriptors chosen for the data were mostly useful, but not all of them were discussed in the text (e.g. theta_nu). Other descriptors were less apparently useful (e.g. theta_i,ret for the matte ceramic and opal glass must surely have very large uncertainties). It would have been great for the paper to have some discussion of the implications of the non-lambertian behaviour for specific measurements, and to identify which standards are most suited for which measurements.

Quality Comment

Validity

The data sets and analysis of them given in this paper are important for metrology, having application in a wide range of measurements using white standards - in particular directional and broad band measurements which rely on lambertian behaviour of reference materials. Uncertainty associated with such measurements can be much more accurately assessed using this information. The data sets can also be used to inform choices of standard for maintaining and disseminating diffuse reflectance scales. The validity of the various figures of merit associated with each standard (delta_il etc) is however difficult to gauge without some indication of the uncertainty associated with the values. Although the uncertainty of the brdf measurement is given (and seems to be independent of geometry?), the propagation of this uncertainty to the descriptor values is not given, meaning that their full potential to inform uncertainty budgets of the above measurements is not realised.

Minor Issues

A quick proof read by a native English speaker would have made this easier to read. That being said, meaning was not lost. The second incidence of 'negative' in the sentence 2 of section 3 should read 'positive'.

Summary

Reviewer

Annette Koo, MSL, Callaghan Innovation

Expertise

I am responsible for realising and maintaining the scale of diffuse reflectance at NZ's NMI. I have been in need of exactly this type of data in my work.