Ocean Optics offers two types of diffuse reflectance standards. The WS-1 uses a PTFE optical diffuser, a Lambertian material distinguished by its white matte finish and reflectivity >98% from 250-1500 nm and >95% from 250-2200 nm.
For field work or unclean environments, the WS-1-SL can be a good option. Its Spectralon diffusing material can be smoothed, flattened and cleaned if it gets pitted or dirty. Reflectivity is 99% from 400-1500 nm and >96% (250-2000 nm).