What reflectance standard should I use?
A reflectance system is not complete without a standard for reference. Reflectance measurements are a ratio of the reflected light spectrum to the incident light spectrum. Since there is no way to directly collect all of the light incident on a surface, reflectivity is usually measured relative to a reference standard. Except that multiple reflectance standards exist.
The standard chosen should be similar in reflectivity to the sample to keep signal levels about the same during measurement and thereby achieve the best S:N. The WS-1 diffuse reflectance standard is most popular, since it is matte white in color and is >98% reflective from 250 to 1500 nm (then >95% reflective up to 2200 nm). The WS-1-SL can be a good choice when working in the field or in dirty environments, since it can be smoothed, flattened and cleaned if it gets pitted or dirty.
The STAN-SSH high reflectivity specular reference standard is the best choice when measuring very shiny surfaces, but it varies in reflectivity from 85 to 98% over its range of 250 to 2500 nm. This can be corrected in OceanView; just upload the reflectance values and correct for the reflectivity of the standard. This data comes automatically with the STAN-SSH-NIST calibrated reference standard. If no correction is applied, OceanView will assume the standard is 100% reflective at all wavelengths, giving distorted data.
At the other extreme, the STAN-SSL low reflectivity specular reflectance standard is best for surfaces with low specular reflectance values like thin film coatings, anti-reflective coatings, blocking filters and substrates. It has just ~4.0% reflectance from 200 to 2500 nm. It is also possible to purchase calibrated “gray” standards (diffuse reflectance standards at a variety of intermediate values) from our sister company, Labsphere.