Blocking in an Optical Filter
To understand blocking, one has to understand the concept of optical density (OD). In an interference filter, OD is generated by reflecting unwanted light back instead of letting it go through the filter.
Gradient Index Film Fabrication Using Optical Control Techniques
Proc. SPIE 2046, Inhomogeneous and Quasi-Inhomogeneous Optical Coatings, 224 (November 2, 1993); https://dx.doi.org/10.1117/12.163553
Measuring Sharp Spectral Edges to High Optical Density
56th Annual Technical Conference Proceedings of the Society of Vacuum Coaters, (2013). Reproduced by permission of the Society of Vacuum Coaters.
Measurement and Alignment of Linear Variable Filters
Proc. SPIE 10539, Photonic Instrumentation Engineering V, 105390Y (2018); https://dx.doi.org/10.1117/12.2290689
Laser-based Assessment of Optical Interference Filters with Sharp Spectral Edges and High Optical Density
Surface & Coatings Technology 241 (2014), 54-58 (Oct 5, 2013); https://dx.doi.org/10.1016/j.surfcoat.2013.09.054