The diffuser’s distance to fibre distance is matched to the fibre’s N.A. There is a slight slope in the diffuser’s surface to drain the water through a drainage hole. An 'o'-ring gives additional protection against rain.
The diffuser can be made in almost any size. Here is a slightly smaller model:
Drawing with dimensions of diffuser BAT 6
Characteristics of the smaller and bigger diffuser are almost the same.
Good Cosine response with known correction factors.
Reasonably good random polarised transmission independent to temperature changes.