It seems odd that no one has yet exploited the possible advantages of fluoride prisms?
my logic is thus...surely the manufacture of lenses is a much more costly and laboured procedure than a triangular chunk of prism that only needs to be finely polished flat on three sides.
the remaining optical lenses could be made from cheaper/regular optical glass
and still have all the usuall multi coatings etc.
not sure wether the majority of light has to pass through the combined mass of optical lenses or prisms,but surely those prisms count for a lot...
so why not make em in fluoride?
matt
ps..this question just popped strait off the top of my head with no research what so ever,so apologies to any optics designers who already make such things or if indeed zeiss also include fluoride glass into the manufacture of their prisms!
BaK4 prisms work better than BK7 prisms because BaK4 has a higher index of refraction. In green light the index for BaK4 is 1.5712451, while BK7 is 1.51872218. The index for CaF2 (fluorite) is 1.43494007. Prisms with such a low index of refraction would not work well.
Also, the improved color correction in a lens using fluorite comes about because there are crown glasses that nicely match the partial indices of fluorite. It takes two to tango, and fluorite by itself in a lens or prism would not provide good color correction.
In low power binoculars fluorite is really just a marketing tool anyway. It is not needed.
Clear skies, Alan
Optically there would be no great benefit from making the prisms from fluorite glass, as there is no refraction at the air/glass interface. The prism is deviating the light path by total internal reflection rather than by refraction. The lenses do gain optically by being made from fluorite materials and having top notch multi-layer anti reflection coatings on them. Problems like chromatic abberation are lower with the higher refractive index fluorite materials.
So as the cost of using fluorite for the prisms would bump up the price and the weight without giving any optical benefit, it's not used.
The thing I want to know is why some of the newer optical pastics materials aren't being investigated for scopes and bins...for the internal elements at least.