Not a good answer but I know that an original pair of Blue lensed
RayBan sunglasses will bring mucho-dinaras on the Bay. Whatever
mineral that was added to the glass ain't as easy to get as it once
was. Think African country.
Probably didymium blue:
from <http://en.wikipedia.org/wiki/Didymium :
Didymium (Greek: twin element) is a mixture of the elements praseodymium
and neodymium. Ever since the invention by Sir William Crookes, it is
used in safety glasses for glassblowing and blacksmithing, especially
when a gas (propane) powered forge is used, where it provides a filter
which selectively blocks the yellowish light at 589 nm emitted by the hot
sodium in the glass, without having a detrimental effect on general
vision, unlike dark welder's glasses. Blocked also is the strong
ultraviolet light emitted by the superheated forge gases and insulation
lining the forge walls thereby saving the crafters' eyes from serious
cumulative damage. (See also arc eye, also known as welder's flash or
-- end quote --
Didymium blue filters were once used for OA welding and brazing, but were
banned from that use well before the mid 60's when I learned OA, because
of inadequate blockage in the IR. While blue looks cool, it is the least
useful color for human vision, with the eye having approximately 1% as
many blue cones as red and green as well as not properly focusing blue.
No way would I consider using an old didymium blue filter for welding,
even if it was free. (They are still good for glassblowing, however.)
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