Ordinary glass used for making household windows, mirrors, etc.
Such glass contains many impurities compared to the far more
transparent optical glass used by better manufacturers for
making optical components. This is easily demonstrated by
holding a piece of window glass and looking at the edge. The
glass will appear to have a greenish tint - it wont be clear.
Unfortunately, many cheap lens filters are made with poor-
quality window glass. cf. optical glass, transparent. Entry last
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