I just came across this post from Roland Christian of Astro-Physics in the Yahoo groups. He is answering a members questions, which are,
What is it that makes a good optical glass ?
What was the innovation that led to the development of the ED glass that is commonly used to make APO lenses ?
Why is it so hard to find this glass, and are there no domestic manufacturers or suppliers ?
"Good optical glass is very homogeneous thruout and can thus transfer the light beam accurately to focus. Bad glass has internal striae which bend the light away from perfect focus. Best optical glass for homogeneity is low index ED glass and hard crown like BK7, BSL7 etc. Worst glass for homogeneity is heavy dense flints and lanthanum glasses. Reason is that during melting, they need to be constantly stirred to prevent the heavy components from sinking. This stirring can cause striae. I worked in the glass plant at B&L in the early 70's and talked to the opticians about quality glass. It was difficult to make quality glass in heavy dense types. They always complained about the poor quality flints they had to work with for the specialty cameras that went into the U2 spy planes and other military projects.
ED glass became available in sizes up to 8 - 9 inches when Ohara started melting FPL51 and FPL52. The latter glass was of superb quality and could be mated with hard crown glass to produce essentially color-free triplets. Before Ohara, Schott had a few special glasses that were not of low enough dispersion to make apos, and not of large enough sizes to be interesting, and of course at very high cost. The camera industry in Japan was the driving force for ED glass, to be used in super telephotos for sports events and for commercial TV cameras.
There are no domestic manufacturers of optical glass. The ovens needed for melting high purity glass require platinum lining inside. When catalytic converters were added to automobiles, the price of platinum soared. Companies like B&L, Kodak and Corning tore down their furnaces and sold all their platinum. Japanese companies like Ohara did not sell their platinum, even though it would have boosted their earnings.
When Marj and I toured the Ohara glass plant we saw the same electric furnaces that B&L had torn down to get at the platinum. We were told that a typical company like Ohara has a planning horizon of something like 50 years, compared to a US company that looks 5 years forward, and sometimes not even that. No way was Ohara going to sell their platinum for a momentary gain. I think you'll see that the Chinese companies are even more disciplined and plan way into the future. Just think, where is Kodak, B&L and Corning today? Scattered to the winds they are.
What a very sad commentary of our times, seems we have made our beds and now we must lie in them. When will we learn that the all mighty dollar is not the end-all-be-all of our existence.