In about 1985 both (or maybe all three?) manufacturers
of plastic blank trifocal lens had a maximum intermediate
lens height of 7mm, which meant I could see about half
vertical my 14-inch monitor. With some geometry, I
calculated that a 14mm intermediate would let me see
the whole screen, and had Pearle Vision fabricate true
trifocal glasses (glass, not plastic) with a 14mm
intermediate. Perfect for full screen vision, but very
heavy in glass, and strange looking, stairstepped
lenses. About $700 as I recall.
Less than a month after I received my glasses,
Pearle called to say that at least one of the
blank manufacturers had come out with a plastic
trifocal with a 14mm intermediate, and marketed
it as "CRT Glasses". I concluded the reason was
that the CEO of that company got a monitor on
his desk!
Replaced the glasses with plastic 14mm, worked fine
and much lighter.
About four years ago, I began to notice throbbing
in my left arm, and tension in my neck, erratically.
After a MD visit said it wasn't my heart, we together
figured out the cause:
I had replaced my small monitor with a 20-inch monitor,
which was now only half-screen visible, and I was cocking
my head back to raise up the intermediate lens to see
the top half of the screen, putting pressure on the
nerves and muscles in my neck.
I replaced the trifocals with a separate set of intermediate
only glasses, see full screen of the big screen, and my
arm and neck no longer startle me.
Barry, W5GN
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