20 years ago 95% of photochromic /photochromatic lenses were "Photogrey" brand and were glass made by Corning Glass Co.
They were indeed slow to change back to clear especially as they got older...but in the cockpit environment a certain percentage of UV light which made them go dark is filtered out by Perspex screens or even the heat treated glass screens of a DC9...its a bit hard to believe they would be so dark as to cause an accident...another myth perhaps?
Unfortunately the CASA guidelines were written about 20 yrs ago ...so this myth is perpetuated...the guidelines disappeared off the website about 2 yrs ago pending re-writing and have not made it back yet!!!
I quite happily wear my Transitions brand photochromic lenses flying day and night and only put sunglasses on if landing directly into a low western sun...the tinting brand is available across several different lens making companies and works best if combined with a good quality anti-reflection coating...the coating makes them go darker faster ,lighten up faster, and makes any residual tint virtually disappear completely indoors or at night.
"Transitions" [a registered brand] is a "treatment" for most types of plastic used in lenses including so called "hi-index" materials which reduce thickness and weight in modern lenses[ which are 99% plastic these days] .
In the cockpit they are about 50% tinted behind the Perspex and go to 85% within 36secs of stepping out into daylight.
My 2 cents worth.
Flopt