If UV light stimulates trichome production then why.....

[Douglas.Curtis]

Perhaps a hepa filtered enviro would show the same or greater improvement if this is the case.

I've grown in hepa filtered air for 15+ years. While there may be something there it will be minimal. I've found significantly higher trichome density when flowering in cool/dry conditions. When I flower cuts this way, most growers who supplied the cuts are unable to recognize the end flowers. The difference in trichome density and terpene retention is usually ridiculous.

At higher altitudes (where higher UV is present outdoor), there is less CO2 available and the temps are generally cooler and drier. This results in less plant growing under the same number of trichomes. Like 100 people standing on a carpet. The larger the carpet, the more spread out the people are. Grow less carpet.