I sniffed around and found this reference too:
Here’s a segment from a high times article:
“Dr. Paul G. Mahlberg who found that THC in the resin head is at it’s peak when it’s still clear - not opaque and certainly not amber. Dr. Mahlberg stuck a tiny syringe into the bulbous head of the trichome and extracted it’s contents, then analyzed it for THC. The result: Without a doubt, THC is at it’s highest before the trichome turns opaque.”
I googled his name and found a number of links… an interview, articles, etc.
*His lab had one of the few DEA permits to work with Cannabis since 1970.