Just by sunning the Dried Shiitake that you purchased can increase the Vitamin D content by 24 times. The average dried Shiitake (includes both artificially and naturally grown) contains 12.7μg of Vitamin D2 (Japanese Ministry of Education Data). Forest-grown Dried Shiitake contains 87μg or 7 times the average Dried Shiitake. Recent studies show Vitamin D2 of Shiitake has the same effect as Vitamin D3 Supplements.
(The U.S. National Center of Biology Institute (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3897585/)
If you place the Dried Shiitake under a strong UV Index greater than 6, it is enough to increase the Vitamin D2 content by 24 times.
A Kobe Women’s University of Pharmacy study has documented that under 3 hours of Sunshine at 14.6mW/cm2 (using 1 Dried Shiitake having Vitamin D2 contents of 20IU):
After 1 hour exposed to sunlight increased to 488IU (24x)
After 3 hours exposed to Sunshine increased to 548 IU (27x)
The difference between one and two hours does not appear significant, so I believe one hour is enough.
For your information, we also use UVB to increase natural vitamin D of our Shiitake.
Our Shiitake contains the highest amount of Vitamin D.
Donko: 6880 IU (172mcg) / 100g
Koshin: 9680 IU (242mcg) / 100g