Mycelium Running by Paul Stamets 2005 has unique information about all aspects of the science of mushrooms.
He describes his vitamin D2 experiments with mushrooms on pages 202-204.
The following are IU of vitamin D2 in 100 grams dried weight of mushrooms
Indoor-grown dried indoors had 110 IU
Outdoor-grown dried indoors had 1,620 IU
Outdoor-grown dried in the sun with gills down had 10,900 IU
Outdoor-grown dried in the sun with gills up had 46,000 IU
Indoor-gown, dried indoors 460 IU
Indoor-gown, dried indoors then exposed to sun for 6 hours 31,900 IU
It appears that the main reason that mushroom suppliers had not been supercharging dried mushrooms was the Institute of Medicine limitation of 2,000 IU per day, which would provided by just 10 grams (1/3 of an ounce).
Stamets states that in 1998 Mau in the Journal of Agricultural Food Chemistry (pg 5269-5272) also found the mushroom ability to be supercharged with vitamin D2 with a small amount of light. When Mau applied artificial UV for just 1 minute the amount of vitamin D2 tripled. However, increasing the UV to 2 hours decreased the D2 by 12%, “because the radiation began breaking down the vitamin”
Stamets published his findings in "Notes on Nutritional Properties of Culinary-Medicinal Mushrooms", in the International Journal of Medicinal Mushrooms, vol. 7: 103–110. 2005
His website is http://www.fungi.com/index.html
An article about his work is http://www.motherearthnews.com/Natural-Health/Mushrooms-Health-Benefits.aspx
Example of further research is at http://www.ars.usda.gov/IS/pr/2008/081112.htm
Wikipedia has a small page http://en.wikipedia.org/wiki/Effect_of_sunlight_on_mushrooms