I last posted last fall about my use of the Sperti Vitamin D lamp:
https://www.drmcdougall.com/forums/viewtopic.php?f=1&t=50240&start=45#p516865
https://www.drmcdougall.com/forums/viewtopic.php?f=1&t=50240&start=60#p517189
https://www.drmcdougall.com/forums/viewtopic.php?f=1&t=50240&start=60#p517439
I got tested again in April 2016, after near-daily use during the cold season for 4 minutes per day. 35 ng/ml. No skin reddening.
A year ago, I tested at 30 ng/ml, after using the lamp about every other day during the cold season.