How effective would this source be for indoor growing? A room full of Tubular Skyligts. Can anyone figure out how many would be needed for a 15'x 15' room?
Depends what you're growing. Do the plants require partial shade? If so, the indirect light you'll get might work.
You'll never get vegetables to grow in there. Vegetables need 6-10 hours of DIRECT sunlight a day.
Tricky thing with plants, they need 10-100 times as much light as we do! We can quite happily read in less than 100 foot-candles, whereas full sun plants need several thousand foot-candles to do well. You can grow them with maybe 1,000 foot-candles continuously for 14-16 hours each day, but lighting supplied through the lighting tubes would not achieve that. It would be more practical for low-light plants such as Ficus, Impatiens, Afrian Violets, or other typical houseplants.