My FRTs get about 8 hours of indirect sunlight a day. I don't have to put them in an outdoor pond to provide this since my hatchery utilizes a great invention... windows.
I open the windows in the afternoon. Closed, there's still some UV getting through. I have double insulated acrylic low E windows that filter out 65% of the UV rays.
Keep in mind that normal glass window filter out almost all UVB, and screens can also reduce the amount of UVB reaching your turtle. Unless you have specialized windos like oddball I would suggest geting a reptile UVB bulb. Blindness should not be a big concern as the amount of UV is greatly diminished 12 inches and further from the buld. Although I could be wrong, I do not have much experience with aquatic turtles.
Keep in mind that normal glass window filter out almost all UVB, and screens can also reduce the amount of UVB reaching your turtle. Unless you have specialized windos like oddball I would suggest geting a reptile UVB bulb. Blindness should not be a big concern as the amount of UV is greatly diminished 12 inches and further from the buld. Although I could be wrong, I do not have much experience with aquatic turtles.