This whole issue is kind of controversial. I am still not even sure where I stand on it (despite years of research). I can tell you that Noodles didn't go outside for many years, as she was not harness trained and I couldn't see a way to safely do it. I take her out with me sometimes these days, if the weather is nice, but it's important to know that even a clipped bird (which Noodles is not) can still fly away with the right wind or height etc...So don't ever assume that clipping is a solution to getting them more sun. I did use a craft light for a period of time with a UV/fill spectrum bulb, but the wavelengths etc all are important when considering this-- it's super complicated..There is UVA, UVB and more (so it just gets super tricky).
Sun is natural, and they would get a lot of it in the wild...however, I am still torn as to whether sun lamps are a good thing (I think they can be, but I am not sure..). Some people swear by them, but others insist that they are dangerous and over-hyped...All I can say is research a ton and make sure that if you do get a sun lamp, that your spacing is safe..There are many people who swear by them, but there is also a risk of retinal burns, electrocution etc....so just keep reading. If I knew the answer, I would share it with you, but, sadly, I do not.
There are some who say that they cannot process certain vitamins without certain types of light, but then others insist that this is false. There is science on both ends of the argument, which makes it tricky to decide for sure...