Well, I do the window thing with one of mine, but I still supplement lightly. The difference is that outside they get uvb even in the shade, but with a window they only get uvb through the window and maybe a little if it bounces off of anything (but dont count on it.) Plus you have to take into consideration that, depending on your location, uvb will differ with the time of day, the positioning of the sun and window with each other, weather conditions, and the time of year. That means that each person has to make their own individual judgement based on their circumstances and research.