It's no longer recommended to use UV-B light as a source for Vitamin D, because it increases your risk of skin cancer.

Instead, Vitamin D can be obtained from certain foods, though the number of foods that contain Vitamin D naturally is quite small[0]. Hence why supplements are often needed.

[0] Vitamin D is added to milk, though it doesn't occur in milk naturally. And the quantities included in milk may not be sufficient for a lot of people.