Tanning is stupid. You’re exposed to uncontrolled UV light. The body needs exposure to UVB 280nm-315nm. LEDs come with strong 295nm, 310nm and various others. Exposure to these wavelengths is much more effective than eating foods rich in vitamin D. However, this is not for tanning. We are literally talking 60 seconds to 3 minutes or better even just a few seconds every few minutes sparsely. One idea I had was an LED placed in a hallway or the stairway with an ir detector and timer. Keep the exposures to say 6 seconds and reset the timer after 5 minutes. That would give you a comparable dose to getting it from the sun. The problem is the eye protection. Most damage comes from UVA to form cataracts. However UVB is not better.

Anyway before you palm your face again, the sun give you the same dose as an equally calibrated dose from an LED. But the LED is better because it removes the UVC and UVA from the picture. There’s a study showing actual tests on pig eyeballs. UVA gives you cataracts. So a better idea is a specifically installed light with an activation timer that you must start by pressing a button. So you get your self yellow glasses to protect your eyes and you press the button and wait for say 15 minutes while reading a book or eating. This gives you enough UV to not need supplements. But it would not be a continuous exposure up close to a source. It would be a controlled exposure a couple of meters away from the LED and for only a few seconds every minute for 15 minutes. A prescription. Just like any other prescription.