Our skin produces Vitamin by exposure to sunlight. However, it is the Ultraviolet rays B (or UVB) that produces the Vitamin D in the skin.
Glass used in Windows allows UVA to pass thru but blocks UVB. In extremely cold weather, the only way to get sunlight is via Windows, but that does not work.
If we can identify transparent material similar to glass, but one that is affordable as well as one that allows UVB rays to pass thru, we can design “Vitamin D Windows”. Seeking information, skills and experience in this regard, you may email me firstname.lastname@example.org or reply here via chat below (disqus).