Re: What frequency of light actually stimulates Vitamin D production in humans?

Vitamin D (or Cholecalciferol) is essential for the absorption of Calcium in the intestines, and was first identified for its ability to prevent Rickets. The major source of Vitamin D comes from the skin, where sunlight converts 7-dehydrocholesterol into Vitamin D. Further modification in the liver can convert Vitamin D into Vitamin D₂ (or Calciferol). The conversion of 7-dehydrocholesterol into Vitamin D requires ultraviolet (UV) irradiation between 275 - 310 nm (nanometers: 1 nm equals 1 millionth of a millimeter); however, Vitamin D is susceptible to degradation from UV irradiation between 260 - 290 nm, so the optimal wavelengths for Vitamin D production are between 290 - 300 nm.

Light is almost always measured in wavelengths, since the short wavelengths result in very high frequencies, e.g. 300nm correlates to 1 GigaHertz. Since the product of the wavelength to the frequency is constant (c, the speed of light) for all elect romagnetic radiation, only one value is needed for identification. For long waves, like radio waves, frequency is used, but for short waves, like light and heat, wavelength is used.

Below are citations for the first paper to identify UV conversion of Vitamin D (Hess and Weinstock) and one of R.J. Wurtman's early studies on using artificial light to supplement UV exposure for people prone to Vitamin D deficiency.

Neer RM, Davis TR, Walcott A, Koski S, Schepis P, Taylor I. Thorington L, and Wurtman RJ, "Stimulation by artificial lighting of calcium absorption in elderly human subjects," Nature 229: 255 (1971).

Hess AF, and Weinstock M, "Study of light waves in their relation to Rickets," J. Amer. Med. Assn. 80: 687 (1923).