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Dauben et al., J. Am. Chem, Soc. 104:5780-5781 (1982); J. Am. Chem. Soc. 104:355-356 (1982), disclose the effect of wavelength on the photochemistry of provitamin D3 and the effect of wavelength on the production of previtamin D3. The authors found that when provitamin D3 is exposed to light in the range of 254 nm, it is converted to a variety of photoproducts, the major portion being about 75% tachysterol. This mixture was then exposed to either 300 nm of light, broad-band 350 nm light or 355 nm light to give a build up of previtamin D3. Dauben et al. conclude that if provitamin D3 is first irradiated at 0° C. with 254 nm light to give a quasi photostationary state of provitamin D3, previtamin D3, tachysterol and lumisterol, and the mixture is thereafter irradiated (0° C.) with 350 nm light, a maximum of 83% previtamin D3 is produced.

1. custom355 nm light to give
2. customis first irradiated at 0° C. with 254 nm light