Vitamin D

2

also called ergocalciferol

has important effect on adjusting the in vivo metabolize of calcium and phosphorus. Vitamin D

2

is manufactured by irradiation of ergosterol (E). Many studies have been done on the reaction

showing that UVirradiation of ergosterol produces many isomers: vitamin D

2

pre-vitamin D

2

(P)

tachysterol

lumisterol and that the best irradiation wavelength is in the range of 270~310 nm for the conversion from ergosterol to pre-vitamin D

2

which is not photochemically but thermally isomerized into vitamin D

2

. The purpose of this paper is to find a new light source with a better result for the photoreaction with a low cost. The tube glass of the lamp could absorb most of the spectrum range below 254 nm which gives more by-products

so there was not any filter solution used. Firstly a certain amount of ergosterol was dissolved in ethanol to make a reactant solution (1.0 mg/mL). This solution was put in the photochemical reaction apparatus and irradiated by low pressure Hg lamp. The reactant solution samples taken at different time were put in brown bottles

blown off ethanol with nitrogen gas. The dry sample was stored in refrigerator and was dissolved with n-heptane for liquid chromatography analysis. The intensity of the lamp was also measured at intervals. Through experiments

not only the relation between the transformation rates of photoreaction

but also the time of irradiation were studied. The decay curve of the light intensity within 300 h was also obtained. The result is that

the conversion ratio of the ergosterol was 65.5% in 65 min

and the selectivity of pre-vitamin D

2

was 60.4%. The decay of the light intensity was only 13% in 300 h. Not any filter solution was needed in the photochemical reaction

this could decrease the whole cost in factory. And most important thing is that this lamp is very cheap

which shows a quite satisfying potential prospect for the industrialisation of vitamin D

2

.