What he received only have 30 to 35 percent of watts of ultraviolet radiation. In other words, he will only receives 1.2 to 1.4 watts of ultraviolet radiation. Add this to the distance of receiving ultraviolet radiation dimished as he moves away from the bulb. So he only gets intensity of power divide by (4 times pi times radius squared).
See Intensity (physics). See also at Ultraviolet fluorescent lamps.