Why bluish purple light when uv led strip l started working here?

UV electromagnetic spectrum wavelength from 10nm to 400nm, is not visible We can’t be seen any ul lights in the region,in Conventional thinking The UV led should not be visible, so why we use UV led strip is visible blue-violet light? in general hospital use of sterilization lamps (low pressure mercury lamp), there are blue-violet light it?
We know that the main peak of ultraviolet germicidal lamps as  254nm .the power range of ultraviolet sterilization lamp from 4W ~ 100W, the main function is to sterilization, disinfection, clean air. UV is not visible light, in fact the ultraviolet light able to penetrate the ultraviolet quartz glass tube, the low-pressure mercury lamps that emit mercury spectra after electron bombardment, in addition the mercury spectrum ultraviolet, there are have another visible to the human eye .as such 405,435, 545nm light into the visible range, which includes the blue and violet light, so the ultraviolet light look very pretty pale blue.
And the real effect is not visible. in the ultraviolet 254nm. Sterilizing lamp Can be made into the whole ultraviolet ray in the technology,why not do it? short-term large dose, long-term small dose hurting people.