When an oxygen-rich flame reacts with glass containing sodium the result is a bright yellow flame. The brightness of sodium flare can make it difficult for a lamp or flame worker to see their work.
Though not itself harmful, this sodium flare is typically accompanied by both ultraviolet (UV) light and infrared radiation (IR) -- both of which can be damaging to one's eyes.
A good pair of didymium glasses will filter out UV, IR and sodium flare.
Technical note: The wavelength for sodium flare is 589nm -- well within the visible light spectrum of approximately 380nm to 750nm. Immediately below this range are UV waves at 290nm to 380nm. IR starts about where visible light stops at 760nm.