The Periodic Spiral assigns the color yellow to helium, calcium, chromium, nickel, germanium, krypton, radium, seaborgium, darmstadtium, ununquadium, and (the recently discovered and provisionally named, but subsequently disclaimed) ununoctium, element 118. Helium was detected in the gaseous atmosphere surrounding the Sun by its bright yellow chromospheric spectral line (described by its discoverer as a "glorious yellow effulgence, and consequently named after the sun god Helios. Calcium metal exposed to air forms a yellow coating of its oxide, hydroxide, and nitride. Chromium is the base for such valued yellow pigments as lead chromate (chrome yellow) and zinc chromate (zinc yellow). On the US roadway, chromium pigments are essential to the yellow lines indicating traffic lanes or the edge of the road. Nickel is the basis for another series of yellow pigments, notably nickel dioxine yellow, nickel azo yellow, and nickel titanate yellow. Yellow Krypton and Yellow & Red Krypton lasers are used in fluorescence microscopic imaging.