An electric arc that is formed in mercury vapor through which an electric current flows. The intensity of the illumination of the arc depends mostly on its design and the conditions under which it operates. The mercury spectrum is rich in blue and ultraviolet, but weak in red. Although the ghastly hue of the mercury arc prevents it from being widely used for general lighting, the ability of its bright spectral lines to be isolated easily renders the arc suitable for the optical laboratory. Mercury arcs with glass tubes have sometimes been used in photography.