Farther on they came suddenly upon a spectrally luminous figure just in their path.
1718, "capable of seeing spectres;" 1815, "ghostly;" from spectre + -al (1). Meaning "pertaining to a spectrum" is 1832, from stem of spectrum + -al (1).
spectral spec·tral (spěk'trəl)adj. Of, relating to, or produced by a spectrum.