1718, "capable of seeing spectres;" 1815, "ghostly;" from spectre + -al (1). Meaning "pertaining to a spectrum" is 1832, from stem of spectrum + -al (1). Related: Spectrally.
Entries linking to spectral
spectre n.
chiefly British English spelling of specter (q.v.); for spelling, see -re.
-al 1
suffix forming adjectives from nouns or other adjectives, "of, like, related to, pertaining to," Middle English -al, -el, from French or directly from Latin -alis (see -al (2)).
spectrum n.
1610s, "apparition, specter," from Latin spectrum (plural spectra) "an appearance, image, apparition, specter," from specere "to look at, view" (from PIE root *spek- "to observe"). Meaning "visible band showing the successive colors, formed from a beam of light passed through a prism" first recorded 1670s. Figurative sense of "entire range (of something)" is from 1936.