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.