Spectrum

google

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early 17th century (in the sense ‘spectre’): from Latin, literally ‘image, apparition’, from specere ‘to look’.

wiktionary

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From Latin spectrum(“appearance, image, apparition”), from speciō(“look at, view”). Doublet of specter. See also scope.

etymonline

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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.