Spectral means ghostly in appearance. A spectral figure, spectral light, spectral silence.
It is literary and belongs to ghost stories and to descriptions of pale unreal light. The scientific sense is entirely different and much commoner in technical writing: spectral analysis breaks light into its component wavelengths, and spectral lines are how the composition of stars is read. Note spectre, spectrum; and that both come from the Latin for an appearance.
1adjectivespectralGhostly in appearance.
“A spectral figure in a white dress drifted along the corridor at midnight.”
2adjectivespectrumTo do with the spectrum of light.
“A prism splits white light into its spectral colours, from red to violet.”
spectral is a quality of ghostliness or light.
Spectre plus -al, from Latin spectrum, an appearance or apparition, from specere, to look. Spectrum, spectacle and inspect come from the same looking.
From spectr(e) + al.
Etymology from Wiktionary, CC BY-SA
Practised in A Song of Ice and Fire