Spectrum, Spectra, Spectre
(Latin, specto, to behold). In optics a spectrum is the image of a sunbeam beheld on a screen, after refraction by one or more prisms. Spectra are the images of objects left on the eye after the objects themselves are removed from sight. A spectre is the apparition of a person no longer living or not bodily present.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894