The best replacement for spectrum is spectra.

The best alternatives to spectrum are rated as following:
★★★★★  spectra
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★★  narrow spectrum antibiotics
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★★  line spectrum
★  narrow spectrum
★  absorption spectrum
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★  continuous spectrum
★  lucid
★  htc rezound
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Your alternative for spectrum is not listed? Suggest!
Speaking about spectrum, one probably shouldn't forget about an absorption spectrum.

Other search terms related to spectrum and their ratings are:
★★★★★ an absorption spectrum
★★ diffraction grating
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Google Trends

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spectrum
spectra
broad spectrum