Spectrum has been in English around 500 years,1 yet because it comes from Latin, many writers insist on pluralizing it as a Latin word. Indeed, spectra is one of a handful of Latin plurals that prevail over their English alternatives by significant margins. Still, speakers of English aren’t required to know the rules of Latin grammar, and the English plural, spectrums, is not wrong. It has been gaining ground over the last century, though it still tends to give way to the Latin form, especially where Latin forms typically prevail—i.e., in scientific writing, legal writing, and a few other formal contexts.
Examples
Spectra
Its NMR signal can dominate spectra of biological samples such as blood. [Chemical & Engineering News]
The closest researchers have come is examining the spectra of a super-Earth—called GJ 1214b—that has a radius about 2.6 times that of Earth’s. [Nature]
Spectrums
This runs in contrast to the hierarchical, resume-intense and self-serving leadership that dominates the U.S. across all spectrums. [Washington Post]
At present, people who own digital radios have to navigate between the digital and analogue spectrums by pushing a button.[Telegraph]
But the demonstrators are drawn from across the age and political spectrums. [Los Angeles Times]
Reference
1. http://www.oed.com/view/Entry/186105 (subscription required) ^
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