| 1 | English | spectro- | optical spectra |
| 2 | English | Spectrum | — |
| 3 | Latin | spectrum | appearance, image; apparition, specter |
| 4 | Ancient Greek | σκοπέω | I look, look at, behold; I examine, inspect; 413... |
| 5 | Ancient Greek | σκοπός | watcher, lookout; protector, guardian; spy, scout |
| 6 | Ancient Greek | σκέπτομαι | I look at, examine; I examine, consider, think |
| 7 | Proto-Hellenic | sképťomai | to look at |
| 8 | Proto-Indo-European | spéḱyeti | to be looking at, to keep looking at |
| 9 | Proto-Indo-European | speḱ- | to see, to look, to observe |