spectrally

spectral +‎ -ly

spectrally (comparative more spectrally, superlative most spectrally)

1. In a spectral manner; like a ghost.
   • 1899 February, Joseph Conrad, “The Heart of Darkness”, in Blackwood’s Edinburgh Magazine, volume CLXV, number M, New York, N.Y.: The Leonard Scott Publishing Company, […], →OCLC, part I, page 215:

     Beyond the fence the forest stood up spectrally in the moonlight, and through the dim stir, through the faint sounds of that lamentable courtyard, the silence of the land went home to one’s very heart, - its mystery, its greatness, the amazing reality of its concealed life.

2. In terms of a spectrum.

• cepstrally