Range of wavelengths (4)
(Other definitions for band that I've seen before include "Ring or musical group (4)" , "Group; frequency range" , "troop" , "See 23" , "Outline drawing" .)
(Other definitions for spectrum that I've seen before include "Array, range" , "'Broad range, scope (8)'" , "Bands of colour in crumpets" , "All the bands of colour seen in a rainbow" , "Wide range" .)