Band head

In spectroscopy a band head is the abrupt edge of a spectroscopic band. When a band is described as degrading to the violet, it means that for wavelengths above the band head wavelength, the spectrum is dark, and below the wavelength the brightness of the band weakens. A band that degrades to the red, conversely means that the band head is a lower limit on wavelength for the band, and it fades off toward longer wavelengths, which in visible light is the red end of the spectrum.