Spectrometers are optical instruments that can detect spectral lines and measure their wavelength or intensity. Spectrometers can also be used to test the efficiency of an optical filter in order to determine whether a filter has properly blocked or transmitted specific wavelengths.
A perfectly flat transmission curve would yield a perfectly color neutral result. And while there is no such thing as a perfectly neutral ND filter (contrary to a filter manufacturers claim) the X4 ND is the most color neutral and has the flattest transmission curve of any ND filter, guaranteed.
Neutral density filters manufactured using the traditional method also have strong transmission in the infrared wavelength starting at 700nm, whereas the X4 ND maintains a very well controlled and flat transmission all the way throughout the visible spectrum and into IR.