In mini-spectrometers, gratings – the wavelength dispersive element – generate wavelengths at higher orders than the primary wavelengths to be detected. These higher order wavelengths are fractions of the primary wavelength, and, if the spectrometer covers less than one octave, are spatially separated by angle. If the ratio of upper to lower wavelength is larger than two, the orders begin to overlap and must be removed or sorted out before they reach the detector.   Read More