A monochromator comprising: a light source ; an entrance slit positioned to pass through light from the light source ; a first monochromator mirror positioned to reflect light passed through the entrance slit ; a diffractive surface positioned to receive and diffract light reflected by the first monochromator mirror ; a second monochromator mirror positioned to reflect light diffracted by the diffractive surface ; an exit slit positioned to pass through light reflected by the second monochromator mirror ; a cuvette positioned to pass through light passed through the exit slit ; a long-pass interference filter positioned to receive light from the light source , reflect light that has a wavelength below a selected value, and pass through light having a wavelength above the selected value; and a first sample detector positioned to receive light reflected by the long-pass interference filter ; characterized by : a filter assembly , the filter assembly including the long-pass interference filter and a plurality of additional long-pass interference filters (62A-62N), the long-pass interference filter and the plurality of additional long-pass interference filters (62A-62N) configured to be interchangeably positioned between the light source and the first sample detector ; the filter assembly further including a rotatable filter wheel , the long-pass interference filter and the plurality of additional long-pass interference filters (62A-62N) being disposed around the rotatable filter wheel . a light source ; an entrance slit positioned to pass through light from the light source ; a first monochromator mirror positioned to reflect light passed through the entrance slit ; a diffractive surface positioned to receive and diffract light reflected by the first monochromator mirror ; a second monochromator mirror positioned to reflect light diffracted by the diffractive surface ; an exit slit positioned to pass through light reflected by the second monochromator mirror ; a cuvette positioned to pass through light passed through the exit slit ; a long-pass interference filter positioned to receive light from the light source , reflect light that has a wavelength below a selected value, and pass through light having a wavelength above the selected value; and a first sample detector positioned to receive light reflected by the long-pass interference filter ; characterized by : a filter assembly , the filter assembly including the long-pass interference filter and a plurality of additional long-pass interference filters (62A-62N), the long-pass interference filter and the plurality of additional long-pass interference filters (62A-62N) configured to be interchangeably positioned between the light source and the first sample detector ; the filter assembly further including a rotatable filter wheel , the long-pass interference filter and the plurality of additional long-pass interference filters (62A-62N) being disposed around the rotatable filter wheel . a filter assembly , the filter assembly including the long-pass interference filter and a plurality of additional long-pass interference filters (62A-62N), the long-pass interference filter and the plurality of additional long-pass interference filters (62A-62N) configured to be interchangeably positioned between the light source and the first sample detector ; the filter assembly further including a rotatable filter wheel , the long-pass interference filter and the plurality of additional long-pass interference filters (62A-62N) being disposed around the rotatable filter wheel . <EOS>