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.