Monochromator with stray light reduction

A monochromator comprising:

• a housing;
• a light source positioned in the housing;
• a source mirror positioned to reflect light from the light source;
• an entrance slit positioned to pass through light reflected by the source mirror;
• 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 sample mirror positioned to reflect light passed through the exit slit;
• a cuvette positioned to pass through light reflected by the sample mirror;
• 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.