Abstract: A spectrometer or multiple wavelength absorbance detection method and apparatus providing improved accuracy for an array of measurements at different wavelengths. The spectrometer utilizes a multiple wavelength illumination system with an array of independent detectors with different pathlength cells, where each cell is illuminated with predominately monochromatic light after separation by a light dispersing element. Each sample cell has an optical pathlength, optics and photodetection device that are optimized for its particular wavelength to accurately measure absorbance through an expected substance.