A system and method of correcting reflectance comprises determining a reflectance constant for a test product at a first wavelength for which reflectance does not substantially change with the presence of a test substance, with the test product loaded with the test substance, determining a reflectance at a second wavelength for which signal-to-noise ratio is maximized and determining a measured reflectance at the first wavelength, and determining a corrected reflectance as the product of the reflectance with a ratio of the reflectance constant to the measured reflectance.