Abstract: A method and device for high speed spectroscopic constituent verification. The method includes the steps of illuminating a sample with broadband light and measuring two preselected wavelengths of reflected light: a first narrow-range of wavelengths .lambda.1 that is not significantly absorbed by the constituent of interest, and a second narrow-range of wavelengths .lambda.2 that is substantially absorbed by the constituent of interest. Given the two measurements of reflection, upper and lower thresholds are determined (the latter based on a percentage of the measured baseline reflected light of wavelengths .lambda.1). Finally, the presence of the constituent of interest is indicated if the measure of the reflected discriminant wavelength .lambda.1 is within te upper and lower threshold measures of reflected baseline light .lambda.1.