The Kemtrak DCP007-NIR photometer measures at two wavelengths. A primary "absorbing" wavelength is used to measure the concentration of the chemical being monitored while a "non absorbing" reference wavelength is used to compensate for turbidity, air bubbles and/or fouling. The Kemtrak DCP007-NIR photometer uses precision fiber optics to shine a precisely focused monochromatic light through the process medium and back to a photo-detector. The attenuation of the transmitted light beam caused by NIR absorbing substances in the process medium is mathematically described by the Beer-Lambert law. Applying the Beer-Lambert law (below), the concentration of the sample can be calculated. 
Where: A = absorbance (= - log transmittance) Ε = molar absorptivity coefficient l = optical path length c = concentration of absorbing species
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