Near Infrared (NIR) measurements are based on specific absorption bands in the electromagnetic spectrum between 800 and 2500 nanometers (nm). While full spectrum analyzers may utilize the entire spectrum and multivariate mathematical treatments such as Principal Component Regression (PCR), Partial Least Squares (PLS) or neural networks, photometers use “slices” of the spectrum that correspond to specific molecular overtones and associated vibrational absorptions.
Photometers utilize narrow band pass interference filters that have a specified band width typically 50 nm wide, though the band width is sometimes application dependent. The NIR has broad bands as opposed to the sharp peaks in the Infrared region. Common bonds are O-H (1940 and 1420 nm) in water, C-H (2340 nm) in organics and oils and N-H in proteins. The absorbance level at these specific wavelengths is proportional to the quantity of the constituent to be measured.
Shining light at these wavelengths causes the corresponding chemical bond to vibrate and absorb energy. Think of different sections of a stadium crowd called upon to shout and jump when their section is highlighted on the stadium Jumbotron.
Photometers compare near infrared energies at measure and reference wavelengths and are typically calibrated with a simple linear regression analysis though non-linear algorithms can be employed for specialized applications or for wide dynamic ranges. Photometers are easy to install and maintain and they are used in process, at-line and in the laboratory for rapid product analysis. Typical applications are found in the food, pharmaceutical, tobacco, converting, chemical, mineral and wood processing industries.