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Machine Vision Industry Focus: Putting Light to Work

Nov. 22, 2008
Special fiber optics now let spectroscopy migrate out of the laboratory and into inhospitable environments.

Spectrometers and other sophisticated light-measurement equipment once were consigned to test labs. These instruments have made a debut on plant floors in recent years because the manufactured quality of many products now involves some kind of light measurement. Moreover, these measurements can be relatively sophisticated, sometimes involving light in the UV and IR range.

Who, What, Where
Authored by Cheryl Provost
CeramOptec Industries Inc.
East Longmeadow, Mass.

Edited by Leland Teschler [email protected]

Key Point
Spectroscopy has become rugged and portable enough to work in the harsh regimes characterizing production settings and use in the field.
The fiber optics used to feed signals to these measurement instruments aren't telecom fiber, but rather a special type optimized to transmit light in the bands of interest.


ASD Inc.
Mettler-Toledo Inc.

Spectrometer basics at Wikipedia,

But the only way to take such measurements is to get the light to the spectrometer. Field spectrometers sit in environmentally resistant enclosures that let them work in hot, cold, dirty and wash-down conditions. The problem is that this sort of enclosure precludes the usual testing method of placing a cuvette containing the sample into a special chamber in the instrument.

Instead, portable spectrometers pick up light from a target that is transmitted through a flexible optical fiber. These fibers are not the kind used to transmit telecom signals. They are specialty fibers designed specifically to pass different light qualities that depend on the application at hand. These fiber assemblies are also hardened to survive the harsh chemical environment arising in, say, the cleaning of food-processing equipment.

Specialty fibers of this sort have been available for many years. But they have only become more widely used as it became customary to see sophisticated instruments in factories and engineering departments. It now pays to understand the optical and physical qualities of the fibers employed in these situations. For spectroscopy, not just any fiber will do.

Expanding spectroscopy’s range
Spectroscopy provides a chemical fingerprint of a sample, allowing both quan- titative and qualitative material analysis. Spec- troscopy is nondestructive and fast. It is the re- cent development of compact spectrometers that has let these instruments work beyond the lab. Suppliers have developed a number of dif- ferent types of spectroscopy, including absorption, reflectance, and Fourier transform IR methods, each of which handles a range of analytical work.

However, there is a misconcep- tion that telecom fibers can serve in these portable spectroscopy applications. Unfortunately, these in- expensive pennies- per-meter fibers lack the quali- ties needed for accurate measurements on the shop floor. Accurate spectrographic analysis demands properties that can include the ability to carry light at a wide range of wavelengths — from deep-UV, through the visible, to the near IR, and to the mid-IR — with low loss.

Telecom fiber typically can’t pass light wavelengths over this extreme range. It is optimized for a single mode of light transmission. Spectroscopy demands specialty fibers over a wide variety of core diameters and different numerical apertures (NAs) which determine the acceptance angle in which light can be collected by the fiber.

Nor are all specialty fiber products created equal. Many are special only in terms of size: Companies buy standard preforms (the glass from which optical fiber is drawn) and then draw the fiber to the sizes needed. But the properties of the fiber itself doesn’t change. In contrast, it is possible to get better-performing fiber by controlling the makeup of the preform. For example, my company offers a wide range of NAs by changing the dopants in the core and cladding. We control spectral performance by changing the OH content.

Silica is a good material for fiber in general, in terms of both its optical and thermal properties. It can be produced with ultrahigh purity and has little absorption across wavelengths ranging from about 200 to over 2,400 nm. But sometimes other materials are more appropriate. For some high-NA solutions, CeramOptec uses hard plastic rather than silica for the fiber cladding. And for mid-IR solutions, silver-halide fibers are appropriate.

Specialty fibers and fiber assemblies specifically designed for portable spectroscopy are available with a range of optical and physical qualities. They can be made from silica or other rugged materials, optimized for specific wavelengths, designed to accept light easily and be both flexible and robust.

Specialty fibers are made with tightly constrained processes that control everything from creating the preform to incorporating the fiber into an assembly. However, regardless of the applications, the key qualities for specialty optical fibers used for spectroscopy are OH (or water) content and numerical aperture.

OH content relates to the core material of the preform from which the fiber is drawn and controls its spectral performance. Fiber with a relatively high percentage of OH (a few thousand ppm) transmits UV light better. Fiber used to carry the near-IR range must be “dry” with an extremely low OH level of only a few parts per million. By manipulating OH content, silica fibers can be optimized to carry light over wavelength from 190 to 1,200 nm in the UV-VIS range as well as from the edge of the visible spectrum at 350 nm to the near infrared at 2,400 nm.

Numerical aperture is also controlled by the preform. Fibers typically have an NA of 0.22, but some fibers used for spectroscopy can have NAs as high as 0.53 and as low as 0.06.

The main benefit of high NA is that the fiber can be thinner while still collecting and delivering light efficiently. Smaller fibers are more flexible and more fatigue resistant, occupy less space, and weigh less.

Fibers can be customized with nonsilica cladding (such as hard plastic cladding), or for performance qualities including operation at very high (380°C) or low (–190°C) temperatures.

Some spectrometers, such as portable near-IR reflectance spectrometers made by Analytical Spectral Devices (ASD) Inc., Boulder, Colo., use fiber internally as well. The company uses a hybrid approach, splitting the incoming signal among three spectrometers to cover a range from 350 to 2,500 nm. Shorter wavelengths are limited to fibers 2-m long at most, while visible wavelengths can be transmitted through longer fibers. Spectroscopy in the field In the bright glare of a meat-processing plant in a seemingly endless line of sides of beef roll by on a conveyor. As they reach the black box sitting astride the belt, a light flashes across the surface, setting in motion a sequence of events that directs the meat to the proper processing and packaging area, and ensures the right labels are applied. It’s not a scene from yet another “Rocky” sequel, but the latest application for field-hardened spectroscopy equipment made possible through the use of specialty optical fibers. The spectrometer is part of a system that provides a real-time measure of beef tenderness that is a quantum improvement over conventional measurement techniques. While tenderness is just one aspect of grading beef, it is difficult to perform in real time. The conventional test requires cutting off a piece of meat, aging it 14 days, cooking it, and then measuring the force required to cut it. Trouble is, by the time the test is done, the beef has long since been shipped to supermarkets, having been graded by visual techniques.

To provide a more accurate determination of beef tenderness, a clever designer developed a test that uses near infrared (NIR) reflectance spectroscopy. The spectrometer mounts above a conveyor belt within a hardened case that can handle the harsh cleaning methods required to ensure food safety. It correlates a light signal to a tenderness rating that is then used to determine how the meat is cut, packaged and labeled.

With the use of specialty fibers, portable spectroscopy equipment is finding more applications in the field. For example, near-IR reflectance spectrometry is being used as a secondary method for in-field analysis. Here, it provides “ground truth” that can be compared to either remote sensing (such as satellite data) or chromatography or X-ray testing later on in a lab.

The battery-operated instruments can perform realtime measurements of nearly anything organic: pharmaceuticals, food, paper or wood, as well as mineral products, such as for mining. A light spectrum can be gathered in tenths of a second. This level of performance could allow the sampling of every product passing through a process.

In another case, researchers and process engineers needed to investigate the chemical composition of materials inside a reaction chamber during the process. This problem was solved by Mettler-Toledo Autochem, Columbia, Md., by coupling a lab-based FTIR absorption spectroscope to a mid-IR-transmitting fiber, encased in a tough probe that can withstand low or high pH, temperatures as hot as 300°C and pressures as high as 5,000 psi.

The instrument uses a silver-halide fiber that transmits light from 4 to 16 μm with low attenuation. The nonhygroscopic material resists water absorption, operates at temperatures from –80 to 150°C and has a minimum bend radius of 100 times the diameter of the fiber.

Previous methods for analyzing spectra from these environments used mirrored conduits or chalcogenide fibers. But silver-halide fiber is both thinner and more flexible. In addition, the fiber provides about five times the sensitivity of chalcogenide fiber. This facilitates measurement of lower material concentrations in a reaction vessel. And of course, the instruments can be used in production environments.

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