Medical Device Link.

Originally Published PMPN November 2004

Machine Vision

Figure 1. Wavebands for mainstay imaging systems. Note that various InGaAs formulations cover the entire SWIR range. The labeled bar across the bottom represents the wavelengths that work through glass. Above that range, more-expensive silicon and germanium optics are required (click to enlarge).

by Doug Malchow
Sensors Unlimited

Shortwave infrared (SWIR) imaging is gaining popularity in pharmaceutical packaging applications and will likely continue to expand in importance.

Sometimes referred to as near-infrared (NIR) imagers, SWIR imagers and line-array cameras can see objects and events that visible and thermal cameras cannot, covering the waveband from 900 to 2500 nm. They are smaller and lighter than thermal cameras and cost far less than most (see Table I).

Furthermore, most InGaAs (Indium-Gallium-Arsenide) SWIR cameras, which cover the 900- to 700-nm heart of the SWIR waveband, are all solid-state. Therefore they need no shutters, cooling systems, or other moving parts. Some come factory-set, never needing uniformity corrections over their entire service lives. All SWIR imagers work with plain-glass optics, avoiding the expense of silicon or germanium lenses costing 10 times as much (see Figure 1). In essence, today’s SWIR camera is as smart and simple to use as a video camcorder.

Moreover, the installed base for InGaAs SWIR detection is rising steadily in pharmaceutical packaging quality assurance (QA). SWIR-based machine vision systems have proven successful in hundreds of pharmaceutical and medical product QA applications.

The following applications illustrate how imaging in the shortwave infrared with InGaAs can work.

Fill Levels, Container Integrity

A principal success has been in monitoring liquid fill levels within opaque containers and verifying integrity of glass containers themselves. Many liquid medicines are dispensed into milky white plastic bottles, rendering visual sensing of fill level impossible. As water in the liquid absorbs light strongly in the 1440 and 1940 nm bands, a SWIR camera with incandescent backlighting easily reveals the fill level inside.

Glass medical bottles and other medical glassware must meet stringent dimensional and quality standards. On-line inspection with a SWIR camera permits monitoring of the entire bottle shape, inside and out, and temperature during cooling—all from one side. Thermal cameras are not able to see through glass, while a single SWIR camera can inspect the wall thickness and even see stress defects on the far side of the package. On-line SWIR imaging also detects glass filaments inside the cavity, which may break off and contaminate the product with dangerous shards.

Table I. Comparison of detector technologies for machine vision in medical and pharmaceutical packaging (click to enlarge).

Another growing application is real-time analysis of water, proteins, carbohydrates, fats and oils, and various hydrocarbon molecules in process streams as they move on belts or through pipes. SWIR spectroscopic imaging is a step up from FTIR-NIR spectroscopy, bringing more speed, simplicity, capacity, and information content to a proven constituent monitoring methodology. You can cover a wider belt or a faster-moving process stream by imaging with a compact SWIR array in place of an FTIR spectroscopy system, and with a much smaller instrumentation package. A camera that fits in the palm of your hand replaces a benchtop instrument.

Also growing is the use of SWIR arrays for NIR-Raman spectroscopy of coatings and tablet ingredients. SWIR line cameras work well for Raman applications due to their linearity, high dynamic range, sensitivity, and anti-blooming characteristic. Combining both techniques benefits the inspection of feedstocks and packaging coatings as well as verifying that the correct product is inside the package.

Coatings, Authenticity

Many products use high-performance coatings to ensure freshness while remaining transparent to the eye. While visible-wavelength cameras cannot capture the transparent coating, SWIR cameras render them clearly.

And in a new application to stay a step ahead of counterfeiters, SWIR cameras monitor the placement and readability of NIR-fluorescing authenticity marks on-line. Current UV-stimulated fluorescence marks are no longer unique. SWIR provides an excellent solution.

Recycling

Packaging companies are concerned about recycling of their manufacturing scrap and about the impact of their products on their customers’ waste stream. In Europe, and now in the United States, recyclers have come to depend on SWIR-based spectroscopy for sorting plastics from the waste stream into useful commodities. Inexpensive SWIR line-scan cameras quickly identify the type of polymer in a container passing down a sortation conveyor.

Alternatives

Certainly there are other imaging options besides InGaAs SWIR for pharmaceutical and packaging machine vision systems. In fact, the bulk of applications still involve CCD or CMOS imagers in the visible and thermal cameras for remote temperature measurement. These are complementary technologies, and each has its place.

Certainly, too, there has been progress in all three camera classes. To varying degrees, all have grown smaller, lighter, more sensitive, more rugged, more feature rich, and affordable. Microbolometers, for instance, now perform thermal imaging without the need for cooling. They also cost less than SWIR cameras. While they may someday have a place in pharmaceutical machine vision, they are slow, disqualifying them from any high-speed, flash, or pulsed-illumination applications. Bolometers also still need mechanical shutters and silicon or germanium lenses.

CCDs and CMOS visible-range imagers have evolved, too, featuring smaller pixel sizes, greater sensitivity, and still lower cost. For objects or events visible to the eye, CCDs and CMOS visible-range imagers remain the choice.

Some Facts Don’t Change

The fact remains that SWIR InGaAs cameras offer a combination of intrinsic benefits that thermal and visible-spectrum cameras do not. Dispensing with expensive lenses, shutters, and cryogenic cooling adds reliability, vibration immunity, and cost/size/ weight advantages not available in most IR cameras or in SWIR cameras with other types of detector arrays.

All things considered, some of the greatest progress lately has been in InGaAs SWIR cameras. First, linear arrays up to 1024 pixels on a 25µm pitch provide higher resolution for a wider field of vision so fewer cameras cover more area. Second, today’s InGaAs SWIR cameras provide simultaneous selectable analog and digital output as a standard feature.

For these reasons, a strong case can be made to explore the SWIR waveband for your real-time imaging and process monitoring needs. Virtually every object or event of interest reflects or emits some SWIR radiation. The result can be a more stable, reliable, robust, and lower-cost process that will make the company controller your best friend.

Doug Malchow is an application engineer for Sensors Unlimited (Princeton, NJ).

Copyright ©2004 Pharmaceutical & Medical Packaging News