Click on the advert above to visit the company web site

Product category: Optical sensors and vision systems
News Release from: Sensors Unlimited | Subject: SWIR imaging
Edited by the Processingtalk Editorial Team on 12 October 2005

Principles and practice of Shortwave IR
imaging

Shortwave infrared (SWIR) imaging is establishing a place for itself in manufacturing and process control applications where machine vision provides benefits

Shortwave infrared (SWIR) imaging is establishing a place for itself in manufacturing and process control applications that call for machine vision SWIR imagers, sometimes also referred to as near-infrared (NIR) imagers, can see objects and events that visible light and thermal cameras cannot, are smaller and lighter than all thermal cameras, and cost far less than many of them

SWIR imagers work in a manner similar to that of the human eye or visible-light cameras, but detect energy in the shortwave range of the spectrum (from 900 to 1600 nm, also known as the NIR waveband).

Virtually all objects emit or reflect energy in that waveband, even under starlight.

And when no natural light is present, SWIR emitters can illuminate the object to permit detection by an SWIR camera.

The technology's other major advantage is that plain glass is transparent to SWIR energy but opaque to IR or thermal energy.

Detector arrays in SWIR cameras work much as do silicon detector arrays in visible (CMOS or CCD) or IR cameras, but they are based on wafers made of InxGa1-xAs, InSb, or HgCdTe.

Of these three materials, only InGaAs needs no cooling and offers uniform response over the entire SWIR waveband.

Most InGaAs SWIR cameras are solid state - no shutters, cooling systems, or other moving parts.

Some come factory set and need no tweaking for their entire service lives.

The imagers work with plain glass optics, whereas thermal cameras require silicon or germanium lenses that cost ten times as much.

Furthermore, these imagers are characterised by low noise, minimal cooling and electronic overhead, and simplicity of operation, making them similar to silicon CCDs and CMOS devices.

A look at some typical applications will illustrate the versatility of SWIR technology.

SWIR at work with molten metal.

Emissivity differences between hot metal and slag show up more clearly in the 900 to 1700 nm bandwidth than in the visible or IR range.

An analogue image of the two materials tells operators when to end the process, maximising yield of metals uncontaminated with slag.

The ability of SWIR imagers to work through glass allows them to monitor the 1899F to 3000F process from protective enclosures.

Glassmaking.

Manufacturers of glass hollow-ware have long sought a way to pick out defective product at the "hot end" of their process, while the pieces are still at 200C to 700C.

At that stage, rejects can be shunted aside and reprocessed efficiently, greatly reducing scrap.

It turns out that the peak performance range of lattice-matched InGaAs SWIR cameras falls right within this temperature band.

Because the imagers work through glass, the camera can inspect the bottles from one side.

They can also be calibrated to accurately measure emmisivity vs temperature and thus monitor the object temperature uniformity and cooling rate.

These data allow glassmakers to prevent shattering due to uneven cooling.

Sometimes a manuracturing defect creates a web of hair-like glass filaments that crisscross the inside of the bottle.

When cooled and hardened, these filaments fracture into shards that fall to the bottom of the container, where they can remain through the end of the fill process.

When SWIR imagers detect a temperature differential between filaments and bottle wall, they activate an error signal that can modify the production equipment appropriately.

Pharmaceuticals.

SWIR-based machine vision and hyperspectral spectroscopy systems monitor liquid fill levels in opaque containers and deliver real-time chemical analyses on products moving on belts or through pipes.

For example, many liquid drugs are dispensed in white plastic bottles that make visual camera inspection of fill level impossible.

Since water in the liquid absorbs light strongly in the 1440 and 1940 nm bands, a SWIR camera in combination with incandescent backlighting of the vial easily reveals fill levels.

SWIR or NIR spectroscopy is also used for real-time or continuous measurement of the presence of water, proteins, carbohydrates, fats and oils, and various hydrocarbon molecules.

SWIR arrays also enable 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 design.

Furthermore, they can monitor the placement and readability of NIR fluorescing authenticity marks online.

Recycling.

First in Europe and now in the US, recyclers are coming to depend on SWIR-based spectroscopy for sorting plastics in the waste stream.

Inexpensive SWIR 1024 1 or 512 1 linescan cameras with wavelength sensitivities ranging from 1100 to 2200 nm are mounted on spectrographs.

They quickly identify the type of polymer in a container passing down a sortation conveyor, and the output triggers a dam or paddle that diverts each container into its proper bin.

Agriculture, Textiles, and Forest Products.

Moisture is a key indicator of process control and quality in agriculture, textile processing, and the forest- product industry.

Because water is opaque to SWIR illumination, detecting its presence or absence can be useful in gauging crop health and product ripeness or dryness.

For instance, the technology can detect flaws, such as bruises under the skin of fruits passing by on an inspection line, that are invisible to the eye.

Though perfectly suitable for many processed foods, bruises are not acceptable in fresh produce.

Moisture content is also an indicator of when a dyed fabric is dry enough for the next step, and of whether dye coverage in a particular area is correct.

In particle-board manufacture, online SWIR-based machine vision systems measure moisture in chips and the data are used to regulate heating and drying operations downstream.

In response to growing military demands, the technology is scaling up to production volumes, lowering the cost to ROI-justifiable levels.

Soon a single SWIR detector will add the datacom laser wavelengths of 780 and 850 nm to those previously noted, and new detectors will be available to profile the longer wavelength lasers being developed in the 1800 to 2200 nm wavelengths for LIDAR.

Alternatives.

Most industrial machine vision systems still incorporate CCD or CMOS imagers operating in the visible wavelengths.

Thermal cameras are generally used for remote temperature measurement in industrial settings.

These are complementary sensing ranges, as each can detect contrast between objects that the others cannot.

And there has been progress in all three classes of camera.

To varying degrees, they have grown smaller, lighter, more sensitive, more rugged, more feature rich, and lower in price.

Microbolometers, for instance, cost less than SWIR cameras and can perform imaging in the thermal range without needing cooling.

They are emerging from military applications into automotive night vision and may have a place in some machine vision systems.

They are slow, however, which disqualifies them from highspeed, flash, or pulsed-illumination machine vision applications.

Bolometers also require mechanical shutters and silicon or germanium lenses.

SWIR InGaAs cameras offer a combination of benefits that thermal and visible-spectrum cameras don't.

In gritty industrial applications, for instance, lenses will inevitably be damaged and need replacement.

It is far more economical to use glass than germanium or sapphire.

As do visiblelight CCD and CMOS detectors, SWIR devices respond to reflected light, which yields high-resolution images.

IR cameras sense only heat or thermal differentials, and so have lower resolution.

InGaAs cameras and arrays are as simple to operate as a silicon CMOS imager, but they can image different wavelengths, critical in many applications.

With no mechanical shutters or large cooling systems on board, they are also relatively immune to vibrational error.

Bigger Arrays, Smaller Packages.

Some of the greatest imaging progress of late has been in InGaAs SWIR cameras.

Linear arrays up to 1024 pixels on a 25 mm pitch provide higher resolution for a wider field of vision, so fewer cameras cover more area.

These cameras offer selectable analogue and digital output as a standard feature.

And they are being made smaller, lighter, and much more economical than IR or other SWIR imagers.

Recently for the military, we demonstrated a solid-state microcamera measuring only 5.9 x 2.8 x 1.7 cm, weighing only 70 g, and featuring factory-set nonuniformity corrections.

The camera outputs both 12-bit digital and RS-170 analogue video.

Such compact devices should be of interest for machine vision applications that require squeezing multiple cameras into small, hard-to-reach places.

Acknowledgments.

I wish to thank my colleagues at Sensors Unlimited for their direct help in preparing this article: Martin Ettenberg, Robert Struthers, Robert Jones, John Sudol, as well as others who contributed in various ways.

The author, Doug Malchow, is an Application Engineer at Sensors Unlimited, in Princeton, New Jersey.

• Sensors Unlimited: contact details and other news
• Email this article to a colleague
• Register for the free Processingtalk email newsletter
• Processingtalk Home Page

Search the Pro-Talk network of sites

Put your news on Processingtalk  |   Advertise  |   Get the free Processingtalk newsletter  |   Processingtalk Home
About Processingtalk  |   Copyright © 2000-2008 Pro-Talk Ltd, UK