A multispectral world view

Camera vendors are seeking to add value to their products by developing systems that image objects in more than one frequency spectrum.

Dec 1st, 2006
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To extract more information from images, camera and system vendors are turning to multispectral imaging.

Conard Holton

Camera vendors are seeking to add value to their products by developing systems that image objects in more than one frequency spectrum. While visible-light cameras use the 400 to 700 nm spectrum to image objects, infrared (IR) cameras operating from the short-wavelength-infrared (SWIR; 0.7 µm) to the long-wavelength IR (LWIR; to 15 µm) spectrum are useful in detecting specific characteristics of objects that may be difficult to image in the visible spectrum. Combining these spectra into a multispectral system can be especially useful in applications such as food inspection, medical, and military imaging systems.

The term “multispectral” is often misunderstood. While many microbolometer-based IR cameras do image scenes across the entire 0.5 to 15 µm spectrum, their sensitivity is somewhat less than specialized cameras based on indium gallium arsenide (InGaAs), mercury cadmium telluride (MCT), or quantum-well infrared photodetectors (QWIPs). So the term multispectral best refers to the ability of an imaging system to detect electromagnetic energy in at least two or more individual spectral bands.

While capturing multispectral data from both IR and visible wavelengths can imply the use of multiple detectors, the opposite can also be true. Sensors Unlimited, a division of Goodrich (SUI; Princeton, NJ) and FLIR Systems (Billerica, MA), for example, both offer InGaAs sensors and cameras that operate in the visible and near-IR spectrum.

Other methods can offer similar results. One of the most promising is the development of dual-color QWIP devices. Qmagiq (Nashua, NH) offers a dual-color IR camera based on a 320 × 256 dual-color or dual-wavelength detector array. Because the device can detect IR images in both the 4 to 5 μm IR and 8 to 9 µm spectral bands simultaneously, the device is especially useful in military applications that previously required two different IR detectors.

It is also possible for one camera to acquire images in the visible spectrum while another acquires images in the IR. At the Beth Israel Deaconess Medical Center (Boston, MA), John Frangioni and his colleagues have developed a near-IR florescence imaging system to allow surgeons to visualize optical images and near-IR fluorescence simultaneously. Frangioni says that by maintaining separation of the two spectra, it is possible to simultaneously acquire visible anatomical images and near-IR florescence images that show the vessel, nerve, and tumor location and overlay the two in real time.

To ensure correct registration of the images, light emanating from the surgical field is split into color video and near-IR fluorescence emission components using a dichroic mirror. A Hitachi (Wadsworth, OH) three-chip CCD camera recorded the color; to record the near-IR fluorescence images, an Orca-ER interline CCD camera from Hamamatsu (Bridgewater, NJ) was used.

Assessing vegetables

For industrial applications, many dual-band inspections require that the two images be automatically spatially and temporally registered-data from the same resolved spot on an object must be collected for both bands at the same time. For customers in the food-inspection industry, Princeton Lightwave (Princeton, NJ) has developed a dual-band linescan camera that uses two independent sensors optically coupled to allow simultaneous imaging in both the 400 to 900 nm and 1000 to 1700 nm wavelengths.

While visible sensing is provided by a 2k × 1 CCD, IR is detected by a 512 × 1 element InGaAs array. Pixels are optically aligned, enabling the simultaneous imaging of both spectrums at a 4:1 resolution. Each sensor provides two analog output streams that are processed using correlated double sampling, dark-level correction, and analog gain. The streams are converted to 12-bit digital data and buffered into dual-port RAM for synchronization and further digital processing.

These multispectral cameras command a premium in the current image-processing and machine-vision applications market. The benefit they provide, however, is allowing sensor, camera, and systems suppliers to add greater value to their products while avoiding commodity markets such as low-cost security and surveillance applications. Slowly, as the demand for sophisticated sensors rises, multispectral imaging will become more commonplace.

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CONARD HOLTON is editor in chief of Vision Systems Design; e-mail: cholton@pennwell.com.

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