Advanced Imaging


Advanced Imaging Magazine

Updated: July 8th, 2008 05:26 PM CDT

Imaging in the Shortwave Infrared (SWIR) for Industrial Applications

When Visible Imaging Comes Up Short, SWIR Can Be the Answer
Quantum Efficiency Graph
Fig. 1 - Quantum Efficiency Graph.
Fig. 2 Visible apple image (left) and SWIR apple image (right) clearly shows bruising.
Fig. 3 - Experimental Setup.
Fig. 4: Raw spectra taken at several times after glue drip reached field of view.
Fig. 4: Spectral response normalized by spectra from white box adjacent to drip.
Fig. 5 - (left to right) a)Visible image with glue, after 20 minutes; b) Imaged with 1440 nm bandpass filter; c) Imaged with illumination from 1550 LED laser); d) SWIR image filtered at 1550 nm.; e) SWIR image (no filters).

By Douglas Malchow, SUI (Sensors Unlimited, Inc.), part of Goodrich Corp.

Visible-camera machine vision of industrial processes saves time, money and resources for manufacturers all over the world. Identifying drift in a manufacturing process at the point of origin saves millions of dollars in scrap product down the line. But what do you do if the production of goods you are trying to control provides no observable or visible cues for monitoring the process or detecting defects? The challenge to the process engineer is to find viable alternatives that will work with the inspection infrastructure that already exists.

This article explores the use of invisible light to solve some everyday industrial imaging challenges and illustrates the method by developing a solution for imaging the placement of white glue on white cardboard boxes.

Invisible Light

The human eye generally responds to light with wavelengths from 380 to 720 nm. Visible light cameras can be filtered to match the eye response, including imaging in color. These cameras are the dominate type used in machine vision systems because, in most cases, they will capture the product or process characteristics that are essential to the product quality. Outside this spectral range, the light is invisible to the eye, with wavelengths below 400 nm referred to as ultraviolet (UV) and those above 700 nm as the infrared (IR). The infrared spectrum is further broken down into sub-sections: near-infrared (NIR) or short-wave infrared (SWIR), mid-wave infrared (MWIR) and long-wave infrared (LWIR).

Invisible Light Cameras for Machine Vision

Light from the UV, NIR and SWIR wavelength bands interact with surfaces in much the same way as visible light, so system integrators can set up machine vision systems for these wavelengths in the same way they would set up for visible-responding systems. Compact, un-cooled cameras for these bands are readily available with standard Camera Link® interfaces for machine vision applications and are easy to use. The same illumination concepts apply for both visible and the UV, NIR and SWIR bands; it is important to use diffuse light and avoid direct reflections of the surface of the objects into the camera.

LWIR cameras primarily respond to the thermal emissions of objects in the scene, while MWIR cameras can be used for thermal or reflected light applications. These cameras require cryogenic operating temperatures, creating reliability and power consumption concerns for industrial applications. Uncooled LWIR cameras are available for temperature uniformity applications and monitoring machinery for hot-spots.

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