BARNARD MICROSYSTEMS LIMITED

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Multi-spectral imaging systems

Multi-spectral  infra-red, polarisation sensitive, imaging is used to see:

• human beings on the ground at night, as part of a search and rescue operation
• in very low light level conditions
• in darkened rooms or caves illuminated by eye-safe 1550 nm laser light that cannot be detected by the human eye of by a silicon CCD imager
• regions of diseased crops, in otherwise healthy crops

Topics

• Atmospheric transmittance
• Near infra-red imaging system
• Infra-red  imaging systems
• The InGaAs short wavelength infra-red  camera
• The Goodrich Sensors Unlimited InGaAs camera

from http://upload.wikimedia.org/wikipedia/commons/5/5b/Atmospheric_transmittance_infrared.gif

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DALSA Pantera SA camera, from http://www.dalsa.com/

Responsivity of a DALSA Pantera camera containing a DALSA 2.0 MPixel (1,600 x 1,200 pixel) CCD imager. We estimate about 20% of the total light lies in the wavelength region above 700 nm.

Spectral response of a HOYA R72 infra red filter, from the Hoya web site catalog  http://www.hoyafilter.com/pdf/HOYACatalog.pdf

The HOYA R72 filter allows in infra red light with a wavelength that is greater than 720 nm. There is some detectivity of infra red light with a wavelength greater than 720 nm, as can be seen on the above spectral responsivity plot. Note that on commercial digital cameras:

1. the cover glass over the CCD filters out the infra red light
2. the red filter on the CCD element itself also filters out the infra red light

Consequently, use of an N72 filter in front of a digital SLR camera lens will not in general constitute  an effective way of imaging in the near infra red.

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- images  from www.flir.com

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from http://www.indigosystems.com/PDF/articles/CSMarReprint.pdf

Several unique capabilities and characteristics of Short Wavelength IR (SWIR) make it a valuable asset. The following  text, with some modifications,  has been taken from http://www.sensorsinc.com/ISR.html

• Low Light/Night Vision Imaging: SWIR cameras can operate down to starlight conditions, receiving adequate illumination from the weak natural phenomena known as atmospheric nightglow. Nightglow is created by hydroxyl ion emissions in the short wavelength infrared portion of the spectrum. Unlike mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR), SWIR cameras can image through the windshield and thus be mounted in the driver's compartment for a " driver's eye" view of the way ahead.

• Recognition of Target: Whereas long and medium wavelength infrared sensors are able to spot targets based solely on their heat emissions, SWIR cameras and sensors see reflected light in the shorter wavelengths just beyond the visible range. As a result, the images are close representations of what is seen in the visible spectrum. Humans are very recognizable, with the typical difference being that all hair shows as white due to the lack of moisture in hair conversely, skin shows darker, due to its high moisture content. It is said that long and medium wave sensors provide detection, while SWIR and visible sensors provide recognition. The beauty of SWIR is that recognition can be acheived covertly in darkness.
• Eye-Safe Laser Illumination: Lasers operating in the SWIR portion of the spectrum are eye-safe, meaning these wavelengths of light will not easily penetrate the cornea of the human eye. Thus, illumination by lasers at the common 1550 nanometer wavelength may be used to safely illuminate humans and targets.

• Lasers are used on the modern battlefield in many applications, from detecting the range of an object to designating a target for another weapon system to attack. The most common battlefield lasers operate at 850 nm, 1060 nm, and approximately 1500 nm. The first two laser types are visible to current night vision goggles. The 1550 nm laser, a modern eye-safe wavelength device, is not visible with current night vision technology. 1550 nm is within the short wavelength infrared  region, and is therefore considered covert, invisible to opposing forces. The 1550 nm laser emission is, however,  highly visible to  SWIR cameras. Covert to enemies, but highly visible to troops equipped with SWIR imaging devices, such lasers are increasingly important on the battlefield.

• Covert Illumination: In turn, 1550 nm light is invisible to the human eye,   but is easily seen by SWIR cameras and sensors. An entire area may be lit with SWIR illumination that is totally invisible to the human eye, yet bright and clear to SWIR cameras. Similarly, a single target of interest might be illuminated by a focused laser beam for easy viewing or tracking. This is useful for designating specific targets for range finding, as well as other military-specific applications.

Short wavelength infrared  imagers generally use reflected light to image a scene. In many instances there are no natural sources of illumination available, i.e. in caves, tunnels, basements and so on. In these situations, covert active illumination sources that are not detectable by  the human eye, by night vision goggles, or by silicon CCD or CMOS based cameras are needed to illuminate the area without detection from those common imaging technologies. Short wavelength  infrared cameras  can image scenes illuminated by these covert illumination sources the camera's high signal-to-noise ratio is an added benefit. This fundamental ability of InGaAs cameras to image areas illuminated by  SWIR  light is an important military capability.

A landscape covered in fog, taken with a visible camera. The inset shows the advantage of SWIR cameras in foggy or hazy conditions.

The potential drawbacks

Excellent technology. Professional, helpful  staff at Sensors Unlimited Inc. Two potential problems:

• A minimum of three months wait to get an export licence granted if you live outside of the USA (if it is granted, that is).
• $ 20,000 (budgetary figure as on 12^th December 2007) for the SU320M-1.7RT (see below for a description of this camera) or $ 24,000 (again, budgetary figure as on 12^th December, 2007) for the higher sensitivity SU320MX-1.7RT. Gulp (!)

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The Goodrich SU320M-1.7RT InGaAs camera. This MiniCamera with a 320x240 pixel imager is the most popular camera in our camera line. Capable of imaging from 0.9 µm to 1.7 µm in daylight or typical indoor room lighting conditions, it has proven very effective for industrial processing and laser beam analysis. It features simultaneous analog video and 12-bit digital video outputs. Shipping now to customers in the USA.

Information from http://www.sensorsinc.com/downloads/4110-0005_E_SU320M-1.7RT.pdf

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Additional text © Barnard Microsystems Limited 2006 - 2008