NIR Cameras Add Visible Spectrum to Security, Military and Industrial Imaging
| By: Winn Hardin, Contributing Editor
Security and military applications have long been the champions of infrared (IR) imaging technology because infrared is stealthy - invisible to the unassisted human eye while capable of finding targets without the assistance of active illumination. Since the first night vision system was introduced, industry has found uses for IR imaging, from control of thermal processes, such as smelting, to evaluating the effectiveness and coverage of transparent materials, such as glue and water.
Unfortunately, the majority of IR-sensitive materials used in imaging sensors are only sensitive to segments of the IR spectrum, and generally do not include sensitivity into the visible spectrum. For security applications, this has meant one camera for daytime surveillance, and another for night. Industrial applications may need a visible camera to verify an object’s color, for instance. The result is added cost, especially for applications where space is at a premium, such as with certain military applications and OEM industrial equipment.
Today, camera manufacturers are developing sensors that can detect both the visible and a portion of the IR spectrum, namely the near- and short-wave infrared, running between 450 nm and 1700 nm. Sensors Unlimited, Inc. (Princeton, New Jersey) and NoblePeak Vision Corporation (Wakefield, Massachusetts) are two vendors taking different paths to a new spectral landscape that includes both traditional imaging and portions of the IR spectrum.
Unlike the mid- and far-wave infrared bands of the electromagnetic spectrum, near infrared (NIR) and short wave IR (SWIR) sensors primarily detect ‘‘reflected’‘ short-wavelength infrared light as opposed to medium and long-wavelength ‘‘thermal’‘ IR, or radiation emitted from black body sources (e.g., heat). The sensitivity to shorter IR wavelengths means NIR and SWIR sensors are able to use smaller pixel sizes. Reflected light imagery also brings more contrast out of the same object due to surface variations and spectral absorbance variations due to the material properties of the object.
In the past, the sensitive materials for NIR and SWIR sensors have been made using complex semiconductor materials –- namely Indium (In) Gallium (Ga) Arsenide (As) Phosphorous (P) combinations as well as Germanium (Ge) –- these materials are less expensive than their mid-wave counterparts and comparably priced to far-wave IR counterparts to manufacture. Additional costs come from having to connect the separate sensor chip to the electronic readout chip. Until recently, each pixel had to be connected to a separate readout cell via bump or wire bonding because of crystal lattice mismatch that prevents growing high-yield IR sensitive semiconductor materials on silicon.
Sensors Unlimited’s standard NIR cameras use a back-illuminated InGaAs sensor bonded to a readout chip to detect radiation between 940 and 1700 nm. Several years ago the company first offered cameras with responsivity through the visible, down to 400 nm.
‘‘With back-illuminated sensors, the light shines through the InGaAs, filtering out light below 940 nm or so,’‘ explains Doug Malchow, Manager of Commercial Business Development at Sensors Unlimited. ‘‘By thinning the substrate, we can take the sensor down to 400 nm, but it does add additional manufacturing costs, which makes it more applicable for military applications. However, sensitivity of this visible wavelength design for night vision surveillance was limited until very recently when Sensors Unlimited introduced the NIR/SWIR InGaAs sensor to see light wavelengths from 0.7 um to 1.7 um. Prior to this development, traditional night vision cameras have detected wavelengths up to roughly 1.0 um.’‘
Each InGaAs NIR/SWIR camera comes with a single-stage thermoelectric cooler integrated inside the camera housing to hold the FPA at 18°C for stability despite changing environmental conditions. Other semiconductor compounds operating in this wavelength range have to be cooled to -70°C. The new InGaAs NIR/SWIR imager is the only camera with the low dark current and high responsivity needed to run at room temperature with low power consumption for lightweight and compact man-portable or small UAV applications.
The company has also recently extended their linear array sensitivity in the other direction up to 2.6 microns. This is achieved by changing the ratio of Indium to Gallium to move the bandgap to longer wavelengths. This change results in much higher dark current due to crystal lattice mismatch with the substrate layer, so the company recently redesigned the readout circuit for the linear array to reduce dark current and make the sensor more sensitive. This allows InGaAs detectors to become practical devices for spectroscopy out to 2.6 microns with only modest thermoelectric cooling compared to Indium Antimonide (InSb) or Mercury Cadmium Telluride (HgCdTe).
‘‘This extended range is going to be very strong for spectroscopy applications because there are strong molecular absorption bands between 1.8 and 2.5 microns for chemical analysis,’‘ Malchow adds. ‘‘Also, by using multiple filters, the new area arrays can become multi-spectral imagers for applications where success depends on specific wavelength sensitivity, such as detecting thin films of water on machined parts, for instance. Pharmaceutical companies also use it to verify that a pill inside a package is the right pill, based spectroscopic analysis of the chemical compounds inside the pill. This is becoming more important as pharmaceutical companies combine different medicines into one pill. You want to be able to discern that you have the right compounds in there, and in the right amounts.’‘ The same logic applies to plastic manufacturers who have to comply with ‘‘through end of life’‘ regulations that require more sorting of recycled plastic materials.
A new camera based on germanium-on-silicon technology created for dense optical switches in telecommunications may soon bring down the price of traditional IR camera solutions.
Researchers at NoblePeak Vision have discovered how to manage the crystal lattice mismatch in growing IR-sensitive Germanium on standard silicon wafers, according to Phil Davies, Vice President of Sales and Marketing. ‘‘We work with the world’s largest CMOS foundry to form the transistors on the silicon wafer. They send the wafer to us, where we grow the Germanium pixels. Then we ship it back to them and they finish it off with the metal layers to finish the sensor.’‘
The result is a single chip visble to SWIR (400nm to 1650nm) sensor that does not need the extra bonding steps of a two-chip solution, or need to thin the chip to be sensitive to both the SWIR and visible bands. Called TriWave, NoblePeak has demonstrated a 128x128 10-micron-pixel array, progressive scan germanium sensor, but has a VGA/NTSC/PAL 744 x 576 TriWave-based evaluation camera for shipment in late May, and plans to add color filters to it’s sensors in early 2009; megapixel arrays will follow in later in the year.
Davies estimates that the NoblePeak VGA-based camera module will sell for approximately $3,000 in 1000 unit quantities, or an order of magnitude less than traditional SWIR cameras based on InGaAs and other architectures.