Vision-Based System Design Part 10 – Vision Beyond the Visible

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By Giles Peckham, Regional Marketing Director at Xilinx,
and Adam Taylor CEng FIET, Embedded Systems Consultant

One of the advantages of embedded vision systems is their ability to observe wavelengths outside those which are visible to humans. This enables the embedded vision system to provide superior performance across a range of applications from low light vision to scientific imaging and analysis.

While imaging systems at higher wavelengths including X Ray and Ultraviolet are used for scientific applications such as astronomy, it is IR wavelengths which are most often deployed in industrial, automotive and security applications. As IR imagers sense the background thermal radiation, they require no scene illumination and provide the ability to see in total darkness, making them ideal for automotive and security applications, while within the industrial sphere, IR systems can also be used in thermo graphic applications where they accurately measure the temperature of the scene contents. For example, in renewable energy, thermal imagers can be combined with drones to monitor the performance of solar arrays and detect early failures due to the increasing temperature of failing elements.

Working outside the visible range requires the correct selection of the imaging device technology. If the system operates within the near-IR spectrum or below, developers can use devices such as Charge Coupled Devices (CCDs) or CMOS (Complementary Metal Oxide Semiconductor) Image Sensors (CIS). However, as developers move into the infrared spectrum they need to use specialized IR detectors.

The need for specialized sensors in the IR domain is in one part due to the excitation energy required for silicon based imagers such as CCD or CIS. These typically require photon energy of 1eV to excite an electron but at IR wavelengths photon energies range from 1.24 meV to 1.7eV. As such, IR imagers tend to be based upon HgCdTe or InSb. These have lower excitation energies and are often combined with a CMOS readout IC called a ROIC to control and readout the sensor.

IR systems fall into two categories, cooled and uncooled. Cooled thermal imagers use image sensor technology based upon HgCdTe or InSb semiconductors. To provide useful images a thermal imager requires the use of a cooling system to reduce the temperature of the sensor to 70 to 100 Kelvin. This is required to reduce the generated thermal noise to below that which is generated by the scene contents. Using a cooled sensor brings with it an increased complexity, cost and weight for the cooling system, the system also takes time (several minutes) to reach operating temperature and generate a useable picture.

Uncooled IR sensors can operate at room temperatures and use micro bolometers in place of an HgCdTe or InSb sensor. A micro bolometer works by each pixel changing resistance when IR radiation strikes it. This resistance change defines the temperatures in the scene. Typically, micro bolometer-based thermal imagers have much-reduced resolution when compared with a cooled imager. However, they do make thermal-imaging systems simpler, lighter and less costly to create.
For this reason, many industrial, security and automotive applications use uncooled image sensors like the FLIR Lepton.

Creating an uncooled thermal imager presents a range of challenges for embedded vision designers, requiring a flexible interfacing capability to interface with the select device and display, while providing the processing capability to implement any additional image processing upon on the video stream. Of course, as many of these devices are hand held or power constrained, power efficiency also becomes a significant driver.

Example Architecture
The FLIR Lepton is a thermal imager which operates in the long wave IR spectrum, it is a self-contained camera module with a resolution of 80 by 60 pixels (Lepton 2) or 160 by 120 pixels (Lepton 3). Configuration of the Lepton is performed by an I2C bus while the video is output over SPI using a Video over SPI (VoSPI) protocol. These interfaces make it ideal for use in many embedded systems which require the ability to image in the IR region.

One example combines the Lepton with a Xilinx Zynq Z7007S device mounted on a MiniZed development board. As the MiniZed board supports WiFi and Bluetooth it is possible to create both IIoT / IoT applications and traditional imaging solutions with a local display, in this case a 7-inch touch display. This example will create a design which interfaces with the FLIR Lepton and outputs the video on a local display.

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