Electronic Military & Defense magazine was developed for engineers, program managers, project managers, and those involved in the design and development of electronic and electro-optic systems for military, defense, and aerospace applications.
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Techniques Choosing The Right Sensor For Your Military Imaging Application Using noise equivalent irradiance to quantify sensitivity By Jean-Edouard Communal C hoosing sensor technology is fundamental to designing cameras for warfighters in the field. But, it can be difficult to compare the merits of cameras using differing sensor tech- nologies using only the specifications provided by the manufacturers. Noise equivalent irradiance (NEI) pro- vides a normalized and unique parameter that allows a direct comparison. It is also fairly simple to calculate from the commercial specifications for a given wave- length and exposure time of interest. In this article, we compare the merits of typical charge-coupled device (CCD), electron-multiplying CCD (EMCCD), comple- mentary metal-oxide-semiconductor (CMOS), and indi- um gallium arsenide (InGaAs) focal plane array (FPA) cameras using NEI as a com- parative parameter. Design Fundamentals When designing cam- eras, one has many variables to consid- er. Purpose, func- tionality, and SWaP (size, weight and power) are generally the main factors for military applications. Assuming the cam- eras will be small, lightweight, and low- power instruments, the most important factor is function. There are many uses for cameras in the field, including night or low-light vision; seeing through fog, rain, and haze; providing dirty battlefield vision; laser detection; and seeing beacons and tags for covert tagging, tracking, and locating. In addition, the quality of the image is also a major consideration. All of these variables require choosing the optimum sensor technology. Performance Parameters And Tradeoffs Before considering the expense of integrating a camera, the first step is usually evaluating how it will perform. The capacity of a camera to detect light is determined by the following parameters: • Quantum efficiency (QE) • Pixel size • Full well capacity in electrons • Exposure time or frame rate • Readout noise per pixel • Dark current per pixel per second. Essentially, more signal can be gathered with a higher quantum efficiency, larger pixel size, longer exposure time, and bigger pixel well depth. On the other hand, the lower the readout and dark current noise, the smaller the detection thresh- old will be. Finally, the dynamic range within a single expo- sure will depend on the pixel full well capacity and detec- tion threshold. In theory, an ideal detector would be able to count the exact position and timing of every sin- gle photon to offer perfect image qual- ity. In practice, one must balance the sometimes conflict- ing parameters of the different sensor types: • Quantum efficiency: This parameter suffers no tradeoffs. The higher it is over the larger spectral range, the better. • Pixel size and full well capacity: A smaller pixel size results in a higher spatial resolution at the expense of sensitivity and full well capacity. • Exposure time: Both the exposure time and frame rate are linked to the readout noise and dark cur- rent. A higher frame rate is desirable for a high tem- poral resolution. However, with a high frame rate readout, noise can increase. Similarly, if temporal Electronic Military & Defense Annual Resource, 4th Edition 38 CCD EMCCD CMOS SWIR Quantum Efficiency 50 to 90% 50 to 90% 50 to 70% 70 to 90% Spectral Range 350 to 1000 nm 180 to 1000 nm 400 to 1000 nm 400 to 1700 nm Pixel size 4.54 µm 8-10 µm 5.5 µm 15 to 30 µm Resolution Large, up to 9 MP Small, <1 MP Large, up to 4 MP Very small, <0.32 MP Exposure time Very long, 1 ms to hours Long, 1 ms to min Short, 1 ms to sec Very short, 500 ns to sec Frame rate 6 Hz 50 Hz 50 Hz 346 Hz Readout noise Low, 3 to 7 e - Very low, <1 e - Low, 7 e - Medium, 50 to 150 e - Dark current Very low, 10 -6 e - /p/s Low, 1 e - /p/s Medium, 9 e - /p/s High, 30,000 e - /p/s Pixel well depth Small 12,000e - Medium 30,000e - Small 12,000e - Large 170,000e - Table 1: The typical characteristics of various sensor types