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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resolution is irrelevant, then long exposure times are desirable. But, in this case, the dark current will increase correspondingly. Each application has its own set of compromises. For instance, the choice of a suitable camera for hyperspectral imaging is primarily driven by the following three param- eters: • Quantum efficiency: The QE combines with the grating efficiency of the spectrometer to provide the total spectral range and efficiency of the instrument. • Pixel size: This parameter translates into spectral resolution according to the spectral dispersion. • Exposure time: The exposure time or frame rate translates into spatial resolution according to the scanning speed. Sensor Technologies CCD, EMCCD, and CMOS are all silicon-based detectors and are typically sensitive in the ultra-violet (UV), visible, and near-infrared (NIR) areas of the spectrum. Short-wave infrared (SWIR) cameras are InGaAs focal plane arrays and, as their name suggests, are sensitive in the SWIR area of the spectrum. Furthermore, each sensor technology has its own particularities (Table 1): • On a CCD, all pixels are read through one analog- to-digital (A/D) output. The readout noise increases with frame rate, but it can be reduced by binning. Sensitivity, speed, and spatial resolution are all linked and subject to compromises and tradeoffs. • On an EMCCD, readout noise can be compensated for by the electron multiplication gain before the A/D output. This makes EMCCD the most sensi- tive silicon-based device and removes the link between sensitivity, speed, and spatial resolution. Unfortunately, very few EMCCD chips are available on the market. • With a CMOS device, all pixels can be read simulta- neously, since each has its own A/D converter. This also removes the link between sensitivity, speed, and spatial resolution. However, each pixel's A/D has slightly different offset, gain, and dark current characteristics, which require a nonuniformity cor- rection to emulate the image quality of a single- output device. In addition, binning won't reduce the readout noise. Irradiance: One Normalized Value With so many parameters, it can be difficult to estimate which camera to select in order to obtain the best pos- sible performance for a given application. A few basic questions regarding the spectral range, spatial resolution, and temporal resolution required can help to narrow the choice. For example, the choice of a suitable camera for hyperspectral imaging is often driven by the QE, pixel size, and exposure time. However, it would be useful to reduce all of these parameters to a single, normalized value. Alternatives To Irradiance Noise equivalent power (NEP) is the minimum power that can be detected and measured in watts. Essentially, the lower the NEP is, the higher the sensitivity. However, it doesn't take into account the pixel surface area. The NEP is usually used for photodetectors and normalized for 1 Hz output bandwidth or 0.5 second integration time [1] . As such, it is not a convenient parameter to compare cameras that cover different ranges, have widely different pixel sizes, or have differing exposure times. Specific detectivity (D*) is the reciprocal of the NEP normalized over the square root of the photosensitive area. It is expressed in cm√Hz/W or Jones [²] , [3] . Unlike the NEP, the higher the specific detectivity is, the higher the sensitivity. However, it still doesn't reflect the wide range of possible exposure times. Noise Equivalent Irradiance (NEI) Irradiance is the power of electromagnetic radiation per unit area (radiative flux) incident on a surface. Its International System of Units (SI) measurement is watts per square meter (W/m 2 ). The NEI is the light flux density required to be equal to the noise of the camera. NEI is usu- ally expressed in W/cm 2 or in photons/(cm 2 ·s) [4] . The benefit of NEI is that it offers a single, normalized quantity representing the sensitivity that can be calculated using the specifications provided by the manufacturer. The lower the NEI is, the higher the sensitivity will be. The general noise equation is the following: Since only the readout and dark current noises are camera dependent, we can define the NEI at a given wavelength as follows: Numerical Example: For a 640 x 512 Vis-SWIR camera in high gain mode at 1550 nm, the typical values are the following: • QE is 80 percent at 1550nm • Pixel size is 15 x 15µm Techniques Electronic Military & Defense Annual Resource, 4th Edition 40 Noise total = Noise readout ² + Noise dark current ² + Noise shot ² √ NEI(Photons/cm 2 . s)= Noise readout ² + Noise dark current ² √ Pixel size x Exposure time x Quantum efficiency