Modulation transfer functions (MTFs) describe how a sensor system transfers spatial frequencies of a scene through an imaging system. For infrared systems, lab measurements are performed in a laboratory setting with a collimated source and a tilted edge target. This method is the standard way to measure a sensor’s performance metric. When these sensors are used for practical applications in the field, factors such as focus, atmospheric turbulence, and path radiance limit the performance of the system. These environmentally induced blurs need to be considered when designing sensor systems to ensure the required performance is met. The effects of these factors on the sensor’s performance can be quantified by measuring an MTF while in the field. By matching laboratory and static field MTFs, the effects of other blurs can be isolated, such as platform dynamics, vibration, and atmospheric turbulence, which will affect the performance of the system. To obtain a field MTF that matches one measured in the laboratory, the variable field conditions need to be well controlled. The effects of MTF target nonuniformity, tilt angle, illumination spectra, integration time, dynamic range, and number of pixels on target were explored as possible environmental factors affecting the quality of field MTF measurements.
KEYWORDS: Modulation transfer functions, Sensors, Light sources and illumination, Long wavelength infrared, Sunlight, Sun, Short wave infrared radiation, Collimators
Modulation Transfer Functions (MTFs) describe how a sensor system transfers spatial frequencies of a scene through an imaging system. For Infrared systems, lab measurements are performed in a laboratory setting with a collimated source and a tilted edge target. This method is the standard way to measure a sensor’s performance metric. When these sensors are used for practical applications in the field, factors such as focus, atmospheric turbulence, and path radiance limit the performance of the system. These environmentally induced blurs need to be considered when designing sensor systems to ensure the required performance is met. The effects of these factors on the sensor’s performance can be quantified by measuring an MTF while in the field. By matching laboratory and static field MTFs, the effects of other blurs can be isolated, such as platform dynamics, vibration, and atmospheric turbulence, which will affect the performance of the system. To obtain a field MTF that matches one measured in the laboratory, the variable field conditions need to be well controlled. The effects of MTF target nonuniformity, tilt angle, illumination spectra, integration time, dynamic range, and number of pixels on target were explored as possible environmental factors affecting the quality of field MTF measurements.
KEYWORDS: Black bodies, Sensors, Calibration, Radiometry, Infrared radiation, Target detection, Nonuniformity corrections, Infrared sensors, Infrared search and track, Signal to noise ratio
The intensity of objects in the infrared is an important quantity for a number of applications. Intensity in watts per steradian is the parameter that is used to describe either small targets or targets that are far away. Intensity is used because these cases are usually presented to a detection sensor where the object is smaller than the sensor detector angular subtense, a situation known as an “unresolved target.” In the military, unresolved targets can be rocket-propelled grenades, man-portable air defense threats, enemy aircraft at long range, or even ground vehicles that are being engaged by ground-to-ground or air-to-ground missiles. Typical “resolved target” metrics such as root-sum-squared differential temperature do not work well for unresolved targets. In addition, a given target intensity coupled with range, atmospheric transmission, and sensor noise equivalent irradiance can provide a quick signal-to-noise estimate of a particular sensor against a particular target. Target intensity can even be a measure of how visible ones platform is to other sensor and can be used to reduce platform signatures. Measurement of intensity is always a difficult procedure, where there is typically a sensor that does not encompass all aspects of measurement parameters. For example, there are very few radiometers that include high-resolution spatial measurements with high-resolution spectral measurements with high-resolution temporal measurements, not to mention polarization. For the few systems that exist that can provide a simultaneous measurement with most of these parameters, the cost is prohibitive. Usually, a spectral radiometer will provide high spectral resolution with no spatial information and a slow temporal rate. These measurements are common. In the case we describe here, the intensity measurement is taken broadband in the midwave or longwave infrared regions with good spatial resolution. This measurement provides a band integrated intensity measurement. We describe an approach for sensor calibration and object intensity measurement that can be used for broadband sensors applications.
An attractive approach to realizing a real-time imaging polarimeter is to integrate an array of polarization- sensitive filers directly onto the focal plane array. This has the advantage of allowing all of the requisite polarization data to be acquired within each image frame. In this paper we discuss the design, fabrication, and performance of a diffractive optical element (DOE) that fulfills this requirement. The DOE consists of an array of broadband form birefringent quarter-wave plates and wire grid polarizers which are designed to allow the measurement of all four Stokes vector components for each image pixel.
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