Sensor Arrays & Imaging System Applications
Electronic imaging equipment based upon CCD and CMOS detector arrays require pixel normalisation, flat fielding and distortion correction to yield high quality data or images. A similar approach is taken with remote sensing spectral imagers.
Pixel Gain Normalisation of Array Detectors
Focal-plane arrays (FPA) are multi-element photosensitive detectors used in electronic imaging. Their applications range from inexpensive video and still cameras for the consumer market to advanced
Once fabricated and packaged, the sensor arrays can suffer from some degree of non-uniform gain and offset coefficients. When exposed to an equal irradiance of light, each pixel in the array does not produce an identical electrical signal. Photoresponse Nonuniformity (PRNU) is due to differences in responsivity (gain) among the pixels in the array and Fixed-Pattern-Noise (FPN) is due to variation in dark current (offset). In the presence of gain and offset variations, the device produces images that have features that do not exist in the original object but are imparted on the image by the array. In other words, a picture of a uniform field is not uniform.
Offset normalization is simply performed by providing zero irradiance on the array and setting the output signal of each pixel to zero. During gain normalization, a uniform light source produces irradiance on the array that is equal at each pixel. The gain, or responsivity, of each pixel is set so that each pixel produces an equal electrical signal. Detector linearity can be measured by irradiating the array with varying light levels and measuring the signal produced.
Uniform irradiance may be produced by placing the FPA in the plane of the exit port of a uniform light source integrating sphere. At the exit port, the array will be irradiated from all directions. If a more limited field is desired, the array can be located some distance from the exit port. This distance, and the size of the exit port must be chosen to provide the required field angle and to ensure adequate uniformity.
It is useful to know the responsivity of an array. A uniform source can provide a known amount of illumination. When the illumination level is varied and the array's response measured, the responsivity, linearity, and dynamic range can be characterised. By introducing narrowband light of various wavelengths, the spectral response can also be measured. A uniform light source system is an excellent tool to measure an array's photon transfer curve. By varying the level of input illumination, one can measure the noise in the array and determine the sources of that noise: noise "floor" under low illumination; shot noise as illuminance increases; and FPN at higher illuminance. This technique also gives the dynamic range of the array, including its associated readout electronics.
Flat Fielding of Imaging Systems
Digital cameras, remote-sensing systems, and other electronic imagers must be normalised in much the same way as bare FPAs. However, one more element is introduced that must be accounted for - the optical imaging system. Imaging systems, whether they be refractive, reflective, or both (catadioptric), suffer from irradiance that varies with field angle. The most common variations are lens vignetting and the angular non uniformity caused by the cos4 law. A procedure similar to that described for simple FPAs will correct for cosine-fourth irradiance falloff and other sources of irradiance variation in the image.
Image distortions introduced by lens vignetting and cos4 irradiance fall-off are easily corrected by viewing a uniform light source. The process of "flat-fielding" of electronic imaging devices provides in addition for pixel gain normailisation and also imparts a traceable radiance, luminance or spectral radiance calibration onto the camera.
Uniform light source spheres are used to calibrate and characterise large, multi-sensor imaging systems such as those used in earth-observing satellite instruments. Calibration of a remote sensing imaging device employs a uniform light source to determine the sensor’s spectral radiance and radiance responsivity. To do this, the field-of-view of the instrument under test is focused at infinity through the integrating sphere’s exit port which must be large enough to overfill the instrument’s field-of-view.
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