A powerful concept for the realization of large, high performance mosaic focal planes has been developed at New England Research Center. The concept is based on the use of ion-milling technology for batch production of fully delineated, infinitely buttible mosaic arrays. The mechanics of this approach, its advantages, and a demonstration of ion-milling technology are the main subjects of this paper. In addition, a hybrid-on-hybrid (HOH) con-cept will be introduced and illustrated, whereby full advantage of independent fabrication, preselection, and optimization of sensor and electronics modules are used to achieve common module mosaic standardization.
A new blooming suppression system for infrared hybrid focal plane arrays is presented and discussed. This system, using a hybrid technique, is based on a charge coupled device (CCD) on a p-silicon substrate. The infrared detectors are p-n photodiodes, for example, HgCdTe, PbSnTe, I nSb, etc. Each detector is connected to a lateral input of an integrator and multiplexer CCD respecting the direct injection technique. A small area input CCD stage has been designed which allows, assuming adequate bias conditions, complete separation of the storage wells where the current of the photodiodes is integrated. Consequently, there is no blooming between the storage wells during the integration time. In addition, this stage, prevents the charge transferred (from the storage well to the CCD well) exceeding the capacity of the CCD well. Assuming that these two conditions are fulfilled, and that there is adequate gate dynamic drive, blooming is avoided in the p-silicon substrate : there is no theoretical limit to this blooming protection. This antiblooming system uses standard CCD technology, two-phases, surface-and buried- channels, and poly-silicon gates. The efficiency of the antiblooming has been demonstrated by connecting infrared photodiodes on a HgCdTe and PbSnTe substrate to a 34-stage CCD The antiblooming device allows a protection of at least 40 dB over blooming ; for values greater than this, care must be taken of diaphoty.
A process for directly integrating photoconductive lead sulfide (PbS) infrared detector material with silicon MOS integrated circuits has been developed primarily for application in long (>10,000 detector elements) linear arrays for pushbroom scanning applications. The processing technology is based on the conventional PMOS and CMOS technologies with a variation in the metallization. Results and measurements on a fully integrated eight-element multiplexer are shown.
The demand for large, high purity, low defect, single crystal CdTe substrates with high IR transmission has accelerated because of their use in liquid phase epitaxy (LPE) of HgCdTe used in IR focal planes. CdTe is nearly lattice matched to Hgl-xCdxTe for all compositions and is chemically similar to Hgl-xCdxTe, allowing thermodynamic equilibrium between liquid and solid to be readily established at growth temperature. HgCdTe and CdTe also have a similar thermal expansion coefficients, and CdTe is optically transparent to IR radiation allowing the fabrication of backside-illuminated devices.
The discovery of a passivant for HgCdTe which might be suitable for the production of hardened mosaic focal plane arrays is a topic of general interest. The native oxide passivation of HgCdTe has been found to be susceptible to radiation effects. The purpose of this paper is to describe a new passivant for HgCdTe, one containing neither native nor non-native oxides. The performance of MWIR and LWIR photodiodes so passivated will be presented. In addition, ion-milled diode data will be presented and shown to be comparable to that of unmilled diodes.
We have developed a technique, using sensor feedback measurements, to adaptively control a tunable optical filter to reject laser energy being transmitted to a focal plane array (FPA). Laser energy at the focal plane can mask a target or spoof a track processor. The technique is based on processing pixel counts (densities of detectors exceeding detection threshold) generated by laser energy at the focal plane. An algorithm, using the pixel count, controls the spectral wavelength and bandwidth of a dual tunable Fabry-Perot (DTFP) optical filter to reduce this undesired energy. This paper describes the algorithm used to control the DTFP and the model and methodology used to verify the DTFP control algorithm performance. Results show the feasibility of using an adaptive spectral selection technique for laser rejection. The importance of control loop sensitivity and out-of-band leakage to achieve the best sensor performance was clearly demonstrated.
When the solid state image devices were introduced into the optical industry, they revolutionized the techniques for image detection, but not without the user tolerating its nonuniformity. This nonuniformity anomaly is usually ignored in many optical imaging applications; accordingly, its existence and effects are known only by a few among the device implementors. In some applications, this anomaly may seriously impair a system design; consequently, the unwary should be informed of its existence. The nonuniformity can be induced electrically and/or optically into a video signal's transmission path, and in turn it produces an arbitrary gray scale on an image space, derived from a uniformly exposed target. This paper defines the nonuniformity in relation to the solid state imaging devices, demonstrates the need to compensate it in low-contrast image application, and discusses an inexpensive network to perform the compensation and the results of its implementation.
Image reading techniques for high speed, high resolution document scanners were studied. The study covered high-speed line scanning using charge coupled device (CCD) image sensors, high intensity illumination system, improvement of modulation transfer function (MTF) of scanning optical system, and reliable document handling mechanism. This paper also discusses a document scanner made to demonstrate the study results. The scanner read documents at 1 second per document and 11.8 picture elements per millimeter (pels/mm) of resolution, and had more than 0.5 of the MTF. A newly developed document feed mechanism reduced the feed error rate to less than 0.05% and assured a document throughput of 60 pages/min.
This paper describes methods used to simulate the performance of Z-package infrared focal plane arrays. Signal generation and transfer, noise generation, and bandpass filtering are discussed for nonspecific (generic) Z-packaged focal plane arrays.
A mathematical model for the two-pole and four-pole analog focal plane filters is formulated which gives the filter pole locations in terms of the circuit parameters. The frequency response and impulse response of the filters are computed, and the impulse response is used to calculate the filter output for targets of various dwell times, assuming a Gaussian model for the optical point spread function. The effects of varying the individual component values in the four-pole filter upon pole locations are also considered.
A number of applications require the precise tracking or position estimation of an object that is unresolved in the system optics. This paper evaluates the performance of several interpolation algorithms designed to make these estimates to subpixel accuracy. The tracking sensor examined was a scanning linear array of infrared detectors which were assumed to be background limited. The optics blur spot was assumed gaussian. The relative change in performance with changes in the array configuration was investigated. The detector size and physical spacing were varied parametrically, but with realistic fabrication constraints, to obtain the optimum configuration. The sources of error considered to affect the performance were the systematic algorithm bias, the random noise, and the post-calibration residual detector responsivity nonuniform-ities. The systematic algorithm error or bias was calculated and its rms value over an array pitch was used as one measure of estimation error. The random noise in the signal was propagated by variance analysis into another source of estimation error. Finally, the residual nonuniformities in the detector responsivities after calibration were propagated by variance analysis into an rms spread in the algorithm error, and provide a third source of error. The interpolation algorithms investigated were the odd N point centroids (N = 3, 5, 7, 9) and the three and five-point quadratic curve fits. A simple coarse search routine of peak signal detection was assumed to determine the origin. The optimum performance of a two-row staggered linear array with square detectors and realistic fabrication constraints (P ≥ L/2) occurs for detector lengths (L) slightly less than 1/3 the blur spot size, as defined by 2.44 λ/D where A is the spectral wavelength and D is the optics aperture. The 3 point centroid performs best as a function of signal-to-noise (SNR), but requires a systematic correction of the algorithm bias. The higher N-point centroids quickly degraded in SNR performance but had negligible algorithm error. The quadratic curve fits were worse in SNR performance than the three-point centroid (except for larger, nonoptimized detector sizes) and still required a correction algorithm. The analysis was confirmed with Monte Carlo simulations. An experimental infrared tracking focal plane was purchased and used in a tracker simulation. The tracker error data, taken as a function of SNR confirmed closely the analysis for all signal-to-noise ratios above four. The track error follows the predicted SNR-1 behavior until limited at high SNR to a constant that corresponds to the algorithm bias error in the absence of correction or to the nonuniformity error if a correction routine is included. With the three point algorithms, an experimental accuracy to smaller than 1/100th a detector (<1/250th a blur spot) was obtained at high signal-to-noise ratios.
Cellular logic has been available to accomplish image processing since the early 1960s. With the rapid decline in the cost of digital circuitry, it is now practical to develop advanced systems employing cellular logic and other types of parallelism that are well suited to image and sensor array analysis. Three recent systems,CLIP4, DAP, angi MPP show use of these principles and can attain, respectively, rates of 109, 1010, and 1011 pixel operations per second. This paper describes: 1) the nature of cellular logic, 2) the relationship of cellular computers to the diverse digital devices available today, 3) the concerns in applying three-dimensional cellular logic to track detection and, 4) track detection experiments.
Staring mosaics and line-array scanners are compared in their clutter suppression characteristics, as measured by signal-to-clutter ratio. Established sensor designs for detecting a moving point source target were selected for the comparison. Factors for target speed and crossing geometry are included in the analysis, but temporally varying background is neglected. Results are given in terms of the line-of-sight drift rate for the mosaic which would yield the same signal-to-clutter ratio as the scanner.