The integration of diffractive optical elements (DOE) into an optical design opens up new possibilities for applications in sensing and illumination. If the resulting optics is used in a larger spectral range we must correct not only the chromatic error of the conventional, refractive, part of the design but also of the DOE. We present a simple but effective strategy to select substrates which allow the minimum etch depths for the DOEs. The selection depends on both the refractive index and the dispersion.
Dual color infrared imaging systems are being developed as missile warning sensors operating within the 3μm to 5μm spectral regime. To demonstrate the sensor performance of such sensors we introduce IR test projectors which provide an optical output within the required spectral band (3µm to 5µm). A bispectral objective serves as the projection optics while also forming a part of a telescope which allows visual alignment of the projection axis with high precision, e.g. by autocollimation. A compact IR source generates the IR radiation by resistive heating with heating and decay times close to 10 ms and a large dynamic range. These characteristics are exploited for the generation of intensity sequences which simulate the IR signature of an approaching missile, accomplished by a programmable control electronics driving the IR source. Results are shown which compare the required design intensity sequence with the measured projector output intensity. As an additional design feature we have also integrated an electrically tunable Fabry-Perot filter into the test projector thus making it a tunable monochromatic IR source. This allows the measurement of the spectral sensitivity of IR sensors which is of particular importance to characterize the sensor for evaluating its performance by simulation.
When the field of operation of precision strike ground/air-to-ground missiles is extended to beyond-line-of-sight missions, autonomous seekers will soon encounter serious difficulties, especially with regard to low signature targets and complex scenarios. We have investigated dual-mode sensors which are conceived to overcome these specific problems by combining an imaging sensor with a semi-active laser seeker. These sensors offer non-line-of-sight target engagement with high reliability and under operator control using a laser target designator while minimizing the active exposure time for target designation by handing over the tracking process, once the passive imaging sensor has locked onto the target. For this purpose a laboratory demonstrator has been built with a standard TV-sensor and an InGaAs 4-quadrant detector mounted on a 2-axes gimbal system. Both detectors use a common objective; the focussed radiation is divided by a spectral beam splitter. The signals of the 4-quadrant detector are digitized and subsequently processed by an FPGA. If the pre-programmed laser pulse characteristic is identified, the position information is evaluated and the gimbal system activated in order to center the laser spot. Subsequently a tracker locks onto the target signature found in the imaging sensor signal. Once lock-on is confirmed the laser can be turned off automatically. We present the results of laboratory and field tests obtained with the dual-mode demonstrator. Based on these results we plan to replace the TV-sensor by an uncooled microbolometer array in the future. The design and expected performance of such a dual-mode sensor will be discussed.
Keywords: dual-mode sensor, semi-active laser seeker, microbolometer array, target engagement
The European anti-tank missile system MILAN has found wide-spread use in numerous countries. Introduced in 1974 it has since undergone several technological upgrades. We report here on the newly developed firing post MILAN ADT ("Advanced Technology") which improves the MILAN system performance substantially while maintaining all operational features to which MILAN operators are accustomed. An even further advanced version of this firing post is now under development in the frame of a range extension of the missile system dubbed MILAN ADT/ER. Being a command-to-line-of-sight system, the new MILAN ADT firing post is equipped with a missile tracking sensor which captures the missile's signature with a wide field-of-view optics and a large CMOS detector covering both gathering and guidance phase. Using adaptive windowing and sub-sampling functions combined with differential imaging modes this sensor tracks the signatures of all MILAN missile types with optimum precision, high resistance against IRCM, and improved signal-to-noise ratio over the entire flight path. An integrated thermal imager replaces the earlier ancillary TIs, MIRA and MILIS. The TI image is displayed on an internal micromonitor and projected into the eyepiece. Optimum axis harmonization between both missile tracking and sighting channels is ensured by projection of reference marks into each optical sensor path from an integrated multispectral projector. An extended range version will also be offered which takes advantage of the missile tracking sensor's enhanced responsivity and the oustanding precision of axis alignment. An integrated color TV sensor is substituted for the bulky direct view telescope, and both TI-/TV-sensor will provide two fields-of-view on the internal micromonitor for surveillance and target identification, respectively.
A new firing post MILAN ADT ("Advanced Technology") is developed by EADS-Lenkflugkoerpersysteme GmbH with the aim to improve the performance of the MILAN weapon system substantially while maintaining all operational features to which MILAN operators are accustomed. The missile tracking sensor of MILAN ADT is now equipped with a single, wide field-of-view optics and a large CMOS detector covering both gathering and guidance phase. Using adaptive windowing and sub-sampling functions of the detector combined with differential imaging modes, all types of MILAN missile are localized with optimum precision over the entire flight path. Another novel feature is the integration of a thermal imager into the optical scheme of the MILAN ADT guidance unit. This replaces the earlier ancillary TIs MIRA and MILIS thus saving the weight of the additional housing and reducing logistic effort. The TI image is displayed on an internal micro-monitor and projected into the eyepiece of the daysight. Optimum boresight harmonization between both missile tracking and sighting channels is ensured by projection of reference marks into each optical sensor path from a common multispectral projector. MILAN ADT is compatible with all existing MILAN missile versions and with MIRA and MILIS TIs; the integrated TI is offered as an option. A planned future range increase of the MILAN weapon system will also be discussed in brief.
In many applications of camera systems such as for surveillance a combination of a large field-of-view (FOV) with a high spatial resolution is desirable. Classical focal plane array (FPA) cameras can only fulfill this requirements with a very large number of pixels. We present a novel method to enlarge the FOV of a camera equipped with an FPA keeping the spatial resolution of the system untouched. Four images taken at different directions of the line of sight (LOS) are stitched together to a large image resulting in a significantly enlarged FOV in both directions. For this purpose two continuously rotating scan elements are placed in front of the optics which move the LOS with a highly non-uniform scan velocity. Image exposure is triggered when the scan velocity is almost zero. The influence of the residual LOS-movement to the image blur is controllable by various parameters of the system. We present the results of the simulation for an enlarged FOV with 584 x 488 pixels obtained with a scanned infrared FPA of 384 x 288 pixels working in the 3-5μm regime. The resulting system modulation transfer function (MTF) is calculated and the correlation of system performance parameters with the design parameters of the scanner is discussed.
A new method for obtaining the geometric resolution of infrared cameras with focal plane array (FPA) detectors is
introduced. Selected values of the modulation transfer function (MTF) are measured in real time on a frame-to-frame
basis. For this purpose a test bar pattern is imaged onto the FPA plane so that the bar image has a spatial frequency
which is detuned with respect to the FPA Nyquist frequency. This generates a beat signal whose amplitude corresponds
to the contrast of the bar image to be measured, without the sampling MTF. If the test bar pattern covers a range of at
least half a beat period the modulation amplitude can be determined independent of the actual phase position. This
method can be adapted to real-time MTF measurements. Examples are presented for shaker excitations of an IR camera
on a platform with means for active and passive stabilization. Sweeping the frequency in sinusoidal excitation mode
exhibits resonance structures deteriorating the camera resolution thus giving valuable information for optimizing the
design of camera and suspension system. Residual stabilization errors are measured under random vibration excitations
for each individual frame, their distribution being conveniantly visualized by bar histograms.