Harry Schlemmer

ATR technique for UV/VIS analytical measurements

SummaryAn ATR technique is presented for UV/VIS analytical measurements featuring a solid glass ATR probe which is connected to a fast diode array spectrometer via optical fibers. It is demonstrated that samples with very high absorption, which are of some practical interest in process and quality control in chemistry and pharmacy, can be directly investigated with an overall reproducibility of 0.1% in concentration.

The infrared-based early warning system for bird strike prevention at Frankfurt airport

Flocks of migratory birds are very often using geographic structures like rivers, valleys or coast lines for orientation. Wherever the preferred migration routes are crossing the approach corridor of an airport there is an increased risk of bird strike. Flocks of birds crossing the runway corridor of the new runway Northwest of the Frankfurt airport are kept under surveillance now with in total three watch towers located at the river Main which in this case is the preferred used line of orientation. Each of the watch towers carries an early warning system which consists of two pairs of stereoscopic thermal imaging cameras sensitive in the mid wavelength infrared range (3 - 5 μm). A stereoscopic pair measures the swarm size, direction of flight and velocity in real time and with high accuracy. From these results an early warning is derived under all relevant weather conditions. The fixed focus thermal imaging cameras are thermally compensated and designed for ultra low image distortion. Each stereoscopic pair is aligned in the sub-pixel range and is controlled by a reference beam to ensure that the alignment is preserved under all environmental conditions and over a very long time. The technical concept is discussed and the design of the realized warning system at the Frankfurt airport is presented.

Characterization of SWIR cameras by MRC measurements

Cameras for the SWIR wavelength range are becoming more and more important because of the better observation range for day-light operation under adverse weather conditions (haze, fog, rain). In order to choose the best suitable SWIR camera or to qualify a camera for a given application, characterization of the camera by means of the Minimum Resolvable Contrast MRC concept is favorable as the MRC comprises all relevant properties of the instrument. With the MRC known for a given camera device the achievable observation range can be calculated for every combination of target size, illumination level or weather conditions. MRC measurements in the SWIR wavelength band can be performed widely along the guidelines of the MRC measurements of a visual camera. Typically measurements are performed with a set of resolution targets (e.g. USAF 1951 target) manufactured with different contrast values from 50% down to less than 1%. For a given illumination level the achievable spatial resolution is then measured for each target. The resulting curve is showing the minimum contrast that is necessary to resolve the structure of a target as a function of spatial frequency. To perform MRC measurements for SWIR cameras at first the irradiation parameters have to be given in radiometric instead of photometric units which are limited in their use to the visible range. In order to do so, SWIR illumination levels for typical daylight and twilight conditions have to be defined. At second, a radiation source is necessary with appropriate emission in the SWIR range (e.g. incandescent lamp) and the irradiance has to be measured in W/m2 instead of Lux = Lumen/m2. At third, the contrast values of the targets have to be calibrated newly for the SWIR range because they typically differ from the values determined for the visual range. Measured MRC values of three cameras are compared to the specified performance data of the devices and the results of a multi-band in-house designed Vis-SWIR camera system are discussed.

Colorimetry and multispectral imaging in the shortwave infrared

The typically used shortwave infrared spectral range (SWIR) between 900 nm and 1700 nm is a spectrally broader wavelengths range than the visible range. Available SWIR cameras generate a gray level image using the intensity over the entire spectral band. However, objects can exhibit completely different spectral behavior in this range. Plants have a high reflection at the lower end of the SWIR range and liquid water has a strong absorption band around 1400 nm, for example. We propose to divide the SWIR range into an appropriate number of spectral channels to extract more details from a captured image. To extract this information the proposal follows a concept similar to color vision of the human eye. Analog to the three types of color receptors of the eye four spectral channels are defined for the SWIR. Each point of the image is attributed now by four “color values” instead of a single gray level. For a comprehensive characterization of an object, a special SWIR colorimetry is possible by selecting appropriate filters with suitable band width and spectral overlap. The spectral sensitivity, the algorithms for calculating SWIR-color values, the discrimination of SWIR-color values by Noise Equivalent Wavelength Difference (NEWD) and spectral coded false color image display is discussed and first results with an existing SWIR camera are presented.

Unified characterization of imaging sensors from visible through longwave IR

Modern reconnaissance strategies are based on gathering information from sensors that operate in several spectral bands. Besides the well-known atmospheric windows at the visible (VIS), medium-wave IR, and longwave-IR (LWIR) wavelengths, today’s detectors can also operate in the 1–1.7 m window known as shortwave IR (SWIR). SWIR cameras are especially useful in the hazy or misty atmospheric conditions typical of a maritime environment. For optimum application of SWIR cameras, as well as detectors in other bands, it would be useful to have a single, uniform method of characterizing the various sensors. One way to provide such characterization is to use minimum resolvable contrast (MRC) measurements. MRC is a measure of a system’s sensitivity and its ability to resolve data. It was pioneered by John Johnson in the late 1950s when he first described the probability of detecting an object as dependent on the object’s effective resolution.1 This intuitive idea showed that the probability of locating a target increases with the number of resolvable cycles across that target. Johnson’s analysis was initially used to assist the design of image intensifier tubes, which increase the intensity of light in optical systems where there is limited light available. Later—with the growing importance of day sight (surveillance) cameras—Johnson’s work was revived by developers who used MRC measurements to assess electrooptical systems. SWIR imaging makes use of the radiation reflected by observed objects in the same way that visible imaging does. It is therefore possible to use MRC methods to characterize SWIR imaging.2 We have employed MRC measurements to determine the ability of a camera system to resolve detail contrast in the visible Figure 1. Setup used for measuring minimum resolvable contrast (MRC).3 The targets are mounted in front of a light source with specified luminance. The outcoming light is collimated by a parabolic mirror and directed into the aperture of the camera under test. The camera is fixed on a rotatable arm to allow measurements under different angles of incidence.

Thermal imagers: from ancient analog video output to state-of-the-art video streaming

The video output of thermal imagers stayed constant over almost two decades. When the famous Common Modules were employed a thermal image at first was presented to the observer in the eye piece only. In the early 1990s TV cameras were attached and the standard output was CCIR. In the civil camera market output standards changed to digital formats a decade ago with digital video streaming being nowadays state-of-the-art. The reasons why the output technique in the thermal world stayed unchanged over such a long time are: the very conservative view of the military community, long planning and turn-around times of programs and a slower growth of pixel number of TIs in comparison to consumer cameras. With megapixel detectors the CCIR output format is not sufficient any longer. The paper discusses the state-of-the-art compression and streaming solutions for TIs.

A modular packaging approach for upgrading tanks with staring thermal imagers

The thermal imager ATTICA was designed to fit into the thermal sights of the new German Infantry tank PUMA. The flexible approach for the optical concept, using different folding mirrors allows meeting the different available space requirements for thermal sights also of other tanks like the main battle tank Leopard 2 and the infantry fighting vehicle Marder. These tanks are going to be upgraded. The flexible concepts of the thermal imager optics as well as the mechanical packing solutions for the different space volumes of the commander and gunner sights of the vehicles are discussed.

Innovative optronics for the new PUMA tank

The new PUMA tank is equipped with a fully stabilized 360° periscope. The thermal imager in the periscope is identical to the imager in the gunner sight. All optronic images of the cameras can be fed on every electronic display within the tank. The thermal imagers operate with a long wave 384x288 MCT starring focal plane array. The high quantum efficiency of MCT provides low NETD values at short integration times. The thermal imager has an image resolution of 768x576 pixels by means of a micro scanner. The MCT detector operates at high temperatures above 75K with high stability in noise and correctibility and offers high reliability (MTTF) values for the complete camera in a very compact design. The paper discusses the principle and functionality of the optronic combination of direct view optical channel, thermal imager and visible camera and discusses in detail the performances of the subcomponents with respect to demands for new tank applications.

Stationary early warning system for bird strike prevention in aviation

In case bird migration routes cross approach corridors near airports bird strike prevention with thermal imaging systems has advantages compared to others technologies i.e. RADAR systems. In our case a stereoscopic thermal imaging system sensitive in the mid wavelength range (3 - 5 μm) with high geometrical (640 × 512 pixel) and high thermal resolution (< 20 mK) measures in real time the swarm size, direction and velocity with high accuracy in order to give an early warning under all relevant weather conditions during day, night and twilight. The system is self-calibrating to keep the relative position of the paired stereoscopic thermal imagers in the sub-pixel range under all environmental conditions. The stereoscopic systems are placed in a sufficient distance to the crossing with the take-off or landing path to enable warning times of several minutes. Moreover the risk potential of the swarm is determined by taking the size of a single bird as well as the number of birds in the swarm into account. By using this information an arrival time of the swarm at the crossing point is determined and provided to the air security controllers together with the risk potential of the swarm.

Near-field observation platform

A miniaturized near-field observation platform is presented comprising a sensitive daylight camera and an uncooled micro-bolometer thermal imager each equipped with a wide angle lens. Both cameras are optimised for a range between a few meters and 200 m. The platform features a stabilised line of sight and can therefore be used also on a vehicle when it is in motion. The line of sight either can be directed manually or the platform can be used in a panoramic mode. The video output is connected to a control panel where algorithms for moving target indication or tracking can be applied in order to support the observer. The near-field platform also can be netted with the vehicle system and the signals can be utilised, e.g. to designate a new target to the main periscope or the weapon sight.