Optical microscanning is a popular method in infrared imaging, providing a relatively cost-efficient means to increase the spatial resolution of the camera system. Here, we discuss the impact of microscan on parameters relevant for thermography applications. Other than imaging applications, thermography is extremely sensitive to changes of the absolute irradiation power caused by the additional microscan optics. Additional reflectance and stray radiation must be avoided or corrected for. Also, current and future developments in detector technology, such as reduced pixel pitch, will pose new challenges to the feasibility of microscan. As a practical example, we will present a microscan implementation adapted to the specific needs of thermography applications with infrared cameras based on a cooled detector. A fast rotating filter wheel with precisely adjusted deflection windows is used to produce the desired image shift. This allows us to utilize microscan while retaining the high imaging speeds usually required in applications employing cooled infrared detectors. Often, calibrated thermographic cameras are used in applications necessitating a wide range of calibrated temperature measuring ranges reaching from -40 °C to <2000 °C. Optimizing the design of the microscan device for compactness opens the possibility to combine the feature with additional optical filters, allowing wide temperature measurement ranges as well as imaging within different spectral windows.
System integrators are confronted with the challenge to implement various integrated detector cooler assemblies into their IR cameras. This paper will provide solutions for adjustable electrical and digital interfaces. The presented system design supports the control and frame processing of small detectors, e.g. with 320x256 pixels, as well as high-end detectors with a resolution up to 1,920x1,536. A 10 GigE camera interface to the PC provides a bandwidth of 10 GBit/s. It offers downwards compatibility to a 1GigE Interface without changing any hardware.
The usage of cooled IR Focal Plane Array detectors in thermographic or radiometric thermal imaging cameras, respectively, leads to special demands on these detectors, which are discussed in this paper. For a radiometric calibration of wide temperature measuring ranges from -40 up to 2,000 °C, a linear and time-stable response of the photodiode array has to be ensured for low as well as high radiation intensities. The maximum detectable photon flux is limited by the allowed shift of the photodiode’s bias that should remain in the linear part of the photodiode’s I(V) curve even for the highest photocurrent. This limits the measurable highest object temperature in practice earlier than the minimum possible integration time. Higher temperature measuring ranges are realized by means of neutral or spectral filters. Defense and Security applications normally provide images at the given ambient temperature with small hot spots. The usage of radiometric thermal imagers for thermography often feature larger objects with a high temperature contrast to the background. This should not generate artifacts in the image, like pixel patterns or stripes. Further issues concern the clock regime or the sub-frame capabilities of the Read-Out-Circuit and the frame rate dependency of the signal. We will briefly describe the demands on the lens design for thermal imaging cameras when using cooled IR Focal Plane Array detectors with large apertures.
Responsive pyroelectric linear arrays are described. After a short representation of the principal detector function, the pyroelectric materials L-alanine doped triglycine sulfate (DTGS:L-A) and lithium niobate (LiNbO3) are characterized, and the system parts pyroelectric chip, CCD-multiplexer, and hybrid arrangement are described in detail. Finally, the measured properties responsivity, noise equivalent power, and modulation transfer function are summarized.