A large-area plate radiator with in situ temperature homogenization for thermographic imager calibration.

Large-area plate radiators with a high emissivity and uniform temperature distribution are needed as reference sources for calibrating infrared imagers and camera systems. However, achieving very uniform temperature distribution over a large area is technically challenging, especially at high temperatures. We present a large-area plate radiator with an improved uniformity in its surface temperature distribution for the calibration of infrared thermographic imagers. It is based on an industrial plate radiator which is temperature homogenized in situ by using the Data Reference Method (DRM) developed at the Physikalisch-Technische Bundesanstalt. The DRM takes three spatially shifted pictures of the plate radiator with a thermographic imager, and using this information, it calculates both the nonuniformity in the temperature distribution of the plate radiator and the nonuniformity in the response of the thermographic imager used for imaging the scene. The surface of the applied plate radiator is 300 mm × 300 mm and it operates in a temperature range from 30 °C to 600 °C. The surface is segmented into 9 different parts of identical size whose temperature can be individually controlled. The in situ controlled plate radiator system developed uses an infrared camera, whose detector is corrected for its response inhomogeneity using the DRM. This camera permanently monitors the plate radiator, and from these data, the temperature distribution of the plate is homogenized. Through this method, the homogeneity of the plate radiator can be considerably improved compared to the non-actively regulated mode of operation. For example, at a nominal temperature of 400 °C, without the in situ homogenization procedure, 90% of the plate area has a radiation temperature in the range from 391.7 °C to 403.6 °C. Applying the in situ homogenization procedure leads to 90% of the plate area having a radiation temperature in the range from 394.2 °C to 401.2 °C.

Nonuniformity correction of imaging systems with a spatially nonhomogeneous radiation source.

We present a novel method of nonuniformity correction of imaging systems in a wide optical spectral range by applying a radiation source with an unknown and spatially nonhomogeneous radiance or radiance temperature distribution. The benefit of this method is that it can be applied with radiation sources of arbitrary spatial radiance or radiance temperature distribution and only requires the sufficient temporal stability of this distribution during the measurement process. The method is based on the recording of several (at least three) images of a radiation source and a purposeful row- and line-shift of these sequent images in relation to the first primary image. The mathematical procedure is explained in detail. Its numerical verification with a source of a predefined nonhomogenous radiance distribution and a thermal imager of a predefined nonuniform focal plane array responsivity is presented.

Optical methods for power measurementof terahertz radiation.

Precision power measurements of terahertz (THz) radiation are required to establish metrological applications in the THz spectral range. However, traceability to the International System of Units (SI) has been missing in the THz region in the past. The Physikalisch-Technische Bundesanstalt (PTB), as the national metrology institute of Germany, determines the spectral responsivity of detectors for THz radiation by using two complementary optical methods: source- and detector-based radiometry. Both approaches have been successfully prototyped, and a pyroelectric THz detector with a well-defined aperture is used to verify the consistency of the two independent calibration methods. These primary investigations led to the design of a new measurement facility for the determination of THz radiant power and the responsivity calibration of THz detectors traceable to the SI.