Shutter-less infrared cameras based on microbolometer focal plane arrays (FPAs) are the most widely used cameras in thermography, in particular in the fields of handheld devices and small distributed sensors. For acceptable measurement uncertainty values the disturbing influences of changing thermal ambient conditions have to be treated corresponding to temperature measurements of the thermal conditions inside the camera. We propose a compensation approach based on calibration measurements where changing external conditions are simulated and all correction parameters are determined. This allows to process the raw infrared data and to consider all disturbing influences. The effects on the pixel responsivity and offset voltage are considered separately. The responsivity correction requires two different, alternating radiation sources. This paper presents the details of the compensation procedure and discusses relevant aspects to gain low temperature measurement uncertainty.
This paper concerns with the problem of disturbing radiation derived from the interior of radiometric
microbolometer-based infrared cameras. The amount of internal radiation depends particularly on the ambient
temperature. Variation of ambient temperature leads to a change of the temperature distribution inside the camera. The
approach proposed here is determining the disturbing radiation without using a shutter by measuring the internal thermal
state with several temperature probes and deducing the disturbing radiation flux. Because of this discrete temperature
measurement it is not possible to determine the present thermal state of the camera interior as precise as performing a
shutter process. Therefore, the position of the temperature measurement is crucial for the significance of the relation
between measured temperature and disturbing radiation flux. Furthermore, the transient thermal behavior during a
cooling or heating period of the camera enclosure is a non-ergodic process . Two approaches facing these problems
The first approach is based on the usage of more than one temperature probe at different positions inside the camera.
Each position of temperature measurement has its own characteristic of heat conductance and convection parameters.
Therefore, the low-pass behavior and the corresponding response time of the measured temperature in relation to the
ambient temperature differ. Developing a thermal model using different probes with a higher significance of the transient
thermal trend reduces the calculation uncertainty.
A second approach is to separate the transient and the steady-state behavior of the calculation model. If the camera is
able to follow a slow change of ambient temperature completely, then it stays always in steady state and the process is
ergodic. Only in case of an abrupt change of ambient temperature the thermal behavior leaves the steady state and a
transient correction factor is necessary. This factor has to take the history of the measured temperature into account.
This work investigates the ability to integrate conductive polymer compounds as self-supporting sensitive layers in
microbolometers. The polymer matrix is a photoresist that can be structured by UV-lithography and hardened to a highly
cross-linked phenolic resin by thermal curing. The electrically conductive filler material is tellurium being synthesized as
(nano)rods with an average diameter of 250 nm and an average length of 5 μm. To fabricate microbolometers pixel
elements an appropriate technology was developed with the motivation to use cost-efficient polymeric materials and
processing steps. It includes a sacrificial layer technique and a dielectrophoretic alignment procedure of the tellurium
nanorods. The resulting electrical conducting network in the polymer matrix has a temperature coefficient of resistance
(TCR) of -1.4 % / K that yield the bolometric effect. The TCR-value and the resistance are determined by the intrinsic
properties of the tellurium nanorods and the characteristics of the hopping conduction occurring between neighboring
tellurium nanorods. The electrical properties can be tailored by the alignment procedure to some extent. However, there
is an interrelation between a high TCR and a high resistance.
Until now Microbolometer cameras have been operated only in the long-wave infrared range (LWIR). Since microbolometers are now available with broadband windows and acceptable absorption in the mid-wave infrared range (MWIR), they are becoming more and more interesting for the MWIR range. Primarily for industrial applications, this wavelength range offers many advantages, e.g., for the measuring of glass temperatures or for supervision of furnace rooms. To achieve a sufficiently high measuring accuracy, such crucial MWIR peculiarities like carbon dioxide absorption lines and water-vapor absorption must be known. Such problems can be avoided by usage of narrowband filters. Usually, they have to be adjusted to the particular measurement task. The newly developed camera system is based on a 320 x 240 pixels LWIR microbolometer camera system. The optical channel had to be adapted to the microbolometer. In addition, special correction and calibrating procedures were implemented for the MWIR.
The camera system is suitable for stationary use in harsh industrial environments. The robust housing may be completed by integrating water-cooling and air purge for the lens system. The camera is equipped with two trigger inputs for the synchronization with the process to be measured.
New uniform camera electronics were developed for different stationary line and 2D infrared cameras for non-contact temperature measurement. The 16bit analog/digital converter used enables a maximum pixel rate of 10 MHz. The electronics are based on a System Of Programmable Chip (SOPC) solution using a PLD with an embedded processor. The ability to reprogram the PLD allows an inexpensive adaptation to different sensor types and various industrial applications. The embedded processor executes the signal processing, including the necessary signal corrections. In addition, the embedded processor controls and monitors the camera head, monitors the operation of the chopper/shutter motor and internal temperature sensors, and can be used to control a number of functions such as triggering or frame rate. The PC communicates with the micro-controller via an asynchronous interface. The other essential components of the digital signal-processing unit include a serializer, a flash memory and an SRAM. The new 16bit camera electronics have been incorporated into the following 2D infrared cameras PYROVIEW 256 with a pyroelectric array (256 x 128 pixel)
PYROVIEW 320 with a microbolometer array (320 x 240 pixel)
The paper will describe technical properties and typical applications in industrial applications for both devices as well as the advantages and disadvantages of pyroelectric vs. microbolometer arrays. In addition, since the newly developed 16bit camera electronics also provide the basis for faster line cameras (PYROLINE with pyroelectric arrays of 128 x 1 and 256 x 1 and an InGaAs line array with 256 x 1), selected applications of these cameras will also be discussed.
The PYROLINE/ MikroLine cameras provide continuous, non-contact measurement of linear temperature distributions. Operation in conjunction with the IR_LINE software provides data recording, real-time graphical analysis, process integration and camera-control capabilities. One system is based on pyroelectric line sensors with either 128 or 256 elements, operating at frame rates of 128 and 544 Hz respectively. Temperatures between 0 and 1300DGRC are measurable in four distinct spectral ranges; 8-14micrometers for low temperatures, 3-5micrometers for medium temperatures, 4.8-5.2micrometers for glass-temperature applications and 1.4-1.8micrometers for high temperatures. A newly developed IR-line camera (HRP 250) based upon a thermoelectrically cooled, 160-element, PbSe detector array operating in the 3 - 5 micrometers spectral range permits the thermal gradients of fast moving targets to be measured in the range 50 - 180 degree(s)C at a maximum frequency of 18kHz. This special system was used to measure temperature distributions on rotating tires at velocities of more than 300 km/h (190 mph). A modified version of this device was used for real-time measurement of disk-brake rotors under load. Another line camera consisting a 256 element InGaAs array was developed for the spectral range of 1.4 - 1.8 micrometers to detect impurities of polypropylene and polyethylene in raw cotton at frequencies of 2.5 - 5 kHz.
Pyroelectric infrared detectors have among other things the known advantages of being able to be used at room temperature and of being provided with a sufficient signal- to-noise ratio for a number of applications, a spectral responsivity of a relative homogeneity within the infrared range, and properties of a very long-term stability. For extending their range of application both extensive and intensive efforts were made, during the last few years, to considerably improve responsivity, noise equivalent power (NEP) and modulation transfer function of linear arrays. The paper describes the structure and the main properties of newly developed line sensors based on lithium tantalate and containing 128 sensitive elements (size of element: 90 X 100 micrometers <SUP>2</SUP>, pitch 100 micrometers ). It is shown that the thickness of the responsive elements has a strong influence on responsivity. Special techniques for structuring the pyroelectric chip and using a low-noise CMOS read-out circuit made it possible to achieve NEP values below 0.5 nW at a chopper frequency of 40 Hz. Illustrated by the examples of a line scanner and a 2D camera, the paper proves that noise equivalent temperature difference values around 0.1 K are possible.
Choppers are inevitable parts of thermal imager or radiometer systems based upon uncooled pyroelectric focal plane arrays, since pyroelectric detectors require the incident radiation to be modulated. In comparison with other chopper techniques such as oscillator or shutter choppers rotating circular disks are considered superior in both mechanical simplicity and electronic control. However, the use of rotating disks introduces two non-ideal circumstances affecting the system's operation quality. Two-dimensional arrays require a bended chopper edge to approximate an ideal straight edge traveling over the array. Since sampling is performed line- sequentially, a local shift in time occurs between the sampling instant and the change of the optical chopper phase within each line. Furthermore, the chopper can only be placed in front of the entrance window of a detector array and, therefore, modulates the incident radiation in the blurred region of the optical path. This flattens the ideal rectangular modulation shape one would obtain if the chopper were able to run in the focal plane. Both effects attenuate the sensor output signal. The article presents a model for designing optimum chopper disks for 2- D arrays in order to achieve minimal possible signal attenuation. The optimization procedure incorporates all interdependencies between chopper layout, array size, chopper position, and readout velocity. The model also considers the unavoidable distance of the chopper plane from the focal plane and provides a chopper performance prediction method where the varying signal attenuation is discussed as contribution to the array's nonuniformity. Finally, a sample design is given, and measurements are presented.
Uncooled pyroelectric arrays can be advantageously used for contactless measurements of one- and two-dimensional temperature fields. Satisfying values of noise equivalent power NEP, modulation transfer function MTF and long term stability of responsivity, respectively, are necessary for these applications. Linear pyroelectric arrays developed for those purposes are described. The pyroelectric chip based on lithium tantalate contains 128 sensitive elements (element size 90 X 100 micrometers <SUP>2</SUP> with 100 micrometers pitch). A CMOS read-out circuit (low noise preamplifiers, S&H-stages, analog switching structures with digital components, output-amplifier) is specially designed. Pyroelectric chip, read-out circuit, and a PTAT temperature sensor chip, respectively, are mounted in a metal hermetic package with 8...12 micrometers germanium window. Measured NEP values reach 5 nW at a chopping frequency of 128 Hz. The modulation transfer function MTF (128 Hz, 3 lp/mm) measured is typically 60%. Devices for the measurement of temperature distributions based on linear arrays described contain the uncooled array, infrared optics, chopper, control electronics, analog to digital converter, and a comfortable digital processing unit for multi point pattern correction, accumulation, digital filtering and so on. The measuring range of such PYROLINE systems reaches from 0...80 degree(s)C, 50...300 degree(s)C, 200...700 degree(s)C (1500 degree(s)C), respectively.
Starting from a characterization of the used pyroelectric materials LiNbO<SUB>3</SUB> and LiTaO<SUB>3</SUB> the paper describes the hybrid arrangement and essential properties of realized single- element detectors and arrays. It shows that the design and the production technique of pyroelectric chips have a strong influence on the thermal and spatial resolution of the detectors. This technique includes both the thinning process and reticulation of the chips, for which ion beam milling as a universal method was optimized and used. Single-element detectors with extremely thin, self-supporting LiTaO<SUB>3</SUB> chips (dp < 2 micrometers ) were produced. With a responsive element of 2 X 2 mm<SUP>2</SUP> in area, they have a specific detectivity D<SUP>*</SUP> (500 K; 10 Hz; 25 degree(s)C) > 1 X 10<SUP>9</SUP> cmHz<SUP>1/2</SUP>W<SUP>-1</SUP>. Linear arrays with 128 responsive elements of 90 X 100 micrometers <SUP>2</SUP> element size, 100 micrometers pitch, integrated readout circuit, and coated germanium window have a noise equivalent power (NEP) (500 K; 40 Hz; 25 degree(s)C) of 4 nW. The modulation transfer function MTF<SUB>S</SUB> (40 Hz; 31 p/mm; 25 degree(s)C) is 0.15 for pyroelectric chips without isolating grooves and was increased up to 0.45 by means of ion- beam milling of 10 micrometers wide isolating grooves between the responsive elements. First results of two-dimensional arrays with 128 X 128 elements, of 50 micrometers pitch and integrated CCD-readout circuit are presented.
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 (LiNbO<sub>3</sub>) 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.