Low-altitude remote sensing (RS) using unmanned aerial vehicles (UAVs) is a powerful tool in precision agriculture (PA). In that context, thermal RS has many potential uses. The surface temperature of plants changes rapidly under stress conditions, which makes thermal RS a useful tool for real-time detection of plant stress conditions. Current applications of UAV thermal RS include monitoring plant water stress, detecting plant diseases, assessing crop yield estimation, and plant phenotyping. However, the correct use and interpretation of thermal data are based on basic knowledge of the nature of thermal radiation. Therefore, aspects that are related to calibration and ground data collection, in which the use of reference panels is highly recommended, as well as data processing, must be carefully considered. This paper aims to review the state of the art of UAV thermal RS in agriculture, outlining an overview of the latest applications and providing a future research outlook.

Applications of UAV Thermal Imagery in Precision Agriculture: State of the Art and Future Research Outlook

Recently, a lot of attention has been paid to infrared imaging using uncooled microbolometer detectors, primarily because of their low cost, and wide military and civilian applications This paper reviews the basic principles of these detectors and some of the important works done internationally in this field This paper presents some of the research work on titanium microbolometer carried out at Solid State Physics Laboratory, Delhi Defence Science Journal, 2009, 59(6), pp580-589 , DOI:http://dxdoiorg/1014429/dsj591562

http://publications.drdo.gov.in/ojs/index.php/dsj/article/download/1562/690

Uncooled Infrared Microbolometer Arrays and their Characterisation Techniques (Review Paper)

Resistive sensor arrays have been increasingly adopted in wearable electronic applications, which require low-complexity and low-energy circuits. However, current readout strategies for resistive sensor arrays require additional electrical components, such as transistors, diodes, multiplexers, op-amps, switches, current sources, and A/D converters, leading to a considerable increase in circuit complexity, power consumption, system instability, and crosstalk error. To address the problem, this paper proposes a new approach, which determines sensor resistance values by establishing and solving resistance matrix equations of sensor arrays. Unlike conventional approaches, it allows crosstalk currents in arrays to avoid additional components that are originally used for eliminating crosstalk currents and minimizing crosstalk error. Meanwhile, it takes advantage of on-chip resources of wearable platforms, thereby reducing redundant chips. It was implemented on a prototype of 10 × 10 textile resistive sensor array, which was taken in a sensing cushion for sitting pressure monitoring of chair bound people. Experimental results on this array platform showed the new approach achieved a satisfactory accuracy (0.61% ± 0.41%), as well as a low crosstalk error (2.77% ± 0.61%). The fabricated sensing cushion also exhibited a relatively low pressure measurement error (6.30% ± 0.75%). Compared with other approaches, the proposed approach demonstrated the lowest circuit complexity on a microcontroller based wearable platform, and a sufficient sensor capacity. It is ideal for a wide range of applications like wearable or implantable sensing, presenting a reference for the design of low-complexity and low-crosstalk error wearable systems based on resistive sensor arrays.

A New Approach for Readout of Resistive Sensor Arrays for Wearable Electronic Applications

Terahertz frequency (THz) radiation lies in between the microwave and infrared ranges. While it is strongly absorbed by water, it is nonionizing and has low possibility of causing tissue damage as it involves low energy photons. Recent technological progress in developing THz instrumentation, means that commercial THz systems are being produced with improving performance which are easier to operate and more reliable. THz phase imaging, an advanced imaging technology which combines the benefits of THz and commonly used phase imaging techniques, has recently received significant attention. In this paper, the current state of such imaging systems is reviewed. This review deals with both pulsed and continuous-wave (CW) imaging systems. Pulsed THz phase imaging is a coherent measurement, which includes terahertz pulsed imaging (TPI) based on femtosecond laser and holographic imaging in the time domain, both allow phase and amplitude information of the electric field to be recorded. CW THz phase imaging is mainly based on digital holography, interferometry and ptychography. These systems can obtain the complex amplitude by capturing diffraction patterns and applying numerical reconstruction techniques. Biomedical applications of such THz systems are highlighted.

Terahertz phase imaging and biomedical applications

By sharing rows and columns, the 2-D networked resistive sensor arrays simplified the interconnect complexity but suffered from the crosstalk problem among its elements. The crosstalk was mainly affected by the multiplexers' resistor, adjacent column elements, adjacent row elements, and array size. First, we designed an improved isolated drive feedback circuit (IIDFC) using one operational amplifier to reduce the crosstalk caused by the adjacent column elements in the 2-D networked resistive sensor array. Then, a mathematical equivalent resistance expression of the element being tested (EBT) for this circuit was analytically derived. Then, the IIDFC and its equivalent resistance expression of the EBT were evaluated by the simulations. The simulation results show that the IIDFC can greatly reduce the crosstalk error caused by the adjacent column elements in the 2-D networked resistive sensor array with the row-column fashion and its resistance expression of the EBT can be used as a general formula to evaluate the performance of the IIDFC.

Design and Crosstalk Error Analysis of the Circuit for the 2-D Networked Resistive Sensor Array

This paper presents the results of a novel test structure for process control monitor for uncooled IR detector technology of microbolometer arrays. The proposed test structure is based on resistive network configuration. The theoretical model for resistance of this network has been developed using ‘Compensation’ and ‘Superposition’ network theorems. The theoretical results of proposed resistive network have been verified by wired hardware testing as well as using an actual 16x16 networked bolometer array. The proposed structure uses simple two-level metal process and is easy to integrate with standard CMOS process line. The proposed structure can imitate the performance of actual fabricated version of area array closely and it uses only 32 pins instead of 512 using conventional method for a 16x16 array. Further, it has been demonstrated that the defective or faulty elements can be identified vividly using extraction matrix, whose values are quite similar(within the error of 0.1%), which verifies the algorithm in small variation case(~1% variation). For example, an element, intentionally damaged electrically, has been shown to have the difference magnitude much higher than rest of the elements(1.45 a.u. as compared to ~ 0.25 a.u. of others), confirming that it is defective. Further, for the devices having non-uniformity ≤ 10%, both the actual non-uniformity and faults are predicted well. Finally, using our analysis, we have been able to grade(pass or fail) 60 actual devices based on quantitative estimation of non-uniformity ranging from ＜ 5% to ＞ 20%. Additionally, we have been able to identify the number of bad elements ranging from 0 to ＞ 15 in above devices.

A Novel Test Structure for Process Control Monitor for Un-Cooled Bolometer Area Array Detector Technology

We present the characterization of microbolometer area arrays being developed for IR imaging applications. Here, we propose a technique that does not use readout integrated circuit for accessing detector array elements for characterization, which is useful for initial detector technology development. The technique is based on a connection scheme that uses two levels of metal lines in a row–column fashion, thereby, reducing the number of pads and therefore the fan-out complexity from N  ×  M to N  +  M for an array of N  ×  M detectors. The resulting networked structure has been analyzed and the technique has been developed for extracting back the actual signal and noise values from the measured data. We have implemented this technique on a set of 16 × 16 arrays of Ti-microbolometers developed at our laboratory. However, it may be extended to other types as well. The measurement on these arrays and extraction of various performance parameters are discussed here. The results of detailed characterization show that the average detectivity is about 1.44 × 10 8  Jones, TCR is 0.27%, thermal time constant is 9.91 ms and thermal conductivity is 0.16 μW/K in these arrays. These values are very close to the design values.

Characterization of area arrays of microbolometer-based un-cooled IR detectors without using ROIC

In a metal film infrared microbolometer, the responsivity is improved by high bias current to compensate for its low-temperature coefficient of resistance (TCR). However, what are the upper limits of this current without damaging the microbolometer element is not well understood. To study the effects of large bias current, we performed the destructive I-V measurements on an element of a 16 times 16 Ti-microbolometer array developed at our laboratory and report here the experimental observations of its electrical and physical damages. In this study, we performed the I-V measurements repeatedly on a microbolometer element and increased the final bias current in steps of 50 muA in each repetition. The effect of the heating due to I2R power dissipation has been analyzed at each step by monitoring I-V characteristics, specific detectivity and physical health. We report a significant decrease in the detectivity when bias stress is increased beyond 450 muA, which corresponds to the element temperature of 370degC. Further, we found that the resistance started decreasing, when the power dissipated and the element had increased to about 2.5 mW, resulting in a peaked I-V characteristics. This corresponds to the bias stresses more than 650 muA. Using a new I-V model, we extracted the temperature to be about 750degC at these peaks. A further increase in bias stress has resulted in the complete physical damage of the element.

C. R. Jalwania

Papers

Uncooled Infrared Microbolometer Arrays and their Characterisation Techniques (Review Paper)

A Novel Test Structure for Process Control Monitor for Un-Cooled Bolometer Area Array Detector Technology

Characterization of area arrays of microbolometer-based un-cooled IR detectors without using ROIC

Effect of Excessive Bias Heating on a Titanium Microbolometer Infrared Detector