Part one of this article discusses cooled detector technology, including thermal control technology and sensor performance. The second part of the feature will cover uncooled detector technology, and detail the engineering tradeoffs between the two.
By: Christopher J. Alicandro, FLIR Systems Inc.
Focal plane arrays
Thermal imaging systems have become mainstream instruments in a variety of industrial, scientific, process control, and military environments. Since development began in the late 1960's, thermal imaging systems have evolved into very portable, easy to use, and reasonably priced instruments. Although there are many instruments available utilizing a variety of technologies, currently the two main categories are cooled (quantum effect detector based) and uncooled (thermal detector based) infrared (IR) systems (see Figure 1).
Both types of IR systems have their own unique advantages, applications, and performance attributes. With the recent improvements in the uncooled microbolometer systems, many people wonder: uncooled or cooled IR systems, which system is right for me? There are many technical similarities and differences between the two approaches that can affect how the system performs in a specific application. Engineers need to understand these strengths and weaknesses to make the best decision for each case (see Figure 2).
Infrared sensors became popular in commercially available imaging systems in the 1960s. The original systems were extremely large and heavy, precluding portability. Products also suffered from high cost and mediocre performance. As IR technology was new and unique, however, its popularity and application diversity grew quickly.
The majority of infrared systems in the field are based on cooled technology. Such systems incorporate IR sensors with required operating temperatures far below room temperature. There is a variety of cooling methods available, but each affects the performance of the imager, as well as overall portability.
Cooled imagers incorporate quantum effect, or photon, detectors. Such detectors respond to incident radiation on an electronic levelin effect, these types of IR detectors count the quantity of IR energy, or the photons of IR energy, at any given time.
The photons of IR energy impinge upon the infrared detector, changing the electrical capabilities of the material. The majority of quantum effect detectors are photovoltaic in nature, which means that when the photons strike the sensor, a specific voltage is generated. The voltage generated is proportional to the energy of the target and therefore can be easily calibrated for imagery and measurement.
There are many technical advantages to quantum effect detectors, including spatial resolution, high thermal sensitivity, and speed. One of the inherent requirements of quantum detectors is that the sensor must be cooled for operation, typically to between 50 K and 200 K. The reason cooling is necessary is that the sensors either will not operate at room temperature or because the performance is greatly improved at lower temperatures.
The original systems incorporated miniature vacuum bottles, called dewars, which were filled with liquid nitrogen. Obviously, this made the original systems large and bulky, limiting portability. Adding to that the required logistics of procuring and storing liquid nitrogen, it was clear that a better solution was needed.
Gaseous cooling, in which a cryostat bled pressurized gas onto the detector, was also a viable method, but involved its own problems. Unless the cryogenic gas was greater than 99.99% pure, contamination could clog the miniature cryostat; typically, industrial gasses rarely met the purity requirements. In addition, the gas required extremely high pressurization, up to 5000 psi, which made safety a concern.
Thermoelectric (TE) coolers represented a significant advance in the thermal imaging world. This type of cooling system incorporated a solid state device that, in effect, was a thermocouple with a voltage forced though it. This technology, called the Peltier effect or thermoelectric effect, eliminated the need for any cryogenic fluid. The temperatures produced did not reach those of cryogenic fluids, however, so the performance of the systems were limited. In addition, TE coolers consumed large amounts of power.
In the late 1980's, a mechanical devicethe Stirling coolerbecame the defacto temperature control means for IR systems. Based on the Stirling cycle, the 180-year-old method involved the compression and expansion of a cooling gas, typically helium. These miniature refrigeration units provided the high performance associated with liquid nitrogen, while offering minimal power consumption (see Figure 3).
The advent of Stirling coolers allowed thermal imagers based on cooled focal plane arrays to be reduced to hand held proportions. In addition to reduced size and weight, the new cooled systems offered dramatically lower power consumption.
Focal plane arrays
The original quantum detectors were typically single element mercury cadmium telluride (HgCdTe) or indium antimonide (InSb.) The HgCdTe detectors could be optimized for either shortwave (3-5 µm) or longwave (8-12 µm) IR operation. Both types of sensors provided adequate thermal sensitivity. Because they were scanned with mechanical devices, however, the systems faced limits in sensitivity or scan speed.
The most technical innovation to the IR world was the introduction of systems incorporating two-dimensional focal play arrays. Such designs eliminated the need for mechanical scanning, dramatically reducing the size and weight of the systems while improving the spatial and thermal performance.
When two-dimensional Schottky barrier focal plane arrays were introduced, the detector of choice was platinum silicide (PtSi). Platinum silicide proved to be a very stable material, lending itself to repeatable temperature calibration. In addition, PtSi was highly manufacturable, which led to the lowering of its cost.
In general, the quantum efficiency, or the thermal efficiency, of PtSi was fairapproximately 22%and provided excellent sensitivity for the majority of industrial and scientific applications. The typical spectral response is 3.4 – 5.0 µm. Cooled PtSi systems have exhibited noise equivalent temperature difference (NETD) of 70 mK (0.07° C).
A need existed for FPA systems with higher thermal sensitivity, however. Indium antimonide detectors became very popular as a result of high quantum efficiency (80-90%) which led to an NETD of approximately 30 mK. InSb is a very unstable material that is not well suited for temperature calibration, however; relative difficulty in manufacturing also led to higher costs.
Another good choice for detectors was HgCdTe, which offered broad spectral appeal and high quantum efficiency. With the advent of HdCdTe, it was now possible to have a long wave or short wave FPA system with sensitivities of 20 mK or better. Extremely low manufacturing yields made the technology cost-prohibitive, however. In addition, the instability of the HgCdTe material did not lend itself to repeatable temperature measurements.
A recent innovation in cooled detectors is the quantum well infrared photodetector (QWIP) sensor. This sensor is a layered gallium arsenide/aluminum gallium arsenide (GaAs/AlGaAs) compound that offers spectral response from 7.5 to 9 µm. Although the quantum efficiency is not as high as HgCdTe, the stability of the detector lends itself to radiometric design. In addition, QWIP has an extremely large dynamic range that allows temperature ranges from ambient to 1000° C or more.
In part II of this article, Chris Alicandro will discuss uncooled detector technology, and detail the engineering tradeoffs involved in choosing a sensor for a specific application.
This material first appeared in a technical paper presented at the Infraspection Institute´s IR Info '99, Las Vegas, NV.
1. Fundamentals of Infrared Detector Operation and Testing, John David Vincent, 1990.
2. Quantum Effect Infrared Sensing, Sensors Magazine, Patrick Finney, 1995.
3. Understanding Microbolometer Uncooled Infrared Detector Technology, IR/Info, Arthur Stout, 1997.
4. IR Imaging with Uncooled Focal Plane Arrays, Sensors Magazine, Patrick Finney, 1996.
About the author…
Chris Alicandro is new business development manager at FLIR Systems Inc., 16 Esquire Rd., North Billerica, MA 01862. Phone: 978-901-8227; fax: 978-901-8887; e-mail: email@example.com.