Generally speaking, sensor manufacturers are driven to improve performance (i.e. offering greater sensitivity, smaller packages, more features) and lower costs. The prioritization of these sometimes conflicting demands varies with each target sensor application. For detector manufacturers, this translates into an assortment of requirements including high sensitivity, expanded range of operation, more integrated products, and low cost solutions. Final detector selection will depend on which attributes are most important for the target application.
To identify the technologies best suited for a particular application, it is helpful to begin with a general overview of the advantages and disadvantages of different types of detectors and the tradeoffs that each provide.
Some complex detector technologies — including InSb, InAs and HgCdTe — typically operate at very cold temperatures to reduce thermally generated free carriers and achieve adequate performance. These detectors normally provide higher performance with broadband wavelength coverage. Unfortunately they often require extensive cooling and a complex design. The resulting solution is bulkier and more costly than alternative technologies. These detector materials should be considered when high sensitivity is required in conjunction with broadband coverage into the far-IR and cost is not a factor.
If low cost is the highest priority, thermal detectors applying pyroelectric, bolometric or thermopile materials are a viable option. These detectors typically have simple designs, broad wavelength response, and are very cost-effective. However they have low sensitivity and slow response speed compared to photon detectors, so they should be used for applications where these attributes are not an issue.
The detector materials that remain for consideration are InGaAs photodiodes (both standard and extended) and lead salt photoconductors (PbS and PbSe). All of these detectors have reasonable cooling requirements and good response times. Standard InGaAs photodiodes have excellent sensitivity and speed and are generally low-cost as well, making them a strong option for applications that only require a spectral response up to wavelengths of 1.7 μm. Extended InGaAs photodiodes have reasonable sensitivity up to 2.2μm although there is some degradation in performance and increased costs. Some extended InGaAs photodiodes provide spectral coverage up to 2.6μm, however, these often come with significant performance degradation and notably higher cost. InGasAs is not a viable alternative for applications requiring a spectral response beyond 2.6μm.
The remaining alternatives, lead selenide (PbS) and lead sulfide (PbSe) photoconductors, have characteristics that make them excellent candidates for meeting demanding industry requirements including high performance, fast response, smaller packages, and costeffective solutions. PbS and PbSe detectors stand out for their industry-leading sensitivity-versus-cost ratio across the entire 1 to 5.5 micron infrared spectrum. PbS has detectivity values that are at least as good as Extended InGaAs with coverage up to 3μm. PbSe is approximately an order of magnitude less sensitive than PbS, but fully covers the mid-IR spectrum with a 1 to 5μm operating range.
The Evolution of Infrared Lead Salt Technologies
Lead salt detector technology has emerged in roughly three stages. PbS detectors were developed during World War II by the German military for use as heat-seeking sensors for weapons. During the ensuing 50 years, the major volume users of the detectors continued to be primarily military, for applications in missile guidance and surveillance systems. The second stage began around 1985 and marked an era of emerging new commercial applications including spectrometry, protein analysis, fire detection systems, combustion control, and moisture detection and control. The most recent stage, which emerged during the last five years, involves an expansion of commercial applications taking advantage of the extended wavelength coverage provided by PbSe technology. Much of this growth is centered around environmental applications, such as pollution detection and medical applications, such as blood protein and medical gas analysis.
Advances in chemical deposition and post-processing operations allow characteristics like time constant, dark resistance, responsivitity, detectivity, and peak wavelength responses to be designed to meet specific application requirements. Additionally, significant developments in areas like integrated circuit design, photolithography, ion milling, hermetic sealing, and thermoelectric cooling have allowed users to more fully take advantage of the benefits of photoconductive detectors.
Over the last two years, there have been significant developments in PbS and PbSe single element, multi-element, and array sensors as well as in emitter technologies.
Single Element Detectors
New PbS single element detector technology introduced in 2011 takes advantage of process improvements and provides exceptional sensitivity at a great value. Offering typical D* peak from 9 × 1010 to 1.75 × 1011 Jones across the entire one to three micron wavelength range, they are truly cost-effective with high volume pricing of less than $25. Thanks to precise process controls and automated production equipment, they combine industry-leading price/performance with exceptional repeatability and product consistency.
Single element detectors can support a glass window or lens configuration and are typically available in a range of sizes (including 1mm2, 2mm2, and 3mm2 in industry standard TO-46 or TO-5 packages) depending on whether size is a critical concern (as in portable test equipment) or if greater field-of-view or larger element sizes is a priority.
Key applications include fire safety and flame detection, as well as process and quality control applications.
This year has also witnessed advancements in multi-element PbS and PbSe detector technology. Multi-element detectors with a two to four channel configuration provide significant opportunities for cost reduction and design simplification, minimizing the need for multiple individual detectors and complex optics. System costs and footprints can be reduced up to 60% with a four element detector versus alternative single-detector designs.
The latest generation of multi-channel detectors provide market-leading typical D* peak of 1.5 × 1010 Jones for detection of up to four distinct materials/gases with exceptional channel isolation (>99.5%) and superior quality and reliability.
A new feature is the option to integrate a thermistor within the detector package to optimize temperature compensation and maximize measurement precision. Co-locating the temperature sensor and detector material in a hermetically-sealed TO-5 package ensures they both “feel” the same temperature variations and reduce potential effects of external environmental factors.
Multi-element detectors are well-suited for a variety of applications including industrial and medical gas analysis, as well as auto and aviation emissions testing. They are ideal for environmental applications such as stack monitoring, greenhouse gas analysis, and overseeing air quality in confined spaces including tunnels and underground structures.
The latest generation of 256-element PbS and PbSe infrared detector arrays introduced in 2011 feature built-in compensation elements that enhance measurement stability by up to 25%.
This superior measurement stability is made possible by three key factors. First, the 256-element array (consisting of six compensation elements and 250 active elements) provides real-time measurement compensation for environmental changes. The measurement is adjusted based on a comparison of the six compensation elements and three active elements on each side of the array. Secondly, the new array design reduces the effects of system noise by up to 10%, optimizing the measurement resolution. Finally the arrays built-in shielding isolates the array from environmental variables that could reduce measurement accuracy.
A new feature is an optional cooler controller that further optimizes measurement stability by providing the means to fine-tune the temperature set point via included software, allowing the array to be set at the optimal operating temperature for a particular application.
PbS and PbSe arrays are ideally suited for a range of applications including gas analysis, spectroscopy, process and quality control, and thermal imaging/hotspot detection in applications such as manufacturing and assembly process lines or in buildings and railway systems.
High-output, high-pulse rate emitters can be pulsed as a source of blackbody radiation for near-to-mid infrared applications and are compatible with a wide range of infrared detector technologies. Whereas alternative emitter technologies pulse at roughly 10 Hz with 50% modulation, the latest generation of infrared emitter technology introduced in 2010 features a pulse speed that is 18 times faster (typically 180 Hz with 50% modulation depth), allowing users to obtain accurate readings of materials with much lower parts per million concentrations. Furthermore, these high pulse speeds are obtained without the added expense of having to design and implement optical choppers or mechanical modulators, as is often done with incandescent bulb emitters, and covers a much wider spectrum than high-pulsing infrared LEDs, which focus on only very narrow bandwidths.
The newest pulsable emitters come with integrated drive electronics in an industry standard 14 pin-dip IC package. The user-friendly drive electronics are voltage variable and allow users to select from a wide range of frequencies between 1Hz and 200Hz according to their specific application needs. A special feature of the drive electronics is the ability to provide peak pulse temperatures independent of the pulse frequency. This ensures the highest output for any application.
Target applications include industrial and medical gas analysis, environmental monitoring, process control instrumentation, spectroscopy, and plastics sorting.
In conclusion, lead salt detector technology offers significant performance and value benefits compared to alternative infrared technology options. The past two years in particular have witnessed significant enhancement to single-element, multi-element and array lead salt technologies.
When selecting a lead salt detector and emitter supplier, factors to consider include detector quality, process control and integration support. Selecting a supplier with expertise in detectors, emitters, packaging and drive controllers/software helps ensure optimized performance. Ask for a list of recent technology introductions to ensure your provider is keeping their finger on the pulse of developing technologies. Finally, make sure significant product design support is provided via prototyping tools such as digital drive boards and access to technical experts.