Dr. Laura Ronchi Abbozzo of the Ronchi Foundation and formerly the University of Florence, Florence, Italy, has been organizing this event in different, mostly Italian cities, for the last 30 years with the help of a small but prominent group of Italian scientists, most notably Dr. Carlo Corsi. The conference was organized only once outside Europe in Leon, Mexico in 2007. It will take place again in North America in Quebec, Canada on 26–30 September 2017. The most recent Pisa congress attracted over 70 papers with participants coming from nearly every continent. All authors who participated at the conference were contacted and invited to contribute to this special issue. Furthermore, the AITA feature issue was open to contributions from all practitioners of infrared (IR), through direct contact and a call-for-papers published in AO. The manuscript processing office received 45 papers; of those 22 were accepted upon the strict review process involving two reviewers, as it is customary at the Optical Society of America journals. The paper submission deadline was 1 July 2016, with the accepted papers published only five months later. Most were available online within two to three months after submission.

The reasons for the rapid advances in IR technology are abundant, and quite interesting to contemplate when compared with the advances in the visual interval [0.38 µm–0.78 µm]. The IR interval includes the rather wide spectral band between the visible and the terahertz range. The edge of the visible is relatively easy to define: humans to do not see any radiation beyond the red. The onset of the terahertz range is defined by the use of different sources and detectors and, even more, by the specialized advanced electronics.

The materials that are used in IR optical systems have different performance requirements. We include five technology development papers that report on the achievements of distinct sub-systems; of those, two are on coatings achievements, and two on detector optimization.

Yang et al., present an optically stabilized erbium-fiber frequency comb with a broad repetition rate and tuning range based on a hybrid mode-locked oscillator. They use an experimental setup capable of locking two comb modes to narrow line width reference lasers in order to investigate the best performance of control loops. Simultaneous locking of two loops is realized by phase-stabilizing the comb. This facilitates precision dual-comb spectroscopy, laser ranging and timing distribution. The oscillator is housed in a temperature-controlled box with stability of $\pm 0.02\mathrm{K}$. This environment not only keeps high signal-to-noise ratio of the beat frequencies with reference lasers, but also guarantees self-starting at the same mode-locking every time. The authors also provide description of the expected future improvements.

Du et al., from the School of Aeronautic Science and Engineering, Beijing, China, describe tri-band transparent, conductive coating fabricated with indium tin oxide. The authors demonstrate how it is possible to make the indium tin oxide film a promising alternative material to conventional metal mesh for missile domes shielding from the electromagnetic (E&M) radiation. They devise a tri-band conductive indium tin oxide film for the visible, near infrared and mid-infrared intervals on sapphire substrate by radio frequency magnetron sputtering. Such films deposited under optimized conditions exhibit highly tunable properties for Hall mobility, carrier concentration, and sheet resistance. The average transmission of these films is over 80% in the [0.4 µm–1.6 µm] spectral band and 60% in [3 µm–5 µm] region.

Inoue from the Institute of Physics, Taipei, Taiwan, with co-authors form several Japanese institutes, and the University of California, USA, describe two-layer anti-reflection coating for large-diameter, cryogenic IR filters. They are made of alumina, with thermally sprayed mullite and polyimide foam incorporated as antireflection materials. This technology will likely be applied to the next generation cosmic/microwave background measurements.

Shi and coauthors from the Research Center for Advanced Materials and Devices, Shanghai, China, describe a method to grow HgDdTe layers, doped with Cd, grown by molecular beam epitaxy and liquid phase epitaxy. This step is followed by depositing the CdTe and ZnS film as barrier layers using thermal evaporation. The p-on-n photodiodes are fabricated by As implantation and Hg overpressure annealing. So-called barrier layer induced channeling effect (BLICE) is attributed to the ballistic implantation of As ions through the barrier layer with column structure. CdTe barrier layer with layered structure can avoid this effect, and the BLICE effect does not occur in the samples with ZnS barrier. The resulting p-n junctions are referred to as diffused junctions.

Boccardi et al., from the University of Naples Federico II, Naples, Italy, report on the basic temperature correction of QWIP cameras in thermo-elastic-plastic tests of composite materials. The long wavelength IR QWIP SC6000 camera is used to measure small temperature variations related to the thermo-elastic plastic effects in composite materials. The measurement of small temperature variations is difficult due to noisy signals under the normal test conditions. However, it was possible to account for and eliminate most of the noise in an effective way, employing a reference surface. Once the noise has been removed, the temperature variations linked to thermo-elastic plastic effects have been measured successfully.

One of the most attractive features of the IR radiation for the non-destructive, non-obtrusive, non-interfering testing is that anybody at finite temperature emits the radiation. This applies even to Mother Earth that began to be characterized in the near IR wavelengths as early as in the late eighties. The Earth emits radiation in IR and reflects the solar radiation in both the visible and IR. Now that the Earth radiation budget is a pressing issue, more and more Terran remote sensing studies are performed in IR. Three papers report on remote sensing of the Spaceship Earth.

Mahan from the Virginia Polytechnic Institute, Blacksburg, and co-authors from the NASA Langley Research Center, Hampton, USA describe a conceptual design of an instrument to measure the Earth radiation budget. Furthermore, they are interested in determining the diffraction and polarization effects. The latter are significant due to the contribution of the atmosphere and its contamination to the greenhouse effects.

Another aspect of monitoring the Earth surface is to assess the progress of agricultural activities. Haiying and Hongchun, from the University of Science and Technology, Qingdao, China, describe how hyper-spectral remote detection of leaf nitrogen may be used to assess the health and growth progress of winter corn. They propose that the currently measured spectral content may be correlated with the data obtained in previous years. They find that different growth stages from consequent years may indeed be correlated, providing credible monitoring of the growth with the measurement of the leaf nitrogen content.

Only in the 21st century have papers started to appear in refereed journals bringing to our attention the significant temperature changes introduced by cities on their environment. Ghellere et al., from the Italian National Research Council (ITC-CNR), Milan, describe how they use LANDSAT 8 data in IR to characterize key environmental variables for their city. Geo-referenced mapping databases have been created to be used as a benchmark to study the urban heat islands and environmental changes as they spread to the surrounding countryside.

In addition to making sure that the Earth continues to remain healthy for the foreseeable future, scientists are searching nearby stars for a potential earth-like satellite that is eventually feasible for human colonization. Galan et al. propose to place an array of 4 by 4 small-diameter telescopes, with a radius of 1 m, on the far side of the Moon for continuous monitoring of nearby stars for the existence of a planetary companion. The international team includes scientists from the Optics Research Center, Leon, Mexico, and University of Texas, USA. The authors discuss the advantages of the Lunar Planetary Observatory. They furthermore argue that all necessary technology has already been demonstrated. The solid-rock base and the absence of the atmosphere permit long signal integration times and open planet search in the spectral interval from less than 10 microns to 300 microns.

Another interesting feature of IR radiation is its ability to measure work, in the sense of basic mechanics. Work is defined as a scalar product of displacement and force. Thus, work is often accompanied by the generation of thermal energy, which results in temperature increases. We can measure work-related activities by detecting an additional amount of IR radiation, emitted by creatures, including humans. When studying humans, the passive observations afforded by self-emission assure that measurements are indeed non-interfering, non-destructive, and not damaging. We present three different studies of heat generation inside a living creature, each one generated by a different mechanism.

Ludwig et al., from the University of Milan, Milan, and MAPEI Sport Research Centre, Olgiate Olona, Italy, report of the skin temperature increase during dynamic exercise of male elite cyclists. They also discuss uncertainties that arise due to sweat generation.

Yousefi et al., study heat generation in organisms during radiological examination such as magnetic resonance imaging (MRI) for example. They propose an automatic system for detecting and tracking the thermal fluctuations based on a statistical approach using kernelled k-means clustering. The method is able to localize over-heating spots. The proposed approach was tested during experiments under conditions very similar to those used during actual radiology exams. To simulate hot spots occurring during the radiology procedure, the authors utilized a controllable heat source near the body of the subjects (six). The approach proposed by the authors can track the heating spots continuously. The method provides considerable robustness against motion artifacts that tend to accompany most medical radiology procedures.

D’Acunto and collaborators at the Italian National Research Council, the Institute of Science and Technology (CNR-ISTI) and for Materials (CNR-ISM), and the University of Rome, Italy, provide another interesting application of heat generation inside a creature. They are using gold nanoparticles imbedded inside cancer cells to absorb thermal energy of the irradiating laser to burn the cancer cells. The laser radiation passes through the tissue within the therapeutic window and is absorbed by the gold. In the current work, they report on the simulation results, finding that a scanning near-optical microscope is capable of localizing the gold nano-shells for particle apertures of about 100 nm.

Those of us who work in the IR like to think that the IR is that part of the E&M spectrum where all the characteristics that we know about the visible are still present, except the human eye has to be replaced, or better-said, augmented by suitable detectors. The visible CCD camera becomes a CMOS camera, and other even more exotic technologies are substituted. The number of pixels increases slightly less rapidly in IR, while the frame rates may be increased to several thousand per second by sub-framing.

In the visible, imaging with a camera (either film or digital) has evolved along several lines, with some evolving into specialties in their own right. We have machine vision non-destructive testing; wave front testing and reconstruction; image reconstruction with incomplete data; image sampling and reconstruction; detection and visualization of rapid events as in strobe illumination, including the nanosecond laser pulses; and the all-encompassing digital and optical image processing algorithms and techniques. Just transitioning the spectral band from the human-centered (visible) to a machine centered (IR), we observe that the number of problems that can be solved is significantly increased by the special characteristics of the IR radiation.

There is so much information that may be made visible in the world beyond the visible (i.e., the IR). In this research field, some new terminology has been introduced for imaging, “thermography”. This word denotes just about everything that may be accomplished when one is recording a two-dimensional incidence of radiation with the two-dimensional IR detector.

The first part of this word, thermo-, implies that we are recording thermal radiation, same as that recorded by Lord Kelvin when he exposed a thermometer to the solar spectrum beyond red. The second part, -graphy, is defined as a descriptive science, denoting (a method of) writing and recording. Interestingly, one of the editors used the IR imaging (and published work in AO) for measuring temperature distribution and evolution before this word was introduced in our vocabulary. The introduction of this word was actually quite necessary because the IR image has to be recorded into some medium before it may be presented as information to a human. Also in this aspect IR is different from visible: detected thermal images require an additional transducer to make it visible, often some current-voltage combination that drives the display. Three applications of thermography are described next.

IR imaging is indispensable in those applications when visible inspection is either not possible or not feasible. The human veins represent an important area that needs to be made visible in most medical procedures, and its position localized with a high degree of precision. There are hundreds of thousands of needle insertions performed every day to either extract (blood testing) or insert fluid (intravenous delivery of medications and nutrients). This is one area where visible, near IR, and IR methods may be used to find the object of interest (vein). Yet, extensive algorithm development is needed, as reported by Asrar and colleagues, from the Nottingham Trent University and the Airedale General Hospital, Steeton, UK.

The aviation industry has traditionally relied on ultrasound to probe for defects in aircraft. Only recently have the IR non-destructive capabilities started to be applied for inspection of aircraft subsystems. Vavilov and collaborators from the Tomsk Polytechnic University and the Tomsk State University, Tomsk, Russia, report on the feasibility of evaluating the water ingress in composite honeycomb panels. Authors analyze the difference in using 1-D and 3-D models for thermal nondestructive tests (NDT) to detect water in composite honeycombs. The presence of Nomex-made cells in glass fiber reinforced plastic and honeycombs does not significantly change contrast values between the 1-D and 3-D models. The discrepancy between values of observation times may be considerable depending on the cell size and water mass. Thus, 3-D models are preferable when optimizing thermal NDT parameters. The influence of honeycomb cell structure on modeling results is demonstrated, and a criterion for transitioning from the 3-D to 1-D test geometry is introduced.

Swiderski from the Military Institute of Armament Technology, Zielonka, Poland, describes that another interesting application of nondestructive testing is offered by eddy current thermography. This technique is used for the detection of cracks in electro-conductive materials. By combining the inspection methods of NDT and thermography, this technique uses induced eddy currents to heat the samples. Thermography provides visualization of eddy current distribution, distorted in defect sites. The author discusses results of numerical modeling of eddy current thermography applied to marine structures.

The fourth difference between the IR and visible imaging is that we can control the amount of emitted radiation by selectively supplying thermal energy to the tested piece for the purpose of generating the desired signal. This represents an interesting and complicated inverse problem. We know that the time-dependent inhomogeneous heat equation determines the temperature distribution; therefore, we may deposit an appropriate amount of thermal energy. Absorbed energy elevates the local temperature that increases the amount of the emitted radiation, according to the Planck’s law. Four teams explore the relationship between active sample temperature distribution and the thermal images that the sample creates.

Zdeněk and co-authors from the University of West Bohemia, Pilsen, Czech Republic, describe a 3-D model of temperature distribution in space and time, when the heat is deposited with a moving laser beam. The novelty of contribution consists in that the laser beam may follow an arbitrary path as a function of time and with variable dwell time.

Švantner et al., from the same Czech University describe how laser markings change surface optical characteristics. They apply their research to stainless steel to study its corrosion after light-matter interaction. They find that the resistance to corrosion may be increased to some degree by controlling the laser pulse length and frequency.

Another interesting application of thermography that furthermore includes a step response of the heated object to the thermal step input is in determination of tomato freshness. This could be a typical machine vision problem if it could be performed in the visible. Xie and co-authors from the Huazhong Agricultural University, Wuhan, China, in collaboration with the team from the Texas A & M University describe an advanced imaging algorithm. They combine IR thermography with machine learning techniques, incorporating artificial neural networks on a vector machine. They report about 95% accuracy.

Boris Vainer from the Rzhanov Institute of Semiconductor Physics, Novosibirsk, Russia, presents a summary of advantages of using laser heating to facilitate IR thermography for optimal control of experimental parameters. The author concludes that this acts as an indispensable partner to modern thermography, to laser irradiation in numerous biomedical and material processing applications.

The fifth significant difference between the visible and IR imaging is that in IR more options are available in generating the structured radiation by controlling and appropriately stimulating the temperature of the radiation emitting object area. For example, when characterizing reflectance spectrum in the visible, we introduce a mechanical chopper to change the constant radiation into a series of pulses. Those may be detected and analyzed with a lock-in amplifier that records only the first harmonic of the detected radiation. In IR, in contrast, we can stimulate the interrogated object with periodic laser radiation or a set of strong pulsating light/thermal energy emitting lamps.

This flexibility resulted in the creation of a number of special techniques to measure the response of an object, and specifically its exposed surface, to the structured irradiation. In addition to the availability of a laser with precise timing control as a source, a pair of synchronized lamps may also be placed in a symmetrical configuration. These two techniques are often referred to as active thermography. The surface response may be characterized from a series of recorded frames as a pulse phase or lock-in thermography, the former detecting phase and the latter detecting amplitude. Four teams discuss these techniques in diverse applications.

Maierhofer and colleagues from the German Agency for Building Materials in Berlin compare quantitative defect characterization using pulse-phase and lock-in thermography. The authors describe two kinds of temporal excitation techniques (pulse and flash) for active thermography that enables a contactless, remote and non-destructive testing of materials and structures. The first of the two applications is the phase values obtained from flat bottom holes in steel and carbon fiber reinforced plastic. The second one is the spatial resolution measured at crossed notches in steel.

Fernandes et al., apply active thermography to the inspection of carbon fiber composites. The collaborators come from the Federal University of Uberlandia, Brazil, and the Laval University, Quebec, Canada. They study defects via non-destructive testing. Square Kapton pieces of varying sizes and at different depths are inserted into carbon/Polyether ketone laminates. The results are tested with three different techniques: pulsed thermography, vibro-thermography, and line-scan thermography. The finite element method simulating the pulsed thermography experiment agrees well with the numerical predictions.

The same group augmented with the researchers from the Fraunhoffer Development Center in Fuerth, Germany, report on the detection capability of pulsed micro-laser line thermography on sub-millimeter porosities. On behalf of the team, Hai Zhang describes carbon fiber reinforced polymer composite and its porosity. An assessment is made of the probability of porosity detection, first through the simulation and then in an experiment.

Skála et al., from the University of West Bohemia, propose two methods of inspecting the protective glass of laser scanning heads, flash-pulse active thermography and laser active thermography. Upon the thermal excitation of the glass, the authors measure the response with an IR camera. The experimental setup and practical results are described. Authors report that both methods are effective in detecting the contamination of the glass head.

In concluding the presentation of the advancements in the field of IR in this dedicated issue, we can look with a great deal of satisfaction at the breath of applications and contributions of IR technology to the well-being of humanity. We will continue using the IR part of the E & M spectrum as a region where nondestructive testing may be performed to detect material defects and performance anomalies of both inanimate objects and animate creatures. With continued technology developments, the experimental techniques will likewise become more attuned to ever-smaller dimensions of studied features. In this special issue we see that characterization of modern, novel materials, ranging from the carbon fibers to composites to nano-shells, requires utilization of ingenious new imaging and analytical techniques. We invite all who want to see the Future actually happening to join us at the Quebec Meeting, on the grounds of the Laval University in September 2017.

We would like to express our gratitude to Dr. Ronald Driggers, the editor-in-chief of Applied Optics for inviting us to assemble this collection of papers in this special feature, Advanced Infrared Technology and Applications.

Furthermore, we wish to acknowledge the invaluable assistance of Ms. Nicole Williams-Jones, senior journal coordinator, who did a wonderful job assisting along the way with keeping peer review on track and sending out papers for review. We want to express our appreciation to Mr. Dan McDonold, the OSA editorial director, for providing guidance in preparing the call-for-papers and for explaining the established editorial procedures at Applied Optics.

Additionally, we are grateful to all the reviewers who took time from their busy schedules to provide constructive feedback to the authors. We appreciate the efforts that the authors have made in preparing the manuscripts and responding to the suggestions of the editors and reviewers in making the work more understandable to the readership of Applied Optics. Finally, we thank Ms. Michelle Scholl who read the introduction several times, providing critical feedback in clarity of expression and language usage.