Near-IR cameras... R&D and Industrial Applications

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1 R&D and Industrial Applications 1 Near-IR cameras... R&D and Industrial Applications José Bretes (FLIR Advanced Thermal Solutions) jose.bretes@flir.fr / ABSTRACT. Human eye is sensitive to the so-called visible portion of the electromagnetic spectrum, from 350 nm to 750 nm. Infrared spectrum starts above this upper limit. FLIR Systems designs and manufactures infrared imaging systems that cover the infrared spectrum from Near-InfraRed (NIR: 0.9 µm to 1.7 µm) up to the thermal infrared, also called Middle Wave InfraRed (MWIR: 2.5 µm to 5.1 µm) and Long Wave InfraRed (7.7 µm to 14 µm). NIR (also called SWIR Short Wave InfraRed when upper limit is extended to 2.5 µm) imaging systems can be used in several industrial and R&D applications. KEYWORDS: Infrared, camera, NIR, SWIR, MWIR, LWIR, VisGaAs, InGaAs, InSb, MCT, HgCdTe, emissivity, temperature, thermal, thermography, reflectography, FLIR.

2 R&D and Industrial Applications 2 1. Introduction NIR cameras, which are sensitive to the near-infrared spectrum (0.9 to 1.7 µm) are increasingly found in research laboratories and design departments and on factory production lines. Used to view phenomena that cannot be observed with thermal (MWIR/LWIR) imaging systems, NIR cameras have many advantages. For example, they do not require cryogenic cooling, and so are lighter, smaller and less expensive. Most NIR cameras use an InGaAs (Indium Gallium Arsenide) detector that can be used without cooling or combined with a Peltier effect module. Since these cameras have no mobile parts, they are particularly robust and capable of withstanding harsh use conditions such as continuous 24/7 operation. This article presents just a few of the major applications for which NIR cameras are particularly well-suited. The field is vast. Readers are consequently invited to contact the author for details or if they have any questions concerning fields of application not dealt with in this article. 2. NIR cameras: operation and specific characteristics NIR cameras are very similar to traditional CCD cameras. Figure 1 shows a simplified diagram of an NIR camera and an array detector. Figure 1. NIR camera and array detector: simplified diagram

3 R&D and Industrial Applications 3 Key components: - Optical lens: concentrates light flow on the surface of the detector and contributes to the formation of the image. Optics corrected over the entire µm spectrum are best for optimal image quality; - Filter: for applications requiring spectrum selectivity it should be possible to use an interferential filter. Some applications are based on the rejection of a major part of the spectrum in order to maximise the useful information provided; - Detector: detectors, also called FPA (Focal Plane Array), are the core of the camera s detection system. Although most NIR camera detectors are made of InGaAs photovoltaic cells, some are HgCdTe (Mercury Cadmium Telluride) in cases where extended sensitivity (up to 2.35 µm) is required. It should be noted that a modified version of the InGaAs detector, known as VisGaAs, has a sensitivity extending into the visible spectrum: it covers the µm spectrum. The 320x256 pixel detector with a 30 µm pitch is currently the most common. - Electronics module: used to apply a certain number of operations and process the signal from the detector (uniformity corrections, synchronous triggering, etc.); - Connections: NIR cameras available today commonly connect with Giga- Ethernet. Other types of connections, in addition to communications and control, are also available (Trigger In/Out when the camera must control or be controlled by an outside source, Lock-In for applications requiring synchronous demodulation, video output for viewing on a video display, etc.). 3. Applications NIR and SWIR cameras are used in so many applications that it would be impossible to list them all in this article, so readers should not conclude that their use is limited to the applications described here. There are, however, a number of applications sufficiently well-known and integrated in various industrial processes or areas of research that they can be cited as good examples of the potential of these imaging systems.

4 R&D and Industrial Applications R&D Applications NIR/SWIR cameras are often chosen by research laboratories because of the physical and chemical properties of certain materials (solids, liquids or gases) at these wavelengths. This article addresses the following examples: - Imaging through certain coatings: infrared reflectography, - Biochemistry / pharmacology: NIR/SWIR spectroscopy Imaging through coatings: infrared reflectography Infrared reflectography is a method used to investigate drawings beneath easel paintings that was developed in the 1970's. In addition to providing documentary evidence for archives, this technique also helps settle issues relating to the attribution of works of art. This non-destructive expert investigative technique, used by many museums and art research centres, is based on the optical properties (diffusion and absorption) of pigments in the near-infrared range. The example below involves a painting by Hans Memling entitled Triptych of Jan Crabbe (Musei Civici di Vicenza) and, in particular, a detail in the scene showing John the Baptist and a monk (figure 2). Figure 2. Triptych of Jan Crabbe by Hans Memling (Musei Civici di Vicenza) Analysis of the underlying drawing using reflectography shows that the artist tried several approaches (positioning of the monk s head and John the Baptist s right hand figure 3).

5 R&D and Industrial Applications 5 Figure 3. Triptych of Jan Crabbe by Hans Memling (visible and near-ir spectrum) Figure 4 provides an overview of the principle of near-infrared reflectography. An interferential band pass filter is almost always required because the absorptive properties of the pigments are spectrally variable nm Transmission nm Transmission Reflection Absorption Underlying drawing NIR/SWIR camera + filter nm Transmission Reflection «Visible» Imaging System ( nm) Varnish Pigments Preparation Support Figure 4. Analysis of a painting using near-ir reflectography and comparison

6 R&D and Industrial Applications Biochemistry / pharmacology: NIR / SWIR spectroscopy NIR / SWIR cameras with a focal plane array can be used to determine the spectrum of an object or extended component with spatially variable spectral properties (variable composition, localised active components, etc.). For rapid results and greater versatility, the camera should be used with an acousto-optics tunable filter (AOTF) or liquid crystal tunable filter (LCTF), or with an interferometer, depending on the precision of measurement required. Each mono-element in the array (pixel) in this case acts as a multi-spectral detector. A three-dimensional reconstruction can then be made as a function of the wavelength and transmission. NIR / SWIR spectroscopy can be used to measure humidity in a solvent (ethanol, for example) or to determine concentrations of OH -, CO 3 2- and HS - ions in an aqueous solution. In pharmacology, these imaging systems can be used to carry out non-destructive investigations on tablets (to determine the presence or absence of an active ingredient, for example). Figure 5, for example, shows measurements carried out on samples of isopropanol (rubbing alcohol) and natural spring water. Figure 5. Isopropanol (left) and natural spring water (right) - NIR transmission spectrum measured with an InGaAs camera and a variable liquid crystal filter 3.2. Industrial Applications Industrial applications are those in which the imaging system, often an integral part of a more complex system, has a direct or indirect impact on a process (research & development, production, quality control) within a company. Two examples will be described in this article:

7 R&D and Industrial Applications 7 - high-temperature thermography, - inspection and detection of mould in the agri-food industry High-temperature thermography The applications mentioned above are dependent on the reflective properties of the objects under consideration. Thermography, for its part, makes use of emissions from the objects themselves; it is based on the theory of the black body introduced by Gustav Kirchhoff. Traditionally, infrared thermography uses imaging systems operating in the middlewave ( µm) or long-wave ( µm) range depending on the naming systems commonly used by manufacturers. Thermography in the NIR or SWIR range has a number of advantages: - Smaller investment in the camera (no cryostat), - Very high sensitivity to temperatures above 800 C, with measures beginning at 400 C also possible, - Possibility of measuring through windows made of standard glass (BK7, among others). Figure 6 shows two temperature measurements, of a black body at 800 C and 1050 C, and the precision of these measurements.

8 R&D and Industrial Applications 8 Figure 6. Counter-measurements on a black body at two temperatures (800 C and 1050 C). Note the precision and low dispersion of the measure. Conclusion NIR and SWIR cameras are often used in laboratories, R&D departments and up to some extent, on process control lines. Their high quantum efficiency between 900 and 1100 nm (>75%), makes them the best choice compared to EMCCDs for which quantum efficiency at these wavelengths is very low (<20%). Easy to integrate, easy to run, these cameras have opened a new world of exciting investigation. The range of applications is large, and is getting broader every day. With almost 10 years of experience in the design and fabrication of InGaAs detectors and cameras, FLIR has grown an extensive knowledge in this technology, thus offering a first-rate partnership to develop your application.