Although IR optics have long been used for military, surveillance and industrial applications, it is only recently that their full potential is being explored in biomedical settings. Common applications are to study veins in vivo, to image certain cancers, and to sense the absorption of oxygen in blood to determine the hemoglobin content. Below, we explore a few examples of biomedical IR and discuss how to specify or select an IR lens to maximize performance in your own application. Four spectral regions are commonly used: near-IR (0.7 to 0.9 µm), short-wave IR (0.9 to 1.7 µm), midwave IR (3 to 5 µm) and long-wave IR (8 to 12 µm). We refer to these as NIR, SWIR, MWIR and LWIR, respectively. The regions are useful for sensing various features, offering a range of applications. When added to the complement of analytical tools, the IR region of the light spectrum allows for two additional determinants. The first is temperature and the second, imaging within an expanded spectrum (beyond the visible). Temperature and spectrum Temperature historically has been used as a determinant of disease or infection in individuals. In the mid- and long-wave IR, however, thermographic scanning tools can offer abilities such as isolating individuals with elevated temperatures in a crowd. This technique offers broad applications for the control of infectious disease, as demonstrated during recent SARS and H1N1 scares. In airports throughout Asia, IR screening devices scanned for passengers with fever as they crossed security checkpoints. Those with fevers were asked to step aside for further examination. Such identification could minimize the spread of a contagion because those with illness would be prevented from spreading it to other passengers and, ultimately, other locations. A given specimen often will transmit at one wavelength but absorb in another. Sensing in IR allows us to see things that are not in the visible range, and often to do so in a completely noninvasive way. Further, there are differences within the four IR regions, so different information may be provided in one region of the IR spectrum versus another. A good example of how expanded spectrum can be used in biomedical settings is the VeinViewer Vision imaging system from Christie Medical Holdings Inc. of Memphis, Tenn. (Figure 1). This device emits near-IR radiation, which is reflected back by the tissue surrounding a vein. No near-IR light is reflected back from the blood inside the vessel, however, so the differential response between vein and surrounding tissue is clear. A standard digital video camera captures the reflected light, processes it electronically and projects it back onto the body. The result? Hard-to-see veins can be observed clearly. Courtesy of Christie Medical Holdings Inc. Researchers at Cardiff University in the UK provide another example of the opportunities presented by use of an extended spectrum. They have reported using IR for optical “fly-throughs” of the eye, whereby full biopsies can be performed in vivo without cutting into the living tissue. Selecting an IR lens Given the wide range of imaging opportunities between near-IR and long-wave IR, several criteria should be considered when selecting an IR lens for your application: 1. Wavelength - Because the absorption of materials is wavelength-dependent, it is important to determine the range in which you need to image: NIR, SWIR, MWIR, LWIR or a combination thereof. In addition, the optical materials change depending upon the spectrum chosen for analysis, greatly affecting price. 2. Field of View - Do you need to see an entire hallway or just a small portion of an eye? “Instantaneous field of view,” coupled with the standoff distance, fully determines the effective focal length of the lens. Typically, the smaller the field of view, the better the resolution. 3. Zoom Ratio - Many applications require several scales of resolution within a single trial, making it highly desirable to have an IR lens that can zoom in and out. For example, an area of the body may be examined to find an abnormal lesion or hot zone, then magnified for more detail about the nature of the cells in that area. Both continuous and stepped IR zoom lenses can be optimized for a given application. 4. Expected Environment - Environmental concerns such as operating temperature and space constraints will affect both the lens and camera system and should be communicated clearly to the manufacturer or design team. Also important is the desired standoff distance to the sample to be imaged. In public health applications such as airport screenings, the standoff distance is relatively large as compared with clinical settings, where the patient typically can come very close to the lens. 5. Camera Type - In the IR, the camera must be carefully matched to the lens. Whether the camera is cooled or uncooled, the desired wavelength and the available space (as it relates to camera size) are all important. In most cases, this information can be found in an interface control document. As in the visible, pixel size also is significant. In general, the smaller the pixel size, the higher the resolution. The resolution of a 640 x 480 x 17-µm camera, used in a 1:1 imaging system, will be set at 17 µm, meaning that each pixel is 17 µm wide. 6. The f/#-Diffraction Balancing Act - In the visible, diffraction is relatively insignificant. This is not the case, however, in the IR. Understanding the balancing act between f/# and resolution will save time and money when selecting a camera and lens for your application. Diffraction is discussed in terms of the blur circle, which can be approximated using the following formula: Blur circle = (2.44) x (Wavelength) x (f/# of the lens) The f/# equals the effective focal length divided by the diameter, so the lower the f/#, the bigger the lens. From the standpoint of transmission in the IR, a lower f/# is preferred. However, many biomedical applications require small lenses and sensors. As the lens shrinks, the f/# goes up, light throughput drops, and the size of the blur circle increases, limiting resolution. On the contrary, if size is not an issue, the f/# of the lens may be decreased, allowing more light through and enabling a higher resolution. In these cases, resolution often is limited by the camera rather than the lens. Consider the three examples below to see how changes to the f/# of the lens affect system performance and how a good camera-lens match optimizes performance. The black spots – also called Airy disks – shown in Figures 2A, 2B and 2C represent the blur circle, or physical diameter caused by diffractive effects. Figure 2A shows the lens set to f/1.5. In this example, lens performance dominates the spot calculation and serves as the limiting factor. The Airy spot does not play a significant role. In Figure 2B, the lens is set to f/6. Diffractive effects dominate, and the lens geometric aberrations don’t play a part. In this scenario, the system is limited by diffractive effects. If the pixel pitch were 25 µm, the resolution would be at best two pixels. Lastly, in Figure 2C, the lens is set to f/3, making the lens and diffractive effects well-balanced. This would allow for optimal depth of field and smallest size, while still passing as much resolution as possible by the physics of the situation. Note that with the higher f/#, the aberrational control of the lens is better because the light is not going to the maximum aperture. The spot actually decreases in size. Summary Biological applications for the IR region of the spectrum will continue to expand, driving the demand for smaller, more precise and range-specific stock, and custom IR zoom lenses. We are excited by the new work in cancer research, public health and other areas, which continue to push the boundaries of what can be done and seen, using all ranges of the IR spectrum. Meet the author Dr. Jonathan S. Kane is president of Computer Optics Inc. and division leader for the company’s IR division, IRZoom.com; e-mail: jon@irzoom.com.