The word spectroscopy comes from the Latin-Greek hybrid, wherein spectrum is from the Latin word for “an appearance” (specter or apparition = a ghost!), and skopein is the Greek word for seeing or observing. Why does spectroscopy mean observing an appearance or a ghost? One explanation is based on the first systematic observation (among the many contributions to modern mathematics and science made by English physicist and mathematician Sir Isaac Newton [1642–1727]) of how the propagation of sunlight through a prism results in the “appearance” of a band of different colors. A second explanation may be that when one uses light to study an object, the observations are based on the effects of that object on light, so that one does not directly view the object, but one see its appearance, or a “ghost” of it. Finally, one can think of a spectral decomposition of sunlight as being filled with ghosts: the set of dark lines first observed by the English scientist William Wollaston [1766–1828] and the German physicist Joseph Fraunhofer [1787–1826], and the bands of invisible “colors” beyond the red on one side and beyond the violet on the other. The word spectrum has evolved in English to mean a range of ordered values or colors. For our purposes optical spectroscopy may be loosely defined as the study of atoms, molecules and particles by means of their wavelength-dependent interactions with light. Relevant to the methods discussed in this chapter light is broadly defined to encompass the range from the ultraviolet to mid-infrared wavelengths of the electromagnetic spectrum, or about 200 nm to 15 μm, which are transmitted through air with only moderate attenuation (see Figure 4.1). Over this range, the components used for optical instrumentation are most conveniently explained by appealing to the wave-like nature of light.

When it comes to scattering interactions and the diffusion of light in turbid media such as tissue, however, it is often more convenient to treat light as particles, or photons, rather than waves. Scattering can be elastic, in which the photon changes direction but not energy, or inelastic, in which the photon can both exchange energy and change its direction of propagation. Scattering phenomena are often described in probabilistic terms, by specifying the likelihood of a photon being scattered and the probable angular distribution of the scattering.

In Chapters 5, 6, and 7, we will examine in detail the molecular transitions and microscopic-scale structural features of tissue that are responsible for its optical properties on any scale, and that afford opportunities for diagnostic assessment. In this chapter, however, we introduce the primary constituents and structures of tissue that are responsible for its bulk optical properties, with primary attention to the wavelength range of near-UV through visible to near-IR. Of course, the heterogeneity of tissue lends complexity to the interpretation of the interactions of light with tissue, but, prior to addressing the effects of the heterogeneity, it is instructive to understand the factors that govern the averaged optical properties on a macroscopic scale.

The two optical properties that govern light transport in tissue, and that must be understood thoroughly, are absorption and scattering.

Other intrinsic optical properties of tissue that can affect light transport include birefringence and optical activity. These are subtle effects, however, that do not generally affect bulk transport properties of tissue, but can be relevant to microscopy based on phase contrast or differential interference contrast, which will be studied in Chapter 17 (Section 17.2).

Whether absorption or scattering dominates the propagation of light in tissue depends on the specifics of the tissue optical properties, which are generally a function of wavelength, and on their spatial distribution at microscopic and macroscopic levels. Figure 2.1 illustrates two cases, wherein either absorption or scattering dominates.

Absorption and scattering coefficients

We address the bulk optical properties, for which typical scale lengths are long compared to both the wavelength of light and the individual elements of tissue microstructure (cells, for example).

A textbook for a new field based on old concepts

Biomedical optics is a field that is both new and ancient. From the vantage point of the natural sciences and engineering, this is a newly developing interdisciplinary field, dealing with the application of optical science and technology to biological and biomedical problems, including clinical applications. On the other hand, the field has been around for thousands of years in a less quantitative way. Physicians’ eyes have served as optical spectrographs and sensors, with the brain serving as a database repository and providing the computational power (of a massively parallel computer) for pattern recognition. For example, physicians have known for a long time that a Caucasian patient with yellowing of the skin (or of the sclera of the eye) is likely to be suffering from liver disease. If the patient is flushed red, he/she might be running a fever, and if a local tissue area appears flushed and red, an inflammation is indicated; and the bluish appearance of a patient's lips and nail beds might be indicative of hypoxia. Now that the modern approach has become more quantitative and is developing new technologies, however, the field is growing and beginning to have a major impact on bioscience and healthcare. The emerging field combines the observational with the mathematical and computational, benefits from recent advances in optical technologies, and is coupled with a more rigorous physical-science approach that seeks to understand the basic underlying principles.

This textbook provides a broad survey of the field and covers the basics of a quantitative approach to the subtopics, taking advantage of the powerful tools offered by mathematics, physics and engineering. This quantitative approach and the didactic style, coupled with the description of representative applications and problem sets that accompany each chapter, are designed to serve the needs of students and professionals in engineering and the physical sciences. Students of the biological sciences will also find the text useful, especially if they have a good mathematical background. The basic material about general concepts and methods that are directly relevant to biomedical optics, including some topics of medical statistics, are described at an introductory level, whereas selected topics are covered in greater depth and treated at a more advanced level. Consequently, by proper selection of the material, this textbook can be used for upper-level undergraduate courses, as well as more advanced graduate-level courses on biomedical optics.

Chapter 7 introduced theoretical methods for calculating single scattering events of optical fields from individual particles, and illustrated examples of applications for measurement of individual cell properties by backscattering spectroscopy or by measurement of the scattering phase function. Chapters 9–12 will cover the concepts and theoretical framework for measurement of bulk tissue optical properties in the diffusion regime, where source-detector distances are large compared to 1/μ ′s (in most tissues, ≳0.5 cm), to enable application of the diffusion approximation to the radiative transport equation. Chapter 9, especially, rigorously addresses the parameter space for which the diffusion approximation to the transport equation is valid. In this chapter we cover the ground in between: the measurement of light that has scattered a few times, but not so many times that the light can be considered diffuse. The formalisms presented here, nonetheless, can be extended into the beginning of the diffusion regime, and may be advantageous where the P1 approximation holds (to be presented in Section 9.4). For the typical optical properties of tissue, this generally corresponds to distances less than 0.5 cm. Moreover, whereas the diffusion approximation also requires that the absorption coefficient be much lower than the reduced scattering coefficient, in the formalisms for short source-detector distances that are presented in this chapter that restriction is lifted. Consequently, the wavelengths typically used for short source-detector distances may span from the near-UV through the near infrared (NIR). In contrast, for applications in real tissue, the useful wavelength range for the diffusion regime is limited to the red-NIR wavelengths, where absorption by hemoglobin is minimal.

Given these considerations, it may seem confusing that the terms diffuse reflectance and diffuse reflectance spectroscopy (DRS) are commonly used to denote measurements of reflectance at any distance from the light source, including short distances. Such broad application of those terms may be partially justified by the fact that “diffuse” reflectance can be intended simply to distinguish between reflectance from a diffusing medium and single backscattering events or specular reflection (as from a mirror). DRS encompasses terms that more specifically relate to variations of the optical geometry and/or methods of the spectral analysis. These include elastic scattering spectroscopy (Bigio and Mourant, 1997), light scattering spectroscopy (Backman et al., 2001), differential pathlength spectroscopy (Amelink et al., 2004), and optical pharmacokinetics (Mourant et al., 1999) among others.

Introduction

Flexible membranes are the most frequently used components to conceive optical elements with tunable optical functionality, that is tunable micro-optical elements. They can be deformed actively by a variety of techniques, such as hydraulic pressure and thermo-mechanic or piezo-electric actuation. The optical tuning range of the resulting components depends on the amount of deformation generated during the actuation. Next to the optical quality of the membrane material, the strength of the deforming forces as well as the elasticity of the membranes are crucial parameters for the performance of tunable micro-optics. One of the most critical tasks when designing tunable optical elements is the trade-off between stability and tunability of the optical element. Stability is needed to achieve good, reliable, and reproducible optical performance. Elasticity and nonplastic deformation is required to achieve easy and reproducible tuning over a wide tuning range. In order to fulfil the requirements of both, the appropriate choice of material is of utmost importance for the performance of tunable micro-optical components. In this chapter, we discuss nanocrystalline solid materials (such as aluminum nitride, AlN or nanocrystalline diamond, NCD layers) in detail as these represent an excellent basis for tunable optical membranes. These nanocrystalline materials show good optical uniformity and transparency over a broad spectral range. A tailor-made microtechnological process guarantees thin layers that are only a few 100 nm thick. In consequence, the membranes made of the new material possess mechanical properties that are ideally suited for tunable components and offer new perspectives for the development of optical elements and systems.

In this chapter, we will chiefly focus on the processing, performance, and specific material properties of nanocrystalline AlN and NCD membranes. Many of the optical design issues related to the application of such membrane devices in complex optical microsystems for imaging performance will be reported in Chapter 15, “Adaptive Scanning Micro-eye.” To begin with, we will discuss some basic considerations with regard to the optical performance of spherically and non-spherically deformed membrane systems. Some of the performances here have only become possible thanks to the new material properties available by using nanocrystalline solid membranes. In Section 9.3 we address the processing and material characteristics of AlN membranes. We will specifically address the exciting mechanical properties that become feasible, as well as the performance under thermomechanical actuation. NCD membranes will thus be the focus of Section 9.4.

Plenoptic cameras are hybrid imaging systems combining a microlens array with a larger objective lens. In this chapter, we examine the optical limits of these systems and describe techniques to expand them. Before we do so, we start with providing an overview of the history, terminology and capabilities of plenoptic cameras.

History of Light Field Capturing

Plenoptic imaging was invented multiple times during the history of photography, each time changing its name and physical form. Initially, it was called integral photography. The first system was realised with photographic film by the Franco-Luxembourgian scientist and inventor Gabriel Lippmann at the Sorbonne (Lippmann 1908). An integral camera is equipped with multiple lenses arranged side by side in a square grid. Each lens creates a unique image on the film. Each image is slightly different from that of its neighbour. Like in stereoscopy, this difference is caused by the relative displacement of the lenses, an effect known as parallax.

In Lippmann's time, the production and alignment of the lenses was a manual and error-prone process. Imaging quality and light sensitivity were poor. Hence, Lippmann and his assistants could only build lab prototypes. Because no computing technology to process the images was available, the developed film was used as a projection slide, with another array of lenses as the imaging optics. Projection superimposed the individual images optically to form an integral image, which gives the process its name. Rather than being projected on a screen, the image is viewed directly. In the integral image, the observer could perceive depth in the recorded scene through stereoscopy and motion parallax. Throughout the twentieth century, Lippmann's idea was re-discovered every few decades (Ives 1930, Dudnikov 1970), advancing theory and manufacturing technology each time. The current renaissance started in the 1990s and was made possible by contributions from different fields of science and engineering, each with its unique terminology.

Now, light field is the commonly accepted term for the light quantities recorded from a scene. It is an abstraction of the electromagnetic field that describes both intensity and directional distribution of light for every three-dimensional (3D) point in space, but discards polarisation and phase. The term was popularised in computer graphics by Levoy & Hanrahan (1996). Light fields were first applied as a more robust starting point to the problem of view interpolation in image based rendering (Shum & Kang 2000).

Introduction

As delineated in Chapter 2, many optical systems in nature are based on soft and pliable organic tissue. Technical implementations of tunable optical systems, as described in Chapters 1, 3, 6, and 7, also benefit from the use of soft matter in the fabrication of optical components.

Whereas in natural optical systems such as the human eye, actuation (for tuning the focal length) uses flexible muscle tissue, most technical optical systems are actuated with rigid metallic or crystalline semiconductor-based actuators, that are used to deform a pliable optical component. Now, a new family of materials, liquid crystal elastomers (LCEs), represent a useful and novel means for generating force and movement in a micromechanical system and may thus be used to tune an optical system. In this chapter, we show how LCE-based actuators may be conceived to realize compact tunable irises and tunable lenses, all fabricated with soft materials.

The LCE is a polymer-based material that contracts and elongates significantly in direct dependence of temperature changes; it may thus be considered a thermal actuator characterized by a long stroke and able to generate significant force. By microstructuring the LCE and incorporating microfabricated heaters into the material, very compact actuators may be designed, allowing the realization of self-contained, highly miniaturized tunable optical systems.

Bioinspired tunable optical components have been demonstrated before. In the example shown in Figure 14.1a, Carpi et al. (2011) uses a dielectric elastomer to tune a pliable lens, thus imitating the human eye. Alternatively, liquids can be used to realize a completely fluidic tunable iris (Müller et al. 2012), which can be seen in Figure 14.1b; this system is discussed in-depth in Chapter 6.

Following the design of the human eye in both optics and actuation (Schuhladen et al. 2013), one challenge remained: the integration of individual components into a complete imaging system. In the following, we describe means for realizing actuation of soft-matter tunable optical devices using LCEs and the integration of these into an imaging system that includes a tunable lens and variable iris.We begin with an in-depth exposé of the chemistry, fabrication technology, and operating characteristics of LCEs. We then continue to characterize two optical components that rely on the characteristic behavior of this active material: an LCE-based iris and an LCE-actuated tunable lens. The chapter concludes with a discussion of the functionality of a complete imaging system that employs these components.

Introduction

Reflective optics (plane and curved mirrors, gratings, etc.) modify the wavefront of a reflected light beam. Tunable reflective optics bring that wavefront modification under system control. For example, a spherical mirror for which the radius of curvature can be adjusted will have a tunable focal length. A mirror with flexible control of higher-order surface corrugation might compensate aberration to clear up a blurry image. Already, we find tunable reflective optics playing a critical role in barcode and three-dimensional (3D) scanners, compact laser displays, full screen theater projectors, tiny optical fiber modulators, and tunable laser sources, to mention but a few applications. As performance and sophistication continue to increase and costs decrease, we will likely see entirely new capabilities enabled by these remarkable active optical elements.

For some uses, reflective optics might be used interchangeably with transmissive optics. But often there are reasons one may prefer a reflective optic over a transmissive one, or vice versa. For example, for very large apertures such as the primary lens on an astronomical telescope, mirrors are preferred when the sheer weight of a large glass lens becomes unmanageable. Reflective optics also exhibit very low chromatic aberration, helping them to perform well over a broad spectral range. On the other hand, reflective systems offer unique challenges related to obscuration of the optical path, sometimes necessitating the use of beam splitters or divided pupils to get the beam in and out of the system.

There are further characteristics that are important when considering reflection or transmission for tunable optics. One is the possibility of using a large number of actuators, which in the case of a reflective optic can be hidden behind the mirror where they won't interfere with the optical beam. Such mirrors offer precise control over the shape of the optical surface and can form the basis of an aberration compensation mirror, a precise aspheric lens, or a tunable grating. Another feature of reflective optics is the extremely light weight of the movable element, which is often a thin membrane. This optical surface can consequently be moved quickly, leading to very short response times compared to tunable transmissive optics based on redistributing volumes of liquid that are heavier and can cause viscous drag. A third difference is the magnitude of surface motion required to create a particular wavefront sag.

Introduction

In contrast to microfluidic systems, which have reached a high level of sophistication and have been demonstrated in myriad forms, the application of liquids to micromechanical and also micro-optical systems is still at an early stage of development. Yet, as we will see in this chapter, when liquids are employed as a permanent structure, they can greatly enhance the functionality of a micro-system.

Within micro-optics, the use of microscale liquid volumes has consequently resulted in the rapidly developing research field of optofluidics. Optofluidics essentially aims at fusing microfluidics with micro-optics (Psaltis et al. 2006) to achieve tunable and highly compact micro-optical components. As there is a host of physical effects that result from the microscale interaction of fluids with light, more and more scientists and engineers are eager to find ways of exploiting these effects for a large variety of applications. Examples of optofluidic research topics in recent years include novel light sources, such as dye lasers, lateral microlenses, liquid waveguides and filters, tunable gratings, prisms, and photonic crystals, and even metrology systems such as interferometers and refractometers.

One property of liquids that may be employed to great benefit is optical absorption. Since the optical attenuation of a liquid can be easily altered by the addition of colorants or by varying the optical path length, many devices have been conceived which use liquids as variable optical absorbers. For example, optofluidic designs have been developed for displays (either based on tiny droplets of colored oils or aqueous pigment dispersions), for optical switches, for variable optical attenuators, and also for tunable apertures (e.g., irises).

In this chapter, we present recent research from our group and others, aiming at micro-optical apertures that are tunable. It is important to note that in these devices, tunability is achieved by taking advantage of unique properties offered only by liquids. After illuminating the motivation for using liquids in optics, we will take a glance at the current state of development and also at future prospects of commercialization of optofluidic devices. Some basic information on the importance of optical apertures and on fundamental microfluidic principles will be beneficial before we narrow our view to optofluidic approaches for tuning micro-optical apertures. Finally, the remaining and major part of this chapter will be dedicated to the presentation of detailed results of our own research on optofluidic shutters and irises.

Introduction

Over millions of years, evolution drove mammals to adapt to their natural habitat with a perfection and efficiency that is hard to beat. The human eye is one of the most accomplished examples even though – speaking from an optical engineering perspective – mammal eyes are of rather poor optical quality. Still, the vision of mammals is one of the most highly developed senses. This is not simply because the brain works associatively by comparing information to recorded data. Chief factor is the dynamical optical adaptability of the eye as such. This adaptability goes far beyond the mere tuning of the focal length to adjust to specific object distances. The human eye compensates for its shortcoming in achieving high resolution imaging only in a small field of view by constantly scanning and adapting to specific areas in a much larger volume in the object space. In order to build a technical model of a human eye, various dynamic functionalities such as focal length tuning, adaptation of the pupil diameter, as well as scanning have to be integrated.

We present an adaptive scanning imaging system which imitates the complex functions of the human eye. Our technical model comprises microlenses with variable focal length, highly dynamic variable filters and microirises, as well as micro-prisms with variable angles of deflection. The purely electric actuation is meant to reduce the overall size and retain a similar functionality as the biological model.

In Chapter 9, we introduced nanocrystalline aluminumnitride (AlN) and diamond (NCD) as completely new materials for tunable optics. In this chapter, we focus on integration schemes for AlN-based optical components and modules with tunable optical performance for the implementation of the complex functionality of an adaptive scanning micro-eye. In Section 15.2 we present the optical design of our tunable micro-eye and derive the most important features necessary for realizing the desired complex performance. Here, the chief challenge is to adapt the optical design to the performance that is feasible with the available technologies. Sections 15.3 to 15.5 discuss the performance of the individual optical elements with regard to focal length tuning, scanning, aperture stop variation, and dynamic wavefront correction. Various approaches towards the integration of the individual elements are discussed in Section 15.6. We evaluate the performance of submodules for various applications and we present a highly compact tunable scanning micro-eye, the final outcome of our research effort.

Introduction

When considering the structures, fabrication techniques or functionality of miniaturized optics, we see that size does matter. Compared with macroscopic optics, micro-optics is generally manufactured employing different technologies, using different materials, and often relying on entirely different optical effects (Zappe 2012). An upshot of these differences is that micro-optical components and systems may often display functionalities not possible with classical, macroscopic optics. One such functionality is intrinsic tunability, and it is tunable micro-optics that is the subject of this book.

Whereas a macroscopic optical system is usually tuned by a mechanical displacement of components, in micro-optics tunability may often be realized by a controlled change in an intrinsic property of the component itself. Thus the deformation of a soft polymer surface; the change in surface tension of a liquid; or the swelling of surface layers are all mechanisms that may be employed to tune the optical characteristics of micro-optical devices (Friese et al. 2007). The very rich portfolio of effects and materials of which we may take advantage to accomplish tunability is one reason for the broad spectrum of activities in this dynamic discipline.

In this introductory overview chapter, we provide a survey of the current state-of-the art in tunable micro-optics. Using the established knowledge base in micro-optics as a point of departure (Herzig 1998, Sinziger & Jahns 2003, Zappe 2010), we will look at those micro-optical components whose optical characteristics may be tuned using novel mechanisms inapplicable to macroscopic optics.We have organized the following sections by device, allowing for different tuning mechanisms, and will consider tunable lenses; apertures and irises; filters; and diffractive optics. The chapters which follow will address many of the concepts and devices we present here in greater depth.

Microlenses

Microlenses are likely to be the optical components most researched and developed with regard to intrinsic variability (Krogmann et al. 2007, Levy & Shamai 2008, Nguyen 2010, Zeng & Jiang 2013). A wide variety of novel actuation mechanisms for tuning the focal length of microlenses is available, due in large part to the fact that unconventional materials, such as fluids and soft matter structures, may be used to fabricate lenses on the microscale. Since these are generally not useful for manufacturing macroscopic lenses, many of the tuning techniques we will consider are thus usually unique to microlenses.