Linearpolarizationvs circularpolarization

Nomarski differential interference-contrast microscopy. III. Comparison with phase contrast.  ZEISS Information 76: 69-76 (1971).  A discussion of the microscope configuration for phase contrast and DIC with emphasis on similarities and differences between the complementary techniques. Included are specimen thickness, depth of field, birefringence, and refractive index.

Furthermore, laser light is monochromatic and coherent. White light is a jumble of colored light waves. Each color has a different wavelength. If all the wavelengths but one are filtered out, the remaining light is monochromatic. If these waves are all parallel to one another, they are also coherent: the waves travel in a definite phase relationship with one another. In the case of laser light, the wave crests coincide and the troughs coincide. The waves all reinforce one another. It is the monochromaticity and coherency of laser light that makes it ideal for recording data on optical media such as a CD as well as use as a light source for long haul fiber-optic communications.

Linearpolarization

where n is the refractive index of the medium, q(i) is the angle of incidence, and q(r) is the angle of refraction. When incident light is polarized in this way, it is often referred to as glare. On unusually bright days, the glare caused by sunlight on a roadway or a field of snow, may be almost blinding to the human eye.

Another interesting use of light polarization is the liquid crystal display (LCD) utilized in applications such as wristwatches, computer screens, timers, and clocks. These devices are based upon the interaction of rod-like liquid crystalline molecules with an electric field and polarized light waves. The liquid crystalline phase exists in a ground state that is termed cholesteric, in which the molecules are oriented in layers and each successive layer is slightly twisted, forming a spiral pattern. When a polarized light wave interacts with the liquid crystalline phase, the wave is twisted by an angle of approximately 90 degrees with respect to the incident wave. This angle is a function of the helical pitch of the cholesteric liquid crystalline phase, which is dependent upon the chemical composition of the molecules. However, the helical pitch may be fine-tuned if small changes are made to the molecules.

Linearly polarized light

Light Microscopy: Contrast by interference.  Nature Milestones, Milestone 8: (2009).  This brief review focuses on the history of DIC microscopy and provides an excellent set of early references that founded the technique. Also included in the article is a drawing of the DIC microscope optical train and a section highlighting some of the most recent reviews on the topic.

P-polarized light

Polarization of light is also very useful in many aspects of optical microscopy. Microscopes may be configured to use crossed polarizers, in which case the first polarizer, described as the polarizer, is placed below the sample in the light path and the second polarizer, known as the analyzer, is placed above the sample, between the objective and the eyepieces. If the microscope stage is left empty, the analyzer blocks the light polarized by the polarizer and no light is visible. However, when a birefringent, or doubly refracting, sample is placed on the stage between the crossed polarizers, the microscopist can visualize various aspects of the sample. This is because the birefringent sample rotates the light, allowing it to successfully pass through the analyzer.

Nomarski differential interference-contrast microscopy.  ZEISS Information 70: 114-120 (1968).  The first article in a four-part series outlining the basic theory and practice of differential interference contrast. Published by Carl Zeiss in the late 1960s and early 1970s, these articles remain perhaps the best source of information about the technique. This article is an introduction to the theory of DIC microscopy.

When current is applied to the electrodes, however, the liquid crystalline phase aligns with the current and loses the cholesteric spiral pattern. Therefore, light passing through a charged electrode is not twisted and is blocked by Polarizer 2. By coordinating the voltage on the seven positive and negative electrodes, the display is capable of rendering the numbers 0 through 9. In this example, the upper right and lower left electrodes are charged and, consequently, block light from passing through them, which results in the formation of the number "2".

Unpolarizedlaser

A prime example of the basic application of liquid crystals in display devices is the seven-segment LCD numerical display illustrated in Figure 3. Here, the liquid crystalline phase is sandwiched between two glass plates that have seven electrodes, which can be individually charged, attached to them. Although the electrodes appear black in this example, they are transparent to light in real devices. As demonstrated in Figure 3, light passing through Polarizer 1 is polarized in the vertical direction and, when no current is applied to the electrodes, the liquid crystalline phase induces a 90 degree twist of the light and it can pass through Polarizer 2, which is polarized horizontally. This light can then form one of the seven segments on the display.

Circularly polarized light

Even unpolarized incident light, such as natural sunlight, is polarized to a certain degree when it is reflected from an insulating surface like water or a highway. In such cases, the electric field vectors of light parallel to the insulating surface are reflected to a greater degree than vectors with different orientations. However, the optical properties of the surface primarily determine how much of the reflected light is polarized. For instance, the properties of mirrors make them very poor polarizers, while many transparent materials are excellent polarizers if the angle of incident light is within certain limits. The particular angle inducing maximum polarization is known as the Brewster angle and is expressed by the equation:

Differential interference contrast (DIC) converts gradients in specimen optical path length into amplitude differences that can be visualized as improved contrast in the resulting image. The specimen optical path difference is determined by the product of the refractive index difference (between the specimen and its surrounding medium) and the geometrical distance (thickness) traversed by a light beam between two points on the optical path. Images produced in DIC microscopy have a distinctive shadow-cast appearance, as if they were illuminated from a highly oblique light source originating from a single azimuth.

Principles and applications of differential interference contrast light microscopy.  Microscopy and Analysis 20: S9-S11 (2006).  A recently published discussion of DIC, highlighting the origins and advantages of the methodology along with a simplified explanation of how the technique is executed in practice. Discussed are the condenser, objectives, prisms, polarizers, microscope configuration, and image analysis.

Physicist Gordon Gould invented the laser in 1958. The first working model was built in 1960 by T.H. Maiman. It contained a synthetic, cylindrical ruby with a completely reflecting silver layer on one end and a partially reflecting silver layer on the other. Ruby is composed of aluminum oxide with chromium impurities. The chromium atoms absorb blue light and become excited; they then drop first to a metastable level and finally to the ground (unexcited) state, giving off red light. Light from a flash lamp enters the ruby and excites most of the chromium atoms, many of which fall quickly to the metastable level. Some atoms then emit red light and return to the ground state. The light waves strike other excited chromium atoms, stimulating them to emit more red light. The beam bounces back and forth between the silvered ends until it gains enough energy to burst through the partially silvered end as laser light. When most of the chromium atoms are back in the ground state, they absorb light, and the lasing action stops. In continuous-wave lasers, such as the helium-neon laser, electrons emit light by jumping to a lower excited state, forming a new atomic population that does not absorb laser light, rather than to the ground state.

From http://micro.magnet.fsu.edu/optics/lightandcolor/polarization.html �by Mortimer Abramowitz, Shannon Neaves and Michael Davidson and http://technology.niagarac.on.ca/courses/tech238g/Lasers.html by Mark Csele

Nomarski differential interference-contrast microscopy. IV. Applications.  ZEISS Information 77/78: 22-26 (1971).  The final paper in the Carl Zeiss DIC series addresses various applications of DIC microscopy in the biological and material sciences. Among the topics mentioned are cytology, botany, histology, hematology, neurology, metallography, crystallography, mineralogy, and semiconductor applications.

If a wave emitted by one excited atom strikes another, it stimulates the second atom to emit energy in the form of a second wave that travels parallel to and in step with the first wave. This stimulated emission results in amplification of the first wave. If the two waves strike other excited atoms, a large coherent beam builds up. But if they strike unexcited atoms, they are simply absorbed, and the amplification is then lost. In the case of normal matter on Earth, the great majority of atoms are not excited. As more than the usual number of atoms become excited, the probability increases that stimulated emission rather than absorption will take place.

The Zeiss-Nomarski differential interference equipment for transmitted-light microscopy.  Zeitschrift für Wissenschaftliche Mikroskopie und Mikroskopische Technik 69: 193-221 (1969).  Georges Nomarski and associates address the basic fundamentals of differential interference contrast. The authors discuss the wavefront field, shear, contrast, resolution, optical sectioning, amplitude contrast, and provide suggestions.

S-polarization vs p-polarization

The hazard of driving, or performing other daily activities, with a large amount of glare in one's eyes has resulted in the development of polarized sunglasses. The lenses of such sunglasses contain polarizing filters that are oriented vertically with respect to the frames. Below, Figure 2 demonstrates how polarized sunglasses eliminate the glare from the surface of a highway. As illustrated, the electric field vectors of the blue light waves are oriented in the same direction as the polarizing lenses and, therefore, are passed through. In contrast, the red light wave represents glare, which is parallel to the surface of the highway. Since the red wave is perpendicular to the filters in the lenses, it is successfully blocked by the lenses.

Nomarski differential interference-contrast microscopy. II. Formation of the interference image.  ZEISS Information 71: 12-16 (1969).  Review of the wavefront field in differential interference contrast describing Wollaston prism action, interference background, orthogonal symmetry, image formation, and fundamental principles surrounding interpretation of the DIC image.

The Sénarmont compensator: An early application of the geometric phase.  Journal of Modern Optics 40: 2061-2064 (1993).  A review of de Sénarmont compensators and their appliation to alterations of the wavefront field in DIC microscopy. This short review article is recommended reading for anyone who is conducting investigations with de Sénarmont DIC.

High-resolution video-enhanced differential interference contrast (VE-DIC) light microscopy.  Methods in Cell Biology 81: 335-364 (2007).  Drs. Salmon and Tran explain how DIC is used to enhance video microscopy investigations. Discussed are the major features of the VE-DIC image, microscope design, digital and analog contrast enhancement, components, and bias retardation.

Laser light has several features that are significantly different from white light. To begin with, light from most sources spreads out as it travels, so that much less light hits a given area as the distance from the light source increases. Laser light travels as a parallel beam and spreads very little.

The basic concept of polarization is illustrated above in Figure 1. In this example, the electric field vectors of the incident light are vibrating perpendicular to the direction of propagation in an equal distribution of all planes before encountering the first polarizer, a filter containing long-chain polymer molecules that are oriented in a single direction. Only the incident light that is vibrating parallel to the polarization direction is allowed to continue propagating unimpeded. Therefore, since Polarizer 1 is oriented vertically, it only permits the vertical waves in the incident beam to pass. However, the waves that pass through Polarizer 1 are subsequently blocked by Polarizer 2 because it is oriented horizontally and absorbs all of the waves that reach it due to their vertical orientation. The act of using two polarizers oriented at right angles with respect to each other is commonly termed crossed polarization and is fundamental to the practice of polarized light microscopy.

Polarization of laser beamexperiment

Natural sunlight and most forms of artificial illumination transmit light waves whose electric field vectors vibrate equally in all planes perpendicular to the direction of propagation. When their electric field vectors are restricted to a single plane by filtration, however, then the light is polarized with respect to the direction of propagation.

Nomarski's DIC microscopy: A review.  Proceedings of SPIE 1846: 10-25 (1994).  An excellent review by one of the foremost microscopists of the 20th Century. Professor Pluta reviews the basic principles of interference and the DIC wavefront field, covering shear, retardation, the extinction factor, and reflected ight DIC. Also described are the quantitative aspects of DIC imaging.

The laser uses a process called stimulated emission to amplify light waves. (One method of amplification of an electromagnetic beam is to produce additional waves that travel in step with that beam.) A substance normally gives off light by spontaneous emission. One of the electrons of an atom absorbs energy. While it possesses this energy, the atom is in an excited state. If the electron gives off this excess energy (in the form of electromagnetic radiation such as light) with no outside impetus, spontaneous emission has occurred.