Misplaced Pages

#331668

80-455: GDX may refer to: GDx-VCC , a medical diagnostic machine Godwari dialect libGDX , an open source Java game library Sokol Airport , in Magadan, Russia Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with the title GDX . If an internal link led you here, you may wish to change

160-431: A gouty joint will reveal negatively birefringent monosodium urate crystals . Calcium pyrophosphate crystals, in contrast, show weak positive birefringence. Urate crystals appear yellow, and calcium pyrophosphate crystals appear blue when their long axes are aligned parallel to that of a red compensator filter, or a crystal of known birefringence is added to the sample for comparison. The birefringence of tissue inside

240-500: A correlation between standard automated perimetry and GDx VCC measurements in patients with glaucoma, suggesting that GDx VCC measurements relate well with functional loss in glaucoma, in healthy subjects, they found virtually no correlation between perimetry and GDx VCC measurements. This would cast doubt on its predictive value and suggests false positives. see : "The Relationship between Standard Automated Perimetry and GDx VCC Measurements", Nicolaas J. Reus and Hans G. Lemij.... From

320-413: A few ways: The best characterized birefringent materials are crystals . Due to their specific crystal structures their refractive indices are well defined. Depending on the symmetry of a crystal structure (as determined by one of the 32 possible crystallographic point groups ), crystals in that group may be forced to be isotropic (not birefringent), to have uniaxial symmetry, or neither in which case it

400-415: A glass plate to generate an optical vortex and full Poincare beams (optical beams that have every possible polarization state across a cross-section). Birefringence is observed in anisotropic elastic materials. In these materials, the two polarizations split according to their effective refractive indices, which are also sensitive to stress. The study of birefringence in shear waves traveling through

480-456: A less invasive method to diagnose Duchenne muscular dystrophy . Birefringence can be observed in amyloid plaques such as are found in the brains of Alzheimer's patients when stained with a dye such as Congo Red. Modified proteins such as immunoglobulin light chains abnormally accumulate between cells, forming fibrils. Multiple folds of these fibers line up and take on a beta-pleated sheet conformation . Congo red dye intercalates between

560-410: A living human thigh was measured using polarization-sensitive optical coherence tomography at 1310 nm and a single mode fiber in a needle. Skeletal muscle birefringence was Δn = 1.79 × 10 ± 0.18×10 , adipose Δn = 0.07 × 10 ± 0.50 × 10 , superficial aponeurosis Δn = 5.08 × 10 ± 0.73 × 10 and interstitial tissue Δn = 0.65 × 10 ±0.39 × 10 . These measurements may be important for the development of

640-437: A measure of the degree of order within these fluid layers and how this order is disrupted when the layer interacts with other biomolecules. For the 3D measurement of birefringence , a technique based on holographic tomography [1] can be used. Birefringence is used in many optical devices. Liquid-crystal displays , the most common sort of flat-panel display , cause their pixels to become lighter or darker through rotation of

720-449: A plane wave of angular frequency ω can be written in the general form: where r is the position vector, t is time, and E 0 is a vector describing the electric field at r = 0 , t = 0 . Then we shall find the possible wave vectors k . By combining Maxwell's equations for ∇ × E and ∇ × H , we can eliminate H = ⁠ 1 / μ 0 ⁠ B to obtain: With no free charges, Maxwell's equation for

800-449: A polarization perpendicular to that of the ordinary ray, the polarization direction will be partly in the direction of (parallel to) the optic axis, and this extraordinary ray will be governed by a different, direction-dependent refractive index. Because the index of refraction depends on the polarization when unpolarized light enters a uniaxial birefringent material, it is split into two beams travelling in different directions, one having

880-433: A sample is placed between two crossed polarizers, colour patterns can be observed, because polarization of a light ray is rotated after passing through a birefringent material and the amount of rotation is dependent on wavelength. The experimental method called photoelasticity used for analyzing stress distribution in solids is based on the same principle. There has been recent research on using stress-induced birefringence in

SECTION 10

#1732851543332

960-441: Is a biaxial crystal. The crystal structures permitting uniaxial and biaxial birefringence are noted in the two tables, below, listing the two or three principal refractive indices (at wavelength 590 nm) of some better-known crystals. In addition to induced birefringence while under stress, many plastics obtain permanent birefringence during manufacture due to stresses which are "frozen in" due to mechanical forces present when

1040-413: Is a single direction governing the optical anisotropy whereby all directions perpendicular to it (or at a given angle to it) are optically equivalent. Thus rotating the material around this axis does not change its optical behaviour. This special direction is known as the optic axis of the material. Light propagating parallel to the optic axis (whose polarization is always perpendicular to the optic axis)

1120-475: Is aligned with the fast axis of a retarder. In contrast, polarized light travels at lower speed when its electric field vector is aligned with the slow axis of a retarder. In the model, the measuring beam passed through three linear retarders: the corneal compensator (CC), the cornea (C), and a uniform radial retarder (R), that represented birefringent regions in the retina (e.g., peripapillary RNFL or macula). And polarization-preserving reflector (PPR). Firstly,

1200-406: Is at a finite angle from the direction of the wave vector resulting in an additional separation between these beams. So even in the case of normal incidence, where one would compute the angle of refraction as zero (according to Snell's law, regardless of the effective index of refraction), the energy of the extraordinary ray is propagated at an angle. If exiting the crystal through a face parallel to

1280-416: Is called "birefringent" because it will generally refract a single incoming ray in two directions, which we now understand correspond to the two different polarizations. This is true of either a uniaxial or biaxial material. In a uniaxial material, one ray behaves according to the normal law of refraction (corresponding to the ordinary refractive index), so an incoming ray at normal incidence remains normal to

1360-428: Is commonly used in biological tissue, as many biological materials are linearly or circularly birefringent. Collagen, found in cartilage, tendon, bone, corneas, and several other areas in the body, is birefringent and commonly studied with polarized light microscopy. Some proteins are also birefringent, exhibiting form birefringence. Inevitable manufacturing imperfections in optical fiber leads to birefringence, which

1440-510: Is eliminated (in part) by a proprietary ‘corneal compensator’. The amount of retardation of light reflected from the fundus is converted to RNFL thickness. In Retinal scanning laser polarimetry (SLP), the cornea, lens, and retina are all treated as linear retarders (optical elements that introduce retardation to an illuminating beam). A linear retarder has a slow axis and a fast axis, and the two axes are orthogonal to each other. Polarized light travels at higher speed when its electric field vector

1520-400: Is governed by a refractive index n o (for "ordinary") regardless of its specific polarization. For rays with any other propagation direction, there is one linear polarization that is perpendicular to the optic axis, and a ray with that polarization is called an ordinary ray and is governed by the same refractive index value n o . For a ray propagating in the same direction but with

1600-460: Is involved. A material is termed uniaxial when it has a single direction of symmetry in its optical behavior, which we term the optic axis. It also happens to be the axis of symmetry of the index ellipsoid (a spheroid in this case). The index ellipsoid could still be described according to the refractive indices, n α , n β and n γ , along three coordinate axes; in this case two are equal. So if n α = n β corresponding to

1680-403: Is just a scalar (and equal to n ε 0 where n is the index of refraction ). In an anisotropic material exhibiting birefringence, the relationship between D and E must now be described using a tensor equation: where ε is now a 3 × 3 permittivity tensor. We assume linearity and no magnetic permeability in the medium: μ = μ 0 . The electric field of

SECTION 20

#1732851543332

1760-491: Is one cause of pulse broadening in fiber-optic communications . Such imperfections can be geometrical (lack of circular symmetry), or due to unequal lateral stress applied to the optical fibre. Birefringence is intentionally introduced (for instance, by making the cross-section elliptical) in order to produce polarization-maintaining optical fibers . Birefringence can be induced (or corrected) in optical fibers through bending them which causes anisotropy in form and stress given

1840-487: Is responsible for the phenomenon of double refraction whereby a ray of light, when incident upon a birefringent material, is split by polarization into two rays taking slightly different paths. This effect was first described by Danish scientist Rasmus Bartholin in 1669, who observed it in Iceland spar ( calcite ) crystals which have one of the strongest birefringences. In the 19th century Augustin-Jean Fresnel described

1920-484: Is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light . These optically anisotropic materials are described as birefringent or birefractive . The birefringence is often quantified as the maximum difference between refractive indices exhibited by the material. Crystals with non-cubic crystal structures are often birefringent, as are plastics under mechanical stress . Birefringence

2000-488: Is the component for which the material has the higher effective refractive index (slower phase velocity), while the fast ray is the one with a lower effective refractive index. When a beam is incident on such a material from air (or any material with a lower refractive index), the slow ray is thus refracted more towards the normal than the fast ray. In the example figure at top of this page, it can be seen that refracted ray with s polarization (with its electric vibration along

2080-407: Is used to measure thickness of nerve fiber layer in our retina. But, GDx give monochromatic image. Then this system will analyze and give colors for certain various thicknesses. Presents RNFL thickness in colour with thick regions in red and yellow and thin regions in blue and green. For healthy eye, the image will show yellow and red colour in superior and inferior at NFL regions. But, in glaucoma,

2160-441: The x and y axes, then the extraordinary index is n γ corresponding to the z axis, which is also called the optic axis in this case. Materials in which all three refractive indices are different are termed biaxial and the origin of this term is more complicated and frequently misunderstood. In a uniaxial crystal, different polarization components of a beam will travel at different phase velocities, except for rays in

2240-437: The 3 axes) where the refractive indices for different polarizations are again equal. For this reason, these crystals were designated as biaxial , with the two "axes" in this case referring to ray directions in which propagation does not experience birefringence. In a birefringent material, a wave consists of two polarization components which generally are governed by different effective refractive indices. The so-called slow ray

2320-609: The Glaucoma Service, The Rotterdam Eye Hospital, Rotterdam, The Netherlands. For overview, this first prototype of this instrument was developed about 10 years ago, and was first released commercially as the GDx Nerve fiber analyzer (Laser Diagnostic Technologies Inc). The second generation product is called the GDx Access. The field of view is 15 degree and imaging should be performed through an undilated pupil. The polarised laser scans

2400-469: The Henle fiber. As polarized light passes through a form-birefringent medium, one of the two component waves traveling at 90 to each other becomes retarded relative to the other. The degree of the resulting phase shift is directly proportional to the number of microtubules the light passes through, which in turn, is directly proportional to RNFL thickness. The figure above illustrates this process. The RNFL isn't

2480-420: The angle of incidence, the effective refractive index of the extraordinary ray can be tuned in order to achieve phase matching , which is required for the efficient operation of these devices. Birefringence is utilized in medical diagnostics. One powerful accessory used with optical microscopes is a pair of crossed polarizing filters. Light from the source is polarized in the x direction after passing through

GDX - Misplaced Pages Continue

2560-565: The axis around which it is bent and radius of curvature. In addition to anisotropy in the electric polarizability that we have been discussing, anisotropy in the magnetic permeability could be a source of birefringence. At optical frequencies, there is no measurable magnetic polarizability ( μ = μ 0 ) of natural materials, so this is not an actual source of birefringence. Birefringence and other polarization-based optical effects (such as optical rotation and linear or circular dichroism ) can be observed by measuring any change in

2640-407: The birefringence of the optic nerve fiber layer to indirectly quantify its thickness, which is of use in the assessment and monitoring of glaucoma . Polarization-sensitive optical coherence tomography measurements obtained from healthy human subjects have demonstrated a change in birefringence of the retinal nerve fiber layer as a function of location around the optic nerve head. The same technology

2720-494: The birefringent properties of the RNFL. Measurement is obtained from a band 1.75 disc diameters concentric to the disc. It projects a polarized beam of a light into the eye. As this light passes through the NFL tissue, it changes and slow. The detectors measure the change and convert it into thickness units that are graphically displayed. The GDx measure modulation around an ellipse just outside

2800-410: The change in polarization state using such an apparatus is the basis of ellipsometry , by which the optical properties of specular surfaces can be gauged through reflection. Birefringence measurements have been made with phase-modulated systems for examining the transient flow behaviour of fluids. Birefringence of lipid bilayers can be measured using dual-polarization interferometry . This provides

2880-415: The compensator to any value between 0 nm and 120 nm. Rotating the device to any axis can compensate for anterior segment birefringence in any orientation up to 120 nm in magnitude. The slow axis of R was oriented radially, and distance around R was measured from the horizontal nasal meridian by angle β. At each point, therefore, the fast axis of R was R = β + 90°. Radial variation in retardance

2960-427: The cornea. It is also used to analyze the change in the polarization of the reflected radiation. This element consists of a second synchronously rotating quarter-wave retarder and a linear polarizer in front of the photo-detector. The output is then sampled, digitized, and stored by a computer. The GDx nerve fiber analyzers measure the retinal nerve fiber layer (RNFL) thickness with a scanning laser polarimeter based on

3040-453: The crystal is positive (or negative, respectively). In the case of biaxial crystals, all three of the principal axes have different refractive indices, so this designation does not apply. But for any defined ray direction one can just as well designate the fast and slow ray polarizations. While the best known source of birefringence is the entrance of light into an anisotropic crystal, it can result in otherwise optically isotropic materials in

3120-422: The crystal. For most ray directions, both polarizations would be classified as extraordinary rays but with different effective refractive indices. Being extraordinary waves, the direction of power flow is not identical to the direction of the wave vector in either case. The two refractive indices can be determined using the index ellipsoids for given directions of the polarization. Note that for biaxial crystals

3200-424: The direction of the wave vector . This causes an additional shift in that beam, even when launched at normal incidence, as is popularly observed using a crystal of calcite as photographed above. Rotating the calcite crystal will cause one of the two images, that of the extraordinary ray, to rotate slightly around that of the ordinary ray, which remains fixed. When the light propagates either along or orthogonal to

3280-446: The direction of the optic axis, thus called the extraordinary ray ) is the slow ray in given scenario. Using a thin slab of that material at normal incidence, one would implement a waveplate . In this case, there is essentially no spatial separation between the polarizations, the phase of the wave in the parallel polarization (the slow ray) will be retarded with respect to the perpendicular polarization. These directions are thus known as

GDX - Misplaced Pages Continue

3360-509: The direction of what we call the optic axis. Thus the optic axis has the particular property that rays in that direction do not exhibit birefringence, with all polarizations in such a beam experiencing the same index of refraction. It is very different when the three principal refractive indices are all different; then an incoming ray in any of those principal directions will still encounter two different refractive indices. But it turns out that there are two special directions (at an angle to all of

3440-517: The divergence of D vanishes: We can apply the vector identity ∇ × (∇ × A ) = ∇(∇ ⋅ A ) − ∇ A to the left hand side of eq. 3a , and use the spatial dependence in which each differentiation in x (for instance) results in multiplication by ik x to find: The right hand side of eq. 3a can be expressed in terms of E through application of the permittivity tensor ε and noting that differentiation in time results in multiplication by − iω , eq. 3a then becomes: Applying

3520-556: The first polarizer, but above the specimen is a polarizer (a so-called analyzer ) oriented in the y direction. Therefore, no light from the source will be accepted by the analyzer, and the field will appear dark. Areas of the sample possessing birefringence will generally couple some of the x -polarized light into the y polarization; these areas will then appear bright against the dark background. Modifications to this basic principle can differentiate between positive and negative birefringence. For instance, needle aspiration of fluid from

3600-535: The folds and, when observed under polarized light, causes birefringence. In ophthalmology , binocular retinal birefringence screening of the Henle fibers (photoreceptor axons that go radially outward from the fovea) provides a reliable detection of strabismus and possibly also of anisometropic amblyopia . In healthy subjects, the maximum retardation induced by the Henle fiber layer is approximately 22 degrees at 840 nm. Furthermore, scanning laser polarimetry uses

3680-427: The fundus, building a monochromatic image. The state of polarisation of the light is changed (retardation) as it passes through birefringent tissue: cornea and RNFL. Corneal birefringence is eliminated (in part) by a proprietary 'corneal compensator'. The amount of retardation of light reflected from the fundus is converted to RFNL thickness. Sub-optimal compensation of corneal birefringence is currently being addressed by

3760-415: The image from light of either polarization, simply a relative phase shift between the two light waves. Much of the work involving polarization preceded the understanding of light as a transverse electromagnetic wave , and this has affected some terminology in use. Isotropic materials have symmetry in all directions and the refractive index is the same for any polarization direction. An anisotropic material

3840-436: The image is absence of red and yellow colours. Superiorly and inferiorly more uniform blue appearance. Picture indicates that the eye is at the advance stage of the disease. The deviation map reveals the location and magnitude of RNFL thinning relative to a normal value. This normal value was generated as an average of people from various cultures. Defects are colour-coded based on probability of normality (e.g. yellow means that

3920-426: The incoming face, the direction of both rays will be restored, but leaving a shift between the two beams. This is commonly observed using a piece of calcite cut along its natural cleavage, placed above a paper with writing, as in the above photographs. On the contrary, waveplates specifically have their optic axis along the surface of the plate, so that with (approximately) normal incidence there will be no shift in

4000-540: The index ellipsoid will not be an ellipsoid of revolution (" spheroid ") but is described by three unequal principle refractive indices n α , n β and n γ . Thus there is no axis around which a rotation leaves the optical properties invariant (as there is with uniaxial crystals whose index ellipsoid is a spheroid). Although there is no axis of symmetry, there are two optical axes or binormals which are defined as directions along which light may propagate without birefringence, i.e., directions along which

4080-399: The link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=GDX&oldid=932839128 " Category : Disambiguation pages Hidden categories: Short description is different from Wikidata All article disambiguation pages All disambiguation pages GDx-VCC However a Dutch study found that while there is

SECTION 50

#1732851543332

4160-493: The manufacturer with hardware and software modifications. The GDx scanning laser measures the thickness of the retinal nerve fiber layer, which is the very first part of your eye that is damaged by glaucoma. Before we go any further, let us describe the basic GDx instrument. This instrument use a GaAIAs diode laser as a source of light. This diode will emit polarized light. The source is HeNe (632.8 nm) and argon (514 nm). A polarization modulator in this instrument changes

4240-418: The only form-birefringent structure in the eye. Anterior segment structures, such as the cornea, also phase-shift polarized light. So the latest instrument includes a compensating device or compensating corneal which is designed to remove the portion of the signal generated by the anterior segment. This device consists of two optical retarders, which when rotated relative to each other, allow the operator to set

4320-498: The optic axis). In addition, a distinct form of double refraction occurs, even with normal incidence, in cases where the optic axis is not along the refracting surface (nor exactly normal to it); in this case, the dielectric polarization of the birefringent material is not exactly in the direction of the wave's electric field for the extraordinary ray. The direction of power flow (given by the Poynting vector ) for this inhomogenous wave

4400-448: The optic axis, such a lateral shift does not occur. In the first case, both polarizations are perpendicular to the optic axis and see the same effective refractive index, so there is no extraordinary ray. In the second case the extraordinary ray propagates at a different phase velocity (corresponding to n e ) but still has the power flow in the direction of the wave vector . A crystal with its optic axis in this orientation, parallel to

4480-475: The optical surface, may be used to create a waveplate , in which there is no distortion of the image but an intentional modification of the state of polarization of the incident wave. For instance, a quarter-wave plate is commonly used to create circular polarization from a linearly polarized source. The case of so-called biaxial crystals is substantially more complex. These are characterized by three refractive indices corresponding to three principal axes of

4560-424: The optics disc and ratios of the thickest points either superiorly or inferiorly to the temporal or nasal regions. The field of view is 15 degree and imaging should be performed through undilated pupil. The polarized laser scans the fundus and building a monochromatic image. The state of polarization of the light is change (retardation) as it passes through birefringent tissue (cornea and RNFL). Corneal birefringent

4640-414: The ordinary ray is simply described by n o as if there were no birefringence involved. The extraordinary ray, as its name suggests, propagates unlike any wave in an isotropic optical material. Its refraction (and reflection) at a surface can be understood using the effective refractive index (a value in between n o and n e ). Its power flow (given by the Poynting vector ) is not exactly in

4720-412: The phenomenon in terms of polarization, understanding light as a wave with field components in transverse polarization (perpendicular to the direction of the wave vector). A mathematical description of wave propagation in a birefringent medium is presented below . Following is a qualitative explanation of the phenomenon. The simplest type of birefringence is described as uniaxial , meaning that there

4800-420: The plastic is molded or extruded. For example, ordinary cellophane is birefringent. Polarizers are routinely used to detect stress, either applied or frozen-in, in plastics such as polystyrene and polycarbonate . Cotton fiber is birefringent because of high levels of cellulosic material in the fibre's secondary cell wall which is directionally aligned with the cotton fibers. Polarized light microscopy

4880-494: The polarization (circular birefringence) of linearly polarized light as viewed through a sheet polarizer at the screen's surface. Similarly, light modulators modulate the intensity of light through electrically induced birefringence of polarized light followed by a polarizer. The Lyot filter is a specialized narrowband spectral filter employing the wavelength dependence of birefringence. Waveplates are thin birefringent sheets widely used in certain optical equipment for modifying

SECTION 60

#1732851543332

4960-460: The polarization component normal to the optic axis (ordinary ray) and the other linear polarization (extraordinary ray) will be refracted toward somewhat different paths. Natural light, so-called unpolarized light , consists of equal amounts of energy in any two orthogonal polarizations. Even linearly polarized light has some energy in both polarizations, unless aligned along one of the two axes of birefringence. According to Snell's law of refraction,

5040-511: The polarization of light passing through the material. These measurements are known as polarimetry . Polarized light microscopes, which contain two polarizers that are at 90° to each other on either side of the sample, are used to visualize birefringence, since light that has not been affected by birefringence remains in a polarization that is totally rejected by the second polarizer ("analyzer"). The addition of quarter-wave plates permits examination using circularly polarized light. Determination of

5120-464: The polarization of the ordinary ray and the other the polarization of the extraordinary ray. The ordinary ray will always experience a refractive index of n o , whereas the refractive index of the extraordinary ray will be in between n o and n e , depending on the ray direction as described by the index ellipsoid . The magnitude of the difference is quantified by the birefringence The propagation (as well as reflection coefficient ) of

5200-549: The polarization state of light passing through it. To manufacture polarizers with high transmittance, birefringent crystals are used in devices such as the Glan–Thompson prism , Glan–Taylor prism and other variants. Layered birefringent polymer sheets can also be used for this purpose. Birefringence also plays an important role in second-harmonic generation and other nonlinear optical processes . The crystals used for these purposes are almost always birefringent. By adjusting

5280-401: The polarization states of the laser output. The linearly polarized beam from the laser then passes through a rotating quarter-wave retarder. A scanning unit in this instrument is used to move the beam horizontally and vertically on the retina. The focused beam is 35μm in diameter. This instrument also has a polarization detector. It is used to detect polarized light that is reflected back from

5360-529: The probability is below 5% of that RNFL at that location is normal). A healthy eye has a clear deviation map. A further representation is the TSNIT graph. TSNIT is stand for Temporal – Superior – Nasal – Inferior-Temporal. This graph displays the thickness values along the Calculation Circle from T to S, N and back to T. The area of normal values is shaded. Measurements for the left eye are labeled "OS", those for

5440-409: The refracting surface. As explained above, the other polarization can deviate from normal incidence, which cannot be described using the law of refraction. This thus became known as the extraordinary ray . The terms "ordinary" and "extraordinary" are still applied to the polarization components perpendicular to and not perpendicular to the optic axis respectively, even in cases where no double refraction

5520-462: The retardation (i.e., the change in polarization) is proportional to the RNFL thickness. In this instrument, there are four retarders in the measurement beam's path: 1. The first two linear retarders have equal retardance and form a VCC. 2. The third linear retarder is the combination of cornea and lens—the anterior segment 3. The fourth linear retarder, with radially distributed axes, is the retinal birefringent structure (RE; either peripapillary RNFL or

5600-416: The reversal in direction. Each optical component in the model experienced a double pass of the measuring beam. Birefringent is relared or characterized as a double refraction. In this picture we can see calcite crystal laid upon a paper with some letters showing the double refraction. Clinical Interpretation based on results from GDx Nerve Fiber Analyzer from Carl Zeiss Meditec. Firstly, this instrument

5680-499: The right eye "OD". A defect is indicated if a measured value falls below the shaded area. A comprehensive database is essential for accurate glaucoma detection. In this instrument a database from 540 normal eyes is used. The subjects are multi-ethnic and 18–82 years old. The database also contains 262 glaucomatous eyes used by the NFI to discriminate between normal and glaucoma. Birefringent Birefringence means double refraction. It

5760-451: The selection of spermatozoa for intracytoplasmic sperm injection . Likewise, zona imaging uses birefringence on oocytes to select the ones with highest chances of successful pregnancy. Birefringence of particles biopsied from pulmonary nodules indicates silicosis . Dermatologists use dermatoscopes to view skin lesions. Dermoscopes use polarized light, allowing the user to view crystalline structures corresponding to dermal collagen in

5840-403: The skin. These structures may appear as shiny white lines or rosette shapes and are only visible under polarized dermoscopy . Isotropic solids do not exhibit birefringence. When they are under mechanical stress , birefringence results. The stress can be applied externally or is "frozen in" after a birefringent plastic ware is cooled after it is manufactured using injection molding . When such

5920-401: The slow axis and fast axis of the waveplate. Uniaxial birefringence is classified as positive when the extraordinary index of refraction n e is greater than the ordinary index n o . Negative birefringence means that Δ n = n e − n o is less than zero. In other words, the polarization of the fast (or slow) wave is perpendicular to the optic axis when the birefringence of

6000-455: The solid Earth (the Earth's liquid core does not support shear waves) is widely used in seismology . Birefringence is widely used in mineralogy to identify rocks, minerals, and gemstones. In an isotropic medium (including free space) the so-called electric displacement ( D ) is just proportional to the electric field ( E ) according to D = ɛ E where the material's permittivity ε

6080-411: The top of this page, with the optic axis along the surface (and perpendicular to the plane of incidence ), so that the angle of refraction is different for the p polarization (the "ordinary ray" in this case, having its electric vector perpendicular to the optic axis) and the s polarization (the "extraordinary ray" in this case, whose electric field polarization includes a component in the direction of

6160-454: The two angles of refraction are governed by the effective refractive index of each of these two polarizations. This is clearly seen, for instance, in the Wollaston prism which separates incoming light into two linear polarizations using prisms composed of a birefringent material such as calcite . The different angles of refraction for the two polarization components are shown in the figure at

6240-415: The wavelength is independent of polarization. For this reason, birefringent materials with three distinct refractive indices are called biaxial . Additionally, there are two distinct axes known as optical ray axes or biradials along which the group velocity of the light is independent of polarization. When an arbitrary beam of light strikes the surface of a birefringent material at non-normal incidence,

6320-415: Was not analyzed. The measuring beam was reflected at a deeper layer and traveled back through the three retarders to the ellipsometer. Reflection from the ocular fundus exhibits a high degree of polarization preservation, and the reflector in the model (polarization-preserving reflector [PPR]) was assumed to preserve completely the polarization state of the incident beam, except for a 180° phase change due to

6400-501: Was recently applied in the living human retina to quantify the polarization properties of vessel walls near the optic nerve. While retinal vessel walls become thicker and less birefringent in patients who suffer from hypertension, hinting at a decrease in vessel wall condition, the vessel walls of diabetic patients do not experience a change in thickness, but do see an increase in birefringence, presumably due to fibrosis or inflammation. Birefringence characteristics in sperm heads allow

#331668