If Two Samples of a Liquid Give the Same Refractive Index Reading Are They the Same

ALCOHOL | Properties and Determination

A.T. Bakalinsky , M.H. Penner , in Encyclopedia of Food Sciences and Diet (Second Edition), 2003

Refractive Index

The refractive index (RI) of a medium is dependent on its chemical limerick, since under controlled conditions composition dictates its electric and magnetic properties. The RI of a sample is divers as the ratio of the speed of lite in a vacuum to its speed in the sample medium. Consequently, RI values are always greater than i. The RI of aqueous ethanol solutions can be measured directly to indicate ethanol concentration. When interfering compounds are present, the RI of a distillate can exist measured. Since the RI of a liquid is temperature-sensitive, measurements taken at nonstandard temperatures must be corrected by use of conversion tables.

The AOAC has approved RI-based methods for determining ethanol in beers and wines. For wines, the measurement is made on the distillate. Measured RI values for beer are used in conjunction with specific gravity measurements to calculate ethanol content.

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FORENSIC SCIENCES | Drinking glass

J.A. Lambert , in Encyclopedia of Belittling Science (2nd Edition), 2005

Refractive Alphabetize

Refractive alphabetize is the most useful physical belongings in forensic glass characterization. It can be measured to a high caste of accuracy and precision, which is essentially independent of fragment size. The RI of a glass depends upon the combination of raw materials used, together with the nature of the manufacturing process.

The RI of glasses examined in forensic scientific discipline laboratories generally falls in the narrow range of 1.51–1.54, with the exception of borosilicate drinking glass from motor vehicle headlamp lenses, which typically has an RI of 1.475–i.480. As RI measurement tin can discriminate glasses with RI differences of 0.0001, glasses tin can be divided into a large number of groups by this technique.

The inherent variation in RI within the bulk of a single sheet of glass is of the social club of 0.00005–0.0001, so greater accuracy and precision of measurement is unlikely to increment the discriminating ability of this technique.

Electric current techniques of RI measurement are based on the fact that the RI of a liquid changes with temperature at a much greater rate than that of a solid. Typical RI/temperature coefficients are four×ten^−iv  C⁻¹ for a liquid, and 1×10⁻⁶  C⁻¹ for a glass.

A fragment of glass is immersed in a colorless oil and placed on a hotstage that is programmed to change temperature with time at a linear charge per unit. The oil is calibrated by glass standards of known RI. This enables the RI of the oil to be calculated for whatever particular hotstage temperature.

The glass fragment is viewed by transmitted calorie-free through a stage contrast microscope. Monochromatic light from a sodium lamp or filtered light of a narrow range of wavelength produces a bright line around the edge of the fragment. The temperature of the hotstage is varied until the oil has the aforementioned RI as the glass. At this point, known as the lucifer point temperature, the brilliant line around the fragment disappears. The lucifer indicate of a glass fragment tin be detected automatically, although the image of the fragment is displayed on a screen throughout the measuring process.

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Handbook of Pharmaceutical Assay by HPLC

Michael Westward. Dong , in Separation Science and Technology, 2005

B Refractive Alphabetize (Rl)

An RI detector measures the deviation in RI betwixt the sample jail cell containing the eluting analyte and the reference cell (containing pure eluent). It offers universal detection only has lower sensitivity (0.01–0.ane μg) than UV/Vis absorbance detection and is more prone to temperature and flow changes. RI detection is used for components of low chromophoric activities such as sugars, triglycerides, organic acids, pharmaceutical excipients and polymers. It is the standard detector for polymer characterization in gel permeation chromatography (GPC). ²⁵ Modern RI detectors are mostly differential deflection type with a broad RI range of one.00–one.75 RIU (refractive index unit). They have thermostated menstruum-cell assemblies and allow unattended operation via auto-purging the reference flow jail cell. Their sensitivity, baseline stability and reliability have improved significantly in recent years. Their major disadvantages are their low sensitivity and their incompatibility with gradient elution.

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Optofluidics

Y. Sunday , X. Fan , in Applications of Nanoscience in Photomedicine, 2015

viii.3.4 Refractive index detection

Refractive index detection is also chosen label-gratis sensing. It relies on the excess polarizability of the target analyte and thus does not need external labeling. Starting time, the capture molecules are immobilized on a solid surface. The binding of the target molecules results in a refractive index change well-nigh the surface, so generating a sensing point. Different from the aforementioned detection methods, refractive alphabetize change depends on the analyte concentration or surface density instead of the total mass. Therefore, this method is peculiarly attractive to detect analytes in micro-fluidic or nano-fluidic channels where full analyte quantity is limited. Effigy 8.5 illustrates the detection principle of characterization-gratuitous sensing.

Figure 8.5. (a) Principle of label-free detection. Binding of analytes to the substrate changes the local refractive alphabetize (or polarizability) near the surface, thus generating the sensing signal. (b) Case of the characterization-costless sensing signal, which starts to increase upon initiation of analyte binding and saturates when equilibrium is reached between the analytes in solution and the capture molecules on the surface

A plethora of photonic structures can be used for refractive index detection, including plasmonics, ring resonators, and different kinds of interferometers (Mach-Zehnder, Immature, etc.) [6]. The detection limit of a label-complimentary sensor for the refractive index alter is ameliorate than 10^{−   seven} refractive alphabetize units and for surface density change it is   <   1   pg/mm². Recently, unmarried particle detection of a few tens of nanometers in diameter has been achieved using ring resonators [26,27]. Detection of single molecules has also been demonstrated using plasmonic nanostructures [28,29] and hybrid plasmonic–photonic devices [xxx,31].

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Refractive Index Measurements

A.F. Rawle , in Encyclopedia of Spectroscopy and Spectrometry (Third Edition), 2017

Introduction and Background

At that place are no unique issues, just unique solutions. The stop application dictates the reason or reasons for refractive alphabetize (RI) measurement. Common applications cover a wide range from mineral identification in geology to sucrose decision in soft drinks. The awarding besides dictates whether just the existent function of the RI is required or whether both the real and imaginary (absorptive) parts will demand determining, equally there are two parts of the RI:

$\mathrm{Refractive}\mathrm{index}\left(\mathrm{R}\mathrm{I}\right)=\mathrm{due\; north}–\mathrm{i}k$

The real part (n) of the RI indicates the change in velocity in the wave front as light traverses a heterogeneity or boundary. The absorption (or attenuation) of low-cal through a medium is indicated by k and modeled past the so-called imaginary component (use of the foursquare root of −   i). We employ the convention of the negative sign in the previously mentioned equation to signify that the absorptive loss is positive because $\mathrm{i}=\sqrt{-1}$ and, thus, –igrand is positive. An alternative, only distinct convention, is to have a positive sign in the earlier-mentioned equation. The Kramers–Kronig relation (equivalent to the Hilbert transformation) mathematically connects the real and imaginary parts of any circuitous office, and thus, in that location is a relation between the real, n, and imaginary, k, complex parts of the RI.

RI is indirectly linked to a number of other fundamental concrete properties, probably the almost important being every bit follows:

•

Relative permittivity (in the limiting and simplest example, at long wavelength, $n=\sqrt{{\epsilon }_{\mathrm{r}}}$ )

The relative permittivity, ε _r, of a material is the degree by which the electric field between the charges is decreased relative to vacuum as the permittivity expresses the strength between ii indicate charges inside the material.

•

Density

The RI is related to the number of molecules per unit book and hence to the density. In initial theoretical studies (Newton and Laplace), 1000 ₁⋅ ρ  =   (due north ²  −   i) where K ₁ is a constant and ρ the density of the textile. Later, the simpler Gladstone–Dale human relationship found favor peculiarly among geology circles:

$\mathrm{northward}=1+K_2\cdot \rho$

where K _two is another constant of proportionality.

This simple human relationship allows the prediction of RI when the density of the material is known. There is extensive literature on this simple linkage.

•

Concentration

Generally speaking, every bit the concentration of a solution is increased, then there is a corresponding increase in the RI and the specific gravity. This property is one that is extensively utilized in simple, handheld, refractometers. Such refractometers can be calibrated and graduated to indicate, for case, % sugar directly rather than the RI and often use a wide variety of semi-empirical and non-SI units such as Brix, which is actually a specific gravity human relationship: ane°Brix is a 1% by weight sucrose solution.

The earlier-mentioned physical properties may also determine the technique and technology for measurement.

RI is dependent on the wavelength of incident light with both n and k typically increasing with decreasing wavelength, λ. This is commonly called dispersion. A number of empirical relationships (e.thou., the Cauchy and Sellmeier relationships) can be used to interpolate or predict RI values for differing wavelengths. RI values <ane are possible for a number of materials (metals and plasmas) close to the resonance frequencies every bit the RI reflects the phase velocity (crest of the wave) rather than the actual velocity. In that location is also a temperature and force per unit area dependence of RI for a fabric, simply commonly, this is minor in relation to wavelength changes. The most common wavelength for measurement utilizes the sodium D (Na_D) line, which is actually a doublet at ~   589   nm (orange), but it is common to measure and specify a number of different wavelengths.

In certain cases, RI can exist determined to five or more than decimal places, and in other cases, one or ii decimal places may be more than acceptable for the requirements. The degree of precision required for the measurement needs to exist specified in advance in conjunction with the end application.

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MICROSCOPY TECHNIQUES | Low-cal Microscopy

C. Hammond , P.J. Evennett , in Encyclopedia of Analytical Science (2d Edition), 2005

Refractive Index Measurement

The refractive indices of glasses, minerals, plastics, and many other samples may be measured by a range of microscopical methods. The simplest of these is the exam of samples mounted in a range of liquids of which the refractive index is known until the sample–liquid interface becomes invisible. A development of this method involves the apply of a microscope hot phase to heighten the temperature of the liquid and sample gradually. Considering in full general the refractive index of the sample changes more slowly than that of the liquid equally the temperature rises, a specific 'lucifer temperature' can be determined. Either of these methods can be aided by ascertainment of the Becke line, a vivid fringe that appears only within or outside the boundary of the fragment when it is well-nigh in focus. The Becke line appears to motion toward the medium of higher refractive index when the distance betwixt specimen and objective is increased, thus indicating whether the mounting liquid has a higher or lower refractive alphabetize than the fragment.

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Sucrose: Backdrop and Conclusion

M. Peris , in Encyclopedia of Nutrient and Wellness, 2016

Refractometry

Refractive index of a solution depends on the concentration of solute. Therefore, it can exist used to decide the concentration of sucrose solutions and the corresponding instruments (refractometers) may be calibrated with refractive index or directly with sugar concentration scales. For example, they are oftentimes calibrated in Brix, a parameter that is numerically equivalent to %sucrose westward/due west; in this example, these instruments are utilized only with sucrose solutions. When only refractive index scales are available, tables are required to obtain sucrose concentration. Unlike older instruments, modernistic refractometers are electronically controlled and (theoretically) free from operator fault. Measurements are normally made at 20   °C using the sodium D line as the light source. Sucrose is most oftentimes utilized equally the calibration medium for the instruments and the readings expressed every bit %saccharide w/w 'as sucrose' in the food. Obviously, such instruments are only accurate for pure sucrose solutions, although they are also widely used for foods containing other sugars such every bit glucose syrups or invert saccharide, since the reading obtained may be corrected if required; ordinarily, the reading is used equally a reference value to the sugar content (total soluble solids) of the solution. On the other hand, refractive index is heavily influenced by changes of temperature and wavelength of the calorie-free source; thus, control is necessary. Nevertheless, this drawback has been overcome by modern instruments, since their reading is temperature-compensated.

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Polymers for Advanced Functional Materials

J. Grothe , ... A. Leuteritz , in Polymer Science: A Comprehensive Reference, 2012

8.08.4.7 High and Low Refractive Alphabetize in Polymer Nanocomposites

The refractive index is one of the important optical backdrop in focus for nanocomposite inquiry now. Most of the conventional polymers testify refractive indices between one.3 and 1.seven; only few polymers exhibit college refractive indices such as polythiophene with due north  =   2.12. ²²⁸ With the electric current evolution of optoelectronic applications the need to conform the refractive index of polymeric materials is higher than ever earlier. Potential applications range from creating novel composite lenses for charged coupled devices, optical filters, or reflectors to optical waveguides, optical adhesives or encapsulants, antireflection films, or integration in improved efficiency solar cells.

Advances in macromolecular chemical science have disclosed new optical polymers with highest refractive index of 1.76 ²²⁹ by introduction of substituents with high tooth refractions and low tooth volumes. Sulfur-containing groups, effluvious rings, or halogen atoms (except fluorine) may be mentioned in this context to exist capable of highering the refractive alphabetize of polymers significantly. ²³⁰ However, the potential of organic polymers for ultrahigh refractive indices is limited. The integration of loftier refractive index inorganic nanoparticles in organic polymers enables higher due north-values and retaining excellent optical transparency of the composite materials.

The commencement inquiry work on high refractive index nanocomposites was done in the 1990s. ²³¹

Co-ordinate to Rayleigh constabulary the intensity loss of transmitting light due to scattering is proportional to the size of the nanoparticles and to the departure of refractive indexes of the inorganic particles and of the polymeric matrix. Therefore the grooming of transparent loftier refractive alphabetize composites requires small particle diameters far beneath the wavelength of visible light (<   50   nm) and excellent dispersion of nanoparticles within the polymeric matrix.

Withal, working with nanoparticles of that dimension, quantum size furnishings take to exist considered. Amongst other optical properties, such as the colour dependence of semiconductor nanoparticles on their size, absorption coefficient and refractive index bear witness remarkable quantization furnishings. Co-ordinate to Schmitt-Rink et al. the maximum crystallite size for strong quantum solitude is dependent on the cloth and tin be calculated. ²³² For PbS the size dependency was investigated experimentally on PbS/polymer nanocomposite materials by Suter et al. While PbS nanoparticles as big as 25   nm evidence refractive indices close to that of bulk PbS, smaller particles suffer from quantization effects revealing chop-chop decreasing refractive indexes with decreasing particle diameters confirming the theoretical estimation very well. ²³³

For the grooming of nanocomposite materials with extreme refractive indexes (>two) but also depression refractive indexes miscellaneous inorganic materials accept come into consideration. While some of the considered materials such as Os, InP, or Si exhibit high refractive indexes at singled-out wavelengths, merely undergo remarkable fluctuations over a broad wavelength range, PbS with very loftier refractive alphabetize around four and higher over the whole visible range is a very interesting candidate for many nanocomposite materials. Several nanocomposite materials containing PbS nanoparticles have been prepared. Materials with refractive indices ranging from 1.57 to 2.06 ²³⁴ take been realized by Yang et al. when preparing PbS/polythiourethane (PTU) nanocomposite films via moving-picture show casting technique.

Investigating the dependence of refractive index of the nanocomposite materials on the particle content, Zimmermann et al. have establish a linear relationship between the refractive index and the volume fraction of the particles. The human relationship is given by eqn [2]:

[2] ${\mathrm{northward}}_{\text{comp}}={\Phi }_{\text{p}}{n}_{\text{p}}+{\Phi }_{\text{org}}{n}_{\text{org}}$

with the refractive indices n _comp of the composite material, n _p of the inorganic particles, and northward _org of the organic matrix and the book fractions Φ_p of the particles and Φ_org of the organic matrix. ²³⁵ Co-ordinate to eqn [2] the necessary volume fraction of inorganic filler to obtain a certain refractive index can be calculated. For preparation of ultrahigh refractive index materials based on gelatin with PbS, for instance, the volume fraction of particles needs to exist around 0.4 and higher corresponding to a weight fraction of at to the lowest degree 0.eight (using the density of PbS with 7.fifty   thou   cm⁻³ and of gelatine with 1.35   g   cm⁻³). ²³⁶ Considering the necessity of high volume fractions Weibel et al. used an in situ particle generation technique combined with subsequent pic casting, pressing, and boring drying to realize PbS/polyethyleneoxide (PEO) nanocomposites with PbS content of 90% w/west (or ∼50% five/5). The pressed composite films exhibit extremely loftier refractive indices around iii over a broad wavelength range, college than any other polymer blended material. ²³¹

Considering the toxicity of Atomic number 82-containing compounds, nontoxic alternatives have been investigated as well. ZnS with majority refractive index of two.three was used as loftier refractive alphabetize filler material. Lü et al. accept integrated ZnS nanoparticles in poly(urethane-methacrylate macromer) (PUMM) nanocomposite films up to 86   wt.% of ZnS enhancing the refractive alphabetize from one.645 to ane.796 at 632.8   nm. ²³⁷ Afterwards on they developed a novel route to integrate ZnS in bulk poly(N,N-dimethylacrylamide) (PDMAA) highering the refractive index from 1.54 for the polymer matrix to 1.63 for nanocomposite cloth containing 50   wt.% of ZnS. ²³⁸

Since the refractive index of the composite materials is proportional to the book fraction, the high densities of PbS and ZnS bulk materials require extremely high weight fractions of the particles to significantly increment the refractive index.

Comparably loftier volume fractions of crystalline silicon particles can be dispersed at lower weight fractions due to the low density of the material. By dispersing silicone nanoparticles in gelatine polymer solutions and subsequent spin coating of thin films Papadimitrakopoulos et al. have benefitted from the depression density and obtained nanocomposite materials with refractive indices upward to 3.2 at comparably low weight fractions of l%. ²³⁹

During the last years promising results have been achieved using titania particles as nanofillers. Polyimides with intrinsic high refractive indices were used as matrix polymers. ^240–242 Some of the research work combined the integration of moieties with high molar refractions to the polymer, such as sulfur-containing groups, with the addition of nanoparticles, obtaining loftier refractive alphabetize polymers with excellent transparency, high thermal stability, and low birefringence. Standard lithography processes allow for the fabrication of microlenses. ²⁴³ Such microlenses are eligible candidates for the awarding in CMOS image sensors (CIS). Low costs, low power consumption, and pocket-size size are the benefits of CIS compared to CCD sensors. However, their sensitivity is too depression for high-quality images. With high refractive alphabetize microlenses the incident light may be focused and the sensitivity of the sensors tin can exist increased (see Figure 20 ).

Figure 20. Schematic design of a CMOS image sensor with high refractive index microlens.

A further important application mentioned in a higher place is the utilize of high refractive alphabetize polymers as encapsulant for LEDs. Since LEDs are based on high refractive index semiconductors as emitters, at that place is a huge contrast in refractive index at the semiconductor–air interface and most of the emitted low-cal is trapped within the LED. The light extraction efficiency tin be enhanced by encapsulants with refractive indices between that of the emitting semiconductors (typically due north  =   two.five–3.v) and air. Mont et al. have integrated surface-modified TiO_ii nanoparticles in epoxy. Via spin coating on silicon substrates they received films with increasing refractive index depending on loading gene. ²⁴⁴ Fresnel reflection and scattering losses could be minimized by fabricating multiple encapsulant layers with decreasing refractive index values (see Effigy 21 ).

Figure 21. Multiple LED encapsulant layers with decreasing refractive indices.

Compared to loftier refractive index materials the research on depression refractive index materials is less prevalent, merely low refractive index composites are also a matter of interest in order to broaden the spectrum of attainable polymer materials and to achieve graded refractive index materials, for example, in antireflection coatings. ²⁴⁵

In an coordinating way to high refractive index materials, depression refractive index composite materials have also been prepared. Every bit suitable filler materials, metals with low refractive indices beneath 1, such as gilt, were chosen. Zimmermann et al. prepared golden/gelatine nanocomposite films with gilt content up to ∼xc   wt.%, lowering the refractive index of the materials downward to about 1, offer the everyman refractive indices known for polymer composite materials. ²³⁵

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Negative Refractive Alphabetize

W. Park , in Encyclopedia of Materials: Science and Technology, 2010

Introduction

Refractive index is a primal constant that describes the interaction between light and material. It specifies, for example, how fast light travels in a textile or how much light is reflected at an interface. Refractive index is a useful constant equally it is direct related to measurable quantities such as reflectance and absorption. At a more fundamental level, nonetheless, a material's response to external electric and magnetic field, E and H, is specified by permittivity, ɛ, and permeability, μ, respectively. The permittivity and permeability are defined by the constitutive relations, D=ɛɛ _o E and B=μμ _o H, where ɛ _o and μ _o are the permittivity and permeability of vacuum. Amalgam the wave equation from Maxwell's equations then introduces a refractive index, n, as a quantity that specifies the wave velocity in the fabric, resulting in the well-known relation, northward ²=ɛμ.

About natural materials take values of ɛ and μ greater than 1. Therefore, it is mostly causeless that the alphabetize of refraction is institute by taking the positive root of ɛμ, $n=\sqrt{\varepsilon \mu }$ . The permittivity and permeability can, however, go negative. For case, due to the gratis electron response, about metals exhibit negative permittivity at frequencies below their plasma frequency. In additon, when a material exhibits strong electric or magnetic resonance, the permittivity or permeability can get negative at frequencies just higher up the resonance frequency. In natural materials, electric resonances generally occur at much higher frequencies than magnetic resonances. Therefore, even near resonances, a cloth typically has but one of the permittivity and permeability negative. In such cases, the refractive alphabetize becomes imaginary, resulting in exponentially decaying waves instead of propagating waves. The cloth therefore becomes highly reflecting in this frequency region.

In 1968, Veselago considered a hypothetical material with simultaneously negative ɛ and μ. In this case, the refractive index is real, so that the material should support propagating waves. However, the Maxwell equations show that the wave vector, thousand, and the electric and magnetic field vectors, Due east and H, would then form a left-handed set up. Consequently, the Poynting vector, South=E×H, is anti-parallel to the wave vector, k. Thus, if we choose the direction of energy flow, South, as the reference propagation direction, the refractive index must be negative so that the wave vector 1000 is in the contrary direction. It was after shown that simultaneously negative ɛ and μ are not the necessary status for negative index. A more than full general condition for negative alphabetize is ɛ′|μ|+μ′|ɛ|<0 where the prime indicates the real part. Even so, simultaneously negative permittivity and permeability are however preferred because they tend to provide lower loss.

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Chemistry/Trace/Glass

T. Hicks , in Encyclopedia of Forensic Sciences (Second Edition), 2013

Refractive index comparison

Refractive index comparison is the most common examination performed in casework: it provides a loftier degree of discrimination; the method tin be performed on very small-scale fragments, and is quick and relatively inexpensive. The European and North American scientific working groups specialized in forensic glass evidence have published guidelines on the techniques that allow the measurement of the refractive index of glass. When light passes through from one medium to the other, its velocity changes as well every bit its direction (i.due east., it is refracted). The refractive alphabetize tin either be described as the change of the wave'southward velocity in a vacuum to the moving ridge's velocity in a transparent medium (e.g., glass) or the ratio of the sine of the incident angle to the sine of the bending of refraction (see eqn. [1]). The refractive index of a material will vary with wavelength and temperature: the standard wavelength is the sodium D line (589   nm) and the standard temperature 25   °C:

[1] $\begin{array}{l}\text{RI}=\frac{sin{\theta }_I}{sin{\theta }_R}\\ \text{RI}=\frac{5_{\text{Vacum}}}{V_{\text{Drinking glass}}}\end{array}$

In 1892, Friedrich Johann Karl Becke, an Austrian mineralogist, reported the observation of a bright line inside the edge of a mineral that had a refractive index unlike from its surround. By altering the RI of the medium with the addition of a miscible liquid with a different RI, it is possible to reach a point where this line is no longer visible, the refractive index of the two media being similar. With a refractometer, the RI of the oil can be measured and the RI of the drinking glass is determined. This beginning method is time consuming and not very efficient. However, it is useful to recall that the Becke line volition motion to the medium with the highest RI when the altitude from disquisitional focus is increased.

Emmons double variation method consists in varying both the temperature and the wavelength of the calorie-free (iii wavelengths are used: the sodium D line (589   nm), and the hydrogen C and F lines (656   nm and 486   nm)). This technique was accepted as an official method by the Association of Belittling Chemists (Method 973.65). Instrumentation includes a phase dissimilarity microscope, a monochromator, and a controlled hot stage.

In the mid-1980s, Foster and Freeman in collaboration with forensic scientists developed a semiautomatic instrument that was called 'glass refractive index measurement' or GRIM. The glass fragment is immersed in silicon oil and observed using a phase contrast microscope at a fixed wavelength of about 589   nm (a narrow band filter is used). A hot stage allows the variation of the temperature of the microscope slide. Several studies have shown that the instrument is precise, authentic, and has long-term stability. Information technology is a very popular instrument and, until recently, information technology was the merely one on the market: now a few other companies offering like equipment (e.yard., Lucia Forensics, Craic Technologies).

The quality of the measurements can be monitored through the edge tracings and the edge counts (that depend on the contrast of the Becke line). Edge count ranges from 1 to 99, loftier scores indicating high contrast. The dissimilarity of the Becke line will depend on the edge morphology of the fragment and the instrumentation. Debris contamination nowadays on the recovered fragment will also influence the quality of measurements: big fragments will be cleaned, but smaller fragments (e.g., 200   μm in size) typically will not, as there would exist a high risk of losing the fragment. Therefore, control fragments (that are larger and freshly broken) will typically bear witness big border counts and less variation than recovered fragments. An illustration from Newton and coworkers shows the aspect of a clean and a dingy fragment taken from patterned glass ( Figure one ).

Figure 1. GRIM3 images of fragments of glass from the 'dirty' sample (left) and from the 'clean' sample (right).

Reproduced from Newton AWN (2011) An investigation into the variability of the refractive index of glass: Function II – The event of droppings contamination. Forensic Science International 204: 182–185, with permission from Elsevier.

Although the GRIM transmission suggests that fragments with edge count values below x should not be used or be repeated manually, Newton and collaborators showed that the limit was around 30 (the authors do not, all the same, propose this value as a minimum edge count). Ultimately, it is the user's choice to reject or accept a measurement, as this volition depend on dissimilar factors (east.yard., the edge tracing, the Becke line aspect, the lamp, and the instrumentation). When edge counts are low, the mean of the refractive index of the glass remains stable, but the variance is larger. If one considers the added variance when comparison and assessing the glass evidence, then these measurements can still be used.

We take seen that the shape of the fragments, the presence of debris, the contrast of the Becke line, and the instrumentation volition influence refractive index measurements to some degree. The intravariation of this physical characteristic also plays a major role, and studies have been performed in club to know how refractive index varies inside the aforementioned object. Bennett and coworkers concluded that their study "showed that although there was observable variation of refractive indices inside a pane of [float] glass in that location was no evidence of systematic variation or patterning of refractive indices." The applied fallout is that in club to accept a sample that is representative of the cleaved glass object, it is best to sample as much of the control window (or object) equally feasible, taking samples from the frame and glass from the ground. It must be noted that the actual number of fragments to analyze will depend on the number of fragments recovered equally shown by Curran et al.

Refractive index surface anomalies have likewise been reported in the literature and it has been described that float surfaces (enriched in SnO_two) have a different RI than the bulk of the glass. This can be observed when a fragment shows both a surface and a piece of the majority: just parts of the fragment will and then 'disappear' as they accept unlike RI. For command fragments, it is possible to marking using dissimilar colors (e.grand., red, green) both float and antifloat surfaces. One can then sample both surfaces and the bulk. Once cleaned, their refractive alphabetize will be measured.

Newton and collaborators showed that the RI of the float surface has on average a college RI than the bulk and the antifloat surface an RI that is on boilerplate lower than the bulk. However, it does not seem that there are discrete RI surfaces. In add-on, surface fragments are difficult to measure out, and the use of surface fragment RI as a discrimination tool would be very difficult. Other types of glasses (container drinking glass, tableware, nonfloat flat drinking glass) also show surface anomalies. Again, this shows how of import it is to take a representative sample ( Effigy 2 ).

Figure ii. GRIM2® image of drinking glass fragment.

Reproduced from Newton AWN, Curran JM, Triggs CM, and Buckleton JS (2004) The consequences of potentially differing distributions of the refractive indices of glass fragments from control and recovered sources. Forensic Science International 140: 185–193, with permission from Elsevier.

Analogy of a float surface nowadays on a glass fragment observed when measuring its RI from Newton and collaborators: the region on the left side is an original surface of bladder origin (higher RI); the remaining portion of the fragment is the bulk glass (lower RI).

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