April 2005
` DATE
`PAGE 1/15
`
`......
`
`TIE-29: Refractive Index and Dispersion
`
`0. Introduction
`The most important property of optical glass is the refractive index and its dispersion
`behavior.
`This technical information gives an overview of the following topics:
`- Dispersion
`o Principal dispersion (page 2)
`o Secondary spectrum (page 3)
`o Sellmeier dispersion equation (page 4)
`Temperature dependence of refractive index (page 6)
`Influence of the fine annealing process on the refractive index and Abbe number
`(page 9)
`Tolerances (page 12)
`-
`- Refractive index measurement (page 13)
`
`-
`-
`
`1. Refractive Index
`If light enters a non-absorbing homogeneous materials reflection and refraction occurs at the
`boundary surface. The refractive index n is given by the ratio of the velocity of light in
`vacuum c to that of the medium v
`n 
`
`(1-1)
`
`vc
`
`The refractive index data given in the glass catalogue are measured relative to the refractive
`index measured in air. The refractive index of air is very close to 1.
`
`Practically speaking the refractive index is a measure for the strength of deflection occurring
`at the boundary surface due to the refraction of the light beam. The equation describing the
`amount of deflection is called Snell’s law:
`
`n
`1
`
`
`
`α
`sin(
`1
`
`)
`
`
`
`n
`
`2
`
`
`
`α
`sin(
`2
`
`)
`
`(1-2)
`
`The refractive index is a function of the wavelength. The most common characteristic
`quantity for characterization of an optical glass is the refractive index n in the middle range of
`the visible spectrum. This principal refractive index is usually denoted as nd – the refractive
`index at the wavelength 587.56 nm or in many cases as ne at the wavelength 546.07 nm.
`
`2. Wavelength Dependence of Refractive Index: Dispersion
`The dispersion is a measure of the change of the refractive index with wavelength.
`Dispersion can be explained by applying the electromagnetic theory to the molecular
`structure of matter. If an electromagnetic wave impinges on an atom or a molecule the bound
`charges vibrate at the frequency of the incident wave.
`
`TIE-29: Refractive Index and Dispersion
`
`1/15
`
`

`

`......
`
`April 2005
` DATE
`PAGE 2/15
`The bound charges have resonance frequency at a certain wavelength. A plot of the
`refractive index as a function of the wavelength for fused silica can be seen in Figure 2-1. It
`can be seen that in the main spectral transmission region the refractive index increases
`towards shorter wavelength. Additionally the dotted line shows the absorption coefficient as a
`function of the wavelength.
`
`Figure 2-1: Measured optical constants of fused silica (SiO2 glass) [1]
`
`2.1 Principal Dispersion
`The difference (nF – nC) is called the principal dispersion. nF and nC are the refractive indices
`at the 486.13 nm and 656.27 nm wavelengths.
`The most common characterization of the dispersion of optical glasses is the Abbe number.
`The Abbe number is defined as
`
`ν
`
`d
`
`(
`
`n
`d
`
`
`
`/()1
`
`n
`F
`
`
`
`n
`C
`
`)
`
`
`
`Sometimes the Abbe number is defined according to the e line as
`
`ν
`
`e
`
`(
`
`n
`e
`
`
`
`/()1
`
`n
`F
`
` 
`
`n
`C
`
`
`
`)
`
`(2.1-1)
`
`(2.1-2).
`
`Traditionally optical glasses in the range of d > 50 are called crown glasses, the other ones
`as flint glasses.
`
`Glasses having a low refractive index in general also have a low dispersion behaviour e.g. a
`high Abbe number. Glasses having a high refractive index have a high dispersion behaviour
`and a low Abbe number.
`
`TIE-29: Refractive Index and Dispersion
`
`2/15
`
`

`

`April 2005
` DATE
`PAGE 3/15
`
`......
`
`2.2. Secondary Spectrum
`The characterization of optical glass through refractive index and Abbe number alone is
`insufficient for high quality optical systems. A more accurate description of the glass
`properties is achievable with the aid of the relative partial dispersions.
`
`The relative partial dispersion Px,y for the wavelengths x and y is defined by the equation:
`
`P
`,yx
`
`
`
`
`
`(
`
`n
`
`x
`
`
`
`n
`
`y
`
`/()
`
`n
`F
`
`
`
`n
`
`C
`
`)
`
`(2.2-1)
`
`As Abbe demonstrated, the following linear relationship will approximately apply to the
`majority of glasses, the so-called ”normal glasses”
`
`P
`
`
`,yx
`
`a
`
`xy
`
`
`
`b
`
`xy
`
`ν
`
`d
`
`(2.2-2)
`
`axy and bxy are specific constants for the given relative partial dispersion.
`In order to correct the secondary spectrum (i.e. color correction for more than two
`wavelengths) glasses are required which do not conform to this rule. Therefore glass types
`having deviating partial dispersion from Abbe’s empirical rule are especially interesting.
`
`As a measure of the deviation of the partial dispersion from Abbe’s rule the ordinate
`difference P is introduced. Instead of relation (2.2-2) the following generally valid equation
`is used:
`
`P
`
`
`yx,
`
`
`
`a
`
`xy
`
`
`
`b
`xy
`
`ν
`d
`
`
`
`P
`
`
`
`yx,
`
`(2.2-3)
`
`The term Px,y therefore quantitatively describes a dispersion behavior that deviates from
`that of the ”normal glasses.”
`
`The deviations Px,y from the ”normal lines” are listed for the following five relative partial
`dispersions for each glass type in the data sheets.
`
`(2.2-4)
`
`
`
`
`
`
`
`
` /(n)n
`n
`(n
`
`
`C
`F
`t
`C
`/()
`(
`n
`n
`n
`n
`
`
`C
`s
`F
`C
`/()
`(
`n
`n
`n
`n
`
`
`C
`e
`F
`F
`/()
`(
`n
`n
`n
`n
`
`
`C
`F
`F
`g
`)
`/()
`(
`n
`n
`n
`n
`
`
`C
`g
`F
`i
`
`)
`)
`)
`)
`
`P
`C,t
`P
`sC
`P
`eF
`P
`Fg
`P
`,
`gi
`
`,,,
`
`The position of the normal lines is determined based on value pairs of the glass types K7 and
`F2. The explicit formulas for the deviations Px,y of the above-mentioned five relative partial
`dispersions are:
`
`TIE-29: Refractive Index and Dispersion
`
`3/15
`
`

`

`April 2005
` DATE
`PAGE 4/15
`
`
`
`(2.2-5)
`
`)
`)
`)
`)
`
`νννν
`
`d
`
`d
`
`d
`
`C
`
`n
`
`C
`n
`
`n
`
`C
`n
`
`C
`)
`n
`
`C
`
`)
` 5450.0(
`
`
`)
`
` 4029.0(
`
`)
` 4884.0(
`
`
`)
`
` 6438.0(
`
`
` 7241.1(
`
`
`
` 004743.0
`
`
`
`
` 002331.0
`
`
` 000526,0
`
`
`
`
` 001682.0
`
`
`ν
`
` 008382.0
`
`d
`
`d
`)
`
`g
`
`(
`n
`C
`(
`n
`C
`(
`n
`F
`(
`n
`(
`n
`
`i
`
`g
`
`......
`
`
`
`
`
`
`
`P
`tC
`P
`sC
`P
`eF
`P
`Fg
`P
`,
`gi
`
`,,,,
`
`
`
`
`
`
`
`n
`
`t
`n
`
`n
`
`n
`
`n
`
`
`F
`
`s
`
`e
`
`/()
`n
`F
`/()
`n
`/()
`n
`F
`/()
`n
`F
`F
`/()
`n
`F
`
`Figure 2.2-1 shows the Pg,F versus the Abbe number d diagram.
`
`Figure 2.2-1: Pg,F as a function of the Abbe number for Schott’s optical glass sortiment.
`Additionally the normal line is given.
`
`The relative partial dispersions listed in the catalog were calculated from refractive indices to
`6 decimal places. The dispersion formula (2.3-1) can be used to interpolate additional
`unlisted refractive indices and relative partial dispersions (see chapter 2.3).
`
`2.3. Sellmeier Dispersion Equation
`The Sellmeier Equation is especially suitable for the progression of refractive index in the
`wavelength range from the UV through the visible to the IR area (to 2.3 μm). It is derived
`from the classical dispersion theory and allows the description of the progression of refractive
`index over the total transmission region with one set of data and to calculate accurate
`intermediate values.
`
`TIE-29: Refractive Index and Dispersion
`
`4/15
`
`

`

`April 2005
` DATE
`PAGE 5/15
`
`(2.3-1)
`
`......
`
`λ
`2
`C
`
`3
`
`)
`
`
`
`3
`
`B
`λ
`2
`
`(
`
`
`
`λ
`2
`C
`
`2
`
`)
`
`
`
`2
`
`B
`λ
`2
`
`(
`
`
`
`λ
`2
`C
`1
`
`)
`
`
`
`B
`1
`λ
`2
`
`(
`
`2
`
`n
`
`(
`
`λ
`1)
`
`
`The determination of the coefficients was performed for all glass types on the basis of
`precision measurements by fitting the dispersion equation to the measurement values. The
`coefficients are listed in the data sheets.
`
`The dispersion equation is only valid within the spectral region in which refractive indices are
`listed in the data sheets of each glass. Interpolation is possible within these limits. The
`wavelengths used in the equation have to be inserted in μm with the same number of digits
`as listed in Table 2.3-1. For practical purposes Equation 2.3-1 applies to refractive indices in
`air at room temperature. The achievable precision of this calculation is generally better than
`1·10-5 in the visible spectral range. The coefficients of the dispersion equation can be
`reported for individual glass parts upon request. This requires a precision measurement for
`the entire spectral region, provided the glass has sufficient transmission.
`
`Wavelength [nm]
`2325.42
`1970.09
`1529.582
`1060.0
`1013.98
`852.11
`706.5188
`656.2725
`643.8469
`632.8
`589.2938
`
`587.5618
`546.074
`486.1327
`479.9914
`435.8343
`404.6561
`365.0146
`334.1478
`312.5663
`296.7278
`280.4
`248.3
`
`t
`s
`r
`C
`C'
`
`Designation Spectral Line Used
`infrared mercury line
`infrared mercury line
`infrared mercury line
`neodymium glass laser
`infrared mercury line
`infrared cesium line
`red helium line
`red hydrogen line
`red cadmium line
`helium-neon-gas-laser
`yellow sodium line
`(center of the double line)
`yellow helium line
`green mercury line
`blue hydrogen line
`blue cadmium line
`blue mercury line
`violet mercury line
`ultraviolet mercury line
`ultraviolet mercury line
`ultraviolet mercury line
`ultraviolet mercury line
`ultraviolet mercury line
`ultraviolet mercury line
`
`D
`
`d
`e
`F
`F'
`g
`h
`i
`
`Element
`Hg
`Hg
`Hg
`Nd
`Hg
`Cs
`He
`H
`Cd
`He-Ne
`Na
`
`He
`Hg
`H
`Cd
`Hg
`Hg
`Hg
`Hg
`Hg
`Hg
`Hg
`Hg
`
`Table 2.3-1: Wavelengths for a selection of frequently used spectral lines
`
`TIE-29: Refractive Index and Dispersion
`
`5/15
`
`

`

`April 2005
` DATE
`PAGE 6/15
`
`......
`
`3. Temperature Dependence of Refractive Index
`The refractive indices of the glasses are not only dependent on wavelength, but also upon
`temperature. The relationship of refractive index change to temperature change is called the
`temperature coefficient of refractive index. This can be a positive or a negative value. The
`data sheets contain information on the temperature coefficients of refractive index for several
`temperature ranges and wavelengths. The temperature coefficients of the relative refractive
`indices nrel/T apply for an air pressure of 0.10133·106 Pa. The coefficients of the absolute
`refractive indices dnabs/dT apply for vacuum.
`
`The temperature coefficients of the absolute refractive indices can be calculated for other
`temperatures and wavelengths values with the aid of equation (3-1).
`
`dn
`
`)
`
`λ
`,(
`T
`abs
`dT
`
`
`
`n
`2
`2
`
`λ
`,(
`1)
`T
`
`0
`λ
`)
`,(
`Tn
`
`
`0
`
`
`
`(
`
`D
`0
`
`2
`
`
`D
`1
`
`3
`T
`
`
`D
`2
`
`
`
`T
`
`2
`
`
`
`E
`
`0
`
`
`
`2
`E
`
`1
`λλ
`2
`2
`
`TK
`
`T
`
`)
`
`(3-1)
`
`Definitions:
`T0
`Reference temperature (20°C)
`Temperature (in °C)
`T
`Temperature difference versus T0
`T
`Wavelength of the electromagnetic wave in a vacuum (in μm)
`
`D0, D1, D2, E0, E1 and TK: constants depending on glass type
`
`This equation is valid for a temperature range from -40°C to +80°C and wavelengths
`between 0.6438 μm and 0.4358 μm. The constants of the dispersion formula are also
`calculated from the measurement data and listed on the test certificate.
`
`The temperature coefficients in the data sheets are guideline values. Upon request,
`measurements can be performed on individual melts in the temperature range from -100°C
`to +140°C and in the wavelength range from 0.3650 μm to 1.014 μm with a precision better
`than ± 5·10-7/K. The accuracy at the limits of the measurement range is somewhat less than
`in the middle of this interval.
`
`The temperature coefficients of the relative refractive indices nrel/T and the values for nabs
`can be calculated with the help of the equations listed in Technical Information TI Nr. 19
`(available upon request).
`
`Figures 3-1 to 3-4 show the absolute temperature coefficient of refractive index for different
`glasses, temperatures and wavelengths.
`
`TIE-29: Refractive Index and Dispersion
`
`6/15
`
`

`

`April 2005
` DATE
` PAGE 7/15
`
`SF6
`
`N- LASF40
`
`F2
`
`N- LAK8
`N- BK7
`
`N- FK51
`
`N- PK52
`
`20
`
`15
`
`10
`
`5
`
`0
`
`-5
`
`dnabsolut/dT [10-6*K-1]
`
`......
`
`-10
`-100
`
`150
`100
`Temperatur [°C]
`Figure 3-1: Temperature coefficient of the absolute refractive index of several optical
`glasses in relationship to temperature at the 435.8 nm wavelength.
`
`-50
`
`0
`
`50
`
`N- LASF40
`
`SF6
`N- LAK8
`F2
`
`N- BK7
`
`N- FK51
`
`N- PK52
`
`20
`
`15
`
`10
`
`5
`
`0
`
`-5
`
`-10
`
`dnabsolut/dT [10-6 *K-1]
`
`400
`
`500
`
`600
`
`1100
`1000
`wavelength [nm]
`Wellenlänge [nm]
`Figure 3-2: Temperature coefficient of the absolute refractive index of several optical
`glasses in relation to the wavelength at 20°C.
`
`700
`
`800
`
`900
`
`TIE-29: Refractive Index and Dispersion
`
`7/15
`
`

`

`April 2005
` DATE
`PAGE 8/15
`
`......
`
`Figure 3-3: Temperature coefficient of the absolute refractive index in relation to the
`temperature for several wavelengths in the visible spectral range for glass type
`N-FK51.
`
`g (435,8nm)
`
`F' (480,0nm)
`
`e (546,1nm)
`
`d (587,6nm)
`
`C' (643,8nm)
`
`20
`
`15
`
`10
`
`5
`
`dnabsolut/dT [10-6*K-1]
`
`-50
`
`0
`
`50
`
`100
`
`150
`200
`Temperatur [°C]
`Figure 3-4: Temperature coefficient of the absolute refractive index in relation to the
`temperature for several wavelengths in the visible spectral range for glass type
`SF 6.
`
`0
`-100
`
`TIE-29: Refractive Index and Dispersion
`
`8/15
`
`

`

`April 2005
` DATE
` PAGE 9/15
`
`......
`
`4. Influence of the Fine Annealing Process on the Refractive Index and Abbe
`number
`The optical data for a glass type are chiefly determined by the chemical composition and
`thermal treatment of the melt. The annealing rate in the transformation range of the glass
`can be used to influence the refractive index within certain limits (depending on the glass
`type and the allowable stress birefringence). Basically slower annealing rates yield higher
`refractive indices. In practice, the following formula has proven itself.
`
`
`
` (hn
`d
`
`x
`
`)
`
`
`
`
`
` (hn
`0
`d
`
`)
`
`
`
`m
`nd
`
`log(
`
`h
`
`
`
`/
`
`h
`0
`
`)
`
`x
`
`(4-1)
`
`Original annealing rate
`h0
`hx
`New annealing rate
`mnd?? Annealing coefficient for the refractive index depending on the glass type
`
`The refractive index dependence on annealing rate is graphically shown in Figure 4-1.
`
`Figure 4-1:
`Dependence of refractive index on the annealing rate for several glass
`types. Reference annealing rate is 7 K/h
`
`An analogous formula applies to the Abbe number.
`
`ν
`d
`
`(
`
`h
`
`x
`
`)
`
`
`
`ν
`d
`
`(
`
`h
`0
`
`)
`
`
`
`m
`ν
`d
`
`log(
`
`
`
`
`
` / hh
`0
`x
`
`)
`
`(4-2)
`
`md? Annealing coefficient for the Abbe number depending on the glass type
`
`The annealing coefficient md can be calculated with sufficient accuracy with the following
`equation:
`
`m
`ν
`d
`
`
`
`(
`
`m
`nd
`
`
`
`ν
`d
`
`(
`
`
` ) mh
`
`0
`
`/()
`
`n
`
`F
`
`
`
`n
`
`C
`
`)
`
`nF
`
`
`
`nC
`
`(4-3)
`
`The coefficient mnF-nC has to be determined experimentally.
`
`TIE-29: Refractive Index and Dispersion
`
`9/15
`
`

`

`......
`
`April 2005
` DATE
` PAGE 10/15
`Figure 4-2 shows that individual glass types vary greatly in their dependence of the Abbe
`number on the annealing rate. In general also the Abbe number increases with decreasing
`annealing rate. High index lead free glass types like N-SF6 show anomalous behavior.
`Anomalous behaviour means that the Abbe number decreases with decreasing annealing
`rate.
`
`Figure 4-2:
`Abbe number as a function of the annealing rate for several glass types.
`Reference annealing rate is 7 K/h
`
`Values for Annealing coefficients of some optical glasses are shown in Table 4-1. We will
`provide the values for the annealing coefficients of our glasses upon request.
`
`N-BK7
`N-FK51
`SF 6
`N-SF6
`
`mnd
`-0.00087
`-0.00054
`-0.00058
`-0.0025
`
`mnF-nc
`-0.000005
`-0.000002
`+0.000035
`-0.000212
`
`mν d
`-0.0682
`-0.0644
`-0.0464
`0.0904
`
`Table 4-1:
`
`Annealing coefficients for several selected glass types
`
`The annealing rate can be used to adjust the refractive index and Abbe number to the
`desired tolerance range.
`
`In practice the annealing rate influences the refractive index and the Abbe number
`simultaneously. Figure 4-3 shows a diagram of the Abbe number versus the refractive index
`for N-BK7. The rectangular boxes indicate the tolerance limits (steps) for the refractive index
`and the Abbe number. For example the largest box with a dotted frame indicates the
`tolerance borders for step 3 in refractive index and step 4 in Abbe number. The smallest box
`indicates step 1 in refractive index and Abbe number. In the center of the frames is the
`nominal catalog value.
`
`TIE-29: Refractive Index and Dispersion
`
`10/15
`
`

`

`......
`
`April 2005
` DATE
` PAGE 11/15
`After melting the optical glass is cooled down at a high annealing rate. To control the
`refractive index during the melting process samples are taken directly from the melt after
`each casting. These samples are cooled down very fast together with a reference sample of
`the same glass. The reference sample has a known refractive index at an annealing rate of
`2°C/h. By measuring the change in refractive index of the reference sample the refractive
`index of the sample can be measured with moderate accuracy in the range of 10-4.
`
`The annealing rate dependence of the Abbe number and refractive index of each glass is
`represented by a line in the diagram having a slope that is characteristic for the glass type.
`For a given melt the position of the line in the diagram is given by the initial refractive index /
`Abbe number measurement for a cooling rate of 2°C/h as a fix-point together with the glass
`typical slope. The refractive index and Abbe number for a given glass part can be adjusted
`by a fine annealing step along this characteristic line.
`
`Glass for cold processing has to be fine annealed to reduce internal stresses. During this fine
`annealing the annealing rate is in general lower than 2°C/h. The initial refractive index has to
`be adjusted during melting in such a way that the desired tolerances can be reached during
`fine annealing. The initial refractive index of N-BK7 for example is in general lower than the
`target value.
`
`Figure 4-3:
`
`The influence of the annealing rate on the refractive index and Abbe number
`of N-BK7 for different initial refractive indices.
`
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`Glass for hot processing i.e reheat pressing is subjected to much more rapid annealing. The
`heat treatment processes used by the customer in general use annealing rates much higher
`than 2°C/h. Therefore for N-BK7 pressings for example the initial refractive index needs to be
`higher than the target value. For a better visualization in figure 4-3 the annealing line for
`pressings was shifted to higher Abbe numbers. In general it is also possible to achieve step
`1/1 for pressings after hot processing. We deliver an annealing schedule for each batch of
`glass for hot processing purpose. This annealing schedule contains the initial refractive index
`at 2°C/h and the limit annealing rates to stay within the tolerances.
`
`5. Tolerances
`The refractive indices, which are listed to 5 decimal places in the data sheets, represent
`values for a melt with nominal nd-d position for the glass type in question. The refractive
`index data are exact to five decimal places (for  > 2 m: ± 2·10-5). The accuracy of the data
`is less in wavelength regions with limited transmission. All data apply to room temperature
`and normal air pressure (0.10133·10-6 Pa).
`
`Defining tolerances for the refractive index of a glass the customer has to distinguish
`between the refractive index tolerance, the tolerance of refractive index variation within a lot
`and the refractive index homogeneity (figure 5-1).
`
`Figure 5-1: Refractive index variation from within a production sequence.
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`All deliveries of fine annealed block glass and fabricated glass are made in lots of single
`batches. The batch may be a single block or some few strip sections. More information on
`the new lot id system can be found in [3].
`
`The refractive index and Abbe number tolerance is the maximum allowed deviation of a
`single part within the delivery lot from nominal values given in the data sheets of the catalog.
`The refractive index of the delivery lot given in the standard test certificates is given by the
`following formulae:
`
`nlot
`
`
`
`(
`
`n
`max
`
`
`
`n
`
`min
`
`2/)
`
`(5-1)
`
`nmax is the maximum and nmin the minimum refractive index within the lot.
`
`The refractive index variation from part to part within a lot is always smaller than  1*10-4.
`The refractive index homogeneity within a single part is better than  2*10-5 in general [4]. A
`short summary of the refractive index tolerance, variation and homogeneity grades can be
`found in table 5-1. More information is given in the optical glass catalogue [5].
`
`Tolerance
`
`Absolute
`
`Variation
`
`Homogeneity
`
`Grade
`
`Step 4
`Step 3
`Step 2
`Step 1
`SN
`S0
`S1
`H1
`H2
`H3
`H4
`H5
`
`Refractive Index [*10-5]
`
`Abbe Number
`
`--
` 50
` 30
` 20
` 10
` 5
` 2
` 2
` 0.5
` 0.2
` 0.1
` 0.05
`
` 0.8%
` 0.5%
` 0.3%
` 0.2%
`--
`--
`--
`--
`--
`--
`--
`--
`
`Table 5-1: Refractive Index Tolerances
`
`6. Refractive Index Measurement
`For refractive index measurement two different measurement setups are used: the v-block
`refractometer (figure 6-2) and the spectral goniometer. Figure 6-1 shows the principle of the
`v-Block measurement. The samples are shaped in a nearly square shape. One sample is
`about 20x20x5 mm small. The sample will be placed in a v shaped block prism. The
`refractive index of this prism is known very precisely. The refraction of an incoming light
`beam depends on the refractive index difference between the sample and the v-block-prism.
`The advantage of this method is that up to 10 samples can be glued together into one v-
`block stack. Therefore many samples can be measured in a very short time. The relative
`measurement accuracy is very high therefore differences in refractive index within one v-
`block stack can be measured very accurately. Standard measurement temperature is 22°C.
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`......
`
`sample with higher
`refractive index
`
`sample with lower
`refractive index
`
`immersion oil
`
`lightbeam
`
`sample
`
`v-block-prism
`
`Figure 6-1: Refractive index variation from within a production sequence.
`
`Figure 6-2: V-block refractometer.
`
`The spectral goniometric method is based on the measurement of the angle of minimum
`refraction in a prism shaped sample. This is the most accurate absolute refractive index
`measurement method. In our laboratory we have an automated spectral goniometer with
`high accuracy and the ability to measure in the infrared and UV region (figure 6-3).
`
`Figure 6-3: Automated spectral goniometer.
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`With the automated spectral goniometer, the Ultraviolett to infrared Refractive Index
`measurement System (URIS), the refractive index of optical glasses can be measured to an
`accuracy of  4*10-6 . The measurement accuracy for the dispersion (nF-nC) is  2*10-6.
`These measurement accuracies can be achieved independent of the glass type and over
`the complete wavelength range from 185 nm to 2325 nm. The measurement is based on the
`minimum angle of deviation principle. The samples are prism shaped with dimensions of
`about 35 x 35 x 25 mm3. The standard measurements temperature is 22°C. The
`temperature can be varied between 18 to 28°C on request. The standard measurement
`atmosphere is air. On special request also nitrogen is possible.
`
`Table 6-1 shows a summary of the refractive index measurements available at Schott.
`
`Measurement
`
`V-block standard
`
`V-block
`enhanced
`Precision
`spectrometer
`
`Measurement accuracy
`Refractive
`Dispersion
`index
` 30*10-6
`
` 20*10-6
`
` 20*10-6
`
` 4*10-6
`
` 10*10-6
`
` 2*10-6
`
`Wavelengths
`
`Method
`
`g, F’, F, e, d, C’,
`C
`I, h, g, F’, F, e, d,
`C’, C, r, t
`185 nm –
`2325 nm
`
`v-block
`refractometer
`
`URIS
`automatic
`spectral
`goniometer
`
`Table 6-1: Absolute refractive index measurement accuracies
`
`The temperature coefficient of refractive index is measured using the automated spectral
`goniometer and a temperature controlled climate chamber with a temperature range from
`-100°C up to +140°C. The temperature coefficient can be measured with an accuracy of 
`0.5*10-6 K-1.
`7. Literature
`[1] The properties of optical glass; H. Bach & N. Neuroth (Editors), Springer Verlag 1998
`[2] SCHOTT Technical Information No. 19 (available upon request).
`[3] SCHOTT Technical Note No. 4: Test report for delivery lots
`[4] SCHOTT Technical Information No. 26: Homogeneity of optical glass
`[5] SCHOTT Optical Glass Pocket Catalogue
`
`For more information please contact:
`
`Optics for Devices
`SCHOTT North America, Inc.
`400 York Avenue
`Duryea, PA 18642
`USA
`Phone: +1 (570) 457-7485
`Fax:
` +1 (570) 457-7330
`E-mail: sgt@us.schott.com
`www.us.schott.com/optics_devices
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