Supplementary details about optical glass properties

This review sets a mission to familiarize our regular and potential customers with the stock list and basic properties of our optical glasses, with quality indices tolerances and forms of supply aiming on helping our customers to make a correct and economic choice of required products.
Optical clear colorless inorganic glass is the base material for fabrication of lenses, prisms, plates, and other components of optical devices and systems that receive, transmit and transform optical radiation.
Due to the fact that glass compositions include various chemical compounds, these optical glasses possess great diversity of optical and other physicochemical properties enabling creation of optical systems with high degree resolution of image transmission.
High quality of LZOS glasses is ensured by use of advanced production processes, high-purity raw materials and normalization of tolerances for basic parameters of an optical glass.
The glasses, in compliance with customer’s requests, are delivered as blocks, plates, slabs or press - blanks close in shape and size to finished parts.
Detail information:

• Glass Optical properties
• Glass Secondary Spectrum
• Designation system and classification of glasses
• Glass light transmission and resistance to irradiation
• Mechanical properties
• Thermal properties
• Chemical resistance

Refractive index.
Refractive index n is determined as the ratio of the velocity of propagation of electromagnetic radiation in air to such velocity in glass. The refractive index depends on the radiation wavelength.
As the main refractive index for a specific optical glass type is set, in accordance with the ISO 7944-84 the refractive index n_e for the wavelength 546.97 nm of the mercury spectral line e. This line is in the green part of the spectrum and overlaps the region of maximum sensitivity of a human eye.
In the catalogue are given refractive indices n for wavelengths of different spectral lines of chemical elements in the range 365 - 2325.4 nm, as well as for wavelengths generated by lasers of the most often used types.
      Values of the refractive indices refer to glasses gone through the fine annealing procedure (the soaking temperature corresponds to the viscosity of 10*13 dPa/sec, the rate of cooling 2.5 deg/h). Measurement of the refractive indices is carried out at the temperature 20+/- 3 ^oC and at the pressure referred to the atmospheric 760 mm Hg.

Mean dispersion.
Dispersion properties of an optical glass are defined by the value of mean dispersion being the difference of refractive indices for two relatively far separated wavelengths. As the basic mean dispersion the difference of refractive indices n_F' - n_C' for wavelengths of 479.99 and 643.85 nm of the cadmium spectral lines F' and C' bordering the visible region of the spectrum (ISO 7944 - 84) is taken:

where n2 and n3are the refractive indices for the wavelengths bordering some spectral region, and n1 is the refractive index for a wavelength inside this spectral region.

As the main dispersion constant, known also as the Abbe-number (ISO 7944-84) is accepted the following value:

As it has been noted by Abbe, if you plot values of the relative partial dispersions P_x,y versus the dispersion constant n_e, calculated for the same wavelengths, you will find out that for the most glass types the points lie close to some straight line called "Normal Line". The tangent of the angle of slope ò for this line is expressed as the following:

In the review here it is accepted that the "Normal Line" is determined using the values P_x,y and n_e of the types K18 and F13 glasses. The optical parameters of these glasses are given lower on:

glass type	ve	Px,y at the sections
		i - F	y - F	F - e	F - r
K18	60.15	1.697	0.4811	0.5086	1.223
F13	36.09	1.921	0.5168	0.5223	1.205

The necessary condition for an apochromatic objective secondary spectrum DS correction is the difference of relative partial dispersions being minimum and the value of tg t being near zero.
It is real only when there are glasses with "anomalous" properties with parameters off the "Normal Line". Such glasses have got the name "special" and are separated in the catalogue in types.
In the catalogue are given deviations of relative partial dispersions and constants of dispersion off the "Normal Line" for the four areas of the UV and visible regions.

glass group name	short designation
light crown	LK
phosphate crown	FK
dense phosphate crown	TFK
crown	K
barium crown	BK
dense crown	TK
extra dense crown	CTK
special (with special dispersion path) crown	OK
crown flint	KF
barium flint	BF
dense barium flint	TBF
light flint	LF
flint	F
dense flint	TF
extra dense flint	CTF
special (with special dispersion path) flint	OF

As the range of produced glasses widened and other substances - besides silica were used as glass-formers, the necessity arose of both Flint and Crown glasses differentiation into groups and then these groups into types. Optical quality colorless glasses are classified to groups depending on the refractive index and the Abbe-number values.
Types are assigned to certain glasses having definite chemical composition and optical properties. Type designation includes the literal name of the glass group and the number.
For designation of glass types six figures codes are also used with the first three figures corresponding to the three figures after comma in the refractive index ne value and the second three - to the three figures of the dispersion coefficient ne value.
For example, TF5 or 762273 is the Dense Flint group glass, its number is five in this group, the refractive index is ne = 1.76171 and the Abbe-number is ne = 27.32.

For ease of glass groups and types choice the Abbe-diagram (ne/ve diagram) is plotted with the coordinate axis ne and ve. Glasses of each type are placed on the diagram field in the strictly restricted areas, except the glasses OF and OK, which can be in different areas of the diagram field.

Light transmission.
Spectral internal [pure] transmittance T_i is determined as the ratio of exit radiant flux F_ex to input flux F_in, exclusive of reflection losses of boundary surfaces.

Values of T_i for 10 and 25 mm thick glass, at wavelength from 320 up to 1500 nm are presented in this review. These values are the mean values of several melts for given glass type. For some melts they can deviate from the mean values up or down. If a customer desires to have larger transmittance it is necessary to stipulate that when ordering.
Integral internal transmittance factor for white light T_a of a standard source of type A (T = 2856 K) is determined according the attenuation coefficient mu_a, which is an inverse value of a distance , on which radiation flux of type A light source is attenuated as a result of glass absorption and dispersion in 10 times.

Normalized values of the attenuation coefficient mu_a and the integral internal transmittance for white light T_a are stated in the page Standard Features of Glass Quality

Glass Resistance to Radiation.
Under exposure of hard radiation most optical glasses get coloured (darken), light transmission reduces to a certain level depending on the radiation dose and glass composition.
Glass resistance to radiation is defined by the optical density increment DD per 1cm of glass thickness after exposure to the dose of gamma radiation of 10"4 and 10"5 Ð from Ñî60 source.

In this catalogue are given the following values: the 10 mm thick glass optical density in previous to irradiation D₀ and the increment delta D after irradiation.
Enhancement of radiation resistance of glasses is achieved with some additives in their compositions, usually cerium oxide CeO₂, preventing colour centers formation. By their optical and physico-chemical properties radiation-resistant glasses practically not differs from their basic glass analogues.
To designate a radiation-resistant glass, a number 100 or 200, depending on a resistance level, is added to the running number of the basic glass type. For example, the radiation-resistant glass of the type F1 is designated as F101.
For those glass types, which have radiation-resistant modifications there are presented in the catalogue of optical colorless glass

Density
p (g/cm³) - is the ratio of a glass mass to its volume.

Relative hardness with respect to grinding H_o
Is defined as the ratio of the ground out with loose abrasive material volume of the type K8 glass to the volume of the given type glass ground under the same conditions. The value H_o is the technological criterion for glass removal rate during grinding.

Strength properties
Strength properties of a glass as a construction material is defined by standard parameters: the modulus of elasticity E and the shear modulus G, connected with each other by the relationship E=2G(1+mu), where mu is the coefficient of lateral deformation (Poisson's ratio).

Photoelasticity properties
Glass photoelasticity is defined by the photoelastic constants C₁ and C₂, that express the increment in the glass refractive index values for rays of light polarized in the directions parallel and perpendicular to the direction of the 10⁵ Pa stress, as well as by the stress-optical coefficient B=C₁ - C₂. The stress-optical coefficient B determines the difference of the optical paths of rays in a glass and characterizes the birefringence resulted from the 10⁵ Pa stress of.

Thermal linear expansion coefficient a_t
At defines relative elongation of a glass sample upon its heating by 1^oC.
The value a_t varies depending on the temperature range, inside which they are measured. In the catalogue the values a_t are given for the temperature ranges +20 to -60 ^oC and +20 to +120 ^oC.

Thermal conductivity coefficient lambda
Characterizes glass ability to transfer heat from heated areas to less heated. It is defined by the quantity of heat (in kcal), which passes the 1m² section in 1 hour provided that the temperature gradient causing the heat flow is equal to 1^oC. Thermal conductivity depends on temperature.

• resistance of a polished surface against humid air without water vapour condensation (~85% relative humidity);
• resistance against staining agents: neutral water, weak-acidic and alkaline aqueous solutions.