Process for forming two different indices of refraction in a glass ceramic

Abstract

Thermally crystallizable glasses of the Na.sub.2 O--Ta.sub.2 O.sub.5 --SiO.sub.2 and the Na.sub.2 O--Li.sub.2 O--Ta.sub.2 O.sub.5 --SiO.sub.2 systems, and glass-ceramics made therefrom which are highly transparent, have high indices of refraction, and excellent strength properties. By varying the heat treatment schedule for crystallization of a glass to a glass-ceramic, a specific high index of refraction coming within a prescribed range may be imparted to the finished, transparent glass-ceramic. A transparent glass-ceramic having two or more different indices of refraction may also be produced.

Description

With the advent of acoustooptical devices, electrooptical devices, and other devices utilizing transparent glasses and glass ceramics, the need for glasses and glass-ceramics having a high index of refraction is becoming more acute. However, it is well-known that the higher the index of refraction of a glass, the more difficult it is to obtain it in optical quality (freedom from bubbles, stones and cords). This is especially true when the index of refraction is about 1.8 or higher.

Acoustooptical devices (sometimes called elastooptical devices) are known in the art and reference is made to an article entitled "A Review of Acousto-optical Deflection and Modulation Devices" by E. I. Gordon, which appeared in the Proceedings of the I. E. E. E. [10] 1391-1401 (1966) and an article entitled "Dielectric Materials for Electrooptic, Elastooptic and Ultrasonic Device Application" by E. G. Spencer et al, appearing in the Proceedings of the I. E. E. E., 55 [12] 2074-2108 (1967).

Electrooptical devices are also well-known and are described in U.S. Pat. No. 3,069,973 to I. Ames and in U.S. Pat. No. 3,467,463 to N. F. Borrelli et al.

While single crystal materials are of high quality and can be used in acoustooptical and electrooptical devices, they are relatively very expensive to obtain, especially in large sizes and moreover one is limited to the precise index of refraction inherent in any particular single crystal.

The present invention is directed to certain limited compositions having specific parameters and coming within the broad definition of alkali-Ta.sub.2 O.sub.5 -SiO.sub.2 systems which can be readily formed into glasses and the glasses subsequently thermally in situ crystallized to form transparent glass-ceramics having selected high indices of refraction. Such glass-ceramics can be formed from the crystallizable glasses even though the glasses do not have any of the customary nucleating agents, such as TiO.sub.2, ZrO.sub.2, P.sub.2 O.sub.5, and the like. Glasses and glass-ceramics of this invention are suitable for use in electrooptical devices, acoustooptical devices, and the like, for example as modulators, laser Q-switches and/or deflectors, etc. Certain of such glasses and glass-ceramics have been found to also have good dielectric properties including very low dielectric losses, which make them suitable for use in a variety of electrical devices, such as capacitors, electroluminescent cells, wave guides, and the like. Dielectric constants of at least 50 (at room temperature and 0.5 MHz) with loss tangents or dissipation factors of less than 3% and preferably less than 1% are obtained with glass-ceramics of the present invention.

By use of the thermally crystallizable glasses of the present invention having a constant composition, each glass can, during an appropriate heat treatment process, be converted to a transparent, inorganic crystalline oxide ceramic material having any desired index of refraction between a selected upper and a selected lower limit. Furthermore, the ceramic material thus formed, also known as a glass-ceramic, exhibits a relatively high optical index of refraction and contains a major portion of very tiny crystals embedded in a minor glassy matrix remaining as a result of the thermal crystallization.

Furthermore, glass-ceramics of this invention can be prepared in the form of small chips, of a size of about 1/4 inch and less, which are suitable for use as support carriers for electronic microcircuits, which chips have extremely high strengths and are difficult to break even after they have been first scribed with a diamond scribe. The strengths of such compositions are relatively high as compared to strengths of glass-ceramic chips having a composition outside of the present invention and which can usually be broken readily with one's fingers.

Within the broad spectrum of the alkali--Ta.sub.2 O.sub.5 --SiO.sub.2 systems, it has been found that a narrow range of a compositions of the Na.sub.2 O--Ta.sub.2 O.sub.5 --SiO.sub.2 system have good glass-forming properties when the three sole essential ingredients are within the following ranges:

______________________________________Ingredient Mole Percent______________________________________SiO.sub.2 37 - 55Ta.sub.2 O.sub.5 23 - 35Na.sub.2 O 20 - 33______________________________________

Glasses thus formed have a high index of refraction, and can be thermally in situ crystallized to mechanically strong, transparent glass-ceramics having a still higher index of refraction.

While the most stable glasses are obtained when the ratio of Na.sub.2 O to Ta.sub.2 O.sub.5 is 1 or slightly less than 1, it has been found that good glasses are also formed when the ratio of Na.sub.2 O to Ta.sub.2 O.sub.5 is from about 0.7:1 up to 1.2:1. When the Ta.sub.2 O.sub.5 is present in a molar amount greater than the Na.sub.2 O, the glass-forming tendency of the composition is increased.

Several compositions were prepared by melting together silica, Ta.sub.2 O.sub.5 and Na.sub.2 O in the mole percents set forth in the following table.

TABLE I______________________________________ Mole Percent CompositionsIngredient A B C D E F G______________________________________SiO.sub.2 40 45 50 35 40 30 30Ta.sub.2 O.sub.5 30 27.5 25 30 20 25 35Na.sub.2 O 30 27.5 25 35 40 45 35______________________________________

Each of the compositions was heated to a temperature sufficient to melt the ingredients and then held at that temperature for about 1 - 24 hours, the time being dependent on the specific composition and how long it took for the melting to be completed and a homogeneous melt to be formed. The melt was periodically stirred during this time. Each molten composition was then poured onto a metal plate and another metal plate immediately placed over it to quench the composition. The glass-forming tendencies of each composition were then judged on the ability of the melt to be quenched into glass chips between the metal plates.

Compositions A, B and C, which come within the composition range of the invention, formed glasses readily, while compositions E, F and G having at least one ingredient outside the composition range of the invention, did not form any glass whatsoever. Composition D, which was just outside the composition range, had very poor glass-forming qualities, and consequently the quenched plates had only small areas of glass.

FIG. 1 represents, in a triangular composition diagram, a glass formation area of the Na.sub.2 O-Ta.sub.2 O.sub.5 -SiO.sub.2 system;

FIG. 2 represents, in a triangular composition diagram, a transparent glass ceramic area of the same glass system in FIG. 1;

FIG. 3 represents, in a triangular composition diagram, a glass formation area of the Na.sub.2 O-Li.sub.2 O-Ta.sub.2 O.sub.5 -SiO.sub.2 system, wherein the molar ratio of Na.sub.2 O to Li.sub.2 O is 1; and

FIG. 4 represents, in a triangular composition diagram, a transparent glass ceramic area of the same glass system as in FIG. 3.

As seen in FIG. 1 of the drawings, compositions A, B, and C come well within the glass formation area, while composition D is on the borderline and compositions E, F, and G are well outside such area. When each of compositions A - C was subjected to temperatures sufficient to first nucleate and then crystallize the glass, very good transparent glass-ceramics were obtained from the glass compositions A, B, and C. As seen in FIG. 2 of the drawing, composition D falls outside of the transparent glass-ceramic area as, of course, do compositions E, F and G. Glasses could not be formed from these latter three compositions, and composition D did not produce a glass which could be heat treated to form a glass-ceramic.

The following table sets forth the heat treatment schedules utilized in thermally in situ crystallizing the glasses to the glass-ceramics as claimed herein, and also shows the indices of refraction of the glasses and glass-ceramics.

TABLE II______________________________________Properties and Composi- Composi- Composi-Treatment tion A tion B tion C______________________________________Index of Refraction(Glass) 1.825 1.817 1.804Heat Treatment 1570(16) 1625(2) 1625(64)Temp. .degree. F 1675(1) 1750(2) 1800(1/4)(Time, hours) 1725(1/4)Index of Refraction(Glass-ceramic) 1.895 1.884 1.858______________________________________

NaTaO.sub.3 was identified as the predominant crystal phase in the glass-ceramics obtained from the three compositions given above.

As evident in the above Table II, the index of refraction of the crystallizable glass is quite high and it has been found that glasses as claimed in my Ser. No. 169,271 now U.S. Pat. No. 3,984,251 application can be prepared having indices of refraction within the range of about 1.80 to 1.95. When the glasses are subjected to the crystallization schedule, the resultant glass-ceramics have a higher index of refraction, and preferably within the range of about 1.83 to 2.00.

The indices of refraction of the glasses and the glass-ceramics of the invention are much higher than comparable indices of refraction of SiO.sub.2 --Na.sub.2 O glasses which have indices within the range of about 1.48 to 1.52. If CaO or BaO is incorporated in the SiO.sub.2--Na.sub.2 O glasses, the indices of refraction come within the ranges of about 1.5 - 1.56 and about 1.5 - 1.59 respectively. See The Properties of Glass by George W. Morey, Second Edition, 1954, published by Reinhold Publishing Corporation, New York, and particularly pages 378-382 thereof.

It is unexpected to find that thermally crystallizable glasses can be formed from the Na.sub.O--Ta.sub.2 O.sub.5 -SiO.sub.2 system wherein each of the three sole essential ingredients comes within a critical range and furthermore that each of such glasses has an index of refraction which is relatively higher than one would expect.

Also within the broad spectrum of the alkali--Ta.sub.2 O.sub.5 --SiO.sub.2 systems, it has been found that certain compositions of the Na.sub.2 O--Li.sub.2 O--Ta.sub.2 O.sub.2 system coming within a narrow critical range also have good glass-forming properties, provided that each of the four essential ingredients is present within the following ranges:

______________________________________Ingredient Mole Percent______________________________________SiO.sub.2 27 - 45Ta.sub.2 O.sub.5 30 - 45Li.sub.2 O + Na.sub.2 0 20 - 35______________________________________

The molar ratio of Li.sub.2 O to said Na.sub.2 O is within the range of from 0.8:1 to 1.2:1 and the molar ratio of Na.sub.2 O + Li.sub.2 O to Ta.sub.2 O.sub.5 is within the range of 0.5:1 to about 1:1.

To show the criticality of having the glass-forming compositions come within the above ranges, a number of compositions were prepared by melting together silica, Ta.sub.2 O.sub.5, Li.sub.2 O and Na.sub.2 O for a period of from 1 to 24 hours with constant stirring. Each of the compositions was then quenched between metal plates as discussed above with respect to the Na.sub.2 O--Ta.sub.2 O.sub.5 --SiO.sub.2 compositions. The following compositions were prepared:

TABLE III__________________________________________________________________________ Mole Percent CompositionsIngredients H I J K L M N O P Q__________________________________________________________________________SiO.sub.2 40 30 30 30 35 35 40 20 30 30Ta.sub.2 O.sub.5 30 35 40 45 35 40 40 40 25 50No.sub.2 O 15 17.5 15 12.5 15 12.5 10 20 22.5 10Li.sub.2 O 15 17.5 15 12.5 15 12.5 10 20 22.5 10n.sub.D 1.875 1.920 1.955 1.914 1.945 1.934__________________________________________________________________________

Glasses were formed from Compositions H through N inclusive, all of which come within the aforementioned critical range, while compositions O, P and Q, falling outside the critical composition range, did not form any glasses whatever. As may be seen in FIG. 3 of the drawing, compositions H through N inclusive fall within the glass-formation area, while compositions O, P and Q are outside of the defined area.

Of the glasses of compositions H through N inclusive, all were heat treated to form transparent glass-ceramics with the exception of the glass of composition H. As seen in FIG. 4 of the accompanying drawing, composition H falls outside of the transparent glass-ceramic area. Thus, while the glasses of this embodiment of the invention come within the critical range set forth above, thermally crystallizable glasses and glass-ceramics made therefrom come within the following critical range:

______________________________________Ingredient Mole Percent______________________________________SiO.sub.2 27 - 45Ta.sub.2 O.sub.5 32 - 45Li.sub.2 O + Na.sub.2 O 20 - 35______________________________________

The molar ratio of Li.sub.2 O to said Na.sub.2 O is within the range of from 0.8:1 to 1.2:1 and the molar ratio of Na.sub.2 O + Li.sub.2 O to Ta.sub.2 O.sub.5 is within the range of 0.5:1 to about 1:1.

Thus, when the Ta.sub.2 O.sub.5 is less than 32 mole percent (it was present in an amount of 30 mole percent in composition H), the glass is not a thermally crystallizable glass and a glass-ceramic cannot be formed therefrom.

Glass compositions I, J, K, and L were subjected to the heat treatment schedule set forth in the following table whereby each of the glasses was thermally in situ crystallized to a transparent glass-ceramic. Various properties of the glasses and glass-ceramics are set forth in the following table:

TABLE IV__________________________________________________________________________ Compositions I J J J K L L__________________________________________________________________________Heat Treatment 1550(2) 1475(2) 1475(2) 1400(2) 1625(2) 1470(2) 1500(3)Temp..degree. F(Time, Hrs.) 1675(1/2) 1650(1/2) 1600(1/4) 1950(1/4) 1750(1/4) 1500(1)Index of Refraction Glass 1.920 1.955 1.955 1.955 1.914 1.914 Glass-Ceramic 2.000 2.015 2.022 1.973 1.967Crystal Phase NaTaO.sub.3 NaTaO.sub.3 NaTaO.sub. 3 NaTaO.sub.3Dielectric Constant(Room temp., 0.5 MHz) 62.5 85.5 66.3Dielectric Constant(Room temp., 1.0 MHz) 99Dissipation Factor %(Room temp., 0.5 MHz) 0.8 0.8 0.68Dissipation Factor %(Room temp., 1.0 MHz) 0.75Temp. Coefficient of Capacitance(-50.degree. C. to 100.degree. C) 0__________________________________________________________________________

NaTaO.sub.3 was identified as a predominant crystal phase in glass-ceramics formed from the compositions given above LiTaO.sub.3 and an unidentified phase are sometimes present as another crystal phase in the glass-ceramics.

Again, as is evident from the above table, the index of refraction of the crystallizable glass is quite high, coming within the range of 1.85 - 2.0. When crystallized to a glass-ceramic, the index is even higher and preferably comes within the range of about 1.90 to 2.10.

Glass-ceramics of the invention as claimed herein coming within the Na.sub.2 O--Li.sub.2 O--Ta.sub.2 O.sub.5 --SiO.sub.2 system also have high dielectric constants of at least 50 and preferably as high as 100 or more. Moreover, such glass-ceramics have a dielectric loss or dissipation factor of less than 3% and preferably less than 1%.

While each of the glasses and glass-ceramics of this system has a very high index of refraction, the index of each glass-ceramic can be varied to a desired figure by varying the heat-treatment schedule when thermally in situ crystallizing the glasses. The amount of increase in the index is dependent upon the heat treatment. This can be seen from the following table wherein the glass of composition J referred to above was subjected to different heat-treating schedules, so that the index of refraction in the final product differed, depending upon the temperature and the times. The heat treatment and the resulting indices of refraction imparted to the resulting glass-ceramics are shown in the following table:

TABLE V______________________________________ Glasses of Composition J No. 1 No. 2 No. 3 No. 4______________________________________Heat TreatmentTemp..degree. F,(Time, hrs) 1425(2) 1475(2) 1475(2) 1475(2) 1500(1/2) 1650(1/2)Index of Refraction Glass 1.955 1.955 1.955 1.955 Glass-Ceramic 1.962 2.004 2.005 2.015______________________________________

These high indices of refraction for the glasses and glass-ceramics, exemplified in Table IV and V, are considerably higher than the indices of refraction of the Li.sub.2 O-Na.sub.2 O-SiO.sub.2 glasses which fall within the range of about 1.5 to 1.54 according to Morey, The Properties of Glass, supra.

While the glasses and glass-ceramics of the invention have been discussed in terms of the sole essential ingredients, it will be evident to those skilled in the art that minor amounts preferably not more than 10% and most preferably no more than 5% by weight of other metal oxides which are compatible with the glass and glass-ceramic compositions can be used. Care should be taken, however, to insure that such other metal oxides do mot materially affect the basic characteristics of the glasses and glass-ceramics of the present invention, including high indices of refraction, high dielectric constant, low dielectric losses, and high relative strengths.

Furthermore, while the glass-ceramics of the invention have been disclosed as being transparent, so that they can be used in many devices where transparency is important, it will be appreciated that if only the high dielectric constant of the glass-ceramic together with its relatively high strength is required, the glass-ceramic can be further heat-treated to form an opaque glass-ceramic.

Transparent glass-ceramics having two or more indices of refraction can be prepared by subjecting different portions of the thermally crystallizable glass to the particular different heating schedules which produce the desired refraction indices. Thus, a crystallizable glass article of Composition J could have one portion subjected to heat schedule No. 1 to Table V and another portion subjected to heat schedule No. 4 of the same Table. The resulting transparent glass-ceramic article would have an index of refraction of 1.962 along the first portion and an index of 2.015 along the second portion.

Lenses, including eyeglass lenses, which are normally convex at one surface, can be made from flat sheets of transparent glass-ceramic wherein the center of the sheet or lens has a certain index of refraction and a number of concentric areas around the center are given different indices of refraction by subjecting such areas to the requisite heat schedules so that the indices of refraction are in ascending or descending magnitudes from the center to the outer periphery of the lens. Thus, using Composition J and the heat treatments set forth in Table V, the center of the transparent glass-ceramic lens of this composition J can have an index of refraction of 2.015 and the three concentric areas around the center can have indices of refraction of 2.005, 2,004, and 1.962, respectively. By utilizing an appropriate temperature gradient from the center to the edge, a smooth and continuous variation of the index of refraction from the center to the edge can be achieved. Changing the temperature gradient will change the resulting index of refraction gradient. This is an advantage over the Fresnel type lens where discrete changes in the index are achieved by gluing together glasses having different indices. Means for applying the gradient temperature to the thermally crystallizable glass are known in the art. Such lenses can be made to produce the same refraction of light rays as presently obtained by grinding the surface of a piece of glass to shape a particular lens. By combining grinding and the index of refraction gradient, even greater refraction of light can be obtained than by utilizing either the grinding or the index gradient alone. The process of imparting concentric indices of refraction to a thermally crystallizable glass to form a glass-ceramic article can be applied to a number of such glass articles, including transparent telescope mirror blanks which are made of glass-ceramic.

Another important application of this embodiment of the invention is in optical circuitry. By heat-treating a glass block or article so as to form a path of any desired shape and configuration within said block, with the glass-ceramic defining such a path having a particular higher index of refraction than the surrounding material, light can be made to follow such a path and can be bent as it travels about such a path. Thus, optical printed circuits can be readily made.

For example, a laser beam can be used to heat treat a particular spot within a thermally crystallizable glass block or chip and cause that portion of the block to form a glass-ceramic having a particular index of refraction. As the laser beam moves along the block, a narrow path defined by a glass-ceramic having a particular index of refraction is formed, since the heat of the laser beam is localized. The index is higher than that of the surrounding glass and light moving along the path stays within the path.

While glasses of the present invention, namely, those within the particularly Na.sub.2 O--Ta.sub.2 O.sub.5 --SiO.sub.2 and Na.sub.2 O--Li.sub.2 O--SiO.sub.2 and Na.sub.2 O--Li.sub.2 O--Ta.sub.2 O.sub.5 --SiO.sub.2 systems are suitable for purposes of forming transparent glass-ceramics having a plurality of indices of refraction, other thermally crystallizable glasses can be used including those within the Na.sub.2 O--K.sub.2 O-Nb.sub.2 O.sub.5 --SiO.sub.2 system, K.sub.2 O--Nb.sub.2 O.sub.5 --SiO.sub.2 system (Na.sub.2 O, Li.sub.2 O, K.sub.2 O)--Nb.sub.2 O.sub.5 --Ta.sub.2 O.sub.5 --GeO.sub.2 systems and SiO.sub.2 --Al.sub.2 O.sub.3 --Li.sub.2 O systems. An example of a thermally crystallizable glass coming within the latter system is as follows, with amounts expressed in weight percent:

______________________________________ SiO.sub.2 66.9 Al.sub.2 O.sub.3 20.9 Li.sub.2 O 3.9 TiO.sub.2 1.8 ZrO.sub.2 2.0 CaO 3.5 Na.sub.2 O 0.7 Sb.sub.2 O.sub.3 0.3______________________________________ Many glasses coming within the latter system and forming transparent glass-ceramics are known in the prior art.

An example of the glasses coming within the K.sub.2 O--Nb.sub.2 O.sub.5 --SiO.sub.2 system suitable for forming glass-ceramics having an index of refraction gradient is as follows:

______________________________________Ingredient Mole Percent______________________________________SiO.sub.2 55Nb.sub.2 O.sub.5 22.5K.sub.2 O 22.5______________________________________

The batch ingredients for a 5 kg melt were melted in a crucible at a temperature of 2200.degree. F for 24 hours, quenched in water and then remelted and quenched several times. The last remelt was held at the temperature of 2200.degree. F for five hours. The crucible containing the molten glass was then placed within an insulated fire brick which had been previously cut to provide an opening at the bottom thereof to permit the underside of the crucible to extend therethrough. The fire brick and crucible were then placed on a one-inch thick steel plate which had been preheated to a temperature of 900.degree. F with the fire brick being supported on 1/8 inch thick blocks and the undersurface of the crucible in contact with the steel plate. The top of the crucible was covered with quartz felt and the molten glass was permitted to cool to 1200.degree. F. The steel plate, fire brick and crucible were then placed in an annealing furnace having a temperature of 1180.degree. F for about a half hour, at the end of which time the furnace was turned off and the glass permitted to cool to ambient temperature at furnace rate.

The annular transparent glass ceramic which had formed had a radial index of refraction gradient. While the index of refraction was 1.767, it varied in a regular manner from the center to the edge with a different in the index of -.002. Very good radial symmetry was observed with respect to the index of refraction gradient.

Claims

1. A method for forming a transparent glass-ceramic body having at least two different indices of refraction for light transmittal therethrough, said glass-ceramic body being formed by thermally in situ crystallizing a thermally crystallizable glass body, comprising:

a. heating a first portion of said glass body for a period of time and at a temperature sufficient to nucleate and crystallize said first portion of said glass body to a transparent glass-ceramic having a first index of refraction; and

b. heating a second and different portion of the same glass body for a period of time and at a temperature sufficient to nucleate and crystallize said second portion to a transparent glass-ceramic having a second index of refraction which is different from said first index of refraction.

2. The method as defined in claim 1 wherein said thermally crystallizable glass body has a glass composition selected from the glass composition systems consisting of Na.sub.2 O--Ta.sub.2 O.sub.5 --SiO.sub.2, Na.sub.2 O--Li.sub.2 O--Ta.sub.2 O.sub.2 --SiO.sub.2, Na.sub.2 O--K.sub.2 O--Nb.sub.2 O.sub.5 --SiO.sub.2, K.sub.2 O--Nb.sub.2 O.sub.5 --SiO.sub.2, (Na.sub.2 O, Li.sub.2 O, K.sub.2 O)--Nb.sub.2 O.sub.5 --Ta.sub.2 O.sub.5 --GeO.sub.2 and SiO.sub.2 --Al.sub.2 O.sub.3 -Li.sub.2 O systems.

3. A method of forming a transparent glass-ceramic body having at least two different indices of refraction comprising the steps of:

a. forming a thermally crystallizable glass body of a predetermined shape;

b. heating a first portion of said glass body at an elevated temperature and for a period of time sufficient to thermally in situ crystallize said first portion of said glass body to a transparent glass-ceramic having a first index of refraction;

c. heating a second and different portion of the same glass body at an elevated temperature and for a period of time sufficient to thermally in situ crystallize said second portion to a transparent glass-ceramic having a second index of refraction which is different from the first index of refraction.

4. A method of forming a transparent glass-ceramic body having a concentric index of refraction gradient in said body, wherein said transparent glass-ceramic is formed by thermal in situ crystallization of a thermally crystallizable glass body, comprising heating concentric portions of a specific glass body at different elevated temperatures for a period of time sufficient to thermally in situ crystallize said concentric portions of said specific glass body to different transparent concentric glass-ceramic portions of the resulting glass-ceramic body having different indices of refraction.

5. A method of forming a desired optical path in a thermally crystallizable transparent glass body comprising heating a localized portion of said glass body in a desired configuration defining a path for a period of time and at an elevated temperature sufficient to thermally in situ crystallize said portion of said same glass body defining said path and thus crystallizing said path to a transparent glass-ceramic having an index of refraction which is higher than the index of refraction of the surrounding material, whereby light can be made to follow said path of higher index of refraction transparent glass-ceramic.

6. A method of forming an optical path in a thermally crystallizable transparent glass block for passage of light along said path comprising heating a portion within said block by directing a laser beam to that portion for a period of time sufficient to form a thermally in situ crystallized transparent glass-ceramic portion and moving said laser beam along said block to heat consecutive adjacent portions of said glass block to thermally in situ crystallize said portions to transparent glass-ceramic portions, said glass-ceramic portions having an index of refraction higher than the index of refraction of said glass surrounding said glass-ceramic portions and defining a continuous optical path for movement of light through said glass-ceramic portions.