TIE-40 Optical glass for precision molding

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PAGE 1/12 TIE-40 Optical glass for precision moling 1 Precision moling Hot processing of coarse anneale glass (also calle reheat pressing) is the preferre processing step for small lenses of stanar

1 PAGE 1/12 TIE-40 Optical glass for precision moling 1 Precision moling Hot processing of coarse anneale glass (also calle reheat pressing) is the preferre processing step for small lenses of stanar quality at high volumes The isavantage of this process is that the surface of these pressings is still rough an the pressings therefore nee aitional grining an polishing To overcome such restrictions an to reuce the processing expense, precision moling technologies for irect pressing of aspherical lenses have been evelope in the past years worlwie The process of precise moling (figure 1) starts from a polishe or firepolishe preform The surface of such a preform must be of very goo quality with respect to surface roughness an efects Such a preform can be a precision gob (a firepolishe preform prouce irectly from the melt without any aitional surface processing) or any other polishe lens preform (ball, isc or near shape generate out of raw glass by conventional hotforming or grining an polishing steps) During a precision moling process, the preform is shape into its final (often aspherical) geometry, while conserving the surface quality of the preform The moling process is a low temperature moling process with typical temperatures between 500 C an 700 C Low temperature processes helps to lengthen the operating lifetime of the mol material Figure 1: Overview on the precision moling process

2 PAGE 2/12 2 Optical Glasses for Precision Moling Precision moling is a state-of-the-art technology for the volume prouction of complex lenses Generally use mol materials in the moling process exhibit a thermal stability of up to 650 C However, a variety of so-calle low transformation temperature glasses (low Tg glasses) is require for the precision moling process Low Tg glasses have a Tg not higher than 550 C Table 1 shows an overview of such glasses n *2 ν *2 Internal transmittance *3 Color coe 80/5 Tg [ C] AT [ C] CTE [10-6 K -1 ] *4 AR *5 WR *6 Density [g/cm 3 ] P-PK53 * / P-SK57 * / P-LASF47 * / P-SF / N-FK51A / N-FK / N-PK52A / N-PK / *1 new evelope glasses, *2 fine anneale = catalog value, *3 at 10mm thickness an 400 nm, *4 value between C *5 AR = aci resistance accoring JOGIS, *6 WR = water resistance accoring JOGIS, Table 1: Optical glasses for precision moling The avantage of using low Tg glasses is that the press process can take place at lower temperatures hence leaing to a longer lifetime for the mol At the same time, the press process time can also be shortene Low Tg glasses in general are glasses with a glass transformation temperature below 550 C Furthermore, Low Tg glass compositions have been evelope to have low tenencies for evitrification an reuce reactions with mol materials at the moling temperature range P-SK57, P-LASF47, P-PK53 an P-SF67 are newly evelope low Tg glasses especially for use in precision moling (see table 1) The letter P inicates that these glasses are prouce for precision moling an are exclusively available as polishe preform or precision gob They are lea an arsenic free P- type glasses, in general, are coarse anneale glasses with tighter optical specifications (referre to as P-quality grae in the following sections) The shown N-type optical glasses can be use for precision moling mainly ue to their low glass transformation temperature These glasses are also available in P-quality grae N- type glass in P-Quality grae are also available only as polishe perform or precision gob The refractive inex an Abbe number ata given in the SCHOTT glass atasheets represent optical values of fine anneale glasses It is necessary to take into account that the cooling rate of a precision moling process is much higher than common fine annealing rates Therefore, the refractive inex of any precision mole lens will be significantly lower than the catalogue values of the glass being use Detaile explanations can be foun in chapter 5

3 PAGE 3/12 3 Specification of precision moling glass quality ( P-quality ) Precision moling glasses (P-type glasses) an traitional N-glasses that are available in Pquality fulfill the following stanar specifications: Optical glasses for precision moling are selecte base on the refractive inex value an Abbe number at a reference annealing rate of 2 K/h Tolerances for refractive inex are ± an for the Abbe number are ± 05% (step 3/3 accoring to the catalog base on a 2 K/h reference annealing rate) Customers will receive preforms with a test certificate certifying the refractive inex an Abbe number of the batch base on a reference annealing of 2 K/h The actual refractive inex of the glass within the elivery batch will iffer from this value but this causes no problem uring moling For all P-type glasses (eg P-SK57, P-PK53, P-LASF47 an P-SF67) these specifications will be automatically fulfille For N-FK5, N-FK51A, N-PK52A an N-PK51 P-quality grae preforms shall be orere In contrary to stanar optical glass specifications, the refractive inex an Abbe number tolerance steps are relate to a reference annealing rate This is necessary for precision moling glass to achieve a reproucible process epenent inex rop The reason is explaine in chapter 5 For internal quality, striae an other specifications, please refer to the stanar optical glass catalogue or contact your local sales representative 4 Precision moling glass preforms Low Tg glasses (P-type or P-quality glasses) will be elivere as (fire-) polishe preforms Several ifferent preform types are available Table 2 summarizes the properties of the four ifferent preform types, which can be chosen accoring to the final geometry of the presse optical element Preform types Diameter in mm Volume in mm³ Glass types Precision Polishe near Polishe isc Polishe ball gob net shape 4 to 40 > 3 > 3 > to 5000 > 50 > 50 > 1 not all glass types all glass types all glass types all glass types Table 2: Typical preform types an their properties suitable for precise moling Table 3 isplays an overview of the glass type availability in the various preform formats

PAGE 4/12 Precision gob Polishe near net shape Polishe isc Polishe ball P-PK53 *1 E E E E P-SK57 *1 E E E E P-LASF47 *1 E E E E P-SF67 x E E E N-FK51A x E E E N-FK5 x E E E N-PK52A x E E E N-PK51 x E

4 PAGE 4/12 Precision gob Polishe near net shape Polishe isc Polishe ball P-PK53 *1 E E E E P-SK57 *1 E E E E P-LASF47 *1 E E E E P-SF67 x E E E N-FK51A x E E E N-FK5 x E E E N-PK52A x E E E N-PK51 x E E E Table 3: Available preform types for the ifferent low Tg glasses (x = not available) 41 Precision gobs For many years, SCHOTT has been a reliable supplier for precision gobs mae of B270, which is a technical glass use for moling automotive healight lenses Since 2004, SCHOTT has also been a supplier of precision gobs mae out of optical glass Precision gobs are semi-finishe preforms with geometries near to the final geometry of the lens an a very smooth fire-polishe surface with excellent surface roughness They are manufacture using a unique continuous glass melting an hot forming process an can be prouce in volumes from 33 mm 3 up to 5000 mm 3 Possible imensions: Convex surface raius from 25 to 30 mm Minimum thickness aroun 38 mm Minimum center thickness/iameter ratio values between 05 an 1, the lower the ratio, the more critical the prouction Rounness between 05 mm an 0005 mm epening on size The exact gob geometry can be calculate by simulation prior to the prouction Figure 2: Precision gob (left) an a typical rawing (right)

PAGE 5/12 Figure 2 shows a typical rawing an picture of a precision gob (rawings can be generate from simulations) The precision gob fabrication is a very economical prouction process but is not

5 PAGE 5/12 Figure 2 shows a typical rawing an picture of a precision gob (rawings can be generate from simulations) The precision gob fabrication is a very economical prouction process but is not suitable for every precision moling glass Currently, only the P-type glasses can be provie as gob preforms (refer to table 3) Typical tolerances for a precision gob: Diameter tolerance: ± 02 mm own to ± 0005 mm Volume tolerance: 05% to 2% Surface quality: 20/10 scratch an igs (MIL O A) Surface roughness: < 1 nm rms Stress birefringence: < 120 nm/cm (suitable for precision moling) Precision gobs feature excellent surface quality Figure 3 shows an example of the result of an AFM (Atomic Force Microscopy) roughness measurement of a precision gob Precision gobs are inspecte an packe uner clean-room conitions, therefore a very goo cleanliness level can be achieve Customer specific cleanliness an packing requirements can also be fulfille upon request Figure 3: AFM roughness measurement results of a precision gob

PAGE 6/12 42 Polishe near net shape an polishe isc preforms The precision gob fabrication is a highly economical process for the mass prouction of preforms Nevertheless, there are geometries that

6 PAGE 6/12 42 Polishe near net shape an polishe isc preforms The precision gob fabrication is a highly economical process for the mass prouction of preforms Nevertheless, there are geometries that cannot be prouce using the precision gob process Currently, the precision gob process is restricte to biconvex shape preforms The size of the gobs is also restricte to a minimum volume of 120 mm 3 In aition, SCHOTT offers lens (near net shape) an isc shape preforms that can be prouce base on iniviual customer esigns using classical lens prouction processes The surface quality is equal or better 40/20 scratch an igs an the roughness is less than < 2 nm rms It is also possible to achieve very tight volume tolerances Please ask your sales representative for more etaile information 43 Polishe ball preforms Ball lens preforms are available in iameters bigger than 08 mm Their main application can be in small cellular phone camera lenses, for instance Typical tolerances for ball preforms are: Diameter tolerance ± 5 µm (smaller tolerances are possible on request) Surface quality: 40/20 scratch an igs (MIL O A) Surface roughness: < 2 nm rms (smaller tolerances are possible on request) Figure 4: 2 mm size polishe ball preform (left) an AFM roughness measurement (right) Customer specific cleanliness an packing requirements can be fulfille upon request Figure 4 shows a typical atomic force microscope roughness evaluation of a 2 mm size ball preform The roughness is in the range of 09 nm rms

7 PAGE 7/12 5 Influence of the moling process on the refractive inex an Abbe number The optical ata for a glass type are etermine by the chemical composition an thermal treatment of the melt The annealing rate in the transformation range of the glass can be use to influence the refractive inex within certain limits (epening on the glass type an the allowable stress birefringence) Basically, slower annealing rates yiel higher refractive inices In practice, the following formula has shown to be reliable: n ( hx ) = n ( h0 ) + mn log( hx / h0 ) (1) ν h ) ν ( h ) + m log( h / ) (2) ( x = 0 ν x h0 h 0 h x m n m ν Original annealing rate New annealing rate Annealing coefficient for the refractive inex epening on the glass type Annealing coefficient for the Abbe number epening on the glass type More etails can be foun in [2] Values for annealing coefficients of some glasses for precision moling are shown in table 4 m n [10-5 ] m ν N-FK51A P-LASF P-SK Table 4: Annealing coefficients for selecte precision moling glasses The annealing rate influences the refractive inex an the Abbe number simultaneously an is the reason for the change of refractive inex after moling This change of refractive inex an Abbe number is calle inex rop 51 Coarse annealing of optical glass After the melting an casting process, the precision moling glass is coole own in a coarse annealing lehr at a high annealing rate The annealing rate epens on the imensions of the strip Typical values are between 50 an 100 K/h In orer to control the refractive inex through the melting an casting process, samples are taken irectly from the melt at a given frequency The refractive inex an Abbe number of the samples will be measure base on a special proceure at a reference annealing rate of 2 K/h Base on this 2 K/h reference, the optical values uring prouction can be controlle in a reliable way The glass for precision moling is selecte in such a way that the 2 K/h reference value of the glass is within step 3/3 of the catalogue value The real value of the glass will be ifferent from this value ue to the reasons given above However, this ifference is not relevant for the application (Please refer to the next chapter)

8 PAGE 8/12 52 Influence of the precision moling process on the optical position of the glass: inex rop As mentione in the beginning of chapter 5, an annealing process influences the refractive inex an the Abbe number of an optical glass, simultaneously The iagram in figure 5 shows schematically the inex rop behavior of N-PK51 base on an exemplary moling process The iagram isplays the Abbe number versus the refractive inex (n ) for N-PK51 The rectangular boxes inicate the tolerance limits of tolerance step 3/3 for the refractive inex an the Abbe number of N-PK51 The catalogue value is locate in the center of these boxes The re rhomb shape ots within the green box are sample 2 K/h reference values of real glasses from ifferent melts The straight line in the iagram characterizes the annealing behavior of N-PK51 The slope is characteristic for each glass type For every N-PK51 glass melt like those inicate with a re rhomb, the annealing slope is the same The annealing rates printe along the line mark the optical position that will be achieve if this glass woul be anneale using the isplaye annealing rate The precision moling glass in general is coarse anneale glass In the iagram, for example, the big re rhomb has a 2 K/h value within the green tolerance limits whereas the real refractive inex of the current glass lies somewhere between 50 K/h an 100 K/h on the annealing line Usual fine annealing rates range from 05 to 2 K/h For pressings, annealing rates between 2 an 10 K/h are use In normal annealing processes, the annealing rate is ajuste to achieve a specific refractive inex range with at the same time low stress birefringence an sometimes also a goo homogeneity Figure 5: Annealing line an inex rop behavior of N-PK51

9 PAGE 9/12 In contrast to these rather low an well efine rates, the annealing in a precision moling process is very fast an it is in most cases iniviual to the process of the customer an the geometry of the glass part In general, there is no aitional annealing process to ajust the refractive inex after moling because of the high risk of ecreasing the surface quality Therefore, the rate is not known in most cases but fixe an reproucible Aitionally, the annealing rate is not necessarily constant uring the cooling process Nevertheless, by estimating the final optical position of the precision mole glass, an average annealing rate can be assigne Typical average annealing rates for precision moling are between 1000 K/h an K/h (or higher) Accoring to the high annealing rate, the refractive inex an Abbe number of N-PK51 is shifte to much lower values This shift is calle inex rop In Figure 5 the values after precision moling are marke by orange triangles The inex rop is efine as the ifference between the final refractive inex an Abbe number after moling an the initial refractive inex an Abbe number reference values base on a 2 K/h annealing: n = n ( 2K / h) n ( after moling) (3) ν = ν ( 2K / h) ν ( after moling) (4) The amount of inex an Abbe number rop for N-PK51 is inicate with orange arrows in figure 5 Figure 5 also shows that the inex rop is approximately the same for all N-PK51 glasses within the initial tolerance range (using the same process) Therefore, the scattering in optical position between the mole glasses also remains the same uner the assumption that the precision moling process is highly reproucible The tolerance winow simply shifts to a new central position It shoul be note that it is essential for the reproucibility to always etermine the inex rop from initial refractive inex values base on the same annealing rate Determination of the refractive inex rop base on the actual refractive inex values of the coarse anneale glass will lea to errors ue to unefine annealing conitions Therefore, it is recommene to always rely on the values base on one reference annealing rate of 2 K/h The inex rop iffers between glass types because every glass has a ifferent annealing slope Figure 6 shows the relative change in refractive inex an Abbe number base on the annealing slopes for the precision moling glasses P-SF67, P-LASF47, P-SK57, P-PK53, N- FK51A, N-PK51, N-PK52A an N-FK5

10 PAGE 10/12 Figure 6: Theoretical inex rop for ifferent precision moling glasses assuming an average annealing rate of 3500 K/h an ientical geometry In the given simplifie example it can be seen that the inex change for an average annealing rate of 3500 K/h ranges, epening on the glass type, from to for the refractive inex n The Abbe number changes in the range of +020 to 027 P-SF67 behaves ifferent compare to the other glasses, because the Abbe number increases with increasing annealing rate As mentione before, this is a simplifie view In reality, there are other factors influencing the real inex rop (eg geometry, moling process, thermal properties of the glass) Nevertheless, a glass with a steep annealing slope will always lea to a large inex rop A etaile list of the inex rop for SCHOTT precision moling glasses at the an e wavelength base on our own moling process an part geometries is isplaye in table 5 Moling temperature Before moling 2 K/h reference catalog values After moling range [ C] n n e ν ν e n n e ν ν e P-PK , , ,22 65,92 1,5232 1, ,0 65,7 P-SK , , ,60 59,36 1,5843 1, ,4 59,1 P-LASF , , ,90 40,66 1,8016 1, ,8 40,5 P-SF , , ,40 21,23 1,8998 1, ,6 21,4 N-FK51A , , ,47 84,07 1,4847 1, ,2 83,8 N-FK , , ,41 70,23 1,4850 1, ,2 70,0 N-PK52A , , ,61 81,21 1,4952 1, ,3 80,9 N-PK , , ,98 76,58 1,5267 1, ,7 76,3 Table 5: List of inex rop base on the SCHOTT moling process

11 PAGE 11/12 The inex rop behavior for other wavelengths can be provie on request 6 Precision moling glass: Test report The stanar test report for precision moling (see example in figure 7) is base on the test report for fine anneale glass with some exceptions Figure 7: Example of a test report for precision moling glass

12 PAGE 12/12 In aition, the test report contains the glass transformation temperature The refractive inex values given are reference values base on a reference annealing rate of 2 K/h The actual refractive inex of the precision moling glass batches will be ifferent For the application it is necessary to always refer to the reference values It also contains the spectral internal transmission at a wavelength of 400 nm an a sample thickness of 10 mm (central value between maximum an minimum) an the color coe 6 Literature [1] R Jaschek, C Klein, C Schenk, K Schneier, J Freun, S Ritter, Development of a new process for manufacturing precision gobs out of new evelope low Tg optical glasses for precise pressing of aspherical lenses, Proc SPIE Vol TD03, pp 50-52, 2005 [2] Technical Information TIE-29 Refractive Inex an Dispersion Optics for Devices SCHOTT North America, Inc 400 York Avenue Duryea, PA USA Phone: +1 (570) Fax: +1 (570) sgt@usschottcom wwwusschottcom/optics_evices

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