Patent Application
20060192306
N/A
N/A
Fabricating optical systems such as head mounted displays often requires assembling several optical components. See for example U.S. Pat. Nos. 6,538,624; 6,462,882; 6,147,807. Some optical system designs include an air gap between the optical components. This creates the necessity for a housing to hold the elements in mechanical alignment as well as a method of protecting the inner surfaces of the components from dust, oil and other contamination. Other optical systems allow the gap to be filled by some other medium. These systems can be built, for example, by embedding reflective, diffractive, polarizing or other optical components into an optically transparent solid medium. See for example U.S. Pat. Nos. 5,886,882, 6,091,546, and 6,384,922. An advantage of this approach is that the resulting system is a monolithic solid part. The relative positions of the elements are permanently fixed and there are no exposed inner surfaces to become contaminated with dust or condensation.
In practice, the actual manufacturing of embedded optical systems may be quite difficult. It is necessary to take into account the differences of the coefficients of thermal expansion in the embedded optical components and embedding medium, adhesion strength between the embedded optical components and embedding medium, birefringence and distortion in the final product, aging processes and so on. The most obvious embedding media are polymer compounds. However, these may have a number of major disadvantages. A critical concern is shrinkage of the liquid monomer or prepolymer during the polymerization and cross-linking step. This can cause optical distortion and change the relative positions of the embedded components. In addition polymerization that initiates on the surface of the embedded components may lead to preferred molecular orientation in the solidified polymer. This may result in birefringence in the completed part.
Preferably, for the purpose of fabricating head mounted display systems, the cured embedding material must have physical and optical properties that are similar to the materials used in the production of ophthalmic lenses. The material must have high transparency in the visible spectra (transmission at least 85%), high Abbe number to avoid chromatic aberrations, good impact strength to pass the FDA ball drop test, low color or yellowishness index, good resistance to static stress and scratch resistance, and low water absorption level. The most common optical polymer currently used for ophthalmic lens production is diethylene glycol bis (allyl carbonate) also known as CR-39. This material has 13-16% shrinkage upon curing, making it challenging to use for embedded systems. The other commercially available polymers for lens casting have shrinkage at least 6% that is also excessive for the fabrication of embedded systems.
There are several approaches to reduce shrinkage on curing in the optical polymers. For example, Herold et al. in U.S. Pat. No. 5,952,441 suggested partially pre-polymerizing a mixture of ethylenically unsaturated compounds prior to casting embedded systems, to minimize shrinkage during the final cure. The pre-polymerization process is not easy to control and polymerization does not stop completely when the desired degree of polymerization had been achieved. Also, due to the requirement for a low viscosity prepolymer material, the cured polymer may still have substantial shrinkage.
Another approach suggested by Soane in U.S. Pat. No. 5,114,632 is to continue feeding liquid material into the mold during the curing process to compensate for the shrinkage. Although it is probably possible to avoid mechanical stress by this approach it will cause variation in the molecular weight of the polymer in the body of the device that will result in optical index variation and image distortion.
Soane and Huston in U.S. Pat. No. 6,380,314 proposed a method of near-net shape casting from a reactive plasticizer within an entangled dead polymer. In this approach solid state fully polymerized material is dissolved in the polymerizable compound or composition used to embed the optical components, thus reducing the amount of shrinkage during subsequent cure. However, in this case the curable mixture is semi-solid and can not be used in embedded optical systems such as for head-mounted displays.
The present invention relates to a method of producing an optical system for head mounted displays that includes inorganic optical components or polymer optical components such as plates, mirrors, or lenses, embedded in the transparent polymeric, liquid or gel matrix (
The steps to create an embedded optical system include cleaning and pretreatment (optional) of the optical elements, positioning of the optical elements prior to encapsulation, mold assembly, a molding or encapsulating process, overcasting (optional), surface finishing or polishing (optional), and surface coating (optional) (
The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
To avoid contamination on the embedded optical parts it may be necessary to clean the optical elements to be embedded prior to the embedding process. See step 1 in
Prior to the molding, the elements to be embedded may be pretreated to improve adhesion by various techniques. Improving the chemical and physical bonding between embedded elements and the embedding substrate prevents delamination and formation of cavities that causes degradation of the optical properties. The embedded optical parts can be treated with corona discharge, flame, plasma, or the surface may be etched with alkali solution, as would be understood by those of skill in the art. Also, primers, surface grafting with siloxane, silane, borate, metallorganic and other coupling agents can be used if necessary, as also would be understood by those of skill in the art.
The optical elements are then positioned for molding. See step 2 in
During positioning, the optical elements can be supported in place by vacuum.
The elements may be mechanically aligned by a variety of ways, for example, using a mechanical fixture, pick-and-place equipment or other replication equipment prior to gluing.
Another way to accomplish the positioning of the optical elements is to place them into openings cut into a lens fabricated by methods described here or known in the art, including casting, injection molding, and/or cutting. This approach is shown in
The initial position of the elements can be adjusted to compensate for shifting due to shrinkage during the curing process. The positioning and alignment of the elements may also utilize optical methods to check the alignment of the elements. For example, a laser beam or autocollimator may be used to check the angle of fold mirrors or the centration of curved surfaces. Optionally, active optical alignment may be used, in which process the mechanical position of the elements may be adjusted while monitoring the optical performance of the system. The optical performance of the aligned system or subsystem during alignment will generally differ from the optical performance of the completed parts. In this case, optical modeling may be necessary to calculate the expected performance of the subassembly on the mold plate and to design appropriate alignment procedures.
In step 3 (
The mold is then filled with a suitable low-shrink polymerizable optical casting compound (step 4,
An acceptable alternative to a highly amorphous, non-birefringent embedding material would be a highly oriented material with carefully controlled birefringence. In this case, it is desirable that the material polymerize along a preferred direction, usually (although not necessarily) parallel to the primary optical axis direction. This type of material may be highly birefringent, but does not affect the direction of polarization of the light or the image quality because all the ray paths see the same optical index distribution. Such an approach is used, for example, in the fabrication of optical fibers, where the fibers are subjected to mechanical stress to orient the material's polymerization direction and preferred optical axis with the direction of propagation of the light. The preferred orientation of the embedding matrix may be established by a variety of methods known in the art, for example prior surface preparation, thermal gradients, pressure or stress gradients, or magnetic or electrical methods. In this case, the casting compound should have a high level of molecular orientation.
Additives may be added to the casting composition to adjust certain properties, as would be known in the art. For example, polymeric and monomeric non-reactive optical plasticizers can be added to the composition to reduce internal stress in the polymer, as would be known in the art. The optional addition of plasticizers can be used to adjust the refractive index, for example, to match the refractive index of the embedded compounds. Examples of such plasticizers include monomeric plasticizers diisononyl phthalate, bis (2-ethylhexyl) cebacate, triisohexyl trimellitate, dipropyleneglycol dibenzoate, 1,2 propanediol dibenzoate, 2-nitrophenyl octyl ester, 2-butoxyethyl adipate, osooctyl tallate, diisodecyl glutarate, dicycloxyethyl phthalate, tricresylphosphate, polymeric plasticizers—epoxidated soybean oil, Bayer's phthalic polyesters such as plasticizer CEL and Ultramol® PP, Bayer's adipic polyesters such as Ultramoll® I and Ultramoll® II. Reactive plasticizers such as polyethylene glycol dioleate, Ultramoll® M and Cardolite® NC-513 can also be used to relieve internal stress-birefringence and adjust the refractive index.
Matching the refractive index and Abbe number dispersion of the embedded elements and the cured casting compound where possible is important for both cosmetic and optical reasons. This is likely to be desirable when the embedded element uses a clear glass or plastic component for mechanical support of a coating or another element. For example, if a glass plate coated with a reflective coating is embedded in the system, using an index matched glass and polymer matrix pair reduces the appearance of the glass and creates the impression of the reflective film floating unsupported within the matrix. Furthermore, an index mismatch between the glass support element and the embedding matrix can create distortions in both the display image and the see-through image because of prismatic and similar optical effects. The refractive index of the optical elements should be at least within 0.1 of the refractive index of the cured filling compound, and preferably within 0.05 and even more preferably within 0.01.
Alternatively, the monomer can also be polymerized to gel consistency to be used in gel filled systems. Those systems could be formed by polymerization, partial polymerization, polymerization in the presence of plasticizers or reactive or non-reactive dilutants or by swelling or dissolving polymer in the plasticizer or solvent.
Preferably, the plasticizer is compatible with the polymer matrix and is used in concentrations that will not cause phase separation or migration of the plasticizer inside the polymer or to the surface. The polymer plasticizer concentration can be 1 to 60%, preferably 3 to 30%, and more preferably 5 to 25%. However for gels the plasticizer concentration can be as high as 95%. A mixture of different plasticizers can also be used in the composition. It is preferable to select plasticizers that will enhance hydrophobic properties in the material. This will reduce moisture absorption in the final polymer, which is important for the environmental stability and prevents refractive index variations.
Other additives may be used to control the polymerization process. To reduce the heat of reaction that may cause stress-birefringence in the material, inhibitors may be added to the polymer composition, the choice of inhibitor depending on the polymer system used, as would be known in the art. Inhibitor concentrations are usually below 5.0%, preferably below 3.0%. For some polymer systems, it may be necessary or desirable to use catalysts to conduct polymerization, achieve high conversion level or accelerate the polymerization process, the choice of catalyst depending on the polymer system used, as would be known in the art. The catalyst concentration in the system should usually fall below 3.5%, and preferably below 1.0%. In some polymer systems, particularly for free radical polymerization, it may be helpful to add a chain transfer agent, the choice depending on the polymer system used, as would be known in the art. Usually their concentrations should be below 0.5%
Stabilizers can be used in the system to prevent changes in the optical, mechanical, or chemical properties of the polymer over time, as would be known in the art. Organosilicone and metal-organic coupling agents may be added to the resin in concentrations that do not affect the visible light transmission of the finished part, as would be known in the art. These additives reduce the mechanical stress in the finished embedded optical system that contributes to refractive index variations and birefringence. Although the usual concentrations of the coupling agents are between 0.3 to 5.0%, they can be added in concentrations up to 35.0% and be incorporated into the polymer by chemical bonding.
To avoid entrapment of air in the polymer, the casting compound should be degassed prior to introduction into the mold, as would be known in the art, and the casting process carried out under pressure. In addition, air-release agents can be added to the casting mixture, as would be known in the art. Preferable concentrations for the above materials are 0.1 to 3.5%.
The casting step (step 4,
Typically, the polymerization process occurs at a temperature greater than room temperature. Differential thermal expansion during the cure cycle can result in locking in mechanical stress as the system returns to room temperature. For heat curing systems, the temperature must be kept at the low end of the allowable solidification temperature to avoid exothermic reactions that may cause optical and mechanical stress. If post-curing is required, the temperature profile must be selected to achieve high conversion level while keeping the heat generated by the exothermic reaction to a minimum. It is desirable to accomplish solidification of the composition at room temperature if possible, or alternatively at the minimum temperature required for the process. Temperature ramps during polymerization, cure, and post-cure processing must be controlled to limit or minimize the introduction of mechanical stress in the finished part, as would be known in the art. The particular temperatures and pressures and process rates depend on the particular polymer system used, as would be appreciated by those of skill in the art.
For radiation curable systems, for example UV curable systems, the energy level must be selected to achieve complete monomer conversion. It is preferable to cure such systems in thin layer increments. In this case, the casting compound is added to the mold assembly in layers, each layer being cured before the next layer is added. The optical elements are in this manner gradually embedded in the casting compound. Referring to
After molding, the cured component or puck may optionally be post-treated in various ways. To prevent the appearance of surface imperfections, the cured puck 1210 can be placed in an overmold 1200 and then overcast with the same casting material or in a different material with optical index matched to the embedding material, as shown in
An optional grinding or polishing step may be desired. (Step 6,
A surface coating step may be desirable (step 7,
A corrective optical element 1310 can be permanently or temporary attached to the above system 1300 as shown in
If the element is molded on the surface of the planar system, a transparent film can be placed on the planar surface between the composite optical system puck and the added optical element by means of gluing or laminating before overmolding the corrective element. This intermediate film allows for the easy removal of the corrective optical element without destroying the planar optical system. Also, in planar optical systems that use total internal reflection (TIR), the intermediate film may have a refractive index that is lower than the refractive index of the planar system, to maintain the optical conditions that allow for TIR.
The invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims.
