WO2001037030A1

WO2001037030A1 - Thermally stable birefringent prism assembly

Info

Publication number: WO2001037030A1
Application number: PCT/IL2000/000746
Authority: WO
Inventors: Boaz Arnon; Inon Berent; Carina Reisin; Rony Reuveni; Michael Gutin
Original assignee: Unic View Ltd.
Priority date: 1999-11-14
Filing date: 2000-11-14
Publication date: 2001-05-25
Also published as: AU1298001A; IL132928A0

Abstract

A thermally stable polarizing optical component, such as a polarizing beam splitter, which overcomes the effects of thermally induced stress birefringence by using one or more elements constructed of a material with a low optical stress coefficient, typically less than 1x10-6 mm2/N. Such a material reduces the termal stresses resulting from passage of the beam itself, either because of a non-uniform beam cross section, which heats some parts of the element more than other parts, or from a uniform beam cross-section, but with non-uniform heat dissipation from the element's surfaces. The presence of thermal stress within the element results in stress-induced birefringence because of the optical stress coefficient. In addition, thermally induced stress birefringence is reduced by the use of materials which reduce the formation of the thermal gradients themselves, or reduce the level of mechanical stess caused by any thermal gradients nevertheless present.

Description

THERMALLY STABLE BIREFRINGENT PRISM ASSEMBLY

FIELD OF THE INVENTION

The present invention relates to the field of prismatic polarizing beam splitter assemblies, especially for use in applications requiring high light intensities.

BACKGROUND OF THE INVENTION

In many applications it is required that light be transmitted through an optical system with a predetermined and stable state of polarization. Examples for such systems are high performance projectors such as LCD projectors, which, for their operation, require light with a determined state of polarization. The polarizer in such a high performance projection system is one of the key system components since changes in the required state of polarization result in a decrease of the system performance and in particular, poor system contrast.

The polarizing and the splitting of the beams in such systems is performed by means of a polarizing beam splitter, which typically consists of a pair of transmissive prisms, between which is sandwiched a thin film stack, or a material with anisotropic optical density, such as an oriented liquid crystal layer or liquid crystal polymeric layer. The purpose of the prisms is to ensure that the incoming light is incident on the thin layer at such an angle as to ensure correct polarization. It is well known that unwanted changes in the polarization can be caused either by residual birefringence arising from non-homogeneous cooling of the transmissive material during manufacture, or by stress birefringence arising from improper mechanical assembly or clamping of the components. Residual birefringence can be minimized by using materials such as highly annealed glasses, which have very low residual birefringence. Stress birefringence can be minimized by using well designed, low-stress mounts for the optical components. Use of such prior art polarizing beam splitters at high light intensities have not generally resulted in sufficient optical performance, as manifested by severe degradation of the extinction ratio of such polarizing beam splitters. The attempts to increase the thermal stability of the beam splitter and thus to overcome this problem have, in the main, been directed at the ability of the polarization- separating layer to withstand the high local temperatures generated by such high power beams. Similarly, methods have also been used of improving the heat resistant properties of the various cements used to attach this layer to the transmissive prisms. As an example, in U.S. Patent No. 5,042,925 to D.J. Broer, A. deVaan and J. Brambring, is described a polarization-sensitive beam splitter constructed of a birefringent layer in the form of a cured synthetic resin layer, which also acts as an adhesive for the two encasing transmissive elements of glass. This produces, according to the inventors, "a beam splitter which is little temperature dependent, and which is also temperature resistant."

In an article entitled "High Performance Thin Film Polarizer for the UV and Visible Spectral Regions" published in Applied Optics, Vol. 20, pp. 111-116, Jan. 1981, the authors J. A. Dobrowolski and A. Waldorf propose replacing the solid prisms hitherto used in such devices, with a hollow prism filled with an immersion liquid with the appropriate refractive index. This is done in order "to overcome the problem of reduction in the degree of polarization caused by the residual birefringence in prism materials". In this way, they are able to use a MacNeille prism polarizer at high laser powers without undue thermal degradation, but the resulting assembly is inconvenient to manufacture and use, and has not thus find widespread use outside of research laboratory applications.

To the best of the inventors' knowledge, such prior art polarizing beam splitters cannot withstand the high level of light intensities currently in use without suffering noticeable degradation in performance, and they therefore need additional polarization filtration. This results in complex system design and still does not restore full system performance. The applicants' own experiences with polarizing beam splitters constructed using different types of glass, such as SF 10 manufactured by Schott Glaswerke of Mainz, Germany, has been that such prisms are unable to maintain their polarizing performance with the very high intensity light beams required by present day, state-of-the-art, projection systems, and this lack of thermal stability makes them unsuitable for commercial use. There therefore exists an urgent need for polarizing beam splitters, and other polarizing optical components, which are sufficiently stable thermally to allow the use of high intensity light without detrimental effects arising from birefringence effects.

The disclosures of all publications mentioned in this section and in the other sections of the specification, are hereby incorporated by reference, each in its entirety.

SUMMARY OF THE INVENTION

An investigation of the effects which cause the decrease in performance in polarizing beam splitters due to thermal instability, even those constructed with all of the heat resistant features known from the prior art mentioned hereinabove, shows that the degradation arises primarily from depolarization of the light when used at high luminous flux. It is likely that a major source of this deterioration of the polarization state is from birefringence introduced into the prisms because of the passage of the beam itself. Even though the prisms are selected from a glass with very low residual birefringence, and are assembled in a stress-free manner, thus reducing mechanically introduced birefringence, the very passage of such a high intensity beam through the prism results in thermal stresses within them. These thermal stresses can result either because the beam does not have a uniform cross section, and thus heats some parts of the prism more than other parts, or even from a uniform beam because the heat dissipation is not geometrically uniform through all of the prism surfaces. The presence of thermal stress within the prisms results in a stress-induced birefringence, by means of the optical stress coefficient. To the best of the inventors' knowledge, none of the polarizing beam splitters described in the prior art, have been constructed to increase thermal stability by taking such thermally induced stress birefringence effects into consideration. Besides the polarizing beam splitters described hereinabove, a number of others have been described in the prior art. such as in U.S. Patent No. 4.702.557 to F. K. Beckmann. H. Dotsch. and W. Hoppe. in U.S. Patent No. 5,042.925, to D.J. Broer. A. deVaan and J. Brambring, in U.S. Patent No. 5,054.888, to S. D. Jacobs, K. L. Marshall and K. A. Cerqua, and in U.S. Patent No. 5.912,762 to L. Li. and J. A. Dobrowolski. In none of these patents, to the best of the present inventors' understanding, is there any specific reference to the opto-mechanical and thermal conductivity properties of the material from which the prism is made. Reference is made only to the general type of the material. For example, in patent No. 4,702.557, it is stated that "(t)he components of each polarization-sensitive beam splitter (the prism and the rhombohedral plate) can now be made of glass or a suitable transparent plastic" and "instead of glass, any other amorphous optically transparent material with a suitable refractive index n, for example a plastic, may be used for the prisms and rhombohedral plates". In patent No. 5,042,925, the inventors refer to the prism material as a "transparent element consist of non birefringent material" and "(t)here elements may be composed of ordinary glass or a synthetic resin having the same single refractive index". In Patent No. 5,054,888 the inventors refer to the material of the prisms as being an "optically transparent substrate" and in Patent No. 5.912,762 the reference is alternatively to "light transmissive substrates", and to the fact that "(i)n the visible, the substrates may be made of various glasses and various plastics".

The present invention thus seeks to provide a new polarizing beam splitter which is thermally stable, as a result of overcoming the effects of thermally induced stress birefringence by means of specifically selecting and using a prism material with a low optical stress coefficient. The interpretation of what constitutes a thermally stable polarizing beam splitter is determined by the absence of significantly noticeable changes in polarization performance when the beam splitter is used in its intended application at its maximum or rated power level.

According to further preferred embodiments of the present invention, certain other parameters of the prism material also need to have suitably selected values. Even though the use of a low optical stress coefficient material enables very high luminous fluxes to be applied to the polarizing beam splitter of the present invention without the resultant thermal stress causing undue deterioration of the optical polarizing properties, other characteristics can also be selected to reduce the formation of the thermal gradients themselves, and yet others to reduce the level of mechanical stress caused by any thermal gradients nevertheless present.

Experimental measurements and theoretical heat flow calculations have shown the effects on the transmission of polarized light on different materials. As a result of the experimental results, a thermally stable polarizing prismatic beam splitter is described, according to another preferred embodiment of the present invention, whose prisms are constructed of a material, preferably having a low optical stress coefficient.

There is thus provided in accordance with a preferred embodiment of the present invention, a thermally stable polarizing beam splitter consisting of two prisms constructed of a material with a low optical stress coefficient, between which is sandwiched an aligned liquid crystal layer or liquid crystal polymeric layer, or a thin film stack. According to further preferred embodiments of the present invention, the material is selected to have at least one additional property, selected from a low thermal expansion coefficient, a low Young's Modulus, and a high thermal diffusion coefficient.

According to further preferred embodiments of the present invention, the prism material is a glass such as SF 6 manufactured by the Schott Glaswerke of Mainz. Germany, or the glass FeD E05-25 manufactured by Corning Optical Glasses of Avon, France, or the glass FD6 manufactured by the HOYA o

Corporation of Japan, or any other glass with similar required physical properties, produced by these or any other manufacturer.

Such a polarizing beam splitter can be successfully utilized in the construction of high performance compact designs of projection systems, such as those described in US Patents Nos. 5.833.338 and 5.946,139. and in the co-pending U.S. Patent Application No. 09/056107. Furthermore, since both polarization channels are of sufficiently high performance, and no additional polarization filtration is needed, the polarizing beam splitter can be utilized in a configuration where it operates both as a polarizer and an analyzer in a single element.

Though unwanted changes in polarization performance is a problem commonly contended with in the design and use of polarizing beam splitters, because of their widespread use in high intensity projection systems, and although it is largely in this context that the problem is analyzed and solutions proposed in the present application, it is to be understood that the problem is one which can affect all polarizing components used in systems transmitting high intensity optical beams, whose performance is sensitive to the level of polarization maintained. It is therefore to be understood that the present invention can equally well be applied for use in polarizing or non-polarizing components other than beam splitters, used for the transmission of high intensity optical beams, and whose operation depends on their polarizing properties, and also when constructed of optical elements other than prisms.

Furthermore, the present invention is also equally applicable for use in recombinant polarizing beam splitters, popularly known as 100% cubes, such as are described in U.S. Patent No. 4,913.529 to J. F. Goldenberg and J. D. Eskin.

In accordance with yet another preferred embodiment of the present invention, there is provided a polarizing optical component, consisting of at least one transparent element having a low optical stress coefficient such that, when used at its rated luminous power level, significantly noticeable thermally induced stress birefringence effects are absent, so that the component is thermally stable. η

There is further provided in accordance with yet another preferred embodiment of the present invention, a polarizing component as described above, and wherein the optical stress coefficient is less than lxlO^"6 mm²/N.

In accordance with still another preferred embodiment of the present invention, there is provided a polarizing component as described above, and containing two transparent elements.

There is further provided in accordance with still another preferred embodiment of the present invention, a polarizing component as described above, and wherein the transparent element also has at least one of the properties selected from the group consisting of high thermal diffusion coefficient, low Young's modulus and low thermal expansion coefficient.

In accordance with further preferred embodiments of the present invention, there is also provided a polarizing component as described above, and wherein the material from which the transparent element is made is a glass, which could be one of the type SF 6. SF 57. FeD E05-25 or FD6.

There is provided in accordance with yet a further preferred embodiment of the present invention a polarizing component as described above, and wherein the glass has a refractive index greater than 1.7 in the visible range.

There is even further provided in accordance with a preferred embodiment of the present invention, a polarizing component as described above, and having a birefringent layer interposed between the two transparent elements. The birefringent layer may preferably be a liquid crystal layer or a polymer liquid crystal layer

There is also provided in accordance with a further preferred embodiment of the present invention, a polarizing component as described above, and having a thin film stack interposed between the two transparent elements.

In accordance with yet another preferred embodiment of the present invention, there is provided a projection system consisting of at least one polarizing beam splitter as described above.

There is further provided in accordance with yet another preferred embodiment of the present invention a polarizing component consisting of at least one transparent element constructed of a glass of the type SF 6.

In accordance with still another preferred embodiment of the present invention, there is provided a projection system consisting of at least one polarizing beam splitter constructed of a glass of one of the types SF 6, SF 57, FeD E05-25 or FD6.

There is further provided in accordance with still another preferred embodiment of the present invention, a method of constructing a transparent element of a polarizing component, consisting of the steps of selecting a set of suitable materials for the transparent element, the materials having a low optical stress coefficient, and selecting from the set. a material additionally having at least one of the parameters selected from the group consisting of a high thermal diffusion coefficient, a low thermal expansion coefficient, and a low Young's modulus.

In accordance with still another preferred embodiment of the present invention, there is further provided a method of increasing the thermal stability of a polarizing element, consisting of the step of constructing the polarizing element using a material having a low optical stress coefficient. Furthermore, the optical stress coefficient preferably has a value of less than lx lO^"6 mm²/N.

In accordance with yet anot er preferred embodiment of the present invention, there is provided a method of increasing the thermal stability of a polarizing element as described above and wherein the material additionally has at least one of the parameters selected from the group consisting of a high thermal diffusion coefficient, a low thermal expansion coefficient, and a low Young's modulus.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:

Fig. 1 schematically illustrates a polarizing beam splitter, constructed and operative according to a preferred embodiment of the present invention:

Fig. 2 shows schematically an experimental arrangement for measuring the change in the polarization of light passing through a glass slab heated at a steady rate to a fixed temperature:

Fig. 3 is a graph showing the results of an experiment comparing the thermally induced stress birefringence in BK 7 glass, and in SF 6 glass: and

Fig. 4 is a table showing the results of experiments comparing the thermally induced stress birefringence in 7 different types of glass.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is now made to Fig. 1, which schematically illustrates a polarizing beam splitter 10, constructed and operative according to a preferred embodiment of the present invention. The embodiment shown in Fig. 1 is used as an example of a class of optical components whose operation depends on their polarizing properties, and which are used for the transmission of high intensity optical beams, where the performance of the system in which they are used is sensitive to the level of polarization maintained.

As is known in the art. the polarizing beam splitter is constructed of two prisms. 12 14. of an optical grade glass. Between the prisms is sandwiched a birefringent layer 16 of anisotropic optical material, such as an aligned liquid crystal layer or liquid crystal polymeric layer, or a thin film stack. An incident beam of light 18 enters the prism 14 and is incident on the birefringent layer 16. where it undergoes polarization. The ordinary beam Po undergoes total internal reflection, and exits from the prism 22 diverted from the incident beam direction. The extraordinary beam Pe. 20, passes straight through the beam splitter undiverted.

In order to achieve a high extinction ratio, the refractive index of the glass must be as close as possible to the high refractive index of the birefringent layer. The difference between the high refractive index and the low refractive index of the birefringent layer is known as Δn. The higher the value of Δn. the larger the acceptance angle at which the ordinary polarized light is reflected, and hence the larger the acceptance angle for which the prism operates as a polarizer. As is known in the art. it is thus desired to use a highly birefringent material for the layer between the two prisms. Use of a layer having a high Δn thus also allows the use of smaller prisms without reducing the active aperture of the component.

Liquid crystal and liquid crystal polymers have large values of Δn and therefore are suitable for this type of polarizing beam splitter, as is known in the art. The value of the low refractive index for these material is of the order of 1.5 and thus to achieve a large value of Δn, the high refractive index should preferably be more than 1.7. In order to ensure that the ordinary beam Po undergoes total internal reflection and to maintain a high acceptance angle for the polarizing beam splitter, the refractive index of the material of the prism must, therefore, also be at least 1.7.

According to a preferred embodiment of the present invention, the glass of the prisms, 12 14, is characterized in that it has been selected to have a low optical stress coefficient K (also known as the Brewster constant), so that thermal stress is prevented from causing undue deterioration of the optical polarizing properties. As a result, the polarizing beam splitter according to the present invention is thermally stable in that it operates at high optical incident powers without showing serious deterioration in the polarizing efficiency. Preferably, to achieve this desired effect, the value of K should be less than approximately 1 x 10^"6 mm²/N .

A review of the properties of common commercially available optical glasses shows that very few glasses fulfil the requirement of both such a low value of K. and a refractive index of 1.7 or more. Specifically the high lead content, dense flint glasses, such as those designated SF 6 and SF 57, j \

manufactured by the Schott Glaswerke of Mainz, Germany, fulfil both of these requirements. Other manufacturers also produce equivalents of SF 6, such as FeD E05-25 manufactured by Corning Optical Glasses of Avon. France, or the glass FD6 manufactured by the HOYA Corporation of Japan. The refractive index of SF 6 is 1.805 for light at a wavelength of 588 run, while that of SF 57 is about 1.847 at 588 run. To the best of the inventors' knowledge, no liquid crystal material is currently readily available with a high refractive index above 1.81 and with good transmission in the whole of the visible range. Consequently, the most suitable material of construction for the prisms according to this preferred embodiment of the present invention, is SF 6, or a similar glass from another manufacturer.

The use of a glass, such as SF 6 or equivalent, is in contrast to the glasses used in prior art polarizing beam splitters, where the use of such glasses has, to the best of the applicants' knowledge, never been reported. The SF 6 types of glass are difficult to handle and to work, both because of their poor chemical properties, such as low acid resistance, and because of their poor physical propeπies, such as low hardness and high density. As a consequence, except where their special properties are required, such as, for example, the need for a very high refractive index, they are not commonly used in optical design.

For the requirements of the present invention, a preferable maximum level for the optical stress coefficient has been selected in the region of 10^"6 mm^"/N, and currently available materials which fulfil this requirement are proposed. It is however anticipated that future material development will make available other materials which suitably fulfil the requirements of the low optical stress coefficient for the implementation of the present invention. It is thus to be understood that the materials mentioned hereinbelow. and the maximum optical stress coefficient defined are only typical of preferred embodiments of the invention, feasible with currently available materials. The invention, as specified and claimed, is intended to cover all such materials and optical stress coefficient levels which enable the present invention to operate as intended. Though the optical stress coefficient is probably the most important factor in the selection of a glass with the minimum level of thermally induced stress birefringence for use in polarizing beam splitters, there are other properties which contribute to the level of this birefringence, and which must therefore be optimized in order to achieve the best possible performance from the component.

Since it is heat generated in the prisms which degrades the polarization quality through thermally induced stress birefringence, in order to fully solve the problem of reducing such birefringence, a combination of properties should be determined, which also takes into account the factors which contribute to the generation of heat gradients and to the resulting stresses.

There are four factors which contribute to thermally induced stress birefringence:

(1) A temperature gradient within the material causes stresses due to non-uniform expansion. For a given temperature change, the thermal expansion of the material is proportional to the coefficient of expansion of the material, α.

(2) The stresses that are produced due to the expansion of the component are proportional to the Young's modulus of the material, E.

(3) The temperature gradient inside the optical element is inversely proportional to the heat diffusion coefficient. D. D is a function of the thermal conductivity of the material, λ, of the material density, p, and of the constant pressure, heat capacity, C_p, and is given by the expression:

D = λ / pC_p

(4) As previously noted, the stress induced by the above-described thermal gradients results in a level of birefringence in the material, proportional to the stress optical coefficient of the material. K.

A parameter P can be defined, proportional to a positive power of α, E and K and to a negative power of D. The lower the value of this parameter, the less the degeneration of the polarization state of the beam passing through the polarizing beam splitter. The power to which each of the arguments in P should be raised depends on the component geometry, on the component heating and cooling rates and on the heat flow pattern inside the component.

Reference is now made to Fig. 2. which shows schematically an experimental arrangement for measuring the change in the polarization of light passing through a glass slab heated at a steady rate to a fixed temperature. The glass slab 30 under test is located on an electric hot plate 32. whose temperature is allowed to rise after switch-on to a final set value. A source of light 34, which can be either a non-polarized laser giving essentially monochromatic light, or an incoherent source, such a lamp, giving a broadband spectrum of light, dispatches a beam 36 through the slab as it heats up. Before passing through the slab, the beam is polarized by means of a polarizing element 38. The polarization is detected by means of a power meter 40 which determines the power level of the light after passing through an analyzer 42, which could be a polarizer crossed with respect to the polarizer 38. Before the commencement of the test, the power read is virtually zero, since the polarizers are essentially crossed, and the slab, if correctly manufactured and annealed, should have negligible residual birefringence. As the glass slab heats up, any change in the birefringence level of the glass results in a significant change in the power detected, as part of the light incident on the analyzer now becomes transmitted therethrough. This apparatus is therefore capable of detecting very slight changes in birefringence.

Reference is now made to Fig. 3. which shows a graph of the results of an experiment comparing the thermally induced stress birefringence in two glasses manufactured bv the the Schott Glaswerke of Mainz. Germanv - BK 7. which is a common borosilicate glass, widely used to construct the prisms used in prior art polarizing beam splitters, and SF 6. as preferably used as the material of the prisms of a polarizing beam splitter, constructed and operative according to a preferred embodiment of the present invention. The graph shows the detected power in watts, as a function of the elapsed time from turning on the hotplate. For reference purposes, it should be noted that the heating surface itself takes about 180 seconds to reach its working temperature.

Immediately after turning on the hotplate, for both of the materials, the thermally induced stress birefringence rises to a high value, represented by a rise in the detected light power measured through the crossed polarizers. This steep rise to a maximum as the block heats up is to be expected, since it is at this stage that the temperature gradients through the slab are at their highest level, and the induced stresses thus at their highest level. For the SF 6 glass sample, the maximum level of thermally induced stress birefringence, as measured by the transmitted power, is only about one tenth of the maximum level for the BK 7 glass sample.

After the initial heating up period, the temperature within the slab becomes more uniform with the passage of time, and the thermally induced stress birefringence gradually falls to a steady level. (The small periodic oscillations seen are an artifact due to the effects of the hotplate thermostat cutting in and out at a regular rate). However, even when thermal stability has been reached, when the only temperature gradients present in the slab are due to the fairly uniform steady state flow of heat from the hotplate surface to the opposite surface of the slab, the BK 7 still shows well over an order of magnitude more power transmission than the SF 6 slab. (These results are not apparent from the linear scaled graph, but can be seen in tabulated results of the experiments.) This signifies that the residual thermally induced stress birefringence in the SF 6 glass is significantly lower than in the BK 7 glass.

In addition to the experiments shown in Fig. 2, where the glass slabs were heated on a hot plate, similar experiments were conducted with the glass slabs heated in a hot air chamber. The experiments were performed on 7 different types of glass. Reference is now made to Fig. 4. which is a table showing the results of these experiments, in addition to the results of the hot plate experiments, performed on 6 of the 7 glasses. The table shows the optical contrast obtained after passage through the crossed polarizers, this being a more useful measure of .

polarizer efficiency than can be gauged from the plots of transmitted power shown in Fig.3.

As is seen from the contrast measurements in this table, the glass with the least thermally induced stress birefringence is SF 57, followed closely by SF 6. Both of these glasses are significantly less sensitive to thermally induced stress birefringence than any of the others tested. For the reasons mentioned above, however, there are limitations to the use of SF 57 because its refractive index is so high. In addition. SF 57 is problematic to use. both because of its comparative mechanical softness and because of its chemical sensitivity. Furthermore, SF 57 has noticeable absorption at the blue end of the visible spectrum, and in the UV. For all of these reasons, the optimum practical material for current use in temperature-stable, polarizing beam-splitters and other polarizing components is thus preferably SF 6. At the power density levels currently used in high performance projectors, polarizing beam splitters made with SF 6 perform without noticeable polarization degradation. However, it is to be understood that if a liquid crystal material were available with a higher value of high refractive index than that of currently available liquid crystal materials, the SF 57 glass may in the future find preferred use in some specific embodiments of the present invention.

A review of the correlation between the values of the contrasts measured and the levels of the four physical parameters of the glass samples seems to indicate that, although all of the four parameters may be active in determining the level of thermally induced stress birefringence present, the dominant factor in this respect would seem to be the optical stress coefficient, K.

If the dominant source for the heating of the prism is the absorption of light, as is the case in a practical polarizing beam splitter used in an optical projection system, then the specific absorption of the material should also be taken into account. When a broadband light source is used, the absorption that should be used is a weighted average over the entire spectrum of the light source. From all of the above experimental data, and using the arguments proposed hereinabove, it is seen that the optimum glass for reduction of thermally induced stress birefringence in current applications is found to be of the SF 6 type. As an example of the application of the present invention, a polarizing beam splitter with an entrance surface of dimensions 55mm x 44 mm, an optical path length of 80 mm and an acceptance angle range of ±10°, was constructed using prisms of SF 6 glass. This polarizing beam splitter showed negligible reduction in performance when subjected to the thermal gradients generated when used inside a high brightness projection system, at an average power density of 2 W/cm . Even at this power level, extinction ratios of greater than 1000:1 and 500:1 were obtained for the transmitted and reflected beams respectively, these results being significantly better than those obtained form prior art beam splitters.

Video or data projectors or other devices, wherein an image is formed by discrimination between the transmittance of light of different polarizations, are improved by the use of beam splitting components which fully utilize the beams of both polarizations. Slight changes in the polarization of either of the beams can result in a degradation of the contrast, color, brightness and other properties of the projector. For this reason, such projection systems and similar devices, are significantly improved by the use of a polarizing beam splitter according to the present invention, wherein the polarization of the transmitted beams are accurately maintained even under conditions of high beam intensity.

It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description and which are not in the prior art.

Claims

CLAIMSWe claim:

1. A polarizing optical component comprising at least one element having an optical stress coefficient such that said component is thermally stable.

2. A polarizing optical component according to claim 1, and wherein said optical stress coefficient is less than l x lO^"6 mm²/N.

3. A polarizing optical component according to either of claim 1 and claim 2 and wherein said at least one transparent element comprises two transparent elements.

4. A polarizing optical component according to either of claim 1 and claim 2 and wherein said transparent element also has at least one of the properties selected from the group consisting of high thermal diffusion coefficient, low Young's modulus and low thermal expansion coefficient.

5. A polarizing optical component according to either of claim 1 and claim 2 and wherein said element is constructed of a glass.

6. A polarizing optical component according to claim 5 and wherein said glass is one of the types selected from the group consisting of SF 6, SF 57, FeD E05-25 and FD6.

7. A polarizing optical component according to claim 5 and wherein said glass has a refractive index greater than 1.7 in the visible range.

8. A polarizing optical component according to any of claims 3 to 7 and having a birefringent layer interposed between said two transparent elements.

9. A polarizing optical component according to claim 8 and wherein said birefringent layer is a liquid crystal layer.

10. A polarizing optical component according to claim 8 and wherein said birefringent layer is a polymer liquid crystal layer.

11. A polarizing optical component according to any of claims 3 to 8 and having a thin film stack interposed between said two transparent elements.

12. A polarizing optical component according to any of claims 1 to 11 and wherein said optical component is a polarizing beam splitter.

13. A polarizing beam splitter according to claim 12, and wherein said polarizing beam splitter is a recombinant polarizing beam splitter.

14. A projection system comprising at least one polarizing beam splitter according to claim 12.

15. A polarizing beam splitter comprising at least one transparent element constructed of a glass of the type selected from the group consisting of SF6, SF57. FeD E05-25 and FD6.

16. A projection system comprising at least one polarizing beam splitter according to claim 15.

17. A method of increasing the thermal stability of a polarizing element, comprising the step of constructing said polarizing element using a material having a low optical stress coefficient.

18. The method of claim 17, and further comprising the step of using a material additionally having at least one of the parameters selected from the group consisting of a high thermal diffusion coefficient, a low thermal expansion coefficient, and a low Young's modulus.

19. The method of either of claims 17 and 18, and wherein said optical stress coefficient has a value of less than lx lO^"6 mm²/N.

20. The method of any of claims 17 to 19, and wherein said polarizing element is a polarizing beam splitter.