Method for the generation of light of a given polarization state

[0001] The present invention relates to a method for the generation of light of a given polarization state in which a polarization state-sensitive beam splitter is acted upon at an input by light, light of the given polarization state is output by the beam splitter at a first output, light with a polarization state differing from the given state is changed with respect to this state and is output. Furthermore, the present invention relates to a polarizer configuration, an illumination configuration, a projection system as well as their uses.

[0002] U.S. Pat. No. 6,139,157 discloses a method of the above described type or an illumination system operating according to this method. After traversing through a light tunnel as well as a lens system, nonpolarized light is supplied to a beam splitter sensitive to a polarization state. While the component of the supplied nonpolarized light, which is polarized in a given polarization state, is output at a first output of the beam splitter, the component of the nonpolarized light with a state differing from a given polarization state is changed with respect to this state and output at a second output of the beam splitter.

[0003] This approach has the following disadvantages:

[0004] Since the light lastly polarized in a given state exits at two separate regions or outputs of the beam splitter, the subsequent illumination optics must be layed out for the precise acceptance of the light, which makes the entire system relatively inflexible and voluminous.

[0005] The present invention addresses the problem of providing a method or a polarizer configuration operating according to this method, which eliminates said disadvantage.

[0006] This is solved with the above described method thereby that the light of polarization change of state acts again on the input of the polarization-sensitive beam splitter.

[0007] Therewith is achieved that virtually the entire nonpolarized light finally with the desired polarization state exits from the beam splitter. This takes place since the light is output and rotated at the second output until it has at least approximately the given polarization state and subsequently also exits the beam splitter at the first output.

[0008] In a preferred embodiment of the method according to the invention which, as will be shown, opens extremely advantageous realization capabilities, the polarization state of the light is changed through reflection, preferably through multiple reflection.

[0009] In a further preferred embodiment, in particular combinable preferably with the above reflection change of state, the polarization state of the light is changed through a given delay, preferably by means of at least one retarder element.

[0010] From U.S. Pat. No. 6,139,157 is known to focus the light, which is supplied to the beam splitter, initially from a light source with reflector onto the aperture of a light tunnel and to homogenize the intensity distribution of the light through the multiple reflection effect of the light tunnel at its peripheral inner faces at the output side. Consequently, it is known to provide for the nonpolarized light supplied to the beam splitter a path with multiple reflection for reasons of intensity homogenization.

[0011] In an extremely advantageous embodiment of the method according to the invention the light, before the action onto the beam splitter sensitive to the polarization state is also guided through a light tunnel structural element, with reflecting periphery and the light with a state perpendicular to the given polarization state is guided from the end opposing the illumination source into the light tunnel structural element and reflected thereon. Therewith the light tunnel structural element provided for the nonpolarized light for reasons of intensity homogenization is simultaneously utilized in order to change the light component not polarized in the given state with respect to its polarization state and supply it again to the input of the beam splitter. Therewith a highly compact and simple realization of the method according to the invention is generated.

[0012] At at least one region in the structural element acted upon by the light preferably at least one element is provided effecting a given defined polarization change of state, preferably at least one retarder plate.

[0013] In a further preferred embodiment the light, with which the input of the beam splitter is acted upon, is generated by means of a light source and a reflector and at least a portion of the light, with a state perpendicular to the given polarization state, at least also is reflected on the reflector and the light reflected there supplied to the input of the polarization state-sensitive beam splitter.

[0014] The last described approach is in particular suitable in combination with provision of the explained light tunnel structural element, thereby that with said reflector virtually a closed volume to the aperture of the light tunnel structural element is provided such that in any case light components which originate from the second output of the beam splitter and exit through the aperture of the light tunnel are not lost but rather are reflected by the reflector back into the light tunnel structural element. The reflector can therein preferably be formed similarly to an Ulbricht sphere.

[0015] In a further preferred embodiment of the last mentioned approach an element effecting a given polarization change of state is also provided on the reflector face.

[0016] A polarizer configuration according to the invention is specified in the claims, with preferred embodiments according to other claims. An illumination configuration according to the invention is specified in still further claims as is a projector system. The configurations according to the invention as well as also the method according to the invention is singularly well suitable in the application in the frame of projectors or illumination apparatus.

[0025] In conjunction with FIG. 1, the fundamental principle of the present invention will be explained first. For this purpose serves an optic function block/signal flow diagram. Nonpolarized light L, such as conventionally is emitted by projector light sources, is supplied to the optic input E₁ of a polarization state-sensitive beam splitter element 1. The polarization direction-sensitive beam splitter 1 outputs at its one optic output A₁ the component L_P of the nonpolarized light L, which has a given polarization state, for example is P-polarized. The component L_S of the nonpolarized light L, which with respect to the polarization state P of the light output at A₁ has a perpendicular polarization state, and, following the example, is thus S-polarized, is output from the beam splitter 1, sensitive to the polarization state, at its second optic output A₂. Conventionally, the one component of said light L, thus for example L_P, is transmitted through a polarization filter layer system at the polarization state-sensitive beam splitter 1, the other, here for example L_S, is reflected.

[0026] At the optic output A₁ light L_P with the desired polarization state exits. The light L_S exiting at output A₂ of the beam splitter 1 sensitive to the polarization state is changed or rotated at a polarization state rotation or changing element 3 in its polarization state and supplied again to the optic input E₁ of the polarization state-sensitive beam splitter 1. Therewith, virtually the entire light 1 supplied to the beam splitter 1 and, initially nonpolarized, exits as L_P with the desired polarization state, for example P.

[0027] As shown in FIG. 2, the polarization state rotation element 3 can preferably be realized thereby that according to FIG. 1 the light L_S exiting at the optic output A₂ is subjected to multiple reflection. As a rule the polarization state of the light is not retained through reflections. Even if the polarization state of light L_S is only slightly rotated, a component is P-polarized and output at output A₁ as a component of L_P. Remaining components require possibly several cycles over configuration 3, until their polarization direction corresponds to the required P polarization.

[0028] As shown in FIG. 2, the rotation of the polarization state at configuration 1 can be augmented preferably additionally by providing discrete polarization direction rotation elements, such as in particular so-called retarder elements, by means of which, according to the principle of FIG. 1, a λ/2 delay of the light L_S is introduced, i.e. a 90° rotation of the polarization direction.

[0029] In block 3 of FIG. 2 are schematically shown, for one, a multiple reflection configuration 6 for light L_S, for another, in dashed lines, a built-in additional discrete polarization direction rotation element, such as for example a retarder plate 8.

[0030] As stated in the introduction, it is often desired in many applications, such as in particular with projectors, to provide a light beam whose intensity distribution is of maximum homogeneity. In this connection it is known, as has also been explained in the introduction, to supply the light originating from a very small region of a light source, first to a light tunnel, whereon, through multiple reflection of the light originating from the light source, is made available at the output side a homogeneous intensity distribution adapted to the form of an image-providing element.

[0031] But if at the input side of the polarization state-sensitive beam splitter 1 a structural element multiply reflecting the input light L for reasons of homogenization is provided, this provided multiply reflecting structural element—as shown again schematically in FIG. 3—is preferably simultaneously employed as polarization direction-rotating structural element, analogous to element 3 of FIGS. 1 and 2.

[0032] According to FIG. 3, light L_e is incident on a multiply reflecting structural element 3a, as stated for example and preferably onto the optic input E_3a of a light tunnel, and, after multiply reflection is, after it is homogenized in terms of intensity, supplied to input E₁ of the polarization state-sensitive beam splitter 1.

[0033] At optic output A₂ which is not identical to the optic input E₁, the light L_S not having the desired polarization state, is conducted back to the output A_3a of the multiply reflecting element 3a, where, after multiple reflection, it is reflected back and, as component of light L, with rotated polarization direction again supplied to the polarization-sensitive beam splitter 1. In this way a multiply reflecting structural element 3a, provided already for reasons of intensity homogenization, is utilized, in which, as will yet be explained, again specific discrete polarization direction-rotating elements can be provided such as retarder plates, the polarization state rotation to be provided according to the invention, as was explained in conjunction of block 3 in FIG. 1 or 2, can also be realized.

[0034] In FIG. 4 a preferred embodiment form of the preferred embodiment principle, explained in conjunction with FIG. 3, is depicted. Light L_e, for example from a projection light source, enters through aperture 7 at input E_3a of light tunnel 3a and, if it does not progress approximately parallel to the optic axis A of the light tunnel, is reflected once or multiply at the highly reflecting internal peripheral walls 9 of light tunnel 3a.

[0035] The incident light or its path of rays is entered in FIG. 4 in dashed lines.

[0036] Preferably immediately at output A_3a of light tunnel 3a the polarization state-sensitive beam splitter 1a is provided at whose optic output A_1a exits the light L_P of desired polarization state and with homogeneously distributed intensity.

[0037] Following the explanations regarding FIG. 3, the light L_s deviating from the desired polarization state—here P—is guided back into light tunnel 3a, as is represented by the light beam depicted by example and with full lines in FIG. 4, where, after multiple reflections, lastly reflected on the light tunnel termination 11 encompassing aperture 7 and developed so as to be reflecting on the inside, it is guided back again at output A_3a of the light tunnel to the polarization state-sensitive beam splitter 1a with the polarization state rotated by the multiple reflection. Therewith, the light polarized after this rotation in the desired direction or in the desired state exits again as a component of light L_P from the beam splitter 1a. The reflecting internal wall of termination 11 is oriented parallel to the reflecting plane or its average position E′ (see FIG. 5) at beam splitter 1a.

[0038] In a preferred embodiment of the realization according to FIG. 4, between the input aperture 7 and the output A_3a of light tunnel 3a or the input of the polarization state-sensitive beam splitter 1a, one or several optic elements are provided which effect a discrete given rotation of the polarization state. As shown in FIG. 4 in dashed lines, such polarization state-rotating elements, such as for example and preferably λ/4 retarder plates, can be provided in cross section planes of the light tunnel 3a, as depicted at 13a, and/or on the peripheral wall of the light tunnel 3a, as shown at 13b, and/or at the input-side termination 11, defining the input aperture 7 of light tunnel 3a, as shown in FIG. 4 at 13c. In any case, providing such a polarization state-rotating element, in particular a λ/4 retarder plate, at the termination 11 encompassing the input aperture 7, as shown at 12c, is preferred. Due to the twofold traversing of a λ/4 retarder path, light L_S undergoes therein a 90° rotation of the polarization state.

[0039] As already represented in FIG. 4, as the beam splitter 1a sensitive to the polarization state advantageously one that is plate-shaped is employed. Depending on the occurring light intensity, the decision has to be made whether such a polarization beam splitter based on synthetic materials can be applied or a beam splitter which is built with cholesteric filters on glass substrates.

[0040] But it is also possible to use a polarization beam splitter 1b, such as is depicted in FIG. 5, realized on the basis of three coated glass prisms.

[0041] If a minimum parallel shift Δ of the light L and L_S incident on the beam splitter 1b is desired, preferably, and as shown in FIG. 6, the beam splitter selective with respect to the polarization state, and now denoted as 1c, is realized as an array of three-prism structures I, II, . . . . The finer its structure, the less said parallel shift Δ.

[0042] In view of FIG. 4, it is furthermore evident that light L_S which after the reflections is not incident on the termination 11 encompassing the input aperture 7, but rather exits the light tunnel through the input aperture 7, is lost. In FIG. 7, within the scope of a projector example according to the invention, the solution of this problem is shown. A reflector 15 focusing light of a light source 17 onto the input aperture 7, is developed such that that from the polarization state-sensitive beam splitter 1a or 1b or 1c cannot escaped from the system and is also reflected on reflector 15 into the light tunnel 3a. The reflector 15 can essentially be layed out corresponding to the shape of an Ulbricht sphere.

[0043] Apart from the already described elements, the projector specifically according to the invention comprises a color shutter 19 as well as a lens system 21, in addition to a further beam splitter 23, which, due to the polarization state-selective beam splitter 1a, according to the invention already provided, does not absolutely need to be developed as a polarization state-sensitive beam splitter. Lastly, 27 denotes a projection lens.

[0044] Furthermore, as shown and described in conjunction with FIG. 7, on reflector 15 a discrete polarization direction-rotating element can be provided, denoted by 13d in FIG. 7.

[0045] With the color shutter preferably provided in FIG. 7, for example with three color-selective transmission ranges (not shown), for example selectively transmitting red, green and blue, the color modulation of light L_P is carried out. The color shutter 19 is preferably developed as color wheel. As the image-providing element 25 preferably an array of light valves operating in reflection is provided, in which by pixels and by providing the color shutter 19 synchronously with the latter, the particular pixel intensities on the projected image are selected.

[0046] The beam splitter sensitive to the polarization state can be an element which transmits light of one wavelength range Δλ₁ in a first polarization state and reflects it in a second polarization state perpendicularly to it, while it transmits light of another wavelength range Δλ₂ in the second polarization state and reflects it in the first.

[0047] Subsequently, from the splitter exits light with wavelength-dependent polarization state. Such a wavelength-dependent polarization distribution is frequently required with projectors, in particular with projectors based on reflecting LCDs. The polarization state-sensitive element can be structured of a retarder stack and a succeeding polarization beam splitter, acting jointly as wavelength-selective polarization state-sensitive beam splitter.