Optical stack

Abstract

The optical stack includes a reflective polarizer, an absorbing polarizer, and a half-wave retarder. The half-wave retarder is disposed between the reflective polarizer and the absorbing polarizer. For substantially perpendicular incident light and a first polarization state, the reflective polarizer reflects at least about 60% of the incident light for at least a first wavelength less than a cutoff wavelength and transmits at least about 50% of the incident light for at least a second wavelength greater than the cutoff wavelength. For substantially perpendicular incident light, the half-wave retarder has a first retardation less than about 250 nanometers (nm) at the first wavelength and a second retardation at the second wavelength. The deviation of the first retardation from the first half-wave retardation corresponding to the first wavelength is less than the deviation of the second retardation from the second half-wave retardation corresponding to the second wavelength.

Description

Technical Field

[0001] The present disclosure generally relates to optical stacks, and more particularly to optical stacks for backlights.

Background Art

[0002] In backlights, reflective polarizers are commonly used to reuse light and increase the ultimate efficiency and brightness of the display. Further, absorption polarizers and the like are used with liquid crystal modules to polarize light for proper modulation by the liquid crystal module. In some manufacturing processes, roll-form reflective polarizers may have a reflection axis (i.e., a blocking axis or a slow axis) in the transverse direction (i.e., the width direction of the roll). Roll-form absorption polarizers may have an absorption axis along the length direction of the roll. In some cases, it may be desirable to align the reflection axis of the reflective polarizer and the absorption axis of the absorption polarizer within the optical stack. In conventional manufacturing processes, one or both of the rolls of polarizers are cut and rotated, adding manufacturing time and process costs.

Summary of the Invention

[0003] In a first aspect, the disclosure provides an optical stack comprising a reflective polarizer, an absorbing polarizer, and a half-wave retarder. The reflective polarizer has mutually orthogonal transmission and reflection axes. The half-wave retarder is positioned between the reflective polarizer and the absorbing polarizer. For substantially perpendicular incident light and a first polarization state, the reflective polarizer includes a transmission band having a cutoff wavelength. The reflective polarizer reflects at least about 60% of the incident light for at least a first wavelength smaller than the cutoff wavelength and transmits at least about 50% of the incident light for at least a second wavelength larger than the cutoff wavelength. For substantially perpendicular incident light and a second orthogonal polarization state, the reflective polarizer transmits at least 70% of the incident light for at least the first wavelength and each of the second wavelengths. For substantially perpendicular incident light, the half-wave retarder has a first phase difference less than about 250 nanometers (nm) at the first wavelength and a second phase difference at the second wavelength. The first phase difference shift from the first half-wave phase difference corresponding to the first wavelength is smaller than the second phase difference shift from the second half-wave phase difference corresponding to the second wavelength. For substantially perpendicular incident light and a first polarization state, the absorbing polarizer has a first transmittance at the first wavelength and a larger second transmittance at the second wavelength.

[0004] In a second aspect, the disclosure provides another optical stack comprising a display panel, a reflective polarizer, an absorbing polarizer, and a half-wave retarder. The display panel is configured to emit display light comprising a blue emission spectrum in which the blue full width at half maximum (FWHM) is defined between the minimum and maximum wavelengths. The reflective polarizer receives the display light from the display panel and is configured to reflect a portion of the display light as reflected polarization. The reflective polarizer has mutually orthogonal transmission and reflection axes. The half-wave retarder is positioned between the reflective polarizer and the absorbing polarizer. For substantially perpendicular incident light and a first polarization state, the reflective polarizer includes a transmission band having a cutoff wavelength greater than the minimum wavelength of the blue emission spectrum of the display panel. The reflective polarizer reflects at least about 60% of the incident light for at least a first wavelength less than the cutoff wavelength and transmits at least about 50% of the incident light for at least a second wavelength greater than the cutoff wavelength. For substantially perpendicular incident light and a second orthogonal polarization state, a reflecting polarizer transmits at least 70% of the incident light for at least the first wavelength and at least the second wavelength. For substantially perpendicular incident light, a half-wave retarder has a first phase difference less than about 250 nm at the first wavelength and a second phase difference at the second wavelength. The deviation of the first phase difference from the first half-wave phase difference corresponding to the first wavelength is smaller than the deviation of the second phase difference from the second half-wave phase difference corresponding to the second wavelength. For substantially perpendicular incident light and a first polarization state, an absorbing polarizer has a first transmittance at the first wavelength and a larger second transmittance at the second wavelength. [Brief explanation of the drawing]

[0005] The exemplary embodiments disclosed herein can be more fully understood by examining the following “Modes for Carrying Out the Invention” in conjunction with the following figures. The figures are not necessarily drawn to scale. Similar numbers used in the drawings indicate similar components. However, it should be understood that the use of numbers to indicate components in a given figure is not intended to limit components in other figures indicated by the same number. [Figure 1] This is a schematic top perspective view of an unfolded optical stack according to one embodiment of the present disclosure. [Figure 2] Figure 1 is a schematic front view of the unfolded optical stack. [Figure 3] This is a schematic diagram of a half-wave retarder for an optical stack according to one embodiment of the present disclosure. [Figure 4] This is a detailed schematic diagram of a reflective polarizer according to one embodiment of the present disclosure. [Figure 5] This is a graph showing the transmittance against wavelength for a reflective polarizer having a transmission band according to one embodiment of the present disclosure. [Figure 6] This is a graph showing the transmittance against wavelength for a reflecting polarizer having a different transmission band according to another embodiment of the present disclosure. [Figure 7] This is a schematic diagram of a half-wave retarder for an optical stack according to one embodiment of the present disclosure. [Figure 8] This is a graph showing the phase difference with respect to wavelength for a half-wave retarder according to one embodiment of the present disclosure. [Figure 9] This graph shows the phase difference shift with respect to wavelength for a half-wave retarder according to one embodiment of the present disclosure. [Figure 10] This is a schematic diagram of an absorbing polarizer in an optical stack according to one embodiment of the present disclosure. [Figure 11] This is a graph showing the transmittance against wavelength for an absorbing polarizer according to one embodiment of the present disclosure. [Modes for carrying out the invention]

[0006] The following description refers to the accompanying drawings, which form part of the description and illustrate various embodiments as examples. It should be understood that other embodiments may be conceived and implemented without departing from the scope or spirit of this disclosure. Therefore, the embodiments for carrying out the following invention should not be construed as restrictive.

[0007] This disclosure relates to an optical stack. Optical stacks can be used as backlights for electronic devices incorporating displays, such as computer monitors, televisions, mobile phones, personal digital assistants (PDAs), wearable devices, and other portable devices. A typical optical stack includes a reflective polarizer and an absorptive polarizer.

[0008] In conventional manufacturing processes and material selections, reflective polarizers are stretched so that their reflection axis (the axis parallel to the polarization state preferentially reflected by the reflective polarizer) aligns with the width direction of the reflective polarizer. Furthermore, conventional absorptive polarizers are usually aligned in the machine direction, resulting in their absorption axis substantially aligning with the length of the absorptive polarizer. Thus, conventional manufacturing processes result in rolls of reflective and absorptive polarizers with transmission axes oriented orthogonally to each other. Consequently, costly conversion steps may be required to provide sheets with absorptive and reflective polarizers whose transmission axes are aligned. Generally, in sheet-roll lamination processes, reflective polarizers are cut and rotated 90 degrees to align their transmission axes. This process is time-consuming and expensive, and sometimes the conversion step can increase the chances of introducing defects, reducing the yield or usable portion of the optical stack. Therefore, it may be desirable to align the reflection axis of reflective polarizers with the absorption axis of absorptive polarizers within the optical stack to save additional manufacturing time and process costs.

[0009] In some designs, a half-wave retarder is placed between the reflective polarizer and the absorbing polarizer. The half-wave retarder may have a slow axis oriented substantially 45 degrees with respect to the transmission axis of the reflective polarizer. The half-wave retarder may rotate the linear polarization between the reflective and absorbing polarizers by 90 degrees to align the reflection axis of the reflective polarizer with the absorption axis of the absorbing polarizer. However, the incorporation of a half-wave retarder can impose the difficulty of uniformly rotating the linear polarization for all visible wavelengths to minimize chromatic aberration or other artifacts at various viewing angles within the optical stack. Chromatic aberration can result from the modulation of the nonlinear wavelength dependence of light in conventional half-wave retarders. In some of these cases, special materials are used to fabricate achromatic half-wave retarders to reduce chromatic aberration. However, even achromatic half-wave retarders may exhibit some chromatic aberration in the conversion of linear polarization. Furthermore, utilizing achromatic half-wave retarders can significantly increase the overall cost of the optical stack.

[0010] This disclosure relates to an optical stack comprising a reflective polarizer, an absorbing polarizer, and a half-wave retarder. The reflective polarizer of this disclosure has mutually orthogonal transmission and reflection axes. The half-wave retarder is positioned between the reflective polarizer and the absorbing polarizer. For substantially perpendicular incident light and a first polarization state, the reflective polarizer of this disclosure includes a transmission band having a cutoff wavelength. For the first polarization state, the reflective polarizer reflects at least about 60% of the incident light for at least a first wavelength smaller than the cutoff wavelength and transmits at least about 50% of the incident light for at least a second wavelength larger than the cutoff wavelength. For substantially perpendicular incident light and an orthogonal second polarization state, the reflective polarizer transmits at least 70% of the incident light for at least the first wavelength and at least each of the second wavelengths.

[0011] Furthermore, the half-wave retarder of this disclosure is designed such that, with respect to substantially perpendicular incident light, the half-wave retarder has a first phase difference less than about 250 nanometers (nm) at a first wavelength and a second phase difference at a second wavelength. The deviation of the first phase difference from the first half-wave phase difference corresponding to the first wavelength is smaller than the deviation of the second phase difference from the second half-wave phase difference corresponding to the second wavelength.

[0012] Such optimization of a half-wave retarder at wavelengths smaller than the cutoff wavelength of the transmission band of the reflective polarizer can reduce optical artifacts such as chromatic aberration. Specifically, the combination of reflective polarizer and half-wave retarder of this disclosure may exhibit improved performance in reducing chromatic aberration. Furthermore, a half-wave retarder optimized in this manner may be easier and less expensive to manufacture compared to a conventional achromatic half-wave retarder.

[0013] Referring here to Figures 1 and 2, an optical stack 100 according to an embodiment of the present disclosure is shown. The optical stack 100 includes a reflective polarizer 102, a half-wave retarder 104, an absorptive polarizer 106, and a display panel 108. In some embodiments, the reflective polarizer 102, the half-wave retarder 104, the absorptive polarizer 106, and the display panel 108 have substantially the same spread or the same in-plane dimensions (i.e., length and width) to each other.

[0014] The optical stack 100 defines mutually orthogonal x, y, and z axes. The x and y axes are in-plane axes of the optical stack 100, while the z axis is a transverse axis aligned along the thickness of the optical stack 100. In other words, the x and y axes are aligned along the plane of the optical stack 100, while the z axis is perpendicular to the plane of the optical stack 100. The reflective polarizer 102, half-wave retarder 104, absorbing polarizer 106, and display panel 108 are arranged adjacent to each other along the z axis of the optical stack 100.

[0015] In some embodiments, the reflective polarizer 102, half-wave retarder 104, and absorptive polarizer 106 of the optical stack 100 may be in optical contact with each other. In some embodiments, the reflective polarizer 102, half-wave retarder 104, and absorptive polarizer 106 of the optical stack 100 are attached by one or more of the following: pressure-sensitive adhesive, optically transparent adhesive, ultraviolet (UV) curable adhesive, or polyvinyl alcohol-based adhesive, lamination, or any other attachment mechanism.

[0016] The reflective polarizer 102 has a transmission axis 122 and a reflection axis 124 that are orthogonal to each other. The reflective polarizer 102 may be a notched reflective polarizer having one or more notches or bands. The reflective polarizer 102 may be any suitable reflective polarizer, including a wire grid polarizer or a multilayer birefringent reflective polarizer. In the illustrated embodiment, the reflective polarizer 102 may have a transmission axis 122 that is generally along the x-axis of the reflective polarizer 102 and a reflection axis 124 that is generally along the y-axis. The reflective polarizer 102 further has a first principal surface 112 and a second principal surface 114 opposite to the first principal surface 112. In some embodiments, the first and second principal surfaces 112 and 114 of the reflective polarizer 102 may each be substantially planar and arranged along the xy plane.

[0017] The half-wave retarder 104 is positioned between the reflective polarizer 102 and the absorbing polarizer 106. The half-wave retarder 104 has a first principal surface 116 and a second principal surface 118 opposite to the first principal surface 116. In some embodiments, the first and second principal surfaces 116 and 118 of the half-wave retarder 104 may each be substantially planar and may be positioned along the xy plane. In the embodiment shown in Figure 2, the absorbing polarizer 106 is positioned adjacent to the first principal surface 116 of the half-wave retarder 104. Furthermore, the second principal surface 118 of the half-wave retarder 104 is positioned adjacent to the first principal surface 112 of the reflective polarizer 102. Generally, the half-wave retarder 104 includes a layer of birefringent material. In some embodiments, the half-wave retarder 104 includes a half-wave cycloolefin polymer (COP) retarder. In some embodiments, the half-wave retarder 104 may include two layers of birefringent material. In some embodiments, the half-wave retarder 104 may be a blend of two materials, namely a positive birefringent material and a negative birefringent material. In some embodiments, the half-wave retarder 104 may include two layers, one of which can rotate linearly polarized light by substantially 15 degrees and the other layer can rotate linearly polarized light by substantially 75 degrees. In some embodiments, the half-wave retarder 104 is a non-achromatic half-wave retarder, i.e., the phase difference of the half-wave retarder 104 is substantially deviated from the ideal half-wave phase difference over at least a portion of the visible wavelength range. Generally, the deviation of the phase difference of an achromatic half-wave retarder from the phase difference of an ideal half-wave retarder is smaller than the deviation of the phase difference of a non-achromatic retarder from the phase difference of an ideal half-wave retarder. Furthermore, the phase difference of a non-achromatic half-wave retarder is generally the same or smaller at higher wavelengths compared to the phase difference at lower wavelengths. On the other hand, the phase difference of an achromatic half-wave retarder generally increases at higher wavelengths.

[0018] The half-wave retarder 104 has a slow axis 126 that is oriented substantially 45 degrees with respect to the transmission axis 122 of the reflective polarizer 102. For the purposes of this application, orientations of substantially 45 degrees and substantially 135 degrees can be considered to be substantially equal, given the bidirectionality of the transmission axis 122. It is understood that being substantially 45 degrees is not limited to exactly 45 degrees; instead, the alignment of the axes can be within 10 degrees, 5 degrees, or 1 degree of 45 degrees. The alignment may, in some cases, be a trade-off between manufacturability (e.g., tolerance) and optical performance, and an appropriate balance is determined according to the application attributes.

[0019] The display panel 108 is disposed under the reflective polarizer 102 on the side opposite to the half-wave retarder 104. In the embodiment shown in FIG. 2, the display panel 108 is disposed adjacent to the second major surface 114 of the reflective polarizer 102. In some embodiments, the display panel 108 includes an organic light-emitting diode (OLED) display panel. In some embodiments, a quarter-wave retarder (not shown) may be disposed between the reflective polarizer 102 and the display panel 108. The quarter-wave retarder can be used to generate circular polarization in order to reduce ambient reflections from the display panel 108. In some other embodiments, the display panel 108 includes a liquid crystal display (LCD) panel.

[0020] The absorption polarizer 106 can be disposed on the half-wave retarder 104 on the side opposite to the reflective polarizer 102. In the embodiment shown in FIG. 2, the bottom surface 120 of the absorption polarizer 106 is disposed on the first major surface 116 of the half-wave retarder 104. The absorption polarizer 106 has a transmission axis 128 that is oriented substantially 90 degrees with respect to the transmission axis 122 of the reflective polarizer 1 and substantially 45 degrees (or 135 degrees) with respect to the slow axis 126 of the half-wave retarder 104. In the illustrated embodiment, the transmission axis 128 of the absorption polarizer 106 is disposed generally along the y-axis. The absorption polarizer 106 can include any suitable material, such as a polymer material. In some embodiments, the absorption polarizer 106 may include polyvinyl alcohol. In some embodiments, the absorption polarizer 106 may include a polarizing element that includes a polarizing or dichroic dye.

[0021] In some embodiments, the optical stack 100 may include additional or intermediate films, layers, or components, such as a diffusion layer, a turning layer, or a substrate layer. The optical stack 100 may be of any suitable thickness as a whole, based on the desired application attributes.

[0022] The display panel 108 is configured to emit display light 10. The reflective polarizer 102 is configured to receive the display light 10 from the display panel 108 and reflect a portion of the display light as polarized light 12 that is reflected. The display light 10 is generally not polarized. However, in some cases, the display light 10 may be at least partially polarized light. For the sake of explanation, the display light 10 can be treated as light having an unknown or arbitrary polarization state or distribution of polarization states. The reflected polarized light 12 is generally polarized along the reflection axis 124 (i.e., the y-axis) of the reflective polarizer 102. Thus, the reflected polarized light 12 can have a first polarization state along the y-axis. The reflective polarizer 102 transmits a first transmitted light 14 having a second polarization state orthogonal to and parallel to the transmission axis 122 (i.e., the x-axis) of the reflective polarizer 102.

[0023] The half-wave retarder 104 receives the first transmitted light 14 from the reflective polarizer 102. The half-wave retarder 104 is configured to rotate or modulate at least a portion of the polarization of the polarized first transmitted light 14 into orthogonally polarized second transmitted light 16. Thus, the second transmitted light 16 can have a first polarization state. The absorption polarizer 106 receives the second transmitted light 16 from the half-wave retarder 104 and emits at least a portion of the second transmitted light as output light 18 having a first polarization state.

[0024] Figures 3 and 4 show a reflective polarizer 102. The reflective polarizer 102 may include one or more of the following: a polymer reflective polarizer, a wire grid reflective polarizer, and a diffuse reflective polarizer. As shown in Figure 4, the reflective polarizer 102 includes a plurality of alternating first polymer layers 302 and second polymer layers 304. In some embodiments, the alternating plurality of first and second polymer layers 302, 304 total at least 40 layers. In some other embodiments, the alternating plurality of first and second polymer layers 302, 304 total at least 50 layers, at least 75 layers, at least 90 layers, at least 95 layers, or at least 100 layers. Each of the first and second polymer layers 302, 304 defines an average thickness "T" along the z-axis. In some embodiments, each of the first and second polymer layers 302, 304 has an average thickness "T" less than about 350 nm. In some embodiments, the first and second polymer layers 302, 304 may each have a thickness "T" less than about 400 nm. In some other embodiments, the average thickness "T" of each of the first and second polymer layers 302, 304 may be less than about 500 nm, less than about 600 nm, or less than about 700 nm. In some embodiments, the reflective polarizer 102 may further include a protective layer 306 disposed on each main surface of a plurality of alternating first and second polymer layers 302, 304.

[0025] Figures 5 and 6 show exemplary graphs 400 and 500, respectively. Graphs 400 and 500 show the transmission percentage against wavelength for different transmission bands of the reflecting polarizer 102. Wavelength is expressed in nanometers (nm) over the visible wavelength range. In Figures 5 and 6, transmittance is expressed as a transmission percentage on the left vertical axis. In Figure 5, reflectance is expressed as a reflection percentage on the right vertical axis. The reflection percentage is complementary to the transmission percentage, i.e., reflection percentage = (100 - transmission percentage).

[0026] Referring to Figures 1, 5, and 6, the display panel 108 is configured to emit display light 10 including a blue emission spectrum 402 in which the blue full width at half maximum (FWHM) 410 is defined between the minimum wavelength 406 and the maximum wavelength 407. In graphs 400 and 500, any suitable energy units may be used to show the blue emission spectrum 402 of the display panel 108. In some embodiments, the minimum wavelength 406 is greater than about 430 nm, and the maximum wavelength 407 is less than about 480 nm. In some other embodiments, the minimum wavelength 406 may be greater than about 400 nm, and the maximum wavelength 407 may be less than about 500 nm.

[0027] Referring to Figures 3, 4, and 5, for substantially perpendicular incident light 202 and a first polarization state, the reflecting polarizer 102 includes a transmission band 404 having a cutoff wavelength 408. Specifically, for substantially perpendicular incident light 202 and a first polarization state, the reflecting polarizer 102 includes a transmission spectrum 403 that includes a transmission band 404 having a cutoff wavelength 408. The minimum wavelength 406 is smaller than the cutoff wavelength 408 of the transmission band 404 of the reflecting polarizer 102. In other words, the reflecting polarizer 102 includes a transmission band 404 having a cutoff wavelength 408 that is greater than the minimum wavelength 406 of the blue emission spectrum 402 of the display panel 108. In the embodiment shown in Figure 5, the maximum wavelength 407 is also smaller than the cutoff wavelength 408 of the transmission band 404 of the reflecting polarizer 102. In other words, the cutoff wavelength 408 of the transmission bandwidth 404 of the reflective polarizer 102 is greater than the maximum wavelength 407 of the blue FWHM 410 of the display panel 108.

[0028] The cutoff wavelength 408 may correspond to the wavelength at which the transmittance is half (50%) of the peak or maximum transmittance. The cutoff wavelength 408 of the transmission band 404 of the reflecting polarizer 102 is greater than about 500 nm. In some other embodiments, the cutoff wavelength 408 of the transmission band 404 of the reflecting polarizer 102 is greater than about 525 nm. In some other embodiments, the cutoff wavelength 408 of the transmission band 404 of the reflecting polarizer 102 is less than about 550 nm.

[0029] For a first polarization state, the reflective polarizer 102 reflects at least about 60% of the incident light 202 to at least a first wavelength W1 smaller than the cutoff wavelength 408, and transmits at least about 50% of the incident light 202 to at least a second wavelength W2 larger than the cutoff wavelength 408. In some other embodiments, for a first polarization state, the reflective polarizer 102 reflects at least about 70%, at least about 80%, at least about 90%, at least about 95%, or at least about 99% of the incident light 202 to at least a first wavelength W1 smaller than the cutoff wavelength 408. In some embodiments, for a first polarization state, the reflective polarizer 102 transmits at least about 55%, at least about 60%, at least about 70%, at least about 80%, or at least about 90% of the incident light 202 to at least a second wavelength W2 larger than the cutoff wavelength 408. The first wavelength W1 is within the blue wavelength range. Therefore, for the first polarization state, the reflective polarizer 102 reflects at least a portion of the wavelengths within the blue wavelength range. In some embodiments, the first wavelength W1 is in the range of about 400 nm to about 500 nm. In the illustrated embodiment, the first wavelength W1 is less than the minimum wavelength 406. In some other embodiments, the first wavelength W1 may be between the minimum wavelength 406 and the maximum wavelength 407. The second wavelength W2 may be in the red wavelength range. In some embodiments, the second wavelength range W2 is in the range of about 600 nm to about 700 nm. In some embodiments, for the first polarization state, the reflective polarizer 102 may reflect at least about 60% of the incident light 202 for one or more wavelengths within the blue wavelength range, and transmit at least about 50% of the incident light 202 for each wavelength in subsequent wavelength ranges such as the green wavelength range and the red wavelength range.

[0030] For substantially perpendicular incident light 202 and a second polarization state, the reflective polarizer 102 transmits at least 70% of the incident light 202 for each of the first and second wavelengths W1 and W2. In some embodiments, for a second polarization state, the reflective polarizer 102 transmits at least about 80%, at least about 85%, or at least about 90% of the incident light 202 for each of the first and second wavelengths W1 and W2. As shown in Figure 5, for substantially perpendicular incident light 202 and a second polarization state, the reflective polarizer 102 includes a transmission spectrum 420. In some embodiments, for a second polarization state, the reflective polarizer 102 may transmit at least about 70% of the incident light 202 for each wavelength in the visible wavelength range of about 400 nm to about 700 nm. In some other embodiments, for a second polarization state, the reflecting polarizer 102 may transmit at least 80%, at least 85%, or at least 90% of the incident light 202 for each wavelength within the visible light wavelength range.

[0031] In some embodiments, the first polarization state is S-polarization and the second polarization state is P-polarization. In some other embodiments, the first polarization state is P-polarization and the second polarization state is S-polarization. The first polarization state is generally aligned with the x-axis, while the second polarization state is generally aligned with the y-axis.

[0032] Referring to Figures 3, 4, and 6, for substantially perpendicular incident light 202 and a first polarization state, the reflecting polarizer 102 includes one of the transmission bands 502, 506, and 510. The transmission bands 502, 506, and 510 correspond to the transmission spectra 522, 524, and 526, respectively, for substantially perpendicular incident light 202 and a first polarization state. The transmission spectrum 420 (shown in Figure 5) for substantially perpendicular incident light 202 and a second polarization state may correspond to the transmission spectra 522, 524, and 526, respectively, for the first polarization state.

[0033] The transmission band 502 has a cutoff wavelength 504. The transmission band 506 has a cutoff wavelength 508. The transmission band 510 has a cutoff wavelength 512. The minimum wavelength 406 is smaller than each of the cutoff wavelengths 504, 508, and 512 of the respective transmission bands 502, 506, and 510. In other words, the cutoff wavelengths 504, 508, and 512 of the respective transmission bands 502, 506, and 510 are greater than the minimum wavelength 406 of the blue FWHM 410 of the display panel 108. In the embodiment shown in Figure 6, the maximum wavelength 407 is smaller than each of the cutoff wavelengths 508 and 512 of the respective transmission bands 506 and 510. In other words, the cutoff wavelengths 508 and 512 of the transmission bands 506 and 510 are greater than the maximum wavelength 407 of the blue FWHM 410 of the display panel 108. However, the cutoff wavelength 504 of the transmission band 502 is smaller than the maximum wavelength 407 of the blue FWHM 410 of the display panel 108. Therefore, the cutoff wavelength 504 of the transmission band 502 is greater than the minimum wavelength 406 and smaller than the maximum wavelength 407. In some embodiments, the cutoff wavelength 504 of the transmission band 502 of the reflecting polarizer 102 is about 450 nm. In some embodiments, the cutoff wavelength 504 of the transmission band 502 of the reflecting polarizer 102 is greater than about 425 nm and smaller than about 475 nm. In some other embodiments, the cutoff wavelength 504 of the transmission band 502 of the reflecting polarizer 102 is greater than about 470 nm. In some embodiments, the cutoff wavelength 508 of the transmission band 506 of the reflecting polarizer 102 is about 525 nm. In some embodiments, the cutoff wavelength 508 of the transmission band 506 of the reflecting polarizer 102 is greater than about 500 nm and smaller than about 550 nm. In some embodiments, the cutoff wavelength 512 of the transmission band 510 of the reflective polarizer 102 is about 475 nm. In some other embodiments, the cutoff wavelength 512 of the transmission band 510 of the reflective polarizer 102 is greater than about 450 nm and less than about 500 nm.

[0034] For a first polarization state, the reflective polarizer 102 reflects at least about 60% of the incident light 202 to at least a first wavelength W1 smaller than each of the cutoff wavelengths 504, 508, and 512, and transmits at least about 50% of the incident light 202 to at least a second wavelength W2 larger than each of the cutoff wavelengths 504, 508, and 512. In some other embodiments, for a first polarization state, the reflective polarizer 102 reflects at least about 70%, at least about 80%, at least about 90%, at least about 95%, or at least about 99% of the incident light 202 to at least a first wavelength W1 smaller than each of the cutoff wavelengths 504, 508, and 512. In some embodiments, for a first polarization state, the reflecting polarizer 102 transmits at least about 50%, at least about 60%, at least about 70%, at least about 80%, or at least about 90% of the incident light 202 for at least a second wavelength W2 that is greater than each of the cutoff wavelengths 504, 508, and 512. The first wavelength W1 is within the blue wavelength range. Therefore, for a first polarization state, the reflecting polarizer 102 reflects a portion of the wavelengths within the blue wavelength range.

[0035] For substantially perpendicular incident light 202 and a second polarization state, the reflecting polarizer 102 transmits at least 70% of the incident light 202 for at least each of the first and second wavelengths W1 and W2. In some other embodiments, for a second polarization state, the reflecting polarizer 102 transmits at least about 80%, at least about 85%, or at least about 90% of the incident light 202 for at least each of the first and second wavelengths W1 and W2.

[0036] Figure 7 shows a half-wave retarder 104. Figure 8 is a graph 600 showing the phase difference with respect to wavelength for a half-wave retarder 104 according to one embodiment of the present disclosure. Graph 600 includes a phase difference curve 602 showing the phase difference of the half-wave retarder 104 at various wavelengths.

[0037] Referring to Figures 7 and 8, with respect to substantially perpendicular incident light 204, the half-wave retarder 104 has a first phase difference R1 less than about 250 nm at a first wavelength W1 and a second phase difference R2 at a second wavelength W2. In some embodiments, the second phase difference R2 is greater than about 200 nm. In some embodiments, the second phase difference R2 is smaller than the first phase difference R1. In other words, the second phase difference R2 is less than about 250 nm.

[0038] Furthermore, as shown in Figure 8, the half-wave retarder 104 has a minimum blue phase difference Rb at the corresponding blue wavelength Wb, a minimum green phase difference Rg at the corresponding green wavelength Wg, and a minimum red phase difference Rr at the corresponding red wavelength Wr. The minimum blue, green, and red phase differences Rb, Rg, and Rr are within 20% of each other's ranges. In some other embodiments, the minimum blue, green, and red phase differences Rb, Rg, and Rr are within 15% of each other's ranges. As shown in Figure 8, the minimum blue, green, and red phase differences Rb, Rg, and Rr are less than 250 nm. In some embodiments, a second phase difference R2 is less than the minimum blue and green phase differences Rb, Rg. In some embodiments, a second phase difference R2 is greater than the minimum red phase difference Rr. In some embodiments, a first phase difference R1 is greater than the minimum blue, green, and red phase differences Rb, Rg, and Rr. Graph 600 may clearly show that the phase difference of the half-wave retarder 104 does not change significantly with the wavelength of the incident light 204. Therefore, the half-wave retarder 104 is a non-achromatic half-wave retarder.

[0039] Referring to Figure 8, the half-wave curve 604 shows the variation in phase difference with wavelength for an ideal half-wave retarder. In other words, the phase difference at each point on the half-wave curve 604 is half the corresponding wavelength (i.e., phase difference = wavelength / 2). The half-wave curve 604 intersects the phase difference curve 602 of the half-wave retarder 104 at only one point C. Therefore, the half-wave retarder 104 has an ideal half-wave phase difference at only one wavelength Wc corresponding to point C. The wavelength Wc can be approximately 450 nm. Otherwise, the phase difference curve 602 deviates from the half-wave curve 604 at all other wavelengths. Furthermore, the phase difference curve 602 generally decreases with increasing wavelength, while the half-wave curve 604 increases with increasing wavelength. The half-wave curve 604 has a first half-wave phase difference H1 at a first wavelength W1 (i.e., H1 = W1 / 2). Furthermore, the half-wave curve 604 has a second half-wave phase difference H2 at the second wavelength W2 (i.e., H2 = W2 / 2).

[0040] Figure 9 is a graph 700 showing the phase difference shift with respect to wavelength for the half-wave retarder 104. Graph 700 includes a phase difference shift curve 702 showing the phase difference shift of the half-wave retarder 104 at various wavelengths.

[0041] Referring to Figures 8 and 9, the phase difference shift curve 702 shows the shift of the phase difference curve 602 of the half-wave retarder 104 from the half-wave curve 604. The shift D1 of the first phase difference R1 from the first half-wave phase difference H1 corresponding to the first wavelength W1 is smaller than the shift D2 of the second phase difference R2 from the second half-wave phase difference H2 corresponding to the second wavelength W2. The first half-wave phase difference H1 corresponding to the first wavelength W1 may be considered as the ideal phase difference of the half-wave retarder 104 at the first wavelength W1. Similarly, the second half-wave phase difference H2 corresponding to the second wavelength W2 may be considered as the ideal phase difference of the half-wave retarder 104 at the second wavelength W2. Graph 700 may reveal that the phase difference shift of the half-wave retarder 104 from the half-wave curve 604 for the wavelength range of approximately 400 nm to approximately 500 nm is smaller than the shift for wavelengths greater than 500 nm (for example, within the red wavelength range). The wavelength range of approximately 400 nm to approximately 500 nm can correspond to the blue wavelength range. Therefore, the half-wave retarder 104 can be optimized for the blue wavelength range. In other words, the half-wave retarder 104 can behave more closely to an ideal half-wave retarder in the wavelength range of approximately 400 nm to approximately 500 nm compared to other wavelengths.

[0042] As described above with respect to Figures 3, 5, and 6, for substantially perpendicular incident light 202 and a first polarization state, the reflecting polarizer 102 reflects at least about 60% of the incident light 202 to at least a first wavelength W1 that is smaller than each of the cutoff wavelengths 408, 504, 508, and 512 of the respective transmission bands 404, 502, 506, and 510, and transmits at least about 50% of the incident light 202 to at least a second wavelength W2 that is larger than each of the cutoff wavelengths 408, 504, 508, and 512 of the respective transmission bands 404, 502, 506, and 510. For substantially perpendicular incident light 204, the half-wave retarder 104 has a first phase difference R1 that is smaller than about 250 nm at the first wavelength W1 and a second phase difference R2 at the second wavelength W2. The second phase difference R2 is smaller than the first phase difference R1. Furthermore, the shift D1 of the first phase difference R1 from the first half-wave phase difference H1 corresponding to the first wavelength W1 is smaller than the shift D2 of the second phase difference R2 from the second half-wave phase difference H2 corresponding to the second wavelength W2.

[0043] Such optimization of the half-wave retarder 104 at wavelengths smaller than the cutoff wavelengths 408, 504, 508, and 512 for each transmission band 404, 502, 506, and 510 can reduce chromatic shift during operation of the optical stack 100 in Figures 1 and 2. Specifically, chromatic shift can be reduced by using a half-wave retarder 104 optimized within the blue wavelength range by a reflective polarizer 102 having the cutoff wavelengths 408, 504, 508, and 512 for each transmission band 404, 502, 506, and 510 within or near the blue wavelength range. Thus, the combination of the reflective polarizer 102 and the half-wave retarder 104 can exhibit improved performance in reducing chromatic shift. Furthermore, a half-wave retarder optimized in this manner may be easier and less expensive to manufacture compared to a conventional achromatic half-wave retarder having multiple layers. It should be noted that one of the possible transmission bands 404, 502, 506, and 510 of the reflective polarizer 102 can be selected based on desired application attributes such as the phase difference characteristics of the half-wave retarder 104 and the luminescence characteristics of the display panel 108.

[0044] Figure 10 shows an absorptive polarizer 106. Figure 11 is a graph 800 showing the transmittance percentage against wavelength for the absorptive polarizer 106. Graph 800 includes a transmittance curve 802 showing the transmittance percentage of the absorptive polarizer 106 at various wavelengths. Referring to Figures 10 and 11, for substantially perpendicular incident light 206 and a first polarization state, the absorptive polarizer 106 has a first transmittance T1 at a first wavelength W1 and a larger second transmittance T2 at a second wavelength W2. In some embodiments, for the first polarization state, the first transmittance T1 of the absorptive polarizer 106 is at least about 60%. In some embodiments, for the first polarization state, the first transmittance T1 of the absorptive polarizer 106 may be at least about 70%. In some embodiments, for the first polarization state, the second transmittance T2 of the absorptive polarizer 106 is at least about 65%. In some other embodiments, for a first polarization state, the second transmittance T2 of the absorbing polarizer 106 may be at least about 75%, at least about 80%, at least about 85%, or at least about 90%. In some embodiments, for substantially perpendicular incident light 206 and a second polarization state, the absorbing polarizer 106 may absorb at least 70% of the incident light 206 in the visible light wavelength range.

[0045] Therefore, the absorbing polarizer 106 can transmit less light in the blue wavelength range compared to other wavelengths. When the absorbing polarizer 106 is combined with the reflective polarizer 102 and the half-wave retarder 104 in the optical stack 100 of Figures 1 and 2, chromatic aberration can be further reduced.

[0046] Unless otherwise indicated, all numbers used in this specification and the claims to represent feature sizes, quantities, and physical properties should be understood as being modified by the term “approximately.” Therefore, unless specifically indicated to the contrary, the numerical parameters described in the above specification and the appended claims are approximations that may vary depending on the desired properties that a person skilled in the art would seek to obtain using the teachings disclosed herein.

[0047] While specific embodiments have been illustrated and described herein, it will be understood by those skilled in the art that various alternative and / or equivalent practices can replace the illustrated and described embodiments without departing from the scope of this disclosure. This application is intended to encompass all adaptable or modified examples of the specific embodiments described herein. Accordingly, this disclosure is limited only by the claims and their equivalents. The following are exemplary embodiments. [Item 1] A reflective polarizer having mutually orthogonal transmission axes and reflection axes, Absorbing polarizer and, The system comprises a half-wave retarder positioned between the reflective polarizer and the absorbing polarizer, and for substantially perpendicular incident light, For a first polarization state, the reflecting polarizer includes a transmission band having a cutoff wavelength, and the reflecting polarizer reflects at least about 60% of the incident light for at least a first wavelength smaller than the cutoff wavelength, and transmits at least about 50% of the incident light for at least a second wavelength larger than the cutoff wavelength. For a second orthogonal polarization state, the reflecting polarizer transmits at least 70% of the incident light for each of the first and second wavelengths. The half-wave retarder has a first phase difference smaller than about 250 nanometers (nm) at the first wavelength and a second phase difference at the second wavelength, wherein the deviation of the first phase difference from the first half-wave phase difference corresponding to the first wavelength is smaller than the deviation of the second phase difference from the second half-wave phase difference corresponding to the second wavelength. An optical stack in which, with respect to the first polarization state, the absorbing polarizer has a first transmittance at the first wavelength and a larger second transmittance at the second wavelength. [Item 2] The optical stack according to item 1, further comprising a display panel positioned below the reflecting polarizer on the opposite side of the half-wave retarder, wherein the display panel is configured to emit display light including a blue emission spectrum in which the blue full width at half maximum (FWHM) is defined between the minimum wavelength and the maximum wavelength, and the minimum wavelength is smaller than the cutoff wavelength of the transmission band of the reflecting polarizer. [Item 3] The optical stack according to item 2, wherein the cutoff wavelength of the transmission band of the reflective polarizer is smaller than the maximum wavelength of the blue FWHM. [Item 4] The optical stack according to item 2, wherein the cutoff wavelength of the transmission band of the reflective polarizer is greater than the maximum wavelength of the blue FWHM. [Item 5] The optical stack according to item 1, wherein the cutoff wavelength of the transmission band of the reflecting polarizer is less than about 550 nm and greater than about 450 nm. [Item 6] The optical stack according to item 1, wherein the reflective polarizer comprises a plurality of alternating first polymer layers and second polymer layers totaling at least 40, each of the first polymer layers and the second polymer layers having an average thickness less than about 350 nm. [Item 7] The optical stack according to item 1, wherein the second phase difference is greater than about 200 nm, and the first transmittance of the absorbing polarizer is at least about 60%. [Item 8] A display panel configured to emit display light including a blue emission spectrum in which the blue full width at half maximum (FWHM) is defined between the minimum wavelength and the maximum wavelength, A reflective polarizer configured to receive the display light from the display panel and reflect a portion of the display light as reflected polarized light, the reflective polarizer having mutually orthogonal transmission axes and reflection axes, Absorbing polarizer and, The system comprises a half-wave retarder positioned between the reflective polarizer and the absorbing polarizer, and for substantially perpendicular incident light, For a first polarization state, the reflective polarizer includes a transmission band having a cutoff wavelength greater than the minimum wavelength of the blue emission spectrum of the display panel, the reflective polarizer reflects at least about 60% of the incident light for at least a first wavelength smaller than the cutoff wavelength, and transmits at least about 50% of the incident light for at least a second wavelength larger than the cutoff wavelength. For a second orthogonal polarization state, the reflecting polarizer transmits at least 70% of the incident light for each of the first and second wavelengths. The half-wave retarder has a first phase difference smaller than approximately 250 nm at the first wavelength and a second phase difference at the second wavelength, wherein the deviation of the first phase difference from the first half-wave phase difference corresponding to the first wavelength is smaller than the deviation of the second phase difference from the second half-wave phase difference corresponding to the second wavelength. An optical stack in which, with respect to the first polarization state, the absorbing polarizer has a first transmittance at the first wavelength and a larger second transmittance at the second wavelength. [Item 9] The optical stack according to item 8, wherein the minimum wavelength is greater than approximately 430 nm, the maximum wavelength is less than approximately 480 nm, and the cutoff wavelength of the transmission band of the reflecting polarizer is less than the maximum wavelength of the blue FWHM. [Item 10] The optical stack according to item 8, wherein the cutoff wavelength of the transmission band of the reflective polarizer is greater than the maximum wavelength of the blue FWHM.

Claims

1. A reflective polarizer having mutually orthogonal transmission axes and reflection axes, Absorbing polarizer and, The system comprises a half-wave retarder positioned between the reflective polarizer and the absorbing polarizer, and for perpendicular incident light, For a first polarization state, the reflective polarizer includes a transmission band having a cutoff wavelength, and the reflective polarizer reflects at least 60% of the incident light for at least a first wavelength smaller than the cutoff wavelength, and transmits at least 50% of the incident light for at least a second wavelength larger than the cutoff wavelength. With respect to the orthogonal second polarization state, the reflecting polarizer transmits at least 70% of the incident light for each of the at least first wavelength and the second wavelength. The half-wave retarder has a first phase difference smaller than 250 nanometers (nm) at the first wavelength and a second phase difference at the second wavelength, wherein the deviation of the first phase difference from the first half-wave phase difference corresponding to the first wavelength is smaller than the deviation of the second phase difference from the second half-wave phase difference corresponding to the second wavelength. With respect to the first polarization state, the absorbing polarizer has a first transmittance at the first wavelength and a larger second transmittance at the second wavelength. An optical stack in which the reflective polarizer and the half-wave retarder are arranged such that they are parallel to each other.

2. The optical stack according to claim 1, further comprising a display panel positioned below the reflecting polarizer on the opposite side of the half-wave retarder, wherein the display panel is configured to emit display light including a blue emission spectrum in which the blue full width at half maximum (FWHM) is defined between the minimum wavelength and the maximum wavelength, and the minimum wavelength is smaller than the cutoff wavelength of the transmission band of the reflecting polarizer.

3. The optical stack according to claim 2, wherein the cutoff wavelength of the transmission band of the reflecting polarizer is smaller than the maximum wavelength of the blue FWHM.

4. The optical stack according to claim 2, wherein the cutoff wavelength of the transmission band of the reflecting polarizer is greater than the maximum wavelength of the blue FWHM.

5. The optical stack according to claim 1, wherein the cutoff wavelength of the transmission band of the reflecting polarizer is less than 550 nm and greater than 450 nm.

6. The optical stack according to claim 1, wherein the reflective polarizer comprises a plurality of alternating first polymer layers and second polymer layers totaling at least 40, and each of the first polymer layers and the second polymer layers has an average thickness less than 350 nm.

7. The optical stack according to claim 1, wherein the second phase difference is greater than 200 nm, and the first transmittance of the absorbing polarizer is at least 60%.

8. A display panel configured to emit display light including a blue emission spectrum in which the blue full width at half maximum (FWHM) is defined between the minimum wavelength and the maximum wavelength, A reflective polarizer configured to receive the display light from the display panel and reflect a portion of the display light as reflected polarized light, the reflective polarizer having mutually orthogonal transmission axes and reflection axes, Absorbing polarizer and, The system comprises a half-wave retarder positioned between the reflective polarizer and the absorbing polarizer, and for perpendicular incident light, For a first polarization state, the reflective polarizer includes a transmission band having a cutoff wavelength greater than the minimum wavelength of the blue emission spectrum of the display panel, and the reflective polarizer reflects at least 60% of the incident light for at least a first wavelength smaller than the cutoff wavelength, and transmits at least 50% of the incident light for at least a second wavelength larger than the cutoff wavelength. With respect to the orthogonal second polarization state, the reflecting polarizer transmits at least 70% of the incident light for each of the at least first wavelength and the second wavelength. The half-wave retarder has a first phase difference smaller than 250 nm at the first wavelength and a second phase difference at the second wavelength, wherein the deviation of the first phase difference from the first half-wave phase difference corresponding to the first wavelength is smaller than the deviation of the second phase difference from the second half-wave phase difference corresponding to the second wavelength. With respect to the first polarization state, the absorbing polarizer has a first transmittance at the first wavelength and a larger second transmittance at the second wavelength. An optical stack in which the reflective polarizer and the half-wave retarder are arranged such that they are parallel to each other.

9. The optical stack according to claim 8, wherein the minimum wavelength is greater than 430 nm, the maximum wavelength is less than 480 nm, and the cutoff wavelength of the transmission band of the reflecting polarizer is less than the maximum wavelength of the blue FWHM.

10. The optical stack according to claim 8, wherein the cutoff wavelength of the transmission band of the reflective polarizer is greater than the maximum wavelength of the blue FWHM.

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