TECHNICAL FIELD

This invention relates to circular polarizers, and more particularly to high durability circular polarizers for use with emissive displays.

BACKGROUND

Unpolarized ambient light waves vibrate in a large number of directions without a single characterizing electromagnetic radiation vector. By contrast, plane polarized light consists of light waves having a direction of vibration along a single electromagnetic radiation vector. Also, circularly polarized light has a direction of vibration along an electromagnetic radiation vector that rotates as the light propagates through space. Polarized light has many applications in electro-optical devices, such as the use of plane and circular polarizing filters to reduce glare in displays.

Much commercial attention has been directed to the development and improvement of flat panel displays, particularly flat panel displays that are thinner and more compact than displays requiring backlighting for luminescence. Such flat panel displays may use emissive or electroluminescent displays, i.e., self-luminous displays for which no backlight is required.

Polarizers in the form of synthetic polarizing films are desirable for their comparative ease of manufacture and handling and the comparative ease with which they may be incorporated into electro-optical devices such as flat panel displays. In general, plane polarizing films have the property of selectively passing radiation vibrating along a given electromagnetic radiation vector and absorbing electromagnetic radiation vibrating along a second electromagnetic radiation vector based on the anisotropic character of the transmitting film medium. Plane polarizing films include dichroic polarizers, which are absorbing plane polarizers utilizing the vectorial anisotropy of their absorption of incident light waves. The term dichroism refers to the property of differential absorption of the components of incident light, depending on the vibration directions of the component light waves. Light entering a dichroic plane polarizing film encounters two different absorption coefficients along transverse planes, one coefficient being high and the other coefficient being low. Light emerging from a dichroic film vibrates predominantly in the plane characterized by the low absorption coefficient.

Dichroic plane polarizing films include iodine, dyestuff and H-type polarizers. An H-type polarizer is a synthetic dichroic sheet polarizer including a polyvinyl alcohol-iodine complex. Such a chemical complex is referred to as a chromophore. The base material of an H-type polarizer is a water-soluble high molecular weight substance, and the resulting film has relatively low moisture and heat resistance and tends to curl, peel or otherwise warp when exposed to ambient atmospheric conditions.

In contrast to H-type polarizers and other synthetic dichroic plane polarizers are K-type polarizers. A K-type polarizer is a synthetic dichroic plane polarizer based on molecularly oriented polyvinyl alcohol (PVA) sheets or films with a balanced concentration of light-absorbing chromophores. A K-type polarizer derives its dichroism from the light absorbing properties of its matrix, not from the light-absorbing properties of dye additives, stains, or suspended crystalline materials. Thus, a K-type polarizer may have both good polarizing efficiency and good heat and moisture resistance.

An improved K-type polarizer, referred to as a KE polarizer, is manufactured by 3M Corp., Norwood, Mass. The KE polarizer has improved polarizer stability under severe environmental conditions, such as high temperatures and high humidity. In contrast to H-type polarizers, in which the light absorption properties are due to the formation of a chromophore between PVA and tri-iodide ion, KE polarizers are made by chemically reacting the PVA by an acid catalyzed, thermal dehydration reaction. The resulting chromophore, referred to as polyvinylene, and the resulting polymer may be viewed as a block copolymer of vinylalcohol and vinylene.

For H-type polarizers, stability is achieved by sandwiching the polarizer between two plastic substrates, such as two layers of cellulose triacetate (CTA), one on each side of the polarizer. However, even in these structures the application of heat, humidity and/or vacuum can adversely affect the properties of the polarizer. By contrast, K-type polarizers such as KE polarizers do not need to be sandwiched between sheets of CTA. The polyvinylene chromophore of the KE polarizer is an extremely stable chemical entity, since the chromophore is intrinsic to the polymer molecule. This chromophore is thermally stable as well as resistant to attack from a wide range of solvents and chemicals.

A K-type polarizer such as a KE polarizer has several advantages over other types of polarizers, e.g., iodine and dyestuff polarizers. K-type polarizers have more durable chromophores, are thinner, and may be designed with variable transmission levels. Most notably, K-type polarizers such as KE polarizers may be used in applications that require high performance under severe environmental conditions, including high temperatures and high humidity, such as 85 C. and 85% relative humidity, for extended periods of time. Under such extreme environmental conditions, the stability of iodine and dyestuff polarizers is greatly reduced, thus limiting their usefulness in applications such as emissive flat panel displays. Due to the inherent chemical stability of K-type polarizers, a wide variety of adhesive formulations, including pressure sensitive adhesives, can be applied directly to K-type polarizers. Further, a single-sided plastic support is adequate to give physical support for K-type polarizers, and since this support can be located outside of the display module, it need not be optically isotropic and lower-cost substrates such as polyethylene terephthalate (PET) are acceptable alternatives. Moreover, the ability to construct single-sided laminates allows the optical structures to be thinner, allowing for additional flexibility in the design and manufacture of flat panel display elements. These advantages of K-type polarizers may be used in optical applications such as flat panel displays.

A circular polarizer may be constructed of a plane polarizer and a quarter-wavelength retarder. A quarter-wavelength retarder shifts the phase of light waves propagating along one plane through the retarder by one-quarter wavelength, but does not shift the phase of light waves propagating through the retarder along a transverse plane. The result of combining light waves that are one-quarter wavelength out of phase and that vibrate along perpendicular planes is circularly polarized light, for which the electromagnetic radiation vector rotates as the combined light waves travel through space.

Circularly polarized light may be described with respect to two distinct polarization states: left-handed (L) and right-handed (R) circularly polarized light. A circular polarizer absorbs light of one of these polarization states and transmits light of the other polarization state. The use of circular polarizers to reduce glare in displays is well known. In particular, light from an emissive display can be selectively transmitted through a circular polarizer, while background ambient light reflected in the display, which causes glare, may be reduced or eliminated.

A typical thin film circular polarizer 10 is shown in FIG. 1. A dichroic plane polarizer 12 such as an iodine polarizer usually has 3 mil cellulose triacetate (CTA) coatings 14, 16 applied to both surfaces of the polarizer prior to laminating the polarizer to a quarter-wavelength retarder 18. The cellulose tri-acetate coatings in this construction protect the dichroic plane polarizer from heat and moisture, which can damage the polarizer.

SUMMARY

In general, in one aspect, the invention features a high durability circular polarizer including an unprotected K-type polarizer and a quarter-wavelength retarder.

Implementations of the invention may also include one or more of the following features. The K-type polarizer may include a KE polarizer sheet. The K-type polarizer and the quarter-wavelength retarder may be thin films.

The K-type polarizer may have a first surface and the quarter-wavelength retarder may have a lower surface, and the circular polarizer may include a first adhesive layer disposed on the first surface of the K-type polarizer to secure the K-type polarizer to the lower surface of the quarter-wavelength retarder. The K-type polarizer may have a second surface, and the circular polarizer may include a support layer disposed on the second surface of the K-type polarizer. The support layer may include polyethylene terephthalate. The quarter-wavelength retarder may have a top surface, and the circular polarizer may include a second adhesive layer disposed on the top surface of the quarter-wavelength retarder for attaching the circular polarizer to a display surface. A removable release liner may be disposed over the second adhesive layer.

In general, in another aspect, the invention features a high durability circular polarizer including a K-type polarizer lacking a protective coating thereon and a quarter-wavelength retarder.

In general, in another aspect, the invention features an optical system, including an emissive display module and a circular polarizer including an unprotected K-type polarizer and a quarter-wavelength retarder.

Implementations of the invention may also include one or more of the following features. The emissive display may be an organic light emitting diode or a plasma display device. The circular polarizer may absorb ambient light to decrease interference of the ambient light with a light signal emitted by the display module.

An advantage of the present invention is that a high durability circular polarizer may be constructed that will be stable in severe environmental conditions such as high temperatures and high humidity. A further advantage of the present invention is that an emissive display may be constructed with such a high durability circular polarizer.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.