Various kinds of retardation films are used to improve the display performance of an image display device, e.g., a liquid crystal display or an organic EL display device. Among these, the retardation films whose in-plane retardation increases toward longer wavelengths (also referred to as “reverse wavelength dispersion films” below) may be applied to a reflective liquid crystal display device, a touch screen panel, or an organic EL as an anti-reflection layer.
When the reverse wavelength dispersion film is used as an anti-reflection layer, the phase difference of the measured wavelength is roughly one quarter of lambda (A), and preferably the ratio of the in-plane retardation at 450 nm to the in-plane retardation at the 550 nm Re(450)/Re(550) approaches 0.81. Looking at examples of thinning display devices, the reverse wavelength dispersion films used for this purpose should be around 20 μm to 50 μm thick. Various kinds of retardation films that fulfill the aforementioned needs have been disclosed.
Japanese Patent Publication No. 2007-121351 (published May 17, 2007) discloses a reverse wavelength dispersion film composed of cellulose acylate and ethyl cellulose. The retardation films disclosed in Japanese Patent Publication No. 2007-121351 all possess a roughly quarter-wave in-plane retardation and are suitable as an anti-reflection layer.
Japanese Patent Publication No. 2006-282885 (published Oct. 19, 2006) discloses a reverse wavelength dispersion film composed of cellulose acylate and ethyl cellulose. The in-plane retardation of the retardation film disclosed therein must be modified in order for the same to be used as an anti-reflection layer. The film is made thicker in order for the in-plane retardation to be proportional to the film thickness whereby the film is capable of achieving a roughly quarter-wave in-plane retardation.
Japanese Patent Publication No. 2011-112842 (published Jun. 11, 2011) discloses a reverse wavelength dispersion film composed of cellulose acylate and ethyl cellulose. The retardation film disclosed in JP2011-112842A exhibits better retardation capabilities than the retardation films in JP2007-121351A and JP2006-282885A, and is thus capable of providing identical in-plane retardation properties while being thinner than the aforementioned films.
Japanese Patent Number 4892313 (published Apr. 24, 2008) discloses a cellulose derivative with various aromatic and aliphatic acylates with different maximum absorption wavelengths and molar attenuation coefficients as substituents. The retardation film disclosed demonstrates the intended reverse wavelength dispersion property.
Japanese Patent Publication No. 2009-132764 (published Jun. 18, 2009) discloses a cellulose derivative with a large retardation toward the thickness of the film (Rth). The aforementioned film material is an aromatic esterified cellulose ether, with inorganic particles added to serve as a matting agent.