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Patent Issued for Optical Film, Surface Light Source Device, and Liquid Crystal Display Device

July 23, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- From Alexandria, Virginia, VerticalNews journalists report that a patent by the inventor Banerjee, Saswatee (Niihama, JP), filed on April 10, 2012, was published online on July 8, 2014.

The patent's assignee for patent number 8773620 is Sumitomo Chemical Company, Limited (Tokyo, JP).

News editors obtained the following quote from the background information supplied by the inventors: "The present invention relates to an optical film, a surface light source device, and a liquid crystal display device.

"As an optical film having a function to enhance luminance when used in a liquid crystal display panel, a reflective polarizing optical film has been proposed (see Japanese Translated International Application Laid-Open No. 11-509331). This optical film is manufactured by alternately stacking 800 or more in total of an optical material layer composed of polyethylene naphthalate and an optical material layer composed of glycol-modified dimethyl cyclohexane terephthalate and stretching them.

"The optical film has polarization separating and wavelength selectivity functions. In the visible light wavelength range of 400 to 700 nm, the optical film transmits therethrough 89.7% of light in a specific polarization direction and reflects a light polarized perpendicular to the former polarization direction. The variability of the transmittance of the optical film in the wavelength range of 400 to 700 nm is 1.05% and thus the optical film can uniformly transmit the visible light.

"The optical film is disposed closer to an observer than is a light source of the liquid crystal display panel. In the light emitted from the light source, the optical film transmits therethrough a light component polarized parallel to the transmission axis of a polarizing plate on the backlight side of the liquid crystal display panel and reflects a light component polarized perpendicular thereto back to the backlight side. The light returned to the backlight side is emitted again to the observer with a partly changed polarization direction by a reflecting plate disposed on the opposite side of the light source from the observer. Hence, the light reflected with a partly changed polarization direction by the reflective part is reused (recycled), so that the polarized light component parallel to the polarization direction of the optical film is transmitted through the optical film and emitted from the screen of the transmissive liquid crystal display panel. Thus, the optical film exhibits a function to enhance the luminance of the liquid crystal display panel."

As a supplement to the background information on this patent, VerticalNews correspondents also obtained the inventor's summary information for this patent: "From the viewpoint of environmental protection, three kinds of (e.g., red, green, and blue) light-emitting diodes have recently come into wider use as backlights for illuminating liquid crystal display panels. In this case, the backlight has higher intensity in the respective wavelength bands of the light components emitted from the light-emitting diodes. The liquid crystal display panel is typically equipped with a color filter, which has an optical characteristic such as a light absorption characteristic responding to the wavelength. Since wavelength dependency thus occurs in the liquid crystal display panel itself and the light illuminating the liquid crystal display panel, there has been a demand for an optical film which can enhance the luminance while taking account of such wavelength dependency.

"It is an object of the present invention to provide an optical film functioning to enhance the luminance when used in a liquid crystal display device while having wavelength selectivity, and a surface light source device and a liquid crystal display device which include the optical film.

"The optical film in accordance with one aspect of the present invention comprises at least one stack having a plurality of basic pairs each constructed by stacking first and second layers having respective refractive indexes in a predetermined direction different from each other. The number of stacks and the refractive index difference in the predetermined direction between the first and second layers, thicknesses of the first layer and the second layer, and number of basic pairs in each of the at least one stack are set such that a reflection spectrum by the at least one stack as a whole conforms to a target reflection spectrum. The target reflection spectrum has, at least one reflection peak region including a spectrum region having a reflectance of at least 50% and a wavelength width of 20 to 60 nm in a reflection spectrum of a first polarized light component polarized in a specific direction in a wavelength range of 400 to 700 nm, while exhibiting a reflectance of 20% or less in a reflection spectrum of a second polarized light component polarized in a direction orthogonal to the polarization direction of the first polarized light component in the wavelength range of 400 to 700 nm.

"This optical film includes at least one stack, while the number of stacks and the refractive index difference in the predetermined direction between the first and second layers, thicknesses of the first layer and the second layer, and number of basic pairs in the stacks are set so as to have a reflection spectrum conforming to the above-mentioned target reflection spectrum. Therefore, when light is incident on the optical film, it selectively reflects the first polarized light, while transmitting the second polarized light therethrough. When such an optical film is employed in a liquid crystal display device, the first polarized light can be recycled, whereby the luminance can be enhanced.

"In one embodiment, letting na,x be the refractive index in a direction parallel to the polarization direction of the first polarized light within a plane of the first layer, and nb,x be the refractive index in a direction parallel to the polarization direction of the first polarized light within a plane of the second layer, |.DELTA.n|=|nb,x-na,x| may be at least 0.02 but 0.23 or less.

"When |.DELTA.n| falls within the range mentioned above, the first and second polarized lights can be separated from each other, while having the wavelength selectivity.

"In one embodiment, letting na,y be the refractive index in a direction parallel to the polarization direction of the second polarized light within the plane of the first layer, and nb,y be the refractive index in a direction parallel to the polarization direction of the second polarized light within the plane of the second layer, |nb,y-na,y| may be less than 0.02.

"When |nb,y-na,y| is less than 0.02, the first and second polarized lights can be separated more reliably from each other.

"In one embodiment, letting na,z be the refractive index in a thickness direction of the first layer, and nb,z be the refractive index in a thickness direction of the second layer, |nb,z-na,z| may be less than 0.02.

"In one embodiment, the number of basic pairs may be 25 to 50.

"In one embodiment, the first and second layers may have a thickness of 5 to 400 nm each.

"In one embodiment, the number of stacks may be at least the number of reflection peak regions in the reflection spectrum of the first polarized light component in the target reflection spectrum.

"In this structure, the stacks are provided by a number not smaller than that of reflection peak regions, whereby at least one stack can be allocated to each reflection peak region. Hence, it will be sufficient if the stack is designed so as to generate its corresponding reflection peak region.

"In one embodiment, the number of stacks may be 1 to 3.

"In one embodiment, the reflection spectrum of the first polarized light in the target reflection spectrum may have one reflection peak region within the range of 430 to 480 nm, one reflection peak region within the range of 510 to 560 nm, and one reflection peak region within the range of 600 to 660 nm.

"This makes it possible to selectively reflect respective wavelengths corresponding to blue, green, and red.

"Another aspect of the present invention relates to a surface light source device. The surface light source device comprises a light source unit; a surface-light-emitting element for converting light from the light source unit into surface light and emitting the surface light from an exit surface part thereof; a reflective part, disposed on the opposite side of the surface-light-emitting element from the exit surface part, for reflecting the light from the surface-light-emitting element to the surface-light-emitting element while changing a polarization state of the light; and the optical film according to one aspect of the present invention, arranged on the exit surface part of the surface-light-emitting element, for receiving the surface light.

"In this structure, the surface light emitted from the surface-light-emitting element impinges as incident light on the optical film. In the incident light, the second polarized light passes through the optical film. On the other hand, the first polarized light in the wavelength range of the reflection peak region is reflected back to the surface-light-emitting element. Thus returned light is emitted from the surface-light-emitting element to the reflective part and reflected by the reflective part, so as to impinge on the optical film through the surface-light-emitting element. The polarization state of the first polarized light component changes upon reflection by the reflective part, so that the light reflected by the reflective part includes the first polarized light and the second polarized light. Therefore, when the light reflected by the reflective part is incident on the optical film, the second polarized light in the incident light tends to pass through the optical film. Thus, the surface light source device can recycle the first polarized light reflected by the optical film. As a result, employing the surface light source device in a liquid crystal display device can improve the luminance. Since the first polarized light reflected by the optical film so as to be recycled has a wavelength range in the reflection peak region, the optical film emits a greater amount of light in the wavelength range of the reflection peak region. Therefore, by setting the reflection peak region according to the characteristic of the light source of the liquid crystal display device or the color filter, the luminance of light in a wavelength range more contributory to displaying images in the liquid crystal display device employing the surface light source device can be improved.

"Still another aspect of the present invention relates to a liquid crystal display device. The liquid crystal display device comprises a light source unit; a surface-light-emitting element for converting light from the light source unit into surface light and emitting the surface light from an exit surface part thereof; a reflective part, disposed on the opposite side of the surface-light-emitting element from the exit surface part, for reflecting the light from the surface-light-emitting element to the surface-light-emitting element while changing a polarization state of the light; the optical film according to one aspect of the present invention, arranged on the exit surface part of the surface-light-emitting element, for receiving the surface light; and a liquid crystal panel arranged on the opposite side of the optical film from the surface-light-emitting element.

"In this structure, the surface light emitted from the surface-light-emitting element impinges as incident light on the optical film. In the incident light, the second polarized light passes through the optical film. On the other hand, the first polarized light in the wavelength range of the reflection peak region is reflected back to the surface-light-emitting element. Thus returned light is emitted from the surface-light-emitting element to the reflective part and reflected by the reflective part, so as to impinge on the optical film through the surface-light-emitting element. The polarization state of the first polarized light changes upon reflection by the reflective part, so that the light reflected by the reflective part includes the first polarized light and the second polarized light. Therefore, when the light reflected by the reflective part is incident on the optical film, the second polarized light in the incident light passes through the optical film and irradiates the liquid crystal panel. Thus, the liquid crystal display device can recycle the first polarized light reflected by the optical film. As a result, the luminance of images displayed by the liquid crystal display device can be improved. Since the first polarized light reflected by the optical film so as to be recycled has a wavelength range in the reflection peak region, the optical film emits a greater amount of light in the wavelength range of the reflection peak region. Therefore, by setting the reflection peak region according to the characteristic of the light source of the liquid crystal display device or the color filter, the luminance of light in a wavelength range more contributory to displaying images in the liquid crystal display device can be improved."

For additional information on this patent, see: Banerjee, Saswatee. Optical Film, Surface Light Source Device, and Liquid Crystal Display Device. U.S. Patent Number 8773620, filed April 10, 2012, and published online on July 8, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8773620.PN.&OS=PN/8773620RS=PN/8773620

Keywords for this news article include: Electronics, Light-emitting Diode, Sumitomo Chemical Company Limited.

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Source: Electronics Newsweekly


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