In recent years, thin display devices such as liquid crystal display devices have been widely used in various types of apparatus such as personal computers, cellar phones, PDAs, and game machines. For example, a liquid crystal display device for providing a translucent display includes a liquid crystal display panel and a backlight unit positioned opposite to a viewer of the liquid crystal display panel. Such a display device displays a desired image by selectively allowing illumination light emitted by the backlight unit to pass through the liquid crystal display panel.
Patent Document 1 describes an optical sheet to be provided to a backlight unit. The optical sheet includes a light diffusion member which emits and diffuses light, an optically functional member which controls properties such as direction and luminance distribution of the light, and an adhesive layer which bonds the light diffusion member and the optically functional member together. An air layer is provided between the light diffusion member and the optically functional member, and light passing through the air layer serves as illumination light to be supplied to the liquid crystal display panel.
The adhesive layer is made of an adhesive composition containing flow-preventive micro particles dispersed therein and is configured to have a glass transition point Tg which is greater than −70° C. and less than 0° C. Patent Document 1 also describes that this configuration can bond the light diffusion member and the optically functional member together, preventing the members from coming unstuck and separating from each other and maintaining the air layer.
Meanwhile, it is known to attach, e.g., a touch panel or a parallax barrier panel on a surface closer to a viewer of a liquid crystal display panel (see, for example, Patent Documents 2 and 3). The touch panel or the parallax barrier panel is attached on the display panel with an adhesive sandwiched between the panels.