Fresnel lens sheet for rear projection screen

Abstract: A Fresnel lens sheet of a multifocal type is intended to be used in combination with a lenticular lens sheet to construct a rear projection screen for a rear projection television receiver. An annular portion (2) of the Fresnel lens sheet at a distance (R) in a range of 0.05h to 0.5 h, where h is the height of the Fresnel lens sheet, from the optical center (C) of the Fresnel lens sheet, have the shortest focal length f min among those of other portions of the Fresnel lens sheet, whereby uniformity in brightness of a screen employing the Fresnel lens sheet can be achieved. Alternatively, a portion where an exit angle () of outgoing light ray relative to a normal to the surface of the Fresnel lens sheet is the greatest, lies at a distance (R) in a range of 0.3h to 0.8h from the optical center (C) of the Fresnel lens sheet, to obtain the same result.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a Fresnel lens sheet to be employed in combination with a lenticular lens sheet in constructing a rear projection sheet for a rear projection television screen, and to a rear projection screen employing the Fresnel lens. Description of the Related Art Fresnel lens sheets are classified into those of a single-focal type which refract all light rays that have fallen thereon so as to collect at a single focus as shown in Fig. 6A, and those of a multifocal type which refract light rays that have fallen on regions nearer to their periphery at focuses at longer focal lengths and refract light rays that have fallen on regions nearer to their center at focuses at shorter focal lengths, respectively as shown in Fig. 6B. The center of a Fresnel lens is an optical center of the Fresnel lens where the lens angle at the lens top relative to a plane parallel to the major plane of the Fresnel lens, is nearly zero. When a rear projection screen employing the conventional Fresnel lens sheet is employed in a rear projection television receiver, most part of light rays collected at an optimum observation point of the rear projection television receiver are those traveling through the central region of the rear projection screen and, consequently, an observer feels that a region around the center of the rear projection screen is brightest. Such a phenomenon is called "hot band phenomenon" which makes the central region of the rear projection screen look very bright and makes the peripheral region of the same look dark, spoiling the uniformity in brightness of the rear projection screen. JP 56 247 140 A discloses a Fresnel lens comprising prism faces in the form of concentric circles formed on one face of the molding mold which are transferred and formed on one face of the transparent plastic to obtain a Fresnel lens. In this case lens tilt angle  is made small in the outside edge part so that the focal length of the light, which passes through each prism and is refracted, for the optical axis of the center of the lens becomes longer gradually toward the outside edge part from the center. SUMMARY OF THE INVENTION It is an object of the present invention to provide a Fresnel lens sheet capable of securing uniformity in brightness of a screen employing the Fresnel lens sheet, and a rear projection screen employing the same Fresnel lens sheet. This object is solved by a Fresnel lens sheet having hat features disclosed in claim 1 and rear projection screens having the features disclosed in claim 2. According to one aspect of the present invention, a Fresnel lens sheet to be used in combination with a lenticular lens sheet to form a rear projection screen for a rear projection television receiver has a portion which is at a distance from an optical center of the Fresnel lens sheet and has a shortest focal length among those portions of the Fresnel lens sheet, the distance being in a range of 0.05h to 0.5h, where h is a height of the rear projection screen. In an another aspect of the Fresnel lens sheet of the present invention, a portion where the exit angle  of outgoing light rays, i.e., the angle of the path of outgoing light rays to a normal to the surface of the Fresnel lens sheet at the exit point, is the greatest among those portions of the Fresnel lens sheet, is at a distance in a range of 0.3h to 0.8h from an optical center of the Fresnel lens sheet, where h is a height of the Fresnel lens. A rear projection screen in accordance with the present invention is constructed by combining each of the above Fresnel lens sheet and a lenticular lens sheet. The above and other objects, features and advantages of the present invention will become more apparent from the following description taken in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a side view of a Fresnel lens sheet of a first embodiment according to the present invention; Fig. 2 is a front view of the Fresnel lens shown in Fig. 1; Fig. 3 is a side view of a Fresnel lens sheet of a second embodiment according to the present invention; Fig. 4 is a graph showing a f-R characteristic of the Fresnel lens sheet of Fig. 1; Fig. 5 is a graph showing a -R characteristic of the Fresnel lens sheet of Fig. 3; and Figs. 6A and 6B are views for assistance in explaining the focal length of conventional Fresnel lens sheets. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figs. 1 and 2 showing a Fresnel lens sheet 1 of a first embodiment according to the present invention to be used in combination with a lenticular lens sheet, an annular portion 2 of the Fresnel lens sheet 1 at a distance R from an optical center C of the Fresnel lens sheet 1 is made to give a shortest focal length f min . According to the present invention, the distance R is in the range of 0.05h to 0.5h, where h is the height of the Fresnel lens sheet 1. If R < 0.05h, the portion 2 having the shortest focal length f min would lie in a "hot band" or central brightest region. If R > 0.5h, outer peripheral region of the Fresnel lens sheet 1 would have too short focal lengths, which increases loss of quantity of light and causes coloring. Accordingly, it is desirable that the distance R of the portion 2 having the shortest focal length f min meets an inequality: 0.05h ≤ R ≤ 0.5h. The Fresnel lens sheet 1 reduces concentration of light rays passing through a central region thereof at an observation point and directs light rays passing through regions, such as the outer peripheral region, of the sheet which look darkish, toward the observation point whereby the uniformity of brightness of the Fresnel lens sheet 1 is improved and the effect of the "hot band phenomenon" is reduced. Referring to Fig. 3 showing a Fresnel lens sheet 2 of a second embodiment according to the present invention, the Fresnel lens sheet 2 is designed so that a path of a light ray outgoing from each of portions of the Fresnel lens sheet 2 at a distance R from the optical center C of the Fresnel lens 2 is inclined at a maximum exit angle  max to a normal N to the surface of the Fresnel lens sheet 2 at the same portion. According to the invention, the distance R is in the range of 0.3h to 0.8h, where h is the height of the Fresnel lens sheet 2. If R < 0.3h, the brightness of the outer peripheral region of the Fresnel lens sheet 2 will be reduced by shading. If R > 0.8h, unbalanced loss of quantity of light would occurs, causing coloring. Accordingly, it is desirable that the distance R meets an inequality: 0.3h ≤ R ≤ 0.8h. The peripheral region of the Fresnel lens sheet 2 direct light rays effectively toward an observation point, so that the brightness of the peripheral portion of the Fresnel lens sheet 2 is enhanced to improve the uniformity of brightness distribution on the Fresnel lens sheet 2. Examples of the Fresnel lens sheets 1 and 2 of the present invention will be described hereinafter. Example 1 A Fresnel lens sheet having an f-R characteristic as shown in Fig. 4(f is the focal length) is Example 1 of the Fresnel lens sheet 1 of Fig. 1. The Fresnel lens sheet in Example 1 is 920 mm in height h and is intended for used in constructing a 60 in. screen. Focal length f (mm) Distance R (mm) 850 0 830 (Minimum) 150 (0.16h) A rear projection screen was fabricated by combining the Fresnel lens sheet in Example 1 and a lenticular lens sheet, and the rear projection screen was incorporated into a rear projection television receiver for the sensory test of the performance of the rear projection screen. The rear projection screen had an excellent uniform brightness distribution. Example 2 A Fresnel lens sheet having a  -R characteristic as shown in Fig. 5 ( is the exit angle) is Example 2 of the Fresnel lens sheet 2 of Fig. 3. The Fresnel lens sheet in Example 2 is 920 mm in height h and is intended for used in constructing a 60 in. screen for a television receiver of 940 mm in projection distance. Exit angle  (Degree) Distance R (mm) 0.8 100 2.5 (Maximum) 490 (0.53h) A rear projection screen was fabricated by combining the Fresnel lens sheet in Example 2 and a lenticular lens sheet, and the rear projection screen was incorporated into a rear projection television receiver for the sensory test of the performance of the rear projection screen. The rear projection screen had a peripheral portion of a satisfactory brightness and an excellent uniform brightness distribution. Although the invention has been described in its preferred forms with a certain degree of particularity, obviously many changes and variations are possible therein. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein without departing from the scope thereof.

Claims: A Fresnel lens sheet to be used in combination with a lenticular lens sheet to form a rear projection screen for a rear projection television receiver, said Fresnel lens sheet (1) having a Fresnel lens with an optical center (C), and each annular portion (2) of said lens, concentric with said optical center, having a focal length which depends on the radius (R) of said annular portion, characterised in that , in order to improve the brightness uniformity characteristics of the screen, the annular portion (2) of said Fresnel lens having the shortest focal length is located at an intermediated distance (R) from the lens center, and the focal length of the other lens portions gradually decreases when moving from the lens center towards said intermediate annular portion (2) and gradually increases when moving from said intermediate annular portion (2) toward the periphery of said Fresnel lens sheet. A rear projection screen comprising a Fresnel lens sheet according to claim 1 and a lenticular lens sheet disposed in face-to-face arrangement thereto.

Inventors: TAKAHASHI KATSUNORI, TAKAHASHI, KATSUNORI, Takahashi, Katsunori, Dai Nippon Printing Co. Ltd.
Applicants: DAINIPPON PRINTING CO LTD, DAI NIPPON PRINTING CO., LTD.
Application year: 1997
Main classification: G03B21
IPC codes: G03B21, G02B3