Patent ID: 7364337

Claim:
A flexible backlight unit for a key of an input device, the key having a character shaped opening portion for enabling light transmission, the unit comprising: a light guide plate formed of flexible transparent resin and having a plurality of island-like light guide pattern portions in which a plurality of engraved or embossed patterns on a predetermined regular basis are formed in a plurality of requirement character shapes so that light can be diffused and scattered at uniform luminance from each necessary light emitting portion of a bottom surface of the key; and at least one light emitting unit installed at a side wall of the light guide plate, wherein a pattern density (D) required for one, which is intended for design, of the island-like light guide pattern portions is defined as in Equation below: S·D=A·T·L· 1/[EXP(−4.4·θ 2 )]·( S t /S C )· S 0 /( S 0 −S P ) where, S: Area of the light guide pattern portion at a position intended for design (mm 2 ), D: Pattern density, A: Constant value (1.7E-7 to 2.0E-2), T: Thickness of the light guide plate (mm), L: Distance from the light emitting unit to the pattern (mm), θ: Angle between an extension line of a central line of the light emitting unit and an imaginary line connecting from the light emitting unit to the light guide pattern portion (radian value: 0 to π/2), S t : Total area sum of the character opening portions of the input device (mm 2 ), S C : Opening portion area of the key character of the top surface of the light guide pattern portion (mm 2 ), S 0 : Space area between center of the light emitting unit and two imaginary lines connecting from the light emitting unit to the light guide pattern portion (mm 2 ), and S P : Areas of other light guide pattern portions provided within space between the center of the light emitting unit and the two imaginary lines connecting from the light emitting unit to the light guide pattern portion (mm 2 ).