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Double sided back light system Disclosure Number: IPCOM000021232D
Publication Date: 2004-Jan-07
Document File: 4 page(s) / 181K

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Double sided back light system

A double-sided backlight is proposed based on outcoupling of polarized light where the direction of the polarized light coupled out on both sides is different. In this backlight the number of components is reduced considerably with respect to currently used backlights. This type of backlight can be used for so-called clamp-shell / flip phones or displays that need to be viewed from both sides (eg. Travel agencies, hospitals)

Description of the problem

In the mobile phone market many types of phones currently carry two displays (flip or clamshell phones): a small display on the outside and a large display on the inside. In order to be able to view the images on both displays under all lighting conditions the used displays are generally transflective LCDs. These types of displays can be viewed in a bright environment by the reflection of the ambient light, whereas under dark conditions a backlight system takes care of the readability.

A backlight normally consists of a lamp + reflector, an incoupling unit, a backlight wave-guide and light management foils (Fig. 1). Two polarizers surround the LCD in order to have the correct direction of the polarized light entering and leaving the display. The bottom polarizer is inserted between the backlight system and the LCD.

Figure 1. Conventional backlight.

In order to use the backlight two sides (Fig. 2), two stacks of light management foils need to be placed. This is expensive and adds to the thickness of the display module. Moreover, as the displays are different in size, it is important for uniformity to couple out light over the entire surface, otherwise at the location of the small display a darker region will be observed in the large display.

In summary the problems to be solved are:

  1. thick double sided backlight due to many light management foils
  2. expensive system because of same
  3. inefficient system due to uniformity requirements

Figure 2. Conventional double sided backlight: many optical foils, non-uniformity.

Proposed solution

Polarized light backlights are known that use a micro structured optically anisotropic foil on top of an isotropic lightguide. The micro structured foil has an index of refraction mismatch with the adhesive for either the s- or the p-polarized light, which results in emission of respectively p- or s-polarized light. Figure 3 shows the principle of a polarized light backlight. We propose to use this principle for a double-sided backlight.

Figure 3. Polarized light backlight based LCD.

In Fig. 4 a double...