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Improving the picture quality of full-colour PLED displays. Disclosure Number: IPCOM000007440D
Publication Date: 2002-Mar-26
Document File: 4 page(s) / 57K

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The Prior Art Database



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1.       Introduction

 In a full-colour polymer light emitting diode (PLED) a straight forward arrangement of the pixels is as shown in Fig.1; a pixel

Fig.1, R(ed), G(reen), and B(lue) columns and a full-colour pixel

(300x300mm2) consists of 3 RGB sub-pixels. The sub-pixels are arranged as columns to facilitate ink-jet printing. If a white area is to be displayed then all three sub-pixels are activated with about the same luminance. However, for a saturated colour area only one of the sub-pixels is emitting. Simulations show that the display of saturated colour areas results in coloured lines that can be resolved by the eye and  this leads to a nasty picture. Figure 2, upper row, shows a white square area on a gray back-ground (left hand picture) that is displayed by a full-colour PLED display as RGB-columns (middle picture)

Fig.2, display of a white and red square by a full-colour PLED.

that look as a white square  at some distance. The saturated red square, lower row in Fig.2, is displayed by red columns separated by two black columns (middle picture) and this turns out to lead to a bad picture. It has been long for a long time that such problems may be solved by using optical spatial filters (herein also referred to as optical grids)which filter out the high (spatial frequency) image detail which is due to the pixelation leaving only the lower (spatial) frequencies corresponding to the actual picture content optical grids. The most simple optical spatial filter appears to work properly for the PLED pixel-structure; this is shown in the right hand pictures of Fig.2. The RGB-columns are transferred to a real white square while the red columns of the red square become a real red square. Both squares have small coloured extensions but this turns out to be not annoying.

Optical grid design

Fig.3, outline of  optical grid, the emitting PLED surface is in direct contact with the lower flat surface.

The optical grid that is fit for the purpose is sketched in Fig.3; a plate (of glass, say) with a flat surface and a surface that consists of prisms and flat areas in between. The emitting PLED stack is in direct contact with lower flat. The ridges of the prism are aligned parallel to the RGB sub-pixel columns. Consider a point source in the PLED stack that emits light in the yz-plane within a certain angle as shown in Fig.4. The left picture shows the rays that are

Fig.4, ray tracing of the emission by a point source in optical contact with the body of the grid structure; left hand figure; emitted rays into air, right hand figure: the 3 images as observed in air.

refracted by the three planes of the glass-air transition of the grid. If the path of these rays are extended into the body of the grid then it appears that three images of the point source are formed above the point source. For a given location of the point source the angle of the prisms can be chosen such that the distance between the imag...