A uniform polarized backlight using a low index intermediate layer
Publication Date: 2002-Sep-23
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A uniform polarized backlight using a low
index intermediate layer
To provide a polarized light backlight with a uniform light distribution
Usually a backlight of an LCD emits non-polarized light. Since an LCD needs polarized light, a large fraction of the light emitted by the backlight is absorbed by the polarizer of the LCD. Therefore, a backlight emitting light of the right polarisation only, may be more efficient than a conventional backlight. A backlight emitting polarized light can be achieved using a scattering foil that scatters only one of the two polarisation directions. This was described in [1,2].
Typically, a conventional backlight is composed of a lamp surrounded by a reflector, and a lightguide. The light is coupled out of the lightguide for example a wedge shape of the lightguide, by diffusive dots, or by (micro-)grooves. These structures change the angle of propagation of a ray in the lightguide, so that it will no longer be guided by total internal reflection. Changing for example the density of the grooves or dots can regulate the uniformity of the light output.
As was described in [1,2] a scattering foil can also be used to extract light from a light guide. This foil extracts light of only one polarization, but it results in a non-uniform light distribution. The extraction efficiency is constant (the foil is the same everywhere), but as close to the lamp light is extracted, further away from the lamp less light will be available, and therefore also less light will be extracted from the lightguide.
Although in principle one could combine the scattering foil with a tapered lightguide, or a lightguide with micro-grooves in order to improve the uniformity, in practice this is difficult. Since the initial outcoupling efficiency is relatively high, very large changes in the angular distribution of the rays are needed to compensate for the loss of light. This will cause the wrong polarization direction to couple out of the lightguide too.
. (a) shows a schematic cross-section of the polarized backlight as described by Jagt et al.[1,2] and an example of a ray propagating through the lightguide. Every time the ray strikes the scattering foil, part of the light is scattered, and this may leave the lightguide. The remaining part is guided by total internal reflection. Since the foil scatters only one polarization, only one polarization is extracted. Close to the lamp the intensity is higher than further away from the lamp. (b) by tapering the lightguide the angle of propagation of the rays is changed, and the smaller thickness at the end causes the rays to interact with the foil more frequently. However, in order to make the backlight uniform the tapering angle needs to be too large to be practically feasible.
Here we propose to include an extra layer between the lightguide and the scattering foil. This layer should have a lower refractive index than the lightguide, so that at le...