Creating a More Efficient Light Guide for a 2-D-Type Local-Dimming LCD Backlight
The realization of an optically dimmable, segment-addressable BLU using a single monolithic light-guide plate (LGP) with a light-control function is challenging but has the potential to cut costs and boost performance. This article describes the development of an edge-lit BLU with a monolithic segmented functional LGP.
by K. Käläntär
THE advantages of 2-D dimming for liquid-crystal-display (LCD) backlights have been widely investigated and also discussed in the pages of this magazine (see, for example, "Adaptive Backlight Dimming for LCD Systems" in the November 2009 issue). The author has developed another variation of the concept by creating a monolithic segmented light-guide panel (LGP) that can control cross-talk between segments and is structured to be used as a local-dimming backlight. This architecture can be mass-produced, thus cutting costs and boosting the performance of region-dimming backlights for high-dynamic-range LCDs.
Power savings and high contrast are especially important goals for designers of LCD- based TVs. An LCD module comprises a power-hungry backlight unit (BLU) and a liquid-crystal panel that modulates the light to create 2-D spatial information. Conventionally, when uniform and high-luminance BLUs have been used for displays, they lead to wasted light energy, especially when the display image is of low-average brightness.
The so-called "dimming technique" is a promising concept for solving light-energy waste.1-3 In its ideal form, the brightness of the backlight is controlled spatially and temporally to match the image's local luminance. The luminance of the BLU is changed in cooperation with the transmission of the LCD.3 When the two are successfully managed together, the resulting operation leads to a reduction in the power consumption of the BLU.
With regard to black levels, the dimming technique improves contrast because the backlight can dim the darkened area in the same region as the dark pixels of the image. In addition, the dimming technique enhances moving-image quality because it presents the possibility of inserting the black segment(s) in each frame picture.
Monolithic Segmented BLU Structure
To achieve a power-efficient LCD with high-quality images, a local-dimming backlight is indispensable. The conventional design method has been to use an array of distinct edge-lit backlights, an array of segments of light chambers, or a single light guide and programmed control system to shape the light distribution on the LCD panel.2,3 One of the more recent challenges is the creation of a thin backlight with a monolithic light guide for a local-dimming display.
As shown in Fig. 1(a), the backlight is a combination of a segmented functional LGP, an inverted single prism, a reflector, and 16 pseudo-white LEDs; eight LEDs at each short side. Seven recessed V-shaped grooves are used to make the segments in the light guide [Fig. 1(b)]. The grooves are along the light guide for light isolation between the segments.4-7 An array of rounded prisms is applied to the light-incident surface of each segment for controlling the light distribution inside the light guide, i.e., to eliminate the hot spots. An array of louver prisms is used on the back surface of the light guide to get proper light distribution and propagation. An array of lenticular lenses on the LGP collimates and shapes the emergent light from the LGP. A single inverted prism film is used for directing the emergent light toward the backlight surface normal to the back surface of the LCD.
Fig. 1: (a) Left: the structure of a monolithic BLU. The BLU is a combination of a functional LGP, 16 pseudo-white LEDs, a LGP reflector film, and an inverted prism. (b) Right: An LGP with light-controlling features on the light injection surface, back surface, and "V-groove" optical partition.
Monolithic Light-Guide Plate
An array of rounded prisms is structured on the light-injection surface for forming a uniform light distribution inside and on the LGP close to the light incident surface. The function of the optical microstructures on the light-injection surface is basically to widen the injected light distribution inside the LGP.8 The widening of light-distribution results in reducing the dim regions and increasing the luminance uniformity of the light guide near the light source.4,5 Each microstructure is a rounded prism, i.e., a combination of semi- "V"-prism and round curvature. The light sources are set near the light-injection surface of each segment that face the rounded prisms at a distance of 0.4 mm.
The LED distribution is semi-Lambertian. The light distribution inside the LGP is designed to be collimated at the center and slightly diffused at larger angles. The injected light flux after passing the rounded prisms, i.e., inside the light guide is plotted in Fig. 2.
The features transmit a significant portion of the light and expand some portion up to 170° inside the LGP. This results in illuminating both sides of the light source that are the dark zones inside the LGP and increasing the uniformity on the LGP near the light-injection surface. The light-coupling efficiency is about 93.6%. For comparison, the light distribution on a flat light-injection surface is shown in Fig. 2. The light distribution is limited to about 42° because of the PMMA material and its refractive index (1.492 at the Sodium D-line). The light-coupling efficiency for a flat surface is about 90.6%. In contrast, the round-prism features show how the light is widened and coupling efficiency increased inside the LGP due to the increase in the light-coupling surface.
Fig. 2: The light distributions are shown inside and outside of the LGP. The LED distribution has a semi-Lambertian luminous distribution. The distribution is widened after passing through the light-injection features. For comparison, the light distribution after passing a flat light injection surface is plotted.
Recessed U-Groove as Partition
In a segmented backlight with a 2-D dimmable function, the interaction between the segments due to optical cross-talk has a large impact on the overall performance of segment dimming, i.e., the cross-talk limits the effective spatial backlight modulation.
The control of the light-leakage rate between backlight segments is necessary for the careful handling of the display panel timing. The uniformity in luminance and directionality of the emergent light between the backlight segments are prerequisite for higher-quality displays. The nature of cross-talk between the segments was investigated and several grooves with different geometrical shapes were adopted. A U-groove with fixed width and a gradually increasing height along the y axis of the segment was found to work well to reduce the cross-talk. The gradation in U-groove height corrected the light profile along the length of the segment and resulted in light profiles that were similar to each other, both near and far from the light source along the y axis. The shape of the U-groove is shown in Fig. 3.
Fig. 3: The outline of the recessed "U-groove" is shown on the light-injection surface.
The height of a U-groove is designed to change from 0.1 to 0.5 mm at the center with a width of 0.020 mm as shown the variation along the length of the LGP in Fig. 4. The variation is decided by optimizing the light leakage between the neighboring segments, and in order to obtain a uniform light profile along the segment. The graded height and the fixed width of the U-groove control the amount of light leakage (the cross-talk) between the neighboring segments in order to obtain uniformity and high-quality imagery for a front-of-screen display.
Fig. 4: The height of the "U-groove" increases along the y-axis in order to decrease the leakage of the light to the neighboring segments. The increase of the groove's height is the same as the left side groove. The peak of the graph shows the center of the LGP.
Louver Prism for Light Extraction
In order to distribute and extract the light uniformly in the segment, an array of unilateral micro-prisms that have a louver-shaped cross section is used on the back surface of the LGP. The propagated light (inside the light guide) is reflected onto the prism and a significant amount of the light is directed toward the segment's front surface. The reflected light is controlled by the total internal reflection (TIR) that results in light-energy preservation on reflection. The cross-section of the light guide with a louver prism on the back surface is shown in Fig. 5, where αr and ßr, are the prism's angles and Pr1, Pr2, and Pr3 (= Pr1 + Pr2) are the projected prism's widths.
In order to have uniform luminance on each segment and to reduce the light at the end of the segment, i.e., at the center of the LGP, an angle gradation (αr varies gradually) for the prism array is designed to reflect the propa-gated light. The same gradation is applied to the prism arrays of the other segments to make a uniform luminance on the entire backlight. For making uniform light on the left segments, the louver prism that is shown in Fig. 5(b) is applied.
The surface of a unilateral prism that is confronted to the left light source is adjusted in order to extract the ray that propagates to the right. For extracting light that is injected from the right, the louver prism that is shown in Fig. 5(c) is used. The uniformity on the segment is optimized by changing the αr angle of the louver prism that varies with distance as shown in Fig. 5(d). The angle ßr can be kept constant, so that the angle γr varies with distance. The left half of the graph shown in Fig. 5(d) shows the variation in the prism angle that controls the light propagating toward the right. The left louver prisms reflect the light that propagates only toward the right. The right part of the graph is the variation of the prism angle, containing the light that propagates toward the left.
Since the louver prism is unilateral (e.g., the left segment), the left side surface of the prism does not function as that of the right surface of the prism. The reflection characteristic of the louver prism functions as optical isolation between the left and right segments. This is the reason for controlling the light from both sides.
Fig. 5: (a) Top: the cross-section of the LGP is shown with light-extraction features on the back surface. In (b), the light-extraction features are shown for the left side of the LGP. In (c), the light-extraction features for the right side of the LGP appear. In (d), shown is the angle variation of the light-extraction feature for making uniform luminance distribution on the LGP.
Lenticular Prism for Light-Cone Shaping
An array of lenticular prisms that are part of a cylindrical lens is structured along the y axis on the LGP to deflect the propagated light toward the rear surface of the inverted prism film with a TIR function that is set on the light guide [Fig. 1(a)]. The array functions as low-power light-collimating elements. The cross-section of the array in the x–z plane is shown in Fig. 6.
Fig. 6: The light-cone-shaping feature is shown on the LGP. This is an array of low-power light-collimating lenses.
The prism array has an equal sag of Hf (= 15 μm), a curvature of Rf (= 28.33 μm), and a pitch of Pf (= 50 μm). The prism array of the light-injection surface, the rear prism array, and the front lenticular prism array are designed in combination in order to obtain a uniform luminance on the inverted prism film.
Results for the Monolithic Segmented Light Guide
The dimensions of the monolithic LGP are 41.7 75.0 mm2 and the effective area is about 39.88 x 64.8 mm2. The widths of the second to seventh segments are equal and are designed to be 5 mm. However, the first and eighth segments have widths of 5.85 mm because these segments have only single neighboring segments, and their luminance should be compensated. Therefore, the widths of the first and last segments were designated as 5.85 mm and the effective widths of these segments were set to 5 mm, the same as the other segments. The material of the LGP is PMMA with a refractive index of 1.492 at a Sodium D-line. The backlight is a combination of a monolithic segmented functional light guide, a reflector with a reflectivity of 98% (ESR, Sumitomo 3M Co.), an inverted prism film with a TIR function (M065HS, Mitsubishi Rayon), and 16 pseudo-white LEDs (NSSW206, Nichia Chemical Co.). Each LED has a flux of 6.6 lm at 20 mA.
The luminance uniformity and optical isolation on the backlight were evaluated. All segments were switched on and the profile of light along the width of the backlight was measured. The ripple of the light is about 5%, resulting in a uniformity of more than 95%.
For evaluation of the cross-talk and the light distribution, a single segment was switched on, as shown in Fig. 7(a). The light profiles along the width of the segment at three points were plotted in Fig. 7(b).
Almost the same profiles were obtained at three positions. This means that the light distribution of the segment is uniform along the segment. The cross-talk was evaluated between the confronted segments – that is, an ON segment and an OFF segment as shown in the profile of averaged luminance along the D–D′ line (x axis) in Fig. 7(c). The profile shows that the gradation of the unilateral louver prism could make uniform light on a single segment by controlling the light direction. The average cross-talk was found to be 6:1 in confronted segments of the ON and OFF conditions.
Fig. 7: (a) A single segment is switched "ON" and the leakage light of the confronted segment is evaluated. (b) The profiles of the luminance along the x-axis at three points are plotted (A-A′, B-B′, C-C′) and the same ratios for the leakage light are obtained. (c) The leakage light from the lit segment to the confronted segment is about 6:1.
Ramifications of the New Technology
The author's aim was to optically integrate functional LGP segments to fabricate a monolithic light guide that could be mass-produced, thus cutting costs and boosting the performance of local-dimming backlights for high-dynamic-range LCDs. For this purpose, a monolithic segmented functional light guide (a single plate 85.81 mm on the diagonal) was developed, using characterized recessed U-grooves as partitions, an array of unilateral louver prisms for light extraction, lenticular prisms for extracted light-cone shaping, and rounded prisms for light shaping inside the LGP. This test piece was relatively modest in size due to the cost of creating it, but the concept could be realized more cost effectively in much larger sizes using roll-to-roll technology. The cross-talk between the segments suppressed to 20% and 4% (center-to-center) in the first and second neighboring segments. This amount of cross-talk was an optimized ratio from a vantage point of the evaluation of the uniformity of luminance on the display.
The luminance enhancement was 1.5–2 times that of a conventional BLU, depending on the front-of-screen optical components. In general, sharp cut-off of the illuminated segment enhances the power consumption and the display image. Wide light distribution can result in widening the viewing angle. However, it does not enhance the image. The recessed optical isolators should produce an optimized cross-talk (for the sake of good front-of-screen quality) to enhance the image, while limiting the viewing angle of the display. Possible uses for this monolithic segmented functional LGP are a scanning backlight or a field-sequential-color display with higher frame rates; a scanning-type field alternative 3-D backlight; and a segmented BLU for local dimming, or for local dimming a 3-D display.
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