Turning film having multiple slopes

[0001] This invention generally relates to display illumination articles for enhancing luminance from a surface and more particularly relates to a turning film having multiple slopes that redirects light from a light guiding plate.

[0002] Liquid crystal displays (LCDs) continue to improve in cost and performance, becoming a preferred display type for many computer, instrumentation, and entertainment applications. The transmissive LCD used in conventional laptop computer displays is a type of backlit display, having a light providing surface positioned behind the LCD for directing light outwards, towards the LCD. The challenge of providing a suitable backlight apparatus having brightness that is sufficiently uniform while remaining compact and low cost has been addressed following one of two basic approaches. In the first approach, a light-providing surface is used to provide a highly scattered, essentially Lambertian light distribution, having an essentially constant luminance over a broad range of angles. Following this first approach, with the goal of increasing on-axis and near-axis luminance, a number of brightness enhancement films have been proposed for redirecting a portion of this light having Lambertian distribution in order to provide a more collimated illumination.

[0003] A second approach to providing backlight illumination employs a light guiding plate (LGP) that accepts incident light from a lamp or other light source disposed at the side and guides this light internally using Total Internal Reflection (TIR) so that light is emitted from the LGP over a narrow range of angles. The output light from the LGP is typically at a fairly steep angle with respect to normal, such as 70 degrees or more. With this second approach, a turning film, one type of light redirecting article, is then used to redirect the emitted light output from the LGP toward normal. Directional turning films, broadly termed light-redirecting articles or light-redirecting films, such as that provided with the HSOT (Highly Scattering Optical Transmission) light guide panel available from Clarex, Inc., Baldwin, N.Y., provide an improved solution for providing a uniform backlight of this type, without the need for diffusion films or for dot printing in manufacture. HSOT light guide panels and other types of directional turning films use arrays of prism structures, in various combinations, to redirect light from a light guiding plate toward normal, or toward some other suitable target angle that is typically near normal relative to the two-dimensional surface. As one example, U.S. Pat. No. 6,746,130 (Ohkawa) describes a light control sheet that acts as a turning film for LGP illumination.

[0004] Referring to FIG. 1, the overall function of a light guiding plate 10 in a display apparatus 100 is shown. Light from a light source 12 is incident at an input surface 18 and passes into light guiding plate 10, which is typically wedge-shaped as shown. The light propagates within light guiding plate 10 until Total Internal Reflection (TIR) conditions are frustrated and then, possibly reflected from a reflective surface 142, exits light guiding plate at an output surface 16. This light then goes to a turning film 20 and is directed to illuminate a light-gating device 120 such as an LCD or other type of spatial light modulator or other two-dimensional backlit component that modulates the light. For optimized viewing under most conditions, the emitted light should be provided over a range of relatively narrow angles about a normal V. A polarizer 124 is typically disposed in the illumination path in order to provide light-gating device 120 such as a liquid crystal cell with suitably polarized light for modulation. A reflective polarizer 125 is often provided between absorptive polarizer 124 and turning film 20.

[0005] Referring to FIG. 2, there is shown a schematic cross-sectional view of a conventional turning film 20a used with light guiding plate 10, showing key angles and geometric relationships. Turning film 20a has a number of prismatic structures facing downward toward light guiding plate 10, each structure having a near surface 24 (being near relative to light source 12, as shown in the embodiment of FIG. 1) and a far surface 26, both sides slanted from a film normal direction V as determined by an apex angle α, and base angles β1 and β2, relative to a horizontal S. Light from light guiding plate 10 is incident over a small range of angles about a central input angle θ_in. The output angle θ_out of light delivered to the LC display element at a flat surface 22 of turning film 20a is determined by a number of factors including the central input angle θ_in, the refractive index n of turning film 20a, and the base angle β1 at which far surface 26 is slanted. Output angle θ_out for emitted light is preferably normal with respect to turning film 20a, however output angle θ_out can be considered a target angle, which may be at some inclination with respect to normal for some applications. For most conventional turning films, the target angle is normal. In a typical arrangement, base angles β1 and β2 are about 56 degrees, and apex angle α, 68 degrees. The primary ray 50a having an input angle around θ_in≈70° is redirected to near normal direction. However, some secondary rays 50c, 50c1 having an input angle around θ_in<70° may take paths as shown in FIG. 2. Secondary ray 50c1 is redirected toward a relative large angle from the normal direction. Further, secondary ray 50c is totally reflected back by the light exiting surface 92. Consequently, the light utilization of this existing turning film is not satisfactory.

[0006] Thus, while there have been solutions proposed for turning films suitable for some types of display apparatus and applications, there remains a need for improved turning films.

[0007] The present invention provides a light redirecting article for redirecting light toward a target angle, the light redirecting article comprising: (a) an input surface comprising a plurality of light redirecting structures each light redirecting structure having: (i) a near surface having two slopes, sloping away from normal in one direction as defined by a first inclination base angle β₁, a second inclination angle β₂, and a first half apex angle α₂, for accepting incident illumination over a range of incident angles; (ii) a far surface sloping away from normal, in the opposite direction relative to the input surface, as defined by a second base angle γ₁ and a second half apex angle α₁; and (b) an output surface opposing to the input surface wherein the near and far surfaces are opposed to each other at an angle (α₁+α₂) that is in the range from 60 to 70 degrees, and the base angle β₁ is in the range from 82 to 87 degrees.

[0008] The present invention further provides a light redirecting article for redirecting light toward a target angle, the light redirecting article comprising: (a) an input surface comprising a plurality of light redirecting structures each light redirecting structure having: (i) a near surface having two slopes, sloping away from normal in one direction as defined by a first inclination base angle β₁, a second inclination angle β₂, and a first half apex angle α₂, for accepting incident illumination over a range of incident angles, the first inclination base angle β₁ being the angle nearest to the substrate of the film, the second inclination angle β₂ being the angle farthest from the substrate of the film; (ii) a far surface sloping away from normal, in the opposite direction relative to the input surface, as defined by a second base angle γ₁ and a second half apex angle α₁; and (b) an output surface wherein β₂−β₂ is at least 20 degrees.

[0009] The present invention further provides a light redirecting article for redirecting light toward a target angle, the light redirecting article comprising: (a) an input surface comprising a plurality of light redirecting structures having a pitch P, each light redirecting structure having: (i) a near surface having two slopes, sloping away from normal in one direction as defined by a first inclination base angle β₁, a second inclination angle β₂, a first projection segment onto the substrate having a length L₁, and a second projection segment onto the substrate having a length L₂, and a first half apex angle α₂, for accepting incident illumination over a range of incident angles, the first inclination base angle β₁ being the angle nearest to the substrate of the film, the second inclination angle β₂ being the angle farthest from the substrate of the film; (ii) a far surface sloping away from normal, in the opposite direction relative to the input surface, as defined by a second base angle γ₁ and a second half apex angle α₁; and (b) an output surface wherein the ratio L₁/P is in the range of 0.06 and 0.08 and the ratio L₂/P is in the range of 0.152 and 0.238.

[0021] The apparatus of the present invention uses light-redirecting structures that are generally shaped as prisms. True prisms have at least two planar faces. Because, however, one or more surfaces of the light-redirecting structures need not be planar in all embodiments, but may be curved or have multiple sections, the more general term “light redirecting structure” is used in this specification.

[0022] Referring to FIG. 3A-3D, key features of the improved turning film of the present invention are shown. FIG. 3A shows one unit of a turning film 90a according to the present invention, which comprises a substrate having a light inputting surface 94 and a light exiting surface 92. On the side of the light inputting surface 94 of the film 90a is a prismatic structure which is described by points P1, P2, P3, and P4 and characterized by a near surface 24 and a far surface 26, and the near surface is composed of at least first flat segment 24a and second flat segment 24b, the angle β₂ between the first segment 24a and the horizontal direction S is smaller than the angle β₁ between the second segment 24b and the horizontal direction S. The prismatic structure can be further described by two half apex angles α₁ and α₂, the pitch P and height H, and three projection dimensions L₁, L₂, and L₃. The prismatic structure is made of a material of refractive index n, and the substrate may have its index of refraction greater than, equal to, or less than n. The shape and the refractive index n of the prismatic structure are chosen so that the primary ray 50a from the light guide plate 10, secondary ray 50b having larger incident angle than the primary ray 50a, and secondary ray 50c having smaller incident angle than the primary ray 50a are characterized as following: the primary ray 50a is refracted by the first segment 24a of the near surface 24, subsequently reflected due to the total internal reflection at the far surface 26, and finally emerges out toward the target angle (normally within 5 degrees from the normal of the film); the secondary ray 50b is also refracted by the first segment 24a of the near surface 24, subsequently reflected due to the total internal reflection at the far surface 26, and finally emerges out in a direction that is bent more from its original direction than the primary ray 50a; and the secondary ray 50c is refracted by the second segment 24b of the near surface 24, subsequently reflected due to the total internal reflection at the far surface 26, and finally emerges out in a direction that is closer to the target direction than it would if the second segment 24b has the same slope as the first segment 24a.

[0023] Inventive (denoted as “I”) and comparative examples (denoted as “C”) of turning film 90a are shown in Table 1-Table 5. In all of these examples, refractive index n is held constant at 1.5, and pitch P of the prisms is about 50 μm, though it can be in the range of 15 to 150 μm, preferably in the range of 20 to 75 μm, more preferably in the range of 25 to 50 μm. When n and P are held constant, there are 4 independent parameters to specify the shape of turning film 90a, which are chosen to be L₁/P, L₂/P, β₁, and β₂. The height H and angles can be calculated as

[0024] $H=P\left[l_1\tan \left({\beta }_1\right)+l_2\tan \left({\beta }_2\right)\right],{\alpha }_1={\tan }^{-1}\left(\frac{1-l_1-l_2}{h}\right),{\alpha }_2=90°-{\beta }_2$ $\alpha \equiv {\alpha }_1+{\alpha }_2$ ${\gamma }_1=90°-{\alpha }_1,\mathrm{where}{l}_1\equiv \frac{L_1}{P},l_2\equiv \frac{L_2}{P},h\equiv \frac{H}{P}.$
When α₁=α₂, it follows

[0025] $l_2=\frac{1-l_1}{2}-\frac{l_1}{2}\frac{\tan \left({\beta }_1\right)}{\tan \left({\beta }_2\right)},\mathrm{or}$ $l_1=\frac{1-2l}{1+\frac{\tan \left({\beta }_1\right)}{\tan \left({\beta }_2\right)}}.$

[0026] In Table 1-4, Columns L₁/P, α₁, and α₂ are independent parameters.

[0027] $L_2/P=l_2=\frac{1-l_1}{2}-\frac{l_1}{2}\frac{\tan \left({\beta }_1\right)}{\tan \left({\beta }_2\right)}$
to ensure α₁=α₂=90°−β₂, and α≡2α₁. The four right most columns represent the output of turning film in terms of total power, maximum intensity ratio, maximum intensity angle, and on-axis intensity ratio. The turning film of the present invention has: Power ≧85%, Maximum intensity ratio ≧1.1 and Maximum intensity angle is within −5° and −5°.

[0028]

TABLE 1
Impact of β1
									Maximum	Maximum	On-axis
			β1	β2	α1	α2	α		Intensity	Intensity	Intensity
Ex	L1/P	L2/P	(°)	(°)	(°)	(°)	(°)	Power	Ratio	angle (°)	ratio
C1.1	0.077	0.31422	80	56	34	34	68	0.856	0.967	3.5	0.940
C1.2	0.077	0.27672	82	56	34	34	68	0.872	1.035	4.5	0.920
C1.3	0.077	0.25000	83	56	34	34	68	0.882	1.094	3.5	0.997
I1.1	0.077	0.21443	84	56	34	34	68	0.888	1.154	2.5	1.074
I1.2	0.077	0.16633	85	56	34	34	68	0.892	1.195	0.5	1.172
C1.4	0.077	0.09013	86	56	34	34	68	0.896	1.034	−4.5	0.961

[0029] In Table 1, Ex. C1.1-C1.4 and I1.1-I1.2 show the impact of β₁, given L₁/P=0.077, and β₂=56°. Turning films of inventive examples I1.1 and I1.2 meet the criteria: high power (>0.88), large maximum peak intensity ratio (>1.15), and small maximum intensity angle from the normal (≦±3°). When β₁ is out of the preferred range between 83.5° and 85.5°, other parameters being equal except L₂/P, which is determined by

[0030] $L_2/P=\frac{1-L_1/P}{2}-\frac{L_1/P}{2}\frac{\tan \left({\beta }_1\right)}{\tan \left({\beta }_2\right)}$
to keep
α₁=α₂=α/2, the outputs from comparative examples C1.1-C1.4 do not meet all of the criteria, in terms of power (>0.85), maximum intensity ratio (>1.10), and maximum intensity angle (≦±5°), indicating inferior performance.

[0031]

TABLE 2
Impact of L1/P
									Maximum	Maximum	On-axis
			β1	β2	α1	α2	α		Intensity	Intensity	Intensity
Ex	L1/P	L2/P	(°)	(°)	(°)	(°)	(°)	Power	Ratio	angle (°)	ratio
C2.1	0.04	0.32581	85	56	34	34	68	0.860	0.988	2.5	0.911
C2.2	0.05	0.28226	85	56	34	34	68	0.874	1.075	4.5	0.934
I2.1	0.06	0.23871	85	56	34	34	68	0.886	1.133	3.5	1.010
I2.2	0.062	0.23000	85	56	34	34	68	0.887	1.113	4.5	1.043
I2.3	0.064	0.22129	85	56	34	34	68	0.888	1.133	2.5	1.051
I2.4	0.066	0.21258	85	56	34	34	68	0.889	1.157	2.5	1.069
I2.5	0.068	0.20387	85	56	34	34	68	0.890	1.164	3.5	1.093
I2.6	0.07	0.19516	85	56	34	34	68	0.891	1.185	2.5	1.100
I2.7	0.072	0.18645	85	56	34	34	68	0.892	1.218	2.5	1.134
I2.8	0.074	0.17774	85	56	34	34	68	0.892	1.180	1.5	1.152
I2.9	0.075	0.17339	85	56	34	34	68	0.892	1.204	1.5	1.158
I2.10	0.076	0.16903	85	56	34	34	68	0.892	1.198	0.5	1.165
I2.11	0.077	0.16468	85	56	34	34	68	0.892	1.184	0.5	1.174
I2.12	0.078	0.16032	85	56	34	34	68	0.892	1.188	0.5	1.184
I2.13	0.079	0.15597	85	56	34	34	68	0.893	1.192	−0.5	1.175
I2.14	0.08	0.15161	85	56	34	34	68	0.893	1.171	−0.5	1.167
C2.5	0.09	0.10806	85	56	34	34	68	0.895	1.096	−2.5	1.024
C2.6	0.1	0.06452	85	56	34	34	68	0.898	0.982	−5.5	0.883
C2.7	0.114	0.00355	85	56	34	34	68	0.900	1.089	2.5	1.036
I2.15	0.075	0.22184	84	56	34	34	68	0.888	1.150	3.5	1.056
I2.16	0.076	0.21813	84	56	34	34	68	0.888	1.142	2.5	1.053
I2.17	0.077	0.21443	84	56	34	34	68	0.888	1.154	2.5	1.074
I2.18	0.078	0.21072	84	56	34	34	68	0.888	1.145	2.5	1.071
I2.19	0.079	0.20701	84	56	34	34	68	0.889	1.163	4.5	1.082
I2.20	0.08	0.20330	84	56	34	34	68	0.890	1.165	2.5	1.127

[0032] In Table 2, comparative examples C2.1-C2.7 and inventive examples I2.1-I2.14 show the impact of L₁/P when β₁=85°, β₂=56°. When L₁/P is out of the preferred range between 0.06 and 0.08, other parameters being equal except L₂/P, which is determined by

[0033] $L_2/P=\frac{1-L_1/P}{2}-\frac{L_1/P}{2}\frac{\tan \left({\beta }_1\right)}{\tan \left({\beta }_2\right)}$
to keep α₁=α₂=α/2, the output in terms of power (>0.85), maximum intensity ratio (≧1.10), and maximum intensity angle (≦±5°) is not acceptable. Table 2 also includes inventive examples I2.15-I2.20 when β₁=84°, β₂=56°.

[0034]

TABLE 3
Impact of β2
									Maximum	Maximum	On-axis
			β1	β2	α1	α2	α		Intensity	Intensity	Intensity
Ex	L1/P	L2/P	(°)	(°)	(°)	(°)	(°)	Power	Ratio	angle (°)	ratio
C3.1	0.076	0.21123	85	60	30	30	60	0.895	1.081	−9.5	0.655
C3.2	0.076	0.20102	85	59	31	31	62	0.894	1.124	−7.5	0.859
I3.1	0.076	0.19059	85	58	32	32	64	0.893	1.144	−2.5	1.026
I3.2	0.076	0.17994	85	57	33	33	66	0.892	1.183	−1.5	1.140
I3.3	0.076	0.16903	85	56	34	34	68	0.892	1.198	0.5	1.165
I3.4	0.076	0.15787	85	55	35	35	70	0.893	1.194	2.5	1.027
I3.5	0.076	0.14643	85	54	36	36	72	0.893	1.138	4.5	0.830
C3.3	0.076	0.13470	85	53	37	37	74	0.890	1.075	7.5	0.601
C3.4	0.076	0.12265	85	52	38	38	76	0.888	1.042	9.5	0.421

[0035] In Table 3, comparative examples C3.1-C3.4 and inventive examples I3.1-I3.5 show the impact of β₂ when L₁/P=0.076 and β₁=85°. When L₁/P is out of the preferred range between 58.5° and 53.5°, other parameters being equal except L₂/P, which is determined by

[0036] $L_2/P=\frac{1-L_1/P}{2}-\frac{L_1/P}{2}\frac{\tan \left({\beta }_1\right)}{\tan \left({\beta }_2\right)}$
to keep α₁=α₂α/2, the output in terms of power (>0.85), maximum intensity ratio (≧1.10), and maximum intensity angle (≦±5°) is not acceptable.

[0037]

TABLE 4
Asymmetrical Turning Film Having α1 ≠ α2
									Maximum	Maximum	On-axis
			β1	β2	α1	α2			Intensity	Intensity	Intensity
Ex	L1/P	L2/P	(°)	(°)	(°)	(°)	α (°)	Power	Ratio	angle (°)	ratio
C4.1	0.01	0.17	85	56	65.93	34	99.93	0.732	0.874	27.5	0.031
C4.2	0.03	0.17	85	56	53.36	34	87.36	0.715	0.640	15.5	0.063
C4.3	0.05	0.17	85	56	43.44	34	77.44	0.858	0.983	29.5	0.146
C4.4	0.07	0.17	85	56	35.84	34	69.84	0.891	1.177	6.5	0.837
C4.5	0.09	0.17	85	56	30.02	34	64.02	0.897	1.179	−9.5	0.628
C4.6	0.11	0.17	85	56	25.50	34	59.50	0.898	0.822	−18.5	0.171
C4.7	0.13	0.17	85	56	21.94	34	55.94	0.886	1.011	23.5	0.064
C4.8	0.15	0.17	85	56	19.07	34	53.07	0.878	1.055	28.5	0.038
C4.9	0.17	0.17	85	56	16.73	34	50.73	0.872	1.062	30.5	0.033
C4.10	0.19	0.17	85	56	14.79	34	48.79	0.864	1.038	35.5	0.109
C4.11	0.01	0.19	85	56	63.67	34	97.67	0.687	0.764	23.5	0.035
C4.12	0.03	0.19	85	56	51.31	34	85.31	0.751	0.544	8.5	0.086
C4.13	0.05	0.19	85	56	41.69	34	75.69	0.864	1.033	24.5	0.170
I4.1	0.07	0.19	85	56	34.37	34	68.37	0.891	1.175	3.5	1.075
C4.14	0.09	0.19	85	56	28.79	34	62.79	0.898	1.165	−12.5	0.464
C4.15	0.11	0.19	85	56	24.46	34	58.46	0.898	0.768	15.5	0.131
C4.16	0.13	0.19	85	56	21.04	34	55.04	0.887	1.013	23.5	0.054
C4.17	0.15	0.19	85	56	18.30	34	52.30	0.878	1.111	29.5	0.036
C4.18	0.17	0.19	85	56	16.05	34	50.05	0.870	1.109	33.5	0.039
C4.19	0.19	0.19	85	56	14.18	34	48.18	0.864	1.024	36.5	0.134
I4.2	0.076	0.169	85	50	35.2	40	75.2	0.892	1.124	4.5	0.809
I4.3	0.076	0.169	85	51	35	39	74	0.892	1.131	3.5	0.866
I4.4	0.076	0.169	85	52	34.8	38	72.8	0.893	1.138	3.5	0.940
I4.5	0.076	0.169	85	53	34.6	37	71.6	0.893	1.135	3.5	1.031
I4.6	0.076	0.169	85	54	34.4	36	70.4	0.893	1.159	2.5	1.083
I4.7	0.076	0.169	85	55	34.2	35	69.2	0.892	1.166	1.5	1.135
I4.8	0.076	0.169	85	56	34	34	68	0.892	1.190	0.5	1.168
I4.9	0.076	0.169	85	57	33.8	33	66.8	0.892	1.192	0.5	1.173
I4.10	0.076	0.169	85	58	33.5	32	65.5	0.892	1.191	0.5	1.179
I4.11	0.076	0.169	85	59	33.3	31	64.3	0.892	1.187	−2.5	1.156
I4.12	0.076	0.169	85	60	33	30	63	0.893	1.172	−1.5	1.113
C4.20	0.076	0.1	85	56	39	34	73	0.889	1.045	11.5	0.339
C4.21	0.076	0.12	85	56	37.5	34	71.5	0.891	1.112	7.5	0.616
C4.22	0.076	0.14	85	56	36.1	34	70.1	0.893	1.156	5.5	0.887
I4.13	0.076	0.16	85	56	34.6	34	68.6	0.892	1.179	1.5	1.112
I4.14	0.076	0.18	85	56	33.2	34	67.2	0.891	1.180	0.5	1.177
I4.15	0.076	0.2	85	56	31.9	34	65.9	0.892	1.156	−3.5	1.054
C4.23	0.076	0.22	85	56	30.5	34	64.5	0.894	1.125	−7.5	0.861
C4.24	0.076	0.24	85	56	29.2	34	63.2	0.895	1.092	−9.5	0.631
C4.25	0.076	0.26	85	56	27.9	34	61.9	0.895	1.050	−12.5	0.465
C4.26	0.076	0.28	85	56	26.6	34	60.6	0.895	1.005	−16.5	0.352
C4.27	0.076	0.3	85	56	25.4	34	59.4	0.894	0.968	−22.5	0.261
C4.28	0.076	0.32	85	56	24.2	34	58.2	0.893	0.928	−24.5	0.184
C4.29	0.076	0.34	85	56	23	34	57	0.890	0.902	−25.5	0.127

[0038] Table 4 shows that it is possible to have asymmetrical turning film as shown in Inventive example I4.1-I4.15 to provide acceptable output in terms of power (≧0.85), maximum intensity ratio (≧1.10), and maximum intensity angle (≦±5°), while randomly choosing parameters do not provide satisfactory results, as shown in comparative examples C4.1-C4.29. In general, the difference between α₁ and α₂ is preferably within 5°.

[0039]

TABLE 5
Impact of Refractive Index n
			Maximum	Maximum	On-axis
			Intensity	Intensity	Intensity
Ex	n	Power	Ratio	angle (°)	ratio
C5.1	1.13	0.881	1.053	7.5	0.119
I5.1	1.14	0.880	1.202	6.5	0.147
I5.2	1.15	0.879	1.383	5.5	0.2
I5.3	1.16	0.877	1.482	4.5	0.262
I5.4	1.17	0.877	1.523	4.5	0.297
I5.5	1.18	0.879	1.491	3.5	0.413
I5.6	1.19	0.883	1.633	2.5	0.541
I5.7	1.2	0.887	1.467	1.5	0.938
I5.8	1.21	0.894	1.601	0.5	1.401
I5.9	1.22	0.901	1.690	0.5	1.612
I5.10	1.23	0.909	1.567	−1.5	1.431
I5.11	1.24	0.912	1.528	0.5	1.468
I5.12	1.25	0.915	1.476	0.5	1.427
I5.13	1.26	0.917	1.485	−1.5	1.359
I5.14	1.27	0.921	1.403	0.5	1.399
I5.15	1.28	0.921	1.335	−1.5	1.293
I5.16	1.29	0.923	1.326	−0.5	1.292
I5.17	1.3	0.923	1.269	−0.5	1.243
I5.18	1.31	0.924	1.302	−1.5	1.231
I5.19	1.32	0.924	1.257	−2.5	1.238
I5.20	1.33	0.923	1.287	−1.5	1.243
I5.21	1.34	0.922	1.254	−0.5	1.251
I5.22	1.35	0.921	1.240	0.5	1.238
I5.23	1.36	0.920	1.223	1.5	1.212
I5.24	1.37	0.918	1.227	0.5	1.222
I5.25	1.38	0.917	1.231	−0.5	1.22
I5.26	1.39	0.914	1.219	−0.5	1.204
I5.27	1.4	0.912	1.216	0.5	1.215
I5.28	1.41	0.910	1.231	0.5	1.224
I5.29	1.42	0.909	1.242	0.5	1.226
I5.30	1.43	0.906	1.213	0.5	1.202
I5.31	1.44	0.905	1.238	0.5	1.202
I5.32	1.45	0.903	1.227	0.5	1.199
I5.33	1.46	0.900	1.224	0.5	1.193
I5.34	1.47	0.898	1.205	0.5	1.188
I5.35	1.48	0.896	1.202	1.5	1.179
I5.36	1.49	0.894	1.196	1.5	1.179
I5.37	1.50	0.892	1.204	1.5	1.158
I5.38	1.51	0.889	1.191	1.5	1.140
I5.39	1.52	0.887	1.157	0.5	1.139
I5.40	1.53	0.885	1.155	2.5	1.132
I5.41	1.54	0.884	1.192	1.5	1.120
I5.42	1.55	0.881	1.149	2.5	1.091
I5.43	1.66	0.861	1.113	3.5	1.022
C5.2	1.67	0.860	1.074	4.5	0.991

[0040] Table 5 show the impact of refractive index n, when L₁/P=0.075, β₁=85°, β₂=56°, and

[0041] $L_2/P=\frac{1-L_1/P}{2}-\frac{L_1/P}{2}\frac{\tan \left({\beta }_1\right)}{\tan \left({\beta }_2\right)}=0.17339,$
α₁=α₂=α/2=34°. Inventive examples I5.1-I5.43 show that the index n is preferably in the range between 1.15 and 1.66, more preferably between 1.18 and 1.55, most preferably between 1.19 and 1.31, for a high power, and high maximum intensity ratio, while keeping the maximum intensity angle within 5° from the normal. When n is between 1.15 and 1.28, the maximum intensity ratio is greater than 1.3, sometimes is greater 1.6, which is significantly greater than the maximum intensity ratio of about 1.2 when n=1.5. For comparative examples C5.1 and C5.2, the maximum intensity is lower than 1.1.

[0042] For the turning film 90a according to the present invention, the inclination angle of one of flat surfaces closest to light exit surface, is β₁=85°, and the inclination angle of the other one of flat surfaces farthest from light exit surface, is β₂=56°.

[0043]

Comparison Between Prior Art Examples and Inventive Examples
									Maximum	Maximum	On-axis
			β1	β2	α1	α2	α		Intensity	Intensity	Intensity
Ex	L1/P	L2/P	(°)	(°)	(°)	(°)	(°)	Power	Ratio	angle (°)	ratio
C1	0.5	0	56	56	34	34	68	0.786	1.072	0.5	1.047
C2	0.122	0.0	84.4	84.4	35.2	5.6	40.8	0.902	1.174	−1.5	1.157
I1.1	0.077	0.21443	84	56	34	34	68	0.888	1.154	2.5	1.074
I1.2	0.077	0.16633	85	56	34	34	68	0.892	1.195	0.5	1.172

[0044] Comparative example C1 is the same as inventive example I1.1 except that it has only one slope. The comparative example C1 has much lower power of around 0.79, and lower maximum intensity ratio of about 1.07, compared to the power of about 0.89 and maximum intensity ratio of about 1.15 of inventive example I1.1.

[0045] Comparative example C2 has almost the same power and maximum intensity ratio as inventive examples I1.1 and I1.2. However, comparative example C2 has much smaller apex angle (about 41°) compared to inventive examples I1.1 and I1.2 having apex of about 68°, which means an easier fabrication of the turning films of the present invention.

[0046] The advantage of the turning film of the present invention is its high maximum intensity ratio (optical gain), and its large apex angle which enables its easy manufacture.

Turning Film Having 4 Slopes on the Near Surface

[0047] FIG. 3B shows another embodiment of the turning film 90b according to the present invention. One example is:

[0048]

Ex	L1/P	L2/P	L3/P	L4/P	β1 (°)	β2 (°)	β3 (°)	β4 (°)	α1 (°)	α2 (°)	α (°)
I6	0.06535	0.02678	0.05680	0.09402	85	77.76	63.25	56	34	34	68

When n=1.5,

[0049]

	Maximum Intensity	Maximum Intensity	On-axis
Power	Ratio	angle (°)	Intensity ratio
0.893	1.271	−2.5	1.214

[0050] The inclination angle of one of flat surfaces closest to light exit surface, is β₁=85°, and the inclination angle of the other one of flat surfaces farthest from light exit surface, is β₄=56°. Thus, the difference between them is 29°. In addition, the difference between the other two inclination angles is β₂−β₃=14.51°, which is greater than β₁−β₂=7.24°, β₃−β₄4=7.25°. Moreover, β₂−β₃ is about twice of β₁−β₂ and β₃−β₄.

Turning Film Having 2 Slopes and Curved Surface on the Near Surface

[0051] FIG. 3C shows another embodiment of the turning film 90c according to the present invention. The point P2 in FIG. 3A is now replaced with a curved surface of radius R2 (N=1.5).

[0052]

L1/P	L2/P	β1 (°)	β2 (°)	α1 (°)	α2 (°)	α (°)
0.076	0.16903	85	56	34	34	68

[0053]

			Maximum	Maximum	On-axis
			Intensity	Intensity	Intensity
Ex	R2/P	Power	Ratio	angle (°)	ratio
I7.1	0.1	0.892	1.234	−0.5	1.227
I7.2	0.2	0.893	1.259	0.5	1.240
I7.3	0.3	0.893	1.272	−2.5	1.240
I7.4	0.4	0.894	1.292	−1.5	1.232
I7.5	0.5	0.895	1.312	−1.5	1.221
I7.6	0.6	0.895	1.300	−1.5	1.217
I7.7	0.8	0.897	1.274	−1.5	1.226
I7.8	1.0	0.898	1.216	0.5	1.207
I7.9	1.16882	0.899	1.175	−1.5	1.128

[0054] Though all of above inventive examples I7.1-I7.9 meet the criteria, it has been found that R₂/P is preferably in the range of 0.1 to 1.16882, more preferably in the range of 0.3 and 0.8, and most preferably in the range of 0.4 and 0.6 for further enhancement of maximum intensity ratio.

[0055] The point P3 can also be curved with radius of curvature R₃. R₃/P is preferably smaller than 0.2.

Turning Film Having 2 Slopes on the Near Surface and Curved Surface or Two Segments on Far Surface

[0056] FIG. 3D shows another embodiment of turning film 90d according to the present invention. The turning film 90d has the same features on the near surface 24 as one of turning films 90a, 90b, 90c, and it also has one additional surface 26a, which is either flat segment having different inclination as the other portion of 26, or convex or concave curved surface.

Luminous Intensity Distribution for Example Embodiments

[0057] FIG. 4 shows curves of maximum luminous intensity ratio vs polar angle for light passing through the turning films according to inventive example I1.1 and according to comparative example C1. Also shown is the luminous intensity distribution of light coming out from the light guide plate, which then is incident on the turning films. It is clearly seen that the turning film of inventive example I1.1 produces higher luminous intensity at almost every angle than the turning film of comparative example C1.

Display Apparatus and Orientation of Polarizers

[0058] The apparatus and method of the present invention allow a number of possible configurations for support components to provide light for an LCD. FIG. 5 is a schematic cross-sectional view showing a display apparatus 60 using turning film 90 according to the present invention, which can be either of turning films 90a, 90b, 90c, 90d. An LC spatial light modulator 70 modulates light received from light guiding plate 10 and turning film 90. A back polarizer 72 and a front polarizer 73 are provided for LC spatial light modulator 70.

[0059] FIG. 6A is a schematic top view showing polarized light transmission axes 172 and 173 for LC spatial light modulator 70, using a pair of polarizers that are oriented at 45 degrees relative to light redirecting structures 75 and grooves of turning film 90 that extend vertically in the view of FIG. 6A. In this case, the LC spatial light modulator 70 can be a twisted nematic (TN) LCD, which is the dominant mode used in a notebook and monitor display.

[0060] FIG. 6B is a schematic top view showing polarized light transmission axes 172 and 173 for LC spatial light modulator 70, using a pair of polarizers oriented at parallel or perpendicular relative to the grooves and light redirecting structures 75 of turning film 90. In this case, the LC spatial light modulator 70 can use vertically aligned (VA) LCD or IPS LC elements. Rear polarizer transmission axis 172 is parallel to the plane of the cross section.

[0061] In one embodiment the display apparatus comprises a pair of crossed polarizers, wherein the light redirecting structures are elongated in an elongation direction and wherein each of the crossed polarizers is oriented either substantially parallel or perpendicular to the elongation direction of the light redirecting article. In another embodiment the display apparatus comprises a pair of crossed polarizers, wherein the light redirecting structures are elongated in an elongation direction and wherein the polarizers are substantially oriented at +/−45 degrees relative to the elongation direction of the light redirecting article.

[0062] FIG. 6C is a schematic top view showing turning film 90 with arcuately elongated light redirecting structures 75 in another embodiment. This arrangement is advantageous for employing a point light source such as Light Emitting Diode (LED) at one or more corners of light guiding plate 10 in order to have a more compact design. The rear polarizer transmission axis 172 is more or less parallel to the plane of the cross section.

Materials for Forming Turning Film 90a-90d

[0063] Turning film 90a-90d of the present invention can be fabricated using polymeric materials having indices of refraction ranging typically from about 1.40 to about 1.66. Possible polymer compositions include, but are not limited to: poly(methyl methacrylate)s, poly(cyclo olefin)s, polycarbonates, polysulfones and various co-polymers comprising various combinations of acrylate, alicyclic acrylate, carbonate, styrenic, sulfone and other moieties that are known to impart desirable optical properties, particularly high transmittance in the visible range and low level of haze. Various miscible blends of the aforementioned polymers are also possible material combinations that can be used in the present invention. The polymer compositions may be either thermoplastic or thermosetting. The former are manufacturable by an appropriate melt process that requires good melt processability while the latter can be fabricated by an appropriate UV cast and cure process or a thermal cure process.

[0064] Turning film 90a-90d of the present invention may be fabricated using materials having an index of refraction in the range of 1.12 and 1.40. Example materials are inorganic materials, for example, MgF. Also, materials having a grating formed between a common polymeric material having refractive index in the range of 1.48 and 1.59 and air (n=1). Further, a mix of low index materials (n<1.4) and materials having indices of refraction from about 1.40 to 1.50 may be used as well.

Maximum Intensity Ratio (or Optical Gain), Maximum Intensity Angle (or Peak Angle), and Power of a Turning Film

[0065] In general, light distribution is specified in terms of spatial and angular distributions. The spatial distribution of light can be made quite uniform, achieved by careful placement of micro features on top and/or bottom sides of a light guide plate. The angular distribution of light is specified in terms of luminous intensity I as a function of polar angle θ and azimuthal angle. The angular distribution of light is measured with EZ Contrast 160 (available from Eldim, France). Polar angle θ is the angle between the light direction and the normal of the light guide plate V. The azimuthal angle is the angle between the projection of the light onto a plane that is perpendicular to the normal direction V and a direction that is parallel to the length direction of the light guide plate. The length direction of the light guide plate is perpendicular to the light source 12 and the normal direction V. The angular distribution of light can also be specified in terms of luminance L as a function of polar angle θ and azimuthal angle. The luminance L and the luminous intensity I are related by L=I/cos(θ).

[0066] The maximum intensity angle, also referred as peak angle of a light distribution is defined as the polar angle at which the maximum luminous intensity occurs. Each luminous intensity distribution then defines a maximum (or peak) luminous intensity and a maximum intensity (or peak) angle.

[0067] The maximum intensity ratio, also referred as optical gain, or normalized peak intensity, of a turning film, is defined as a ratio of the maximum luminous intensity of the light that is transmitted through the turning film over the maximum luminous intensity of the light that is emitted from a light guide plate. As a result, the maximum intensity ratio of a turning film is not dependent upon the absolute level of the light source, but is primarily dependent upon the turning film design itself.

[0068] The power of a turning film is the ratio of the total amount of light passing through the turning film over the total amount of light incident upon the turning film. Thus, various turning film designs can be compared in terms of two critical quantities: maximum intensity ratio (or optical gain) and maximum intensity angle of the light that is transmitted through the turning film.

• [0069] 10. Light guiding plate
• [0070] 12. Light source
• [0071] 14. End surface
• [0072] 16. Output surface
• [0073] 18. Input surface
• [0074] 20, 20a. Turning film
• [0075] 22. Flat surface
• [0076] 24. Near surface
• [0077] 24a. First flat segment on Near surface
• [0078] 24b. Second flat segment on Near surface
• [0079] 26. Far surface
• [0080] 50a. Primary rays
• [0081] 50b. Secondary rays
• [0082] 50c, 50c1. Secondary rays
• [0083] 52. Reflective surface
• [0084] 60. Display apparatus
• [0085] 70. LC spatial light modulator
• [0086] 72. Rear polarizer
• [0087] 73. Front Polarizer
• [0088] 75. Light redirecting structure
• [0089] 82. Point light source
• [0090] 90, 90a, 90b, 90c, 90d Turning film
• [0091] 92. Light exiting surface
• [0092] 94. Light inputting surface
• [0093] 96. Substrate
• [0094] 100. Display apparatus
• [0095] 120. Light-gating device
• [0096] 124. Polarizer
• [0097] 125. Reflective polarizer
• [0098] 142. Reflective surface
• [0099] 172, 173. Transmission axes
• [0100] α. Apex angle
• [0101] α₁. Half apex angle
• [0102] α₂. Half apex angle
• [0103] β₁. base inclination angle
• [0104] β₂, β₃, β₄. inclination angle
• [0105] γ₁. Base inclination angle
• [0106] n. Refractive index
• [0107] θ_in. Incident angle for a first light guide plate
• [0108] θ_out. Output angle
• [0109] V. Film normal direction
• [0110] S. Horizontal direction