Image display device

Abstract

The present invention aims to provide an image display device including an optical film having a thickness suitable for practical use without including any special inorganic material, wherein the image display device has high color rendering properties and is capable of minimizing the occurrence of blackout and interference colors (rainbow unevenness) even when the image display device includes light sources that emit light having a narrow emission spectrum. The present invention provides an image display device including an optical film having an in-plane birefringence and a polarizer in this order, wherein the optical film and the polarizer are disposed to form an angle of about 45° between a slow axis of the optical film and an absorption axis of the polarizer, the optical film has a retardation of 3000 nm or more, and light incident on the optical film provides at least 50% coverage of ITU-R BT.2020.

Claims

1. An image display device comprising: an optical film having an in-plane birefringence; a polarizer; and a light source configured to produce light that is incident on the optical film, wherein the optical film and the polarizer are disposed to form an angle of about 45° between a slow axis of the optical film and an absorption axis of the polarizer, the optical film is towards a viewer side of the image display device relative to the polarizer, the optical film has a retardation of 3,000 nm to 50,000 nm, the light incident on the optical film provides at least 50% coverage of ITU-R BT.2020, and has a peak in each of the following regions of the emission spectrum: a red region with a wavelength range from 580 nm to 780 nm; a green region with a wavelength range from 480 nm to less than 580 nm; and a blue region with a wavelength range from 380 nm to less than 480 nm, and the half width of the emission spectral peak in the red region is 70 nm or less, the half width of the emission spectral peak in the green region is 60 nm or less, and the half width of the emission spectral peak in the blue region is 40 nm or less.

2. The image display device according to claim 1, wherein the optical film has a light transmittance of 20% or higher at a wavelength of a maximum peak intensity in a wavelength range from 580 nm to 780 nm.

3. The image display device according to claim 1, wherein the optical film has a light transmittance of 20 to 80% at a wavelength of a maximum peak intensity in a wavelength range from 580 nm to 780 nm.

4. The image display device according to claim 1, wherein the light source includes a blue light-emitting diode, a red phosphor, and a green and/or yellow phosphor, and the red phosphor is a fluoride complex phosphor activated with Mn.sup.4+.

5. The image display device according to claim 1, wherein the light source includes an organic electroluminescence element.

6. The image display device according to claim 1, wherein the half width of the emission spectral peak in the red region is 60 nm or less, the half width of the emission spectral peak in the green region is 50 nm or less, and the half width of the emission spectral peak in the blue region is 30 nm or less.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows an emission spectrum of light from light sources including a KSF phosphor as a red phosphor.

(2) FIG. 2 shows an emission spectrum of light from white LEDs.

(3) FIG. 3 shows an emission spectrum of light from CCFLs.

(4) FIG. 4 shows a graph in which the light transmittance is calculated using the equation (A), where θ is 45°, N(λ) is a value obtained by dividing the birefringence of polyethylene terephthalate at a wavelength in the visible light region (wavelength range from 380 nm to 780 nm) by the birefringence of polyethylene terephthalate at a wavelength of 590 nm, and the phase difference Re is 10000 nm between the incident light and light with a wavelength of 590 nm that passed through the optical film.

(5) FIG. 5 shows a graph in which a graph combining FIG. 1 and FIG. 4 is superimposed on the graph of FIG. 1.

(6) FIG. 6 shows a graph in which a graph combining FIG. 2 and FIG. 4 is superimposed on the graph of FIG. 2.

(7) FIG. 7 shows a graph in which spectra of RGB lights of iMac Retina 4K are superimposed.

(8) FIG. 8 shows a graph in which spectra of RGB lights of iPhone (registered trademark) 6 Plus are superimposed.

DESCRIPTION OF EMBODIMENTS

(9) The present invention is more specifically described below with reference to examples and comparative examples, but the present invention is not limited to these examples or comparative examples.

(10) The “part(s)” and “%” in the description are part(s) by mass and percent by mass, respectively, unless otherwise specified.

(11) The retardation of a light-transmitting substrate or the like produced in each of the examples and comparative examples was measured as follows.

(12) (Measurement of Retardation)

(13) The retardation of an optical film having a retardation value of less than 20000 nm was measured with PAM-UHR100 available from Oji Scientific Instruments Co., Ltd.

(14) The retardation of an optical film having a retardation value of more than 20000 nm was measured as follows.

(15) First, as for a stretched optical film, the direction of a polarization axis of the optical film was determined using two polarizing plates. The refractive indexes (nx and ny) of two axes, i.e., the polarization axis and an axis orthogonal to the direction of the polarization axis, at a wavelength of 590 nm were determined with an Abbe refractometer (NAR-4T available from Atago Co., Ltd.). Here, the axis having a higher refractive index is defined as the slow axis. The thickness d (nm) of the optical film was measured with a micrometer (Anritsu Corporation), and the value was expressed in nanometers. The retardation was determined by the product of birefringence (nx−ny) and thickness d (nm).

(16) (Measurement of Emission Spectrum of Light Incident on the Optical Film)

(17) The emission spectrum can be measured with a spectrophotometer. During measurement, a white image is displayed on the image display device, and a photoreceiver of the spectrophotometer is placed perpendicular to a light emission surface of the display device, with a viewing angle set at 1°. As a measurement device, a device such as spectroradiometer “CS-2000” available from Konica Minolta, Inc. or a spectroradiometer “SR-LEDW-5N” or “SR-UL2” available from Topcon Corporation can be used.

(18) (Measurement of Color Gamut of Light Incident on the Optical Film)

(19) The color gamut of light incident on the optical film can be reproduced by mixing three RGB colors, and is represented by a triangle in the CIE 1931 xy chromaticity diagram. The triangle is formed by determining coordinates of the RGB colors as vertices and connecting these vertices. The coordinates of the RGB colors as vertices can be measured with a spectrophotometer. During measurement, the RGB colors are each displayed on the image display device, and a photoreceiver of the spectrophotometer is disposed perpendicular to a light emission surface of the image display device, with a viewing angle set at 1°. As a measurement device, a device such as a spectroradiometer “CS-2000” available from Konica Minolta, Inc. or a spectroradiometer “SR-LEDW-5N” or “SR-UL2” available from Topcon Corporation can be used.

(20) Table 1 shows xy data of the RGB colors according to the BT.2020 standard, xy data of the RGB colors of light from iMac Retina 4K having light sources including a KSF phosphor, and xy data of the RGB colors of light from iPhone (registered trademark) 6 Plus having light sources including white LEDs.

(21) (Coverage of BT.2020)

(22) The coverage of the color gamut was determined by calculating the coverage ratio of the area of the triangle of the color gamut of light incident on the optical film to the area of the triangle defined by ITU-R BT.2020 in the CIE 1931 xy chromaticity diagram. Table 1 shows the results.

(23) TABLE-US-00001 TABLE 1 BT.2020 iMac Retina 4K iPhone 6 Plus x y x y x y R 0.708 0.292 0.681 0.315 0.617 0.331 G 0.17 0.797 0.263 0.694 0.299 0.603 B 0.131 0.046 0.149 0.054 0.160 0.066 Coverage 100% 73% 49%
(Calculation of Half Width)

(24) The RGB colors were each displayed on iMac Retina 4K having light sources including a KSF phosphor, and the emission spectrum of each region was measured by a method similar to the method for measuring the color gamut of light incident on the optical film. The above measurement was similarly performed using iPhone (registered trademark) 6 Plus having light sources including white LEDs.

(25) Table 2 shows the peak wavelength (nm) and its half width (nm) of each of the RGB lights from the light sources of each device. FIG. 7 shows a graph in which spectra of the RGB lights from iMac Retina 4K are superimposed. FIG. 8 shows a graph in which spectra of the RGB lights from iPhone (registered trademark) 6 Plus are superimposed.

(26) TABLE-US-00002 TABLE 2 iMac Retina 4K iPhone 6 Plus Blue-side region Peak wavelength 450 450 Half width 20 19 Green-side region Peak wavelength 532 530 Half width 45 73 Red-side region Peak wavelength 630 610 Half width 9 64
(Production of Optical Film)

(27) A polyethylene terephthalate material was melted at 290° C. and extruded through a film-forming die into a sheet form. The sheet was attached to a water-cooled rotary quenching drum for cooling. Thus, an unstretched film was produced.

(28) The unstretched film was pre-heated for one minute at 120° C., stretched at a stretching ratio of 4.5 times at 120° C., and then stretched at a stretching ratio of 1.5 times in a direction perpendicular to the first stretching direction, using a biaxial stretching tester (Toyo Seiki Seisaku-sho, Ltd.). Thus, an optical film having nx of 1.70, ny of 1.60, and a thickness of 15 μm was obtained. The retardation at a wavelength of 590 nm was 1500 nm.

(29) The above method was employed to produce optical films each having a different thickness and a different retardation (retardation=1500 nm, 2000 nm, 3000 nm, 4100 nm, 6000 nm, 8200 nm, 9000 nm, 10000 nm, 11500 nm, 12980 nm, 13300 nm, 25200 nm, 28000 nm, 28300 nm, 36000 nm, 41300 nm, 101000 nm, and 101400 nm).

Examples 1 to 9

(30) In iMac Retina 4K (Apple Inc.) including the KSF phosphor, members disposed closer to the viewing side than the polarizer near the viewing side were removed to calculate the coverage of ITU-R BT.2020 of light incident on the optical film. The result was 73%. The optical films each having a different retardation of 3000 nm, 6000 nm, 8200 nm, 9000 nm, 11500 nm, 13300 nm, 28300 nm, 36000 nm, or 41300 nm were independently bonded closer to the viewing side than the polarizer via an adhesive layer. Thus, the image display devices were produced. The angle between the absorption axis of the polarizer and the slow axis of the optical film was 45°.

Comparative Examples 1 to 17

(31) Image display devices were produced as in Example 1, except for using an optical film having a retardation of 1500 nm (Comparative Example 1) and 2000 nm (Comparative Example 2).

(32) In iPhone (registered trademark) 6 Plus (Apple Inc.) having light sources including white LEDs, members disposed closer to the viewing side than the polarizer close to the viewing side were removed to calculate the coverage of ITU-R BT.2020 of light incident on the optical film. The result was 49%. The optical films each having a different retardation value shown in Table 3 were independently bonded closer to the viewing side than the polarizer via an adhesive layer. Thus, the image display devices according to Comparative Examples 3 to 17 were produced. The angle between the absorption axis of the polarizer and the slow axis of the optical film was set to 45°.

Reference Examples 1 to 4

(33) Image display devices were produced as in Example 1, except for using an optical film having a retardation of 4100 nm (Reference Example 1), 10000 nm (Reference Example 2), 28000 nm (Reference Example 3), and 101400 nm (Reference Example 4).

(34) (Evaluation of Color Shade)

(35) A white image was displayed on each of the image display devices produced, and the color shade was evaluated through polarized sunglasses. The following criteria were applied for evaluation. Here, the absorption axis of the polarizer of the image display device and the polarized sunglasses were in a parallel Nicols state.

(36) Poor: Interference colors are intense.

(37) Adequate: Interference colors are observed but not at the level unsuitable for practical use.

(38) Good: Interference colors are only slightly observed.

(39) Excellent: No interference colors are observed.

(40) (Method for Measuring the Light Transmittance of the Optical Film)

(41) An intensity A of the wavelength of a maximum peak intensity in a wavelength range from 580 nm to 780 nm in each of the image display devices produced in the examples, comparative examples, and reference examples was measured with a spectroradiometer CS-2000 (Konica Minolta, Inc.). Next, an additional polarizer was disposed on the viewing side of each image display device, and an intensity B was measured when the absorption axis of the polarizer and the absorption axis of the additional polarizer were in a parallel Nicols state. Then, the light transmittance was determined by substituting the measured values into the following formula: (intensity B/intensity A)×100. When the light sources including the KSF phosphor were used, the light transmittance was measured at a wavelength of 630 nm as the wavelength of the maximum peak intensity in a wavelength range from 580 nm to 780 nm. When the light sources consisting of the white LEDs were used, the light transmittance was measured at a wavelength of 605 nm as the wavelength of the maximum peak intensity in a wavelength range from 580 nm to 780 nm.

(42) (Evaluation of Color Rendering Properties)

(43) Color images were displayed on the image display devices having different light sources to which the optical films each having the same retardation value were independently bonded. Fifteen people evaluated which image display devices had high color rendering properties.

(44) Good: Eight or more people said that the color rendering properties were excellent.

(45) Poor: Less than eight people said that the color rendering properties were excellent.

(46) TABLE-US-00003 TABLE 3 Comparative Examples Examples 1 2 3 4 5 6 7 8 9 1 2 Retardation (nm) 3000 6000 8200 9000 11500 13300 28300 36000 41300 1500 2000 Color shade (visual Adequate Adequate Good Excellent Excellent Good Excellent Excellent Adequate Poor Poor observation) Light transmittance  26  23  55  100   91   37   96   98   34  24  88 at a wavelength of 630 nm Color rendering Good Good Good Good Good Good Good Good Good Good Good properties Comparative Examples 3 4 5 6 7 8 9 10 11 Retardation (nm) 1500 2000 3000 4100 6000 8200 9000 10000 11500 Color shade (visual Poor Poor Adequate Adequate Good Good Excellent Excellent Excellent observation) Light transmittance   2  41  94  42  77   3  53   9   82 at a wavelength of 605 nm Color rendering Poor Poor Poor Poor Poor Poor Poor Poor Poor properties Comparative Examples 12 13 14 15 16 17 Retardation (nm) 13300 28000 28300 36000 41300 101400 Color shade (visual Excellent Excellent Excellent Excellent Excellent Excellent observation) Light transmittance   69   94   7   98   46   33 at a wavelength of 605 nm Color rendering Poor Poor Poor Poor Poor Poor properties Reference Examples 1 2 3 4 Retardation (nm) 4100 10000 28000 101400 Color shade (visual Poor Poor Poor Excellent observation) Light transmittance  13   5   9   85 at a wavelength of 630 nm Color rendering Good Good Good Good properties

(47) The image display devices according to the examples each had excellent color rendering properties because light incident on the optical film provided at least 50% coverage of ITU-R BT.2020. In addition, blackout did not occur because the optical film had a retardation of 3000 nm or more, and the optical film and the polarizer were disposed to form an angle of about 45° between the slow axis of the optical film and the absorption axis of the polarizer. As shown in Table 3, the occurrence of interference colors was also reduced because the optical film had a light transmittance of 20% or higher at a wavelength of 630 nm.

(48) As shown in Table 3, the image display devices according to Reference Examples 1 to 3 each had sufficient color rendering properties and blackout did not occur; however, unfortunately, interference colors were observed because the optical film had a light transmittance of lower than 20% at a wavelength of 630 nm.

(49) The results of the examples and the reference examples confirm that the color rendering properties were excellent when light incident on the optical film provided at least 50% coverage of ITU-R BT.2020, and blackout was eliminated when the optical film had a retardation of 3000 nm or more and the optical film and the polarizer were disposed to form an angle of about 45° between the slow axis of the optical film and the absorption axis of the polarizer. However, in the case of the light sources including KSF phosphor that emits light having a narrow half width of the emission spectrum in the red region, the optical film needed to have a light transmittance of 20% or higher at a wavelength of 630 nm in order to reliably eliminate interference colors. In addition, although the image display device according to Reference Example 4 had excellent results in terms of color rendering properties, blackout, and interference colors, the optical film was very thick (1014 μm) and was thus unsuitable for practical use when the intended application requires a thin film. Table 5 shows the relationship between birefringence of the optical film and its thickness.

(50) In contrast, in the cases of the image display devices according to the comparative examples, interference colors were unfortunately observed when the retardation of the optical film was less than 3000 nm (Comparative Examples 1, 2, 3, and 4). When the optical film had a retardation of 3000 nm or more and the optical film and the polarizer were disposed to form an angle of about 45° between the slow axis of the optical film and the absorption axis of the polarizer, interference colors and blackout were prevented regardless of the percentage of the light transmittance at a wavelength of 605 nm because the emission spectrum of light incident on the optical film had a wide half width in the red region; however, the light incident on the optical film provided lower than 50% coverage of ITU-R BT.2020, so that the color rendering properties were poor (Comparative Examples 5 to 17).

Examples 10 to 14

(51) In iMac Retina 4K (Apple Inc.) including the KSF phosphor, members disposed closer to the viewing side than the polarizer near the viewing side were removed to calculate the coverage of ITU-R BT.2020 of light incident on the optical film. The result was 73%. The optical films each having a different retardation of 3000 nm, 6000 nm, 8200 nm, 12980 nm, and 25200 nm were independently bonded closer to the viewing side than the polarizer via an adhesive layer. Thus, the image display devices were produced. The angle between the absorption axis of the polarizer and the slow axis of the optical film was set to 45°.

Comparative Examples 18 to 32

(52) Image display devices were produced as in Example 10, except for using an optical film having a retardation of 1500 nm (Comparative Example 18) and 2000 nm (Comparative Example 19).

(53) In iPhone (registered trademark) 6 Plus (Apple Inc.) having light sources including white LEDs, members disposed closer to the viewing side than the polarizer near the viewing side were removed to calculate the coverage of ITU-R BT.2020 of light incident on the optical film. The result was 49%. The optical films each having a different retardation value shown in Table 4 were independently bonded closer to the viewing side than the polarizer via an adhesive layer. Thus, the image display devices according to Comparative Examples 20 to 32 were produced. The angle between the absorption axis of the polarizer and the slow axis of the optical film was set to 45°.

Reference Examples 5 to 10

(54) Image display devices were produced as in Example 10, except for using an optical film having a retardation of 4100 nm (Reference Example 5), 9000 nm (Reference Example 6), 10000 nm (Reference Example 7), 11500 nm (Reference Example 8), 33000 nm (Reference Example 9), and 101000 nm (Reference Example 10).

(55) (Evaluation of Color Shade)

(56) The color shade was evaluated as follows using the image display devices produced in the examples, comparative examples, and reference examples.

(57) Ten people simultaneously observed the frontal color shade when the angle between the absorption axis of the polarized sunglasses and the absorption axis of the polarizing plate was 0° (parallel Nicols) and when the angle was 90° (crossed Nicols) on a white image displayed on each image display device in a dark place. The following criteria were applied for evaluation.

(58) The same evaluation made by the largest number of people was regarded as the observation result.

(59) Poor: Strong interference colors are observed.

(60) Adequate: Interference colors are observed, but there is no problem in practical use.

(61) Good: Interference colors are only slightly observed.

(62) Excellent: No interference colors are observed.

(63) (Method for Measuring the Light Transmittance of the Optical Film)

(64) The intensity A of the wavelength of a maximum peak intensity in a wavelength range from 580 nm to 780 nm in each of the image display devices produced in the examples, comparative examples, and reference examples was measured with a spectroradiometer CS-2000 (Konica Minolta, Inc.). Next, an additional polarizer was disposed on the viewing side of each image display device, and the intensity B was measured when the absorption axis of the polarizer and the absorption axis of the additional polarizer were in a parallel Nicols state. Then, the light transmittance was determined by substituting the measured values into the following formula: (intensity B/intensity A)×100. When the light sources including the KSF phosphor were used, the light transmittance was measured at a wavelength of 630 nm as the wavelength of the maximum peak intensity in a wavelength range from 580 nm to 780 nm. When the light sources consisting of the white LEDs were used, the light transmittance was measured at a wavelength of 605 nm as the wavelength of the maximum peak intensity in a wavelength range from 580 nm to 780 nm.

(65) When the absorption axis of the polarizer and the absorption axis of the additional polarizer were in a crossed Nicols state, the light transmittance were determined similarly by first measuring the intensity A, subsequently measuring an intensity C when the absorption axis of the polarizer and the absorption axis of the additional polarizer were in a crossed Nicols state, and substituting the measured values into the following formula for calculation: (intensity C/intensity A)×100.

(66) (Evaluation of Color Rendering Properties)

(67) Color images were displayed on the image display devices having different light sources to which the optical films each having the same retardation value were independently bonded. Fifteen people evaluated which image display devices had high color rendering properties.

(68) Good: More than eight people said that the color rendering properties were excellent.

(69) Poor: Less than eight people said that the color rendering properties were excellent.

(70) (Determination of Color Difference)

(71) The image display devices produced in the examples, comparative examples, and reference examples were evaluated for color difference as follows.

(72) Ten people simultaneously observed the frontal color shade when the angle between the absorption axis of the polarized sunglasses and the absorption axis of the polarizing plate was 0° (parallel Nicols) and when the angle was 90° (crossed Nicols) on a white image displayed on each image display device in a dark place. The following criteria were applied for evaluation.

(73) The same evaluation made by the largest number of people was regarded as the observation result.

(74) Excellent: No color difference between parallel Nicols and crossed Nicols is observed.

(75) Good: A slight color difference between parallel Nicols and crossed Nicols is observed.

(76) Adequate: A color difference between parallel Nicols and crossed Nicols is observed, but there is no problem in practical use.

(77) Poor: A significant color difference between parallel Nicols and crossed Nicols is observed, and it is impossible to use.

(78) TABLE-US-00004 TABLE 4 Examples Comparative Examples 10 11 12 13 14 18 19 Retardation (nm) 3000  6000  8200  12980   25200   1500  2000  Color shade (visual Parallel Nicols Adequate Adequate Good Good Good Poor Poor observation) Crossed Nicols Adequate Good Good Adequate Good Poor Poor Determination of Adequate Good Excellent Good Excellent Poor Poor color difference Light ransmittance at Parallel Nicols 26 23 55 63 55 24 88 a wavelength of 630 nm Crossed Nicols 74 77 45 37 45 76 12 Color rendering properties Good Good Good Good Good Good Good Comparative Examples 20 21 22 23 24 25 26 27 Retardation (nm) 1500  2000  3000  4100  6000  8200  9000  10000   Color shade (visual Parallel Nicols Poor Poor Adequate Adequate Good Good Excellent Excellent observation) Crossed Nicols Poor Poor Adequate Adequate Good Good Excellent Excellent Determination of Poor Poor Good Good Good Good Excellent Excellent color difference Light transmittance at Parallel Nicols  2 41 94 42 77  3 53  9 a wavelength of 605 nm Crossed Nicols 98 59  6 58 23 97 47 91 Color rendering properties Poor Poor Poor Poor Poor Poor Poor Poor Comparative Examples 28 29 30 31 32 Retardation (nm) 11500   12980   25200   33000   101000   Color shade (visual Parallel Nicols Excellent Excellent Excellent Excellent Excellent observation) Crossed Nicols Excellent Excellent Excellent Excellent Excellent Determination of Excellent Excellent Excellent Excellent Excellent color difference Light transmittance at Parallel Nicols 82 38 26 86 21 a wavelength of 605 nm Crossed Nicols 18 62 74 14 79 Color rendering properties Poor Poor Poor Poor Poor Reference Examples 5 6 7 8 9 10 Retardation (nm) 4100  9000  10000   11500   33000   101000   Color shade (visual Parallel Nicols Poor Excellent Poor Excellent Poor Good observation) Crossed Nicols Adequate Poor Excellent Poor Excellent Good Determination of Poor Poor Poor Poor Poor Excellent color difference Light transmittance at Parallel Nicols 13 100  5 91 14 49 a wavelength of 630 nm Crossed Nicols 87  0 95  9 86 51 Color rendering properties Good Good Good Good Good Good

(79) The image display devices according to the examples each had excellent color rendering properties because light incident on the optical film provided at least 50% coverage of ITU-R BT.2020. In addition, blackout did not occur because the optical film had a retardation of 3000 nm or more and the optical film and the polarizer were disposed to form an angle of about 45° between the slow axis of the optical film and the absorption axis of the polarizer. As shown in Table 4, the optical film had a light transmittance of 20 to 80% at a wavelength of 630 nm, so that the occurrence of interference colors was also reduced, and the color difference was evaluated with excellent results.

(80) As shown in Table 4, the image display devices according to Reference Examples 5 to 9 each had excellent color rendering properties and blackout did not occur; however, unfortunately, interference colors and color difference were observed because the optical film had a light transmittance outside the range of 20 to 80% at a wavelength of 630 nm.

(81) The results of the examples and Reference Examples 5 to 9 confirm that the color rendering properties were excellent when light incident on the optical film provided at least 50% coverage of ITU-R BT.2020, and blackout was eliminated when the optical film had a retardation of 3000 nm or more and the optical film and the polarizer were disposed to form an angle of about 45° between the slow axis of the optical film and the absorption axis of the polarizer. However, in the case of the light sources including KSF phosphor that emits light having a narrow half width of the emission spectrum in the red region, the optical film needed to have a light transmittance of 20 to 80% at a wavelength of 630 nm in order to reliably eliminate interference colors.

(82) The image display device according to Reference Example 10 had excellent results in terms of color rendering properties, blackout, interference colors, and color difference. However, the optical film was too thick (1010 μm) and unsuitable for practical use. Table 5 shows the relationship between birefringence of the optical film and its thickness.

(83) In contrast, as for the image display devices according to the comparative examples, interference colors were unfortunately observed when the retardation of the optical film was less than 3000 nm (Comparative Examples 18, 19, 20, and 21). In the image display devices according to Comparative Examples 22 to 32, the emission spectrum of light incident on the optical film had a wide half width in the red region. Thus, interference colors, blackout, and color difference were prevented even when the light transmittance at a wavelength of 605 nm was outside the range of 20 to 80%, as long as the optical film had a retardation of 3000 nm or more and the optical film and the polarizer were disposed to form an angle of about 45° between the slow axis of the optical film and the absorption axis of the polarizer; however, the light incident on the optical film provided lower than 50% coverage of ITU-R BT.2020, so that the color rendering properties were poor.

(84) TABLE-US-00005 TABLE 5 Phase Film thickness difference/nm (μm) 1500 15 2000 20 3000 30 4100 41 6000 60 8200 82 9000 90 10000 100 11500 115 12980 129.8 13300 133 25200 252 28000 280 28300 283 33000 330 36000 360 41300 413 101000 1010 101400 1014

INDUSTRIAL APPLICABILITY

(85) Owing to the structure described above, the image display device of the present invention can include an optical film having a thickness suitable for practical use in devices for which thickness reduction is required, without including any special inorganic material, wherein the image display device has high color rendering properties and is capable of minimizing the occurrence of blackout and interference colors (rainbow unevenness) even when the image display device includes light sources that emit light having a narrow emission spectrum.