RELATED APPLICATION  
       [0001]     This applications claims priority, under 35 U.S.C. 119, from Korean Patent Application No. 2002-0040857 filed on Jul. 12, 2002, which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a liquid crystal display devices.  
         [0004]     2. Description of Related Art  
         [0005]     A liquid crystal display (“LCD”) device includes upper and lower panels provided with field-generating electrodes thereon, a liquid crystal layer interposed therebetween, a pair of a polarizer and an analyzer, compensation films, etc. The LCD generates electric field in the liquid crystal layer by applying electric voltages to the field-generating electrodes and adjusts the intensity of the electric field to control the transmittance of light passing through the liquid crystal layer, thereby displaying desired images.  
         [0006]     One of the most widely used types of LCD has a common electrode and a plurality of pixel electrodes provided on respective panels and a plurality of thin film transistors (“TFT”) for switching voltages applied to the pixel electrodes, which is provided on the panel having the pixel electrodes.  
         [0007]     LCDs may operate in one of several modes. An LCD operating in a vertically-aligned (“VA”) mode contains liquid crystal molecules aligned perpendicular to two panels. VA-mode LCDs are sometimes preferred for their high contrast ratio and wide viewing angle.  
         [0008]     LCDs often suffer from light leakage, the severity of which increases with viewing angle. The light leakage, which causes poor visibility from the sides and a narrow viewing angle, is caused by variations in light path and in the effective angle made by the polarizer and the analyzer depending on the viewing directions.  
         [0009]     Compensation films are sometimes used to neutralize the effect of these variations. However, use of compensation films usually significantly increases the cost of the LCD because they are expensive and there is no efficient way to select the compensation film that yields optimal results. A method of determining the optimal parameters of a compensation film without the costly trial-and-error process is needed in order to allow more LCD applications to take advantage of compensation films.  
       SUMMARY  
       [0010]     The invention is a method of improving the viewing angle of a vertically-aligned liquid crystal display device and a display device made using this method. In more detail, the method involves providing liquid crystal molecules positioned between a first glass substrate and a second glass substrate, and coupling a uniaxial compensation film to at least one of the glass substrates such that the uniaxial compensation film provides a retardation value of 200 nm or less for light having a wavelength of about 550 nm. When building a display device, a liquid crystal panel with liquid crystal molecules contained between glass substrates is first provided. The liquid crystal molecules are positioned such that the long axes of the liquid crystal molecules are oriented orthogonal to the glass substrates in the absence of electrical field. Then, a set of compensation films are coupled to at least one of the glass substrates. The set of compensation films includes one or more uniaxial compensation films and provides a total retardation value of less than or equal to 200 nm for light having a wavelength of about 550 nm. A polarization film is coupled to the set of compensation films, and coupling electrodes are coupled to the liquid crystal panel.  
         [0011]     As stated above, the invention also includes a display device built using the above method. More specifically, the invention includes a display device including a liquid crystal layer disposed between glass substrates so that long axes of liquid crystal molecules are oriented orthogonal to the glass substrates, a set of compensation films coupled to at least one of the glass substrates, wherein the set of compensation films are selected based on having a total retardation value less than or equal to 200 nm for light having a wavelength of about 550 nm, a polarization film coupled to the set of compensation films, and a first electrode and a second electrode coupled to the glass substrates.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The above and other advantages of the present invention will-become more apparent by describing preferred embodiments thereof in detail with reference to the accompanying drawings in which:  
         [0013]      FIG. 1  is a sectional view of an LCD according to an embodiment of the present invention;  
         [0014]      FIG. 2  is a layout view of a TFT array panel of an LCD according to an embodiment of the present invention;  
         [0015]      FIG. 3  is a sectional view of the TFT array panel shown in  FIG. 2  taken along the line III-III′;  
         [0016]      FIG. 4  is a layout view of a color filter array panel of an LCD according to an embodiment of the present invention;  
         [0017]      FIG. 5  is a sectional view of an LCD according to another embodiment of the present invention;  
         [0018]      FIG. 6  is a sectional view of a TFT array panel of an LCD according to another embodiment of the present invention;  
         [0019]      FIG. 7  is a sectional view of an LCD according to another embodiment of the present invention;  
         [0020]      FIG. 8  is a layout view of a TFT array panel of an LCD according to another embodiment of the present invention;  
         [0021]      FIG. 9  is a sectional view of the TFT array panel shown  FIG. 8  taken along the line IX-IX′;  
         [0022]      FIG. 10  is a graph showing reflectance as function of applied voltage for a transflective LCD with and without uniaxial (C-plate) compensation films;  
         [0023]      FIG. 11  is a graph showing transmittance as function of applied voltage for a transflective LCD with and without uniaxial (C-plate) compensation films;  
         [0024]      FIGS. 12A  to  12 F are graphs showing isocontrast curves of a reflective type LCD without and with one C-plate attached to the upper panel;  
         [0025]      FIGS. 13A  to  13 F are graphs showing isocontrast curves of a transmissive type LCD without and with one C-plate attached to the upper panel;  
         [0026]      FIGS. 14A  to  14 E are graphs showing isocontrast curves of a reflective type LCD with C-plates attached to both upper and lower panels; and  
         [0027]      FIGS. 15A  to  15 E are graphs showing isocontrast curves of a transmissive type LCD with C-plates attached to both upper and lower panels. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0028]     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.  
         [0029]     In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numerals refer to like elements throughout. It will be understood that when an element such as a layer, film, region, substrate or panel is referred to as being “on” another element, it can be directly on the other element or on one or more intervening elements. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present.  
         [0030]     Then, LCDs according to embodiments of the present invention will be described in detail with reference to the drawings.  
         [0031]      FIG. 1  is a sectional view of a transmissive type LCD according to an embodiment of the present invention.  
         [0032]     An LCD according to this embodiment includes a TFT array panel  1  and a color filter array panel  2  facing each other, and a liquid crystal layer  3  interposed between the two panels  1  and  2 . The LCD also includes first and second polarization films  12  and  22  having nonparallel polarization axes, and first and second protective films  13  and  23  preferably made of TAC (triacetate cellulose) films and attached on the first and the second polarization films  12  and  22  for protecting the polarization films  12   25  and  22 , respectively. The LCD further includes a first uniaxial (C-plate) compensation film  14  inserted between the TFT array panel I and the first protective film  13 , and a second uniaxial compensation film  24  inserted between the color filter array panel  2  and the second protective film  23 .  
         [0033]     The LCD is in a vertically-aligned (VA) mode. That is, the liquid crystal layer  3  of the LCD includes liquid crystal molecules aligned to make their long axes substantially perpendicular to the two panels  1  and  2 .  
         [0034]     The first and the second protective films  13  and  23  generate slight retardation. In addition, the uniaxial compensation films  14  and  24  have negativity and generate retardation in a range between about 0 nm and about 200 nm for the light having a wavelength of 550 nm. Here, the uniaxiality means that nx=ny≠nz and the negativity means that nx=ny&gt;nz, where nx, ny and nz denote the refractive indices of x, y and z directions, respectively.  
         [0035]     The first uniaxial compensation film  14  may be omitted.  
         [0036]     Now, a TFT array panel and a color filter array panel of an LCD according to embodiments are described in more detail.  
         [0037]      FIG. 2  is a layout view of a TFT array panel for an LCD according to an embodiment of the present invention, and  FIG. 3  is a sectional view of the TFT array panel shown in  FIG. 2  taken along the line III-III′.  
         [0038]     As shown in  FIGS. 2 and 3 , a gate wire  121 ,  123  and  125  preferably made of a metal having low resistivity such as aluminum, silver, etc. is formed on a transparent insulating substrate  110 . The gate wire  121 ,  123  and  125  includes a plurality of gate lines  121  extending in a transverse direction and a plurality of gate electrodes  123  connected to the gate lines  121 . An end portion  125  of each gate line  121  is widened for connection with an external circuit.  
         [0039]     A gate insulating layer  140  is formed on the entire surface of the substrate including the gate wire  121 ,  123  and  125 .  
         [0040]     A plurality of semiconductor stripes  151 ,  153  and  159  preferably made of amorphous silicon are formed on the gate insulating layer  140 , and a plurality of ohmic contacts  161 ,  162 ,  163  and  165  preferably made of amorphous silicon heavily doped with n-type impurity are formed on the semiconductor stripes  151 ,  153  and  159 .  
         [0041]     A data wire  171 ,  173 ,  175 ,  177  and  179  preferably made of a metal having low resistivity such as aluminum, silver, etc. is formed on the ohmic contacts  161 ,  162 ,  163  and  165  and the gate insulating layer  140 .  
         [0042]     The data wire  171 ,  173 ,  175 ,  177  and  179  includes a plurality of data lines  171  intersecting the gate lines  121  to define a plurality of pixel areas, a plurality of source electrodes  173  which are branches of the data lines  171  and connected to the ohmic contacts  163 , a plurality of drain electrode  175  separated from the source electrodes  173  and formed on the ohmic contacts  165  opposite to the source electrodes  173  with respect to the gate electrodes  123 , and a plurality of storage electrodes  177  overlapping the gate lines  121  to form storage capacitors. An end portion  179  of each data line  171  is widened for connection with an external circuit.  
         [0043]     A passivation layer  180  is formed on the data wire  171 ,  173 ,  175 ,  177  and  179 . The passivation layer  180  has a plurality of first contact holes  181  exposing the drain electrodes  175 , a plurality of second contact holes  182  exposing the end portions  125  of the gate lines, a plurality of third contact holes  183  exposing the end portions  179  of the data lines  171 , and a plurality of fourth contact holes  184  exposing the storage electrodes  177 .  
         [0044]     A plurality of pixel electrodes  190  and a plurality of contact assistants  95  and  97  are formed on the passivation layer  180 . The pixel electrodes  190  are connected to the drain electrodes  175  and the storage electrodes  177  via the first and the fourth contact holes  181  and  184 , respectively, and the contact assistants  95  and  97  are connected to the exposed end portions  125  of the gate lines  121  and the exposed end portions  179  of the data lines  171  via the second and the third contact holes  182  and  183 , respectively. The pixel electrodes  190  and the contact assistants  95  and  97  are preferably made of transparent material such as ITO (indium tin oxide) or IZO (indium zinc oxide).  
         [0045]      FIG. 4  is a layout view of a color filter array panel of an LCD according to an embodiment of the present invention.  
         [0046]     A black matrix  220  is formed on an insulating substrate  210 , a plurality of color filters  230  are formed on the black matrix  220 , and a common electrode  270  is formed on the color filters  230 . The common electrode  270  is preferably made of a transparent conductive material such as ITO or IZO.  
         [0047]      FIG. 5  is a sectional view of a reflective type LCD without separate light source according to another embodiment of the present invention.  
         [0048]     An LCD according to this embodiment includes a TFT array panel  1  and a color filter array panel  2  facing each other, and a liquid crystal layer  3  interposed between the two panels  1  and  2 . The LCD further includes a polarization film  22  and a protective film  23  attached on the polarization film  22  for protecting the polarization film  22 . The LCD also includes a uniaxial compensation film  24  and a reverse dispersion phase difference film  25  inserted between the color filter array panel  2  and the protective film  23 .  
         [0049]     The LCD is in a VA mode. The protective film  23  generates slight retardation, and the uniaxial compensation film  24  has negativity and generates retardation ranging 0 nm to 200 nm for the light with 550 nm wavelength.  
         [0050]      FIG. 6  is a sectional view of a TFT array panel of an LCD according to another embodiment of the present invention.  
         [0051]     Referring to  FIG. 6 , a gate wire  121 ,  123  and  125 , a gate insulating layer  140 , a plurality of semiconductor stripes  151  and  153 , a plurality of ohmic contacts  161 ,  162 ,  163  and  165 , a data wire  171 ,  173 ,  175 ,  177  and  179 , a passivation layer  180 , a plurality of pixel electrodes  190 , and a plurality of contact assistants  95  and  97  are formed on a substrate  110 .  
         [0052]     The surface of the passivation layer  180  has embossment including prominences/protrusions and depressions, and the pixel electrodes  190  are preferably made of a metal having good reflectance such as aluminum.  
         [0053]      FIG. 7  is a sectional view of a transflective LCD according to another embodiment of the present invention.  
         [0054]     An LCD according to this embodiment includes a TFT array panel  1  and a color filter array panel  2  facing each other, and a liquid crystal layer  3  interposed between the two panels  1  and  2 . The LCD also includes a pair of first and second polarization films  12  and  22 , and a pair of first and second protective films  13  and  23  attached on the polarization films  12  and  22 , respectively. The LCD further includes a first uniaxial (C-plate) compensation film  14  and a first reverse dispersion phase difference film  15  inserted between the TFT array panel  1  and the first protective film  13 , and a second uniaxial (C-plate) compensation film  24  and a second reverse dispersion phase difference film  25  inserted between the color filter array panel  2  and the second protective film  23 .  
         [0055]     The LCD is in a VA mode. The first and the second protective films  13  and  23  generate slight retardation, and the first and the second uniaxial compensation films  14  and  24  have negativity and generate retardation in a range from 0 nm to 200 nm for the light with 550 nm wavelength. The first uniaxial compensation film  14  may be omitted.  
         [0056]      FIG. 8  is a layout view of a TFT array panel of an LCD according to an embodiment of the present invention, and  FIG. 9  is a sectional view of the TFT array panel shown in  FIG. 8  taken along the line IX-IX′.  
         [0057]     Referring to  FIG. 8 , a gate wire  121 ,  123  and  125 , a gate insulating layer  140 , a plurality of semiconductor stripes  151  and  153 , a plurality of ohmic contacts  161 ,. 162 ,  163  and  165 , a data wire  171 ,  173 ,  175 ,  177  and  179 , and a passivation layer  801  are formed on a substrate  110 .  
         [0058]     A plurality of transparent electrodes  90  and a plurality of contact assistants  95  and  97  preferably made of ITO or IZO are formed on the passivation layer  801 . An interlayer insulating layer  802  having an embossed surface is formed on the transparent electrodes  90 . A plurality of reflecting electrodes  80  are formed on the interlayer insulating layer  802 , and each reflecting electrodes  80  has a window  82  for light transmission.  
         [0059]     Various characteristics of various types of LCDs with various types of compensation films were measured.  
         [0060]     The LCDs used for the measurement have conditions shown in TABLE 1 and TABLE 2.  
                                                                             TABLE 1                                       Mode   VA                Dopant   natural pitch of 67   microns           Twist Angle   90   degrees           Pretilt Angle   89   degrees           K11   13.0   pN           K22   5.1   pN           K33   14.7   pN                ε||   3.6           ε⊥   7.4                Cell Gap   2.89   microns                      
 
         [0061]     Here, K11, K22 and K33 are elastic coefficients of spreading, twisting and bending measured in pico-newton (pN) and ε∥ and ε⊥ are permittivity parallel to and perpendicular to the director, respectively.  
                                                                                           TABLE 2                                               Δnd for                   Thickness   550 nm           n ∞     A(nm −2 )   (microns)   wavelength                                    Liquid   VA   ne   1.5369   7651.0   2.89     240 nm       Crystal       no   1.4607   5569.0            Reverse   ne   1.5934   −268.8   52.14   142.86 nm       dispersionλ/   no   1.59   0       4 plate       TAC   nx   ny   nz   80   —           1.4800   1.4798   1.4791       C-Plate   nx   ny   nz   20      80 nm           1.504    1.504   1.500                  
 
         [0062]     Here, ne is the refractive index parallel to the director (i.e. for extraordinary ray) and no is the refractive index perpendicular to the director (i.e. for ordinary ray), while Δn=ne-no. In addition, the dispersion relation is given by:  
           n   ⁡     (   λ   )       =       n   ∞     +     A     λ   2           ,       
 
 where n ∞  is the refractive index for infinite wavelength and A is a constant. 
 
         [0063]      FIGS. 10 and 11  are graphs respectively showing reflectance and transmittance as function of applied voltage for a transflective LCD with and without uniaxial (C-plate) compensation films.  
         [0064]     The curves show that the presence of the uniaxial compensation films hardly affects the reflectance and the transmittance of the LCD.  
         [0065]      FIGS. 12A  to  12 F are graphs showing isocontrast curves of a reflective type LCD without and with one C-plate attached to the upper panel.  FIGS. 12A  to  12 F show tho isocontrast curves for the cases 2 to 7 in the TABLE 3, respectively.  
                                                                                       TABLE 3                                       Number/   Reflective Type                    Thickness           Viewing Angle   Areal               (um)/   Reflec-   Front   up/down/   Isocontrast               Δnd   tance   view   (left/right)   Ratio       Case   Mode   of C plate   (%)   CR   CR 2:1   (CR 10:1)                    1   TN   None   11.7   19.9   47/34/80/66   0.861       2   VA   None   16.9   26.0   68/68/51/51   0.757       3   VA   One/   16.9   25.8   80/80/79/79   1               20/80 nm       4   VA   One/   16.9   24.0   55/55/68/68   1.324               40/160 nm       5   VA   One/   16.9   22.1   42/42/50/50   0.987               60/240 nm       6   VA   One/   16.9   26.6   35/35/42/42   0.723               80/320 nm       7   VA   One/   16.9   26.6   31/31/36/36   0.603               100/400 nm                    
         [0066]      FIGS. 13A  to  13 F are graphs showing isocontrast curves of a transmissive type LCD without and with one C-plate attached to the upper panel.  FIGS. 13A  to  13 F show the isocontrast curves for the cases 2 to 7 in TABLE 4, respectively.  
                                                                                       TABLE 4                                       Number/   Transmissive type                    Thickness           Viewing Angle   Areal               (um)/   Trans-   Front   (up/down/   Isocontrast               Δnd   mittance   View   left/right)   Ratio       Case   Mode   of C plate   (%)   CR   CR 2:1   (CR 10:1)                    1   TN   None   7.4   378.4   59/59/80/80   1.065       2   VA   None   13.0   881.6   80/47/80/80   1.404       3   VA   One/   13.0   880.3   80/40/80/79   1.55               20/               80 nm       4   VA   One/   13.0   881.9   60/34/70/63   1.410               40/               160 nm       5   VA   One/   13.0   880.7   50/31/57/54   1.177               60/               240 nm       6   VA   One/   13.0   881.0   44/30/50/49   0.925               80/               320 nm       7   VA   One/   13.0   882.0   39/27/45/44   0.797               100/               400 nm                    
         [0067]      FIGS. 14A  to  14 E are graphs showing isocontrast curves of a reflective type LCD with two C-plates respectively attached to upper and lower panels.  FIGS. 14A  to  14 E show the isocontrast curves for Cases 8 to 12, respectively.  
                                                                                   TABLE 5                                       Reflective type                    Thickness           Viewing Angle   Areal               (um)/   Reflec-   Front   (up/down/   Isocontrast               Δnd   tivity   view   left/right)   Ratio       Case   Mode   of C plate   (%)   CR   CR 2:1   (CR 10:1)                    8   VA   10/   16.9   23.2   75/78/62/62   0.861               40 nm × 2       9   VA   20/   16.9   25.8   80/80/79/79   1               80 nm × 2       10   VA   30/   16.9   24.3   69/69/80/80   1.209               120 nm × 2       11   VA   40/   16.9   24.0   55/55/68/68   1.324               160 nm × 2       12   VA   50/   16.9   24.3   47/47/58/58   1.242               200 nm × 2                    
         [0068]      FIGS. 15A  to  15 E are graphs showing isocontrast curves of a transmissive type LCD with two C-plates respective attached to upper and lower panels.  FIGS. 15A  to  15 E show the isocontrast curves for Cases 8 to 12 in TABLE 6, respectively.  
                                                                                   TABLE 6                                       Transmissive Type                    Thickness   Trans-       Viewing Angle   Areal               (um)/   mit-   Front   (up/down/   Isocontrast               Δnd   tance   View   left/right)   Ratio       Case   Mode   of C plate   (%)   CR   CR 2:1   (CR 10:1)                    8   VA   10/   13.0   881.0   80/39/80/78   1.565               40 nm × 2       9   VA   20/   13.0   881.8   59/34/75/63   1.430               80 nm × 2       10   VA   30/   13.0   881.2   49/31/58/54   1.215               120 nm × 2       11   VA   40/   13.0   881.6   43/29/51/48   0.958               160 nm × 2       12   VA   50/   13.0   882.1   38/27/46/44   0.817               200 nm × 2                    
         [0069]     In TABLES 3 to 6, the areal isocontrast ratio is an isocontrast area for the contrast ratio of 10:1 divided by that in Case 3 of the reflective type LCD. White and black voltages in VA mode are 3.5V and 1.8V for the reflective type and 4.5V and 1.8V for transmissive type, respectively. The abbreviation “CR” stands for contrast ratio.  
         [0070]     The ratios such as 2:1, 5:1, 10:1, 20:1 and 22:1 in the legends of  FIGS. 12A  to  15 E indicate contrast ratios and the values (for example,  68 / 68 / 51 / 51  in  FIG. 12A ) at the bottom of  FIGS. 12A  to  15 E indicate upper/lower/left/right side viewing angles giving the contrast ratio of 2:1.  
         [0071]     The measurement values of TABLES 3 to 6 shown in  FIGS. 12A  to  12 F,  13 A to  13 F, and  14 A to  15 E can be summarized as follows.  
         [0072]     Without uniaxial compensation film (C-plate), the transmittance, the reflectance, the contrast ration at the front view, and the viewing angle of the VA mode are superior to those of the TN mode.  
         [0073]     Compared with Case 2, which does not include a uniaxial compensation film, Cases 3 and 4 of the VA mode show both improved viewing angle and improved isocontrast curve, and Cases 3 and 4 of the transmissive mode show improved isocontrast curves. In contrast, Cases 5, 6 and 7, each of which includes a compensation film causing a retardation greater than 160 nm, show deteriorated viewing angle and deteriorated isocontrast curves. Data indicates that a uniaxial compensation film providing a retardation value larger than 160 nm has a detrimental effect on the LCD device.  
         [0074]     With uniaxial compensation films attached to both upper and lower panels, the isocontrast curves for the transmissive-type LCDs are improved until the sum of the retardation values of the two compensation films equals to 160 nm. When the combined retardation value exceeds 160 nm, both the isocontrast curves and the viewing angles become worse. For the reflective-type LCDs, only one of the two compensation films contributes to the total retardation since light is not transmitted through both of films. Therefore, the actual retardation values of the compensation films for the cases 8 to 11 are equal to or smaller than 160 nm. This explains why case 12 for the reflective-type LCD shows improved isocontrast curves in spite of having a retardation value of 200 nm. For the transmissive-type LCD, although there is no experimental example for the case of 200 nm retardation, the results shown in TABLES 3 to 6 suggest that the isocontrast curves will be improved where retardation values are equal to or less than 200 nm.  
         [0075]     The above-described experimental results show a correlation between the total thickness of the uniaxial compensation film(s) and the viewing angle and/or the contrast ratio. It appears that the total thickness of the uniaxial compensation film(s) affects the viewing angle and/or the contrast ratio more than the number or the physical arrangement of the uniaxial compensation film(s).  
         [0076]     Of the cases above, Case 3 has optimal characteristics both for the transmissive type and the reflective type LCDs. Although some cases show better contrast ratio at the front view than Case 3, the difference is small. Overall, Case 3 resulted in a better viewing angle than the other cases. Therefore, it can be said that Case 3 is optimized.  
         [0077]     In conclusion, the total retardation of the uniaxial compensation film(s) equal to or under 200 nm improves the isocontrast curve and/or the viewing angle. This improvement is irrelevant to the number of uniaxial compensation films used and the type (reflective or the transmissive) of LCD.  
         [0078]     According to the present invention, uniaxial compensation film(s) generating a predetermined retardation is used to improve the viewing angle of the LCD.
A method of improving the viewing angle of a vertically-aligned liquid crystal display device is presented. The method involves designing a uniaxial compensation film to provide a retardation value of 200 nm or less for light having a wavelength of about 550 nm. Using this uniaxial compensation film, a display device can be built by obtaining a liquid crystal panel with liquid crystal molecules contained between glass substrates, coupling the uniaxial compensation film to at least one of the glass substrates, and coupling a polarization film and electrodes to the compensation film. Preferably, the uniaxial compensation film has a thickness less than or equal to 50 microns. Where there are multiple compensation films, the total thickness and the total retardation values should be considered.