Patent Publication Number: US-10310142-B2

Title: Optical device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation application filed under 35 U.S.C. 111(a) claiming benefit under 35 U.S.C. 120 and 365(c) of PCT International Application No. PCT/JP2014/053218 filed on Feb. 12, 2014 and designating the U.S., which claims priority to Japanese Patent Application No. 2013-030238 filed on Feb. 19, 2013 and Japanese Patent Application No. 2013-082670 filed on Apr. 11, 2013. The entire contents of the foregoing applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical device. 
     2. Description of the Related Art 
     In general, on a side of a display surface of a display device, such as a liquid crystal display (Liquid Crystal Display: LCD), a cover that is formed of a transparent substrate is disposed so as to protect the display device. 
     However, when such a transparent substrate is disposed on the display device, and when a displayed image of the display device is attempted to be viewed through the transparent substrate, often, an object that is disposed in the vicinity can be reflected. When such a reflection occurs on the transparent substrate, it may become difficult for a person viewing the displayed image to view the displayed image, and the person viewing the image may have an unpleasant impression. Especially, for cover glass that is installed inside a vehicle, a strong light source that is represented by sunlight can be reflected. Thus, it can become very difficult to view a displayed image, and it is possible that driving is adversely affected because necessary information cannot be read. 
     Thus, for preventing such a reflection from occurring, for example, a method has been adopted that is for implementing an anti-glare process that is for forming an uneven shape on the surface of the transparent substrate. 
     Note that Patent Document 1 discloses a method of evaluating a reflection on the display device by using a special device.
     Patent Document 1: Japanese Unexamined Patent Publication No. 2007-147343   

     As described above, Patent Document 1 shows the method of evaluating the reflection on the display device using the special device. 
     However, optical characteristics that are required for cover glass that is installed inside a vehicle is not limited to reduction of reflection. Namely, cover glass that is installed inside a vehicle may be required to have predetermined levels of optical characteristics, such as for a resolution, reflection image diffusiveness, and sparkle, respectively. Accordingly, for selecting a transparent substrate, it may be insufficient to consider only one of the optical characteristics. Often, there may arises a need to consider a plurality of optical characteristics at the same time. 
     The resolution that is described here is for representing whether and to what extent an image that matches a displayed image can be obtained when the displayed image is viewed through a transparent substrate. Further, the reflection image diffusiveness is for representing to what extent a reflected image of an object that (e.g., a light) is disposed in the vicinity of the transparent substrate matches the original object. Furthermore, the sparkle is for representing to what extent unevenness of a bright spot is observed that occurs when light (an image) from the display image passes through the transparent substrate, the light is reflected by the surface of the transparent substrate, and the scattered light beams mutually interfere. 
     Among the optical characteristics that are required for the transparent substrate, there are often characteristics that are in tradeoff relationships. For example, in general, in order to enhance the reflection image diffusiveness, the anti-glare process is applied to the surface of the transparent substrate. However, when such an anti-glare process is applied, a resolution of the transparent substrate tends to be lowered. In this manner, when the anti-glare process is to be applied to the transparent substrate based on a plurality of optical characteristics, it may become difficult to select a proper anti-glare process. 
     SUMMARY OF THE INVENTION 
     The present invention has been achieved in view of such a background. An object of the present invention is to provide an optical device having a resolution, reflection image diffusiveness, and an anti-glare property that are suitable for automotive use. 
     According to the present invention, there is provided an optical device including a display device and a transparent substrate that is disposed on a side of a display surface of the display device, wherein the transparent substrate has a first surface and a second surface, and wherein, when the transparent substrate is evaluated by using a three index values of a resolution index value T, a reflection image diffusiveness index value R, and a sparkle index value S that are quantified by following methods, 
     the resolution index value T≤0.2, 
     the reflection image diffusiveness index value R≥0.2, and 
     the sparkle index value S≤60 
     are satisfied. 
     Here, for the resolution index value T, a first light beam is irradiated from a side of the second surface of the transparent substrate in a direction that is parallel to a thickness direction of the transparent substrate (a direction of an angle of 0°), and brightness of a transmitted light beam that is transmitted from the first surface (which is referred to as 0° transmitted light beam) is measured, a light reception angle of receiving the first light beam with respect to the first surface of the transparent substrate is varied in a range from −90° to +90°, brightness of all the transmitted beams that are transmitted from a side of the first surface is measured, and the resolution index value T is calculated from following expression (1):
 
the resolution index value  T =(the brightness of all the transmitted beams−the brightness of the 0° transmitted light beam)/(the brightness of all the transmitted light beams)  expression (1).
 
     For the reflection image diffusiveness index value R, a second light beam is irradiated from the side of the first surface of the transparent substrate in a direction of 45° with respect to the thickness of the transparent substrate, brightness of a light beam that is specularly reflected by the first surface (which is referred to as a 45° regular reflected light beam) is measured, a light reception angle of receiving the reflected beam that is reflected by the first surface is varied in a range from 0° to +90°, brightness of all the reflected beams that are reflected by the first surface is measured, and the reflection image diffusiveness index value R is calculated by following expression (2):
 
the reflection image diffusiveness index value  R =(the brightness of all the reflected beams−the brightness of the 45° regular reflected beam)/(the brightness of all the reflected beams)  expression (2).
 
     For the sparkle index value S, the transparent substrate is arranged at the side of the display surface of the display device so that the second surface is at the side of the display device, a photograph of the transparent substrate is taken from the side of the first surface to obtain an image, the image is analyzed by software EyeScale-4W (a product of I-System Co., Ltd.), and the sparkle index value S is obtained by setting an ISC-A value that is output by the software as the sparkle index value S. 
     Here, in the optical device according to the present invention, the display device may be one device that is selected from a group that includes a liquid crystal display (LCD) device, an organic light-emitting diode (OLED) device, and a plasma display panel (PDP) device. 
     Further, in the optical device according to the present invention, the transparent substrate may be formed of soda-lime glass or aluminosilicate glass. 
     Further, in the optical device according to the present invention, a chemically strengthening process may be applied to at least one of the first surface and the second surface of the transparent substrate. 
     Further, in the optical device according to the present invention, an anti-glare process may be applied to the first surface. 
     Further, the optical device according to the present invention may be an optical device that is to be installed inside a vehicle. 
     According to the present invention, there can be provided an optical device that has a resolution, reflection image diffusiveness, and an anti-glare property that are suitable for installation inside a vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically showing a flow of a method of obtaining a resolution index value of a transparent substrate according to an embodiment of the present invention; 
         FIG. 2  is a diagram schematically showing an example of a measurement device that is used for obtaining the resolution index value; 
         FIG. 3  is a diagram schematically showing a flow of a method of obtaining a reflection image diffusiveness index value of the transparent substrate according to the embodiment of the present invention; 
         FIG. 4  is a diagram schematically showing an example of a measurement device that is used for obtaining the reflection image diffusiveness index value; 
         FIG. 5  is a diagram schematically showing a flow of a method of obtaining a sparkle index value of the transparent substrate according to the embodiment of the present invention; 
         FIG. 6  is a diagram that is obtained by plotting an example of a relationship between the resolution index value T (a horizontal axis) and the reflection image diffusiveness index value R (a vertical axis) that is obtained for each type of the transparent substrate; 
         FIG. 7  is a diagram that is obtained by plotting an example of a relationship between the sparkle index value S (a horizontal axis) and the reflection image diffusiveness index value R (a vertical axis) that is obtained for each type of the transparent substrate; 
         FIG. 8  is a diagram that is obtained by plotting an example of a relationship between the resolution index value T (a horizontal axis) and the sparkle index value S (a vertical axis) that is obtained for each type of the transparent substrate; 
         FIG. 9  is a schematic three-dimensional diagram in which the resolution index value T, the reflection image diffusiveness index value R, and the sparkle index value S are set as axes, respectively; 
         FIG. 10  is a cross-sectional view schematically showing an optical device according to an embodiment; 
         FIG. 11  is a graph showing an example of a relationship between a determination result of a resolution level by a visual observation (a vertical axis) and the resolution index value T (a horizontal axis) that is obtained for each type of the transparent substrate; 
         FIG. 12  is a diagram that collectively shows transparent substrates that have the reflection image diffusiveness index values from level  1  to level  12 , respectively; 
         FIG. 13  is a graph showing an example of a relationship between a level of the reflection image diffusiveness index value by a visual observation (a vertical axis) and the reflection image diffusiveness index value R (a horizontal axis) that is obtained for each type of the transparent substrate; 
         FIG. 14  is a diagram showing transparent substrates that indicate sparkle of level  0  and sparkle of level  7 , respectively; and 
         FIG. 15  is a graph showing an example of a relationship between the sparkle index value S (a vertical axis) and the level of the sparkle by a visual observation (a horizontal axis) that is obtained for each transparent substrate. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is explained in detail below. 
     According to the present invention, there is provided an optical device including a display device and a transparent substrate that is disposed at a side of a display surface of the display device, wherein the transparent substrate has a first surface and a second surface, and wherein, when the transparent substrate is evaluated by using three index values of a resolution index value T, a reflection image diffusiveness index value R, and a sparkle index value S that are quantified by the following method, 
     the resolution index value T≤0.2, 
     the reflection image diffusiveness index value R≥0.2, and 
     the sparkle index value S≤60 
     are satisfied. 
     Here, for the resolution index value T, a first light beam is irradiated from a side of the second surface of the transparent substrate in a direction that is parallel to a thickness direction of the transparent substrate (a direction of an angle of 0°), and brightness of a transmitted light beam that is transmitted from the first surface (which is referred to as 0° transmitted light beam) is measured, a light reception angle of the first light beam with respect to the first surface of the transparent substrate is varied in a range from −90° to +90°, and brightness of all the transmitted beams that are transmitted from a side of the first surface is measured, and the resolution index value T is calculated from following expression (1):
 
the resolution index value  T =(the brightness of all the transmitted beams−the brightness of the 0° transmitted light beam)/(the brightness of all the transmitted light beams)  expression (1).
 
     For the reflection image diffusiveness index value R, a second light beam is irradiated from the side of the first surface of the transparent substrate in a direction of 45° with respect to the thickness of the transparent substrate, brightness of a light beam that is specularly reflected by the first surface (which is referred to as a 45° regular reflected light beam) is measured, a light reception angle of receiving the reflected beam that is reflected by the first surface is varied in a range from 0° to +90°, brightness of all the reflected beams that are reflected by the first surface is measured, and the reflection image diffusiveness index value R is calculated by following expression (2):
 
the reflection image diffusiveness index value  R =(the brightness of all the reflected beams−the brightness of the 45° regular reflected beam)/(the brightness of all the reflected beams)  expression (2).
 
     For the sparkle index value S, the transparent substrate is arranged at the side of the display surface of the display device so that the second surface is at the side of the display device, a photograph of the transparent substrate is taken from the side of the first substrate to obtain an image, the image is analyzed by software EyeScale-4W (a product of I-System Co., Ltd.), and the sparkle index value S is obtained by setting an ISC-A value that is output in this manner as the sparkle index value S. 
     As described above, for a transparent substrate that is disposed on a side of a surface of a display device, various optical characteristics are required, such as a resolution, reflection image diffusiveness, and an anti-glare property. Thus, for selecting a transparent substrate, there are often cases where it is insufficient to consider only a single optical property. 
     In contrast, according to the present invention, three optical characteristics of the transparent substrate that are a resolution index value, a reflection image diffusiveness index value, and a sparkle index value are objects of the determination. 
     In such a method, a transparent substrate can be more properly selected because the transparent substrate can be selected by comprehensively considering three optical characteristics. 
     Further, in the method according to the present invention, values that are expressed in numerical forms are used as the resolution index value, the reflection image diffusiveness index value, and the sparkle index value of the transparent substrate. Consequently, the optical characteristics of the resolution, the reflection image diffusiveness, and the sparkle can be objectively and quantitatively determined without depending on a subjective view and prejudice of an observer. 
     Further, among the optical characteristics that are required for the transparent substrate, there are often optical characteristics that are in tradeoff relationships, such as the resolution and the reflection image diffusiveness. According to related art, in such a case, since there is no index that can be a basis for the selection, it has been difficult to properly select a transparent substrate with which two optical characteristics are satisfied. Furthermore, when three optical characteristics are to be considered, it has been almost impossible to select a transparent substrate that satisfies these optical characteristics. 
     In contrast, in the method according to the present invention, three optical characteristics of a transparent substrate can be quantitatively and comprehensively evaluated. Thus, in the method according to the present invention, a transparent substrate that has the optimum optical characteristics can be properly selected depending on purpose and use. 
     Hereinafter, an embodiment of a method of obtaining a resolution index value, a reflection image diffusiveness index value, and a sparkle index value of a transparent substrate, which can be used in the method according to the present invention, is explained by referring to the drawings. 
     (Regarding the Resolution Index Value) 
       FIG. 1  schematically shows a flow of a method of obtaining a resolution index value of a transparent substrate according to an embodiment of the present invention. 
     As shown in  FIG. 1 , the method of obtaining the resolution index value of the transparent substrate includes 
     (a) a step of irradiating a first light beam from a side of a second surface of the transparent substrate having a first surface and the second surface in a direction that is parallel to a thickness direction of the transparent substrate, and measuring brightness of a transmitted beam (which is also referred to as “0° transmitted light beam,” hereinafter) that is transmitted in a direction that is parallel to the thickness direction of the transparent substrate from the first surface (step S 110 ), 
     (b) a step of varying a light reception angle for receiving the first light beam with respect to a thickness direction of the transparent substrate in a range from −90° to +90°, and measuring brightness of the first light beam (which is also referred to as “all the transmitted beams,” hereinafter) that passes through the transparent substrate and that is emitted from the first surface (step S 120 ), and 
     (c) a step of calculating a resolution index value T by the following expression (1) (step S 130 ):
 
the resolution index value  T =(the brightness of all the transmitted beams−the brightness of the 0° transmitted light beam)/(the brightness of all the transmitted beams)  expression (1).
 
     Hereinafter, each of the steps is explained. 
     (Step S 110 ) 
     First, a transparent substrate is prepared that has a first surface and a second surface that face each other. 
     The transparent substrate may be formed of any material, provided that it is transparent. The transparent substrate may be glass or plastic, for example. 
     When the transparent substrate is formed of glass, composition of the glass is not particularly limited. For example, the glass may be soda-lime glass or aluminosilicate glass. 
     Further, when the transparent substrate is formed of glass, a chemically strengthening process may be applied to the first surface and/or the second surface. 
     Here, the chemically strengthening process is said to be a general term of a technique for replacing an alkali metal (ions) having a small ionic radius that exists on an outermost surface of the glass substrate with an alkali metal (ions) having a large ionic radius that exists in a dissolved salt by immersing a glass substrate in the dissolved salt that includes the alkali metals. In the chemically strengthening process, an alkali metal (ions) having an ionic radius that is greater than that of the original atom is disposed on the surface of the processed glass substrate. Thus, compressive stress can be provided on the surface of the glass substrate, thereby enhancing the strength of the glass substrate (especially, break strength). 
     For example, when the glass substrate includes a sodium ion (Na+), this sodium ion is replaced with a potassium ion (Ka+) by the chemically strengthening process. Alternatively, for example, when the glass substrate includes a lithium ion (Li+), this lithium ion may be replaced with a sodium ion (Na+) and/or a potassium ion (Ka+) by the chemically strengthening process. 
     When the transparent substrate is formed of plastic, composition of the plastic is not particularly limited. For example, the transparent substrate may be a polycarbonate substrate. 
     Note that, prior to step S 110 , a step of applying an anti-glare process to the first surface of the transparent substrate may be implemented. The method of the anti-glare process is not particularly limited. For example, the anti-glare process may be a frost process, an etching process, a sandblast process, a lapping process, or a silica-coating process. 
     After the application of the anti-glare process, the first surface of the transparent substrate may have a surface roughness (an arithmetic average roughness Ra) in a range from 0.05 μm to 0.5 μm, for example. 
     Next, a first light beam is irradiated from a side of the second surface of the transparent substrate in a direction that is parallel to a thickness direction of the transparent substrate, specifically, in a direction of an angle θ=0°±0.5° (which is also referred to as a “direction of the angle 0°”, hereinafter). The first light beam passes through the transparent substrate, and the first light beam is irradiated from the first surface. The 0° transmitted light beam that is irradiated in the direction of the angle 0° from the first surface is received, and its brightness is measured. It is referred to as the “brightness of the 0° transmitted light beam.” 
     (Step S 120 ) 
     Next, an angle θ of receiving the light beam that is irradiated from the first surface is varied in a range from −90° to +90°, and a similar operation is executed. In this manner, a brightness distribution of the light beam that passes through the transparent substrate and that is irradiated from the first surface is measured and summed, thereby defining the “brightness of all the transmitted light beams.” 
     (Step S 130 ) 
     Next, the resolution index value T is calculated from the following expression (1):
 
the resolution index value  T =(the brightness of all the transmitted light beams−the brightness of the 0° transmitted light beam)/(the brightness of all the transmitted light beams)   expression (1).
 
     As described below, it has been verified that the resolution index value T correlates with a determination result of the resolution by visual observation of an observer, and that it behaves like a human visual sense. For example, for a transparent substrate whose resolution index value T indicates a large value (close to 1), the resolution is unfavorable, and conversely, for a transparent substrate whose resolution index value T indicates a small value, the resolution is favorable. Accordingly, this resolution index value T can be used as a quantitative index for determining a resolution of a transparent substrate. 
       FIG. 2  schematically shows an example of a measurement device that is used for obtaining the resolution index value T that is represented by the above-described expression (1). 
     As shown in  FIG. 2 , the measurement device  200  includes a light source  250 ; and a detector  270 . A transparent substrate  210  is disposed in the measurement device  200 . The transparent substrate  210  has a first surface  212  and a second surface  214 . The light source  250  emits a first light beam  262  toward the transparent substrate  210 . The detector  270  receives a transmitted light beam  264  that is emitted from the first surface  212 , and detects its brightness. 
     Note that the transparent substrate  210  is arranged such that the second surface  214  is at the side of the light source  250 , and the first surface  212  is at the side of the detector  270 . Thus, the first light beam that is to be detected by the detector  270  is the transmitted light beam  264  that passes through the transparent substrate  210 . Note that, when an anti-glare process is applied to one of the surfaces of the transparent substrate  210 , the surface to which the anti-glare process is applied is the first surface  212  of the transparent substrate  210 . Namely, in this case, the transparent substrate  210  is arranged in the measurement device  200 , so that the surface to which the anti-glare process is applied is at the side of the detector  270 . 
     Further, the first light beam  262  is irradiated at an angle θ that is parallel to the thickness direction of the transparent substrate  210 . Hereinafter, this angle θ is defined to be 0°. Note that, in this application, by considering an error of the measurement device, the range of θ=0°±0.5° is defined to be the angle 0°. 
     In such a measurement device  200 , the first light beam  262  is irradiated from the light source  250  toward the transparent substrate  210 , and the transmitted light beam  264  that is irradiated from the side of the first surface  212  of the transparent substrate  210  is detected by using the detector  270 . In this manner, the 0° transmitted light beam is detected. 
     Next, the angle θ at which the detector  270  receives the transmitted light beam  264  is varied in a range from −90° to +90°, and the similar operation is executed. 
     In this manner, by using the detector  270 , the transmitted light beam  264  that passes through the transparent substrate  210  and that is irradiated from the first surface  212  is detected in the range from −90° to +90°, namely, “all the transmitted light beams” are detected. 
     From the obtained brightness of the 0° transmitted light beam and the obtained brightness of all the transmitted light beams, the resolution index value T of the transparent substrate  210  can be obtained by the above-described expression (1). 
     Note that such a measurement can be easily implemented by using a commercially available goniometer (a goniophotometer). 
     (Regarding the Reflection Image Diffusiveness Index Value) 
       FIG. 3  schematically shows a flow of a method of obtaining a reflection image diffusiveness index value of a transparent substrate, according to an embodiment of the present invention. 
     As shown in  FIG. 3 , the method of obtaining a reflection image diffusiveness index value of a transparent substrate includes 
     (a′) a step of irradiating a second light beam from a side of a first surface of the transparent substrate having the first surface and a second surface in a 45° direction with respect to a thickness direction of the transparent substrate, and measuring brightness of the light beam that is specularly reflected by the first surface (which is also referred to as the “45° specular reflected light beam,” hereinafter) (step S 210 ), 
     (b′) a step of varying a light reception angle for receiving the light beam that is reflected by the first surface in a range from 0° to +90°, measuring brightness distribution of the second light beam that is reflected by the first surface (which is also referred to as “all the reflected light beams,” hereinafter) (step S 220 ), and measuring the brightness of the 45° specular reflected light beam (step S 210 ), and 
     (c′) a step of calculating the reflection image diffusiveness index value R by the following expression (2) (step S 230 ):
 
the reflection image diffusiveness index value  R =(the brightness of all the reflected light beams−the brightness of the 45° specular reflected light beam)/(the brightness of all the reflected light beams)   expression (2).
 
     Hereinafter, each of the steps is explained. 
     (Step S 210 ) 
     First, a transparent substrate is prepared that has a first surface and a second surface that face each other. 
     Since a material and composition of the transparent substrate are the same as those of step S 110  that are described above, they are not explained here. 
     Next, the second light beam is irradiated from the side of the first surface of the prepared transparent substrate in a direction of 45°±0.5° with respect to the thickness direction of the transparent substrate. The second light beam is specularly reflected by the first surface of the transparent substrate. Among the reflected light beams, the 45° specular reflected light beam is received, its brightness is measured, and thereby the “brightness of the 45° specular reflected light beam” is defined. 
     (Step S 220 ) 
     Next, the light reception angle of the reflected light beam that is reflected by the first surface is varied in a range from 0° to +90°, and the same operation is executed. At this time, a brightness distribution of the brightness of the second light beam that is reflected by the first surface of the transparent substrate and that is irradiated from the first surface is measured and summed, and thereby the “brightness of all the reflected light beams” is defined. 
     (Step S 230 ) 
     Next, the reflection image diffusiveness index value R is calculated by the following formula (2):
 
the reflection image diffusiveness index value  R =(the brightness of all the reflected light beams−the brightness of 45° specular reflected light beam)/(the brightness of all the reflected light beams)   expression (2).
 
     As described below, it has been verified that the reflection image diffusiveness index value R correlates with a determination result of the reflection image diffusiveness by visual observation of an observer, and that it behaves like a human visual sense. For example, for a transparent substrate whose reflection image diffusiveness index value R indicates a large value (close to 1), the reflection image diffusiveness is favorable, and conversely, for a transparent substrate whose reflection image diffusiveness index value R indicates a small value, the reflection image diffusiveness is unfavorable. Accordingly, this reflection image diffusiveness index value R can be used as a quantitative index for determining reflection image diffusiveness of a transparent substrate. 
       FIG. 4  schematically shows an example of a measurement device that is used for obtaining the reflection image diffusiveness index value R that is represented by the above-described expression (2). 
     As shown in  FIG. 4 , the measurement device  300  includes a light source  350  and a detector  370 . The transparent substrate  210  is disposed in the measurement device  300 . The transparent substrate  210  has the first surface  212  and the second surface  214 . The light source  350  emits a second light beam  362  toward the transparent substrate  210 . The detector  370  receives a reflected light beam  364  that is reflected by the first surface  212 , and measures its brightness. 
     Note that the transparent substrate  210  is arranged such that the first surface  212  is at the side of the light source  350  and the detector  370 . Thus, the second light beam that is detected by the detector  370  is the reflected light beam  364  that is reflected by the transparent substrate  210 . Further, when an anti-glare process is applied to one of the surfaces of the transparent substrate  210 , the surface to which the anti-glare process is applied is the first surface  212  of the transparent substrate  210 . Namely, in this case, the transparent substrate  210  is arranged in the measurement device  300 , so that the surface to which the anti-glare process is applied is at the side of the light source  350  and the detector  370 . 
     Further, the second light beam  362  is irradiated at an angle φ that is tilted by 45° with respect to the thickness direction of the transparent substrate  210 . Note that, in the present application, by considering an error of the measurement device, a range of 45°±0.5° is defined to be the angle of 45°. 
     In such a measurement device  300 , the second light beam  362  is emitted from the light source  350  toward the transparent substrate  210 , and the reflected light beam  364  that is reflected by the first surface  212  of the transparent substrate  210  is detected by using the detector  370 . In this manner, the “45° specular reflected light beam” is detected. 
     Next, an angle φ at which the detector  370  measures the reflected light beam  364  is varied in a range from 0° to +90°, and the same operation is executed. 
     At this time, the reflected light beam  364  that is reflected by the first surface  212  of the transparent substrate  210  in the range from 0° to +90°, namely, a brightness distribution of all the reflected light beams is detected and summed by using the detector  370 . 
     From the obtained brightness of the 45° specular reflected light beam and the obtained brightness of all the reflected light beams, the reflection image diffusiveness index value R of the transparent substrate  210  can be obtained by the above-described expression (2). 
     Note that such a measurement can be easily implemented by using a commercially available goniometer (a goniophotometer). 
     (Regarding the Sparkle Index Value) 
       FIG. 5  schematically shows a flow of a method of obtaining a sparkle index value of a transparent substrate, according to an embodiment of the present invention. 
     As shown in  FIG. 5 , the method of obtaining a sparkle index value of a transparent substrate includes 
     (a″) a step of disposing a transparent substrate having a first surface and a second surface at a side of a display surface of a display device so that the second surface of the transparent substrate is at a side of the display device (step S 310 ), 
     (b″) a step of taking a photograph of the transparent substrate from the side of the first surface, and obtaining an image (step S 320 ), and 
     (c″) a step of quantifying the sparkle of the transparent substrate by analyzing the image by software EyeScale-4W (a product of I-System Co., Ltd.) and by using an ISC-A value that is output by the software (step S 330 ). 
     Hereinafter, each of the steps is explained in detail. 
     (Step  310 ) 
     First, a transparent substrate is prepared that has a first surface and a second surface that face each other. 
     Since a material and composition of the transparent substrate are the same as those of step S 110  that are described above, they are not explained here. 
     Next, a display device is prepared. The display device is not particularly limited. For example, the display device may be a liquid crystal display (LCD) device, an organic light emitting diode (OLED) device, or a plasma display panel (PDP) device. At a side of a display surface of the display device, a cover may be provided that is for a purpose of preventing damage or the like. 
     Next, the transparent substrate is disposed on the side of the display surface of the display device. At this time, the transparent substrate is disposed on the display surface of the display device, so that the second surface is at the side of the display device. 
     (Step S 320 ) 
     Next, a photograph of the transparent substrate is taken from the side of the first surface in a state in which the display device is turned on (i.e., a state in which an image is displayed), and an image of the transparent substrate that is disposed on the display device is obtained. 
     The image that is displayed on the display device may preferably be an image in a single color (e.g., green), and may preferably be displayed on the whole display screen of the display device. That is for minimizing an effect, such as a difference in appearance due to a difference in the displayed color. 
     For taking the photograph, a digital camera having, at least, a large pixel number (number of pixels) may preferably be used. It may preferably have an area of an image sensor and a number of pixels that are sufficient for determining, at least, a size that is smaller than the surface unevenness of the transparent substrate after applying the anti-glare process and that is smaller than a pixel pitch. 
     An image of the transparent substrate that is captured is input into an analysis device (e.g., a computer). 
     (Step S 330 ) 
     Next, an image of the transparent substrate that is captured is analyzed by analysis software. As the analysis software, the EyeScale-4W (a product of I-System Co. Ltd.) is used. As a result of the analysis, an ISC-A value is output, and this is defined to be a sparkle index value S. 
     As described below, it has been verified that the sparkle index value S correlates with a determination result of the sparkle by visual observation of an observer, and that it behaves like a human visual sense. For example, for a transparent substrate whose sparkle index value S is a large value, the sparkle tends to be significant, and conversely, for a transparent substrate whose sparkle index value S is a small value, the sparkle tends to be suppressed. Accordingly, this sparkle index value S can be used as a quantitative index for determining sparkle of a transparent substrate. 
     By using the above-described resolution index value T, the reflection image diffusiveness index value R, and the sparkle index value S, optical characteristics of a transparent substrate can be quantitatively evaluated. 
     (Evaluation by Three Indexes) 
     Next, there is explained a method of comprehensively evaluating three optical characteristics of a transparent substrate and its effect. 
     (A Case of Evaluation by Two Indexes) 
     First, as a preliminary explanation, a case is explained in which optical characteristics of a transparent substrate are evaluated by using two indexes. 
     First, for a case of simultaneously evaluating a resolution and reflection image diffusiveness of the transparent substrate, a correlation diagram, such as shown in  FIG. 6 , is used. 
       FIG. 6  is a diagram that is obtained by plotting an example of a relationship between the resolution index value T (the horizontal axis) and the reflection image diffusiveness index value R (the vertical axis) that are obtained for various types of transparent substrates. In the figure, as the resolution index value T of the horizontal axis becomes smaller, the resolution of the transparent substrate is enhanced, and as the reflection image diffusiveness index value R of the vertical axis becomes greater, the reflection image diffusiveness of the transparent substrate is enhanced. 
     Note that, in  FIG. 6 , for reference, an ideal area of the transparent substrate that has both a favorable (high) resolution and favorable reflection image diffusiveness is indicated by a circle mark that is indicated as “Ideal.” 
     Here, as before, when a candidate transparent substrate is to be selected among various types of transparent substrates by only considering a single optical characteristic, for example, by only considering the resolution, transparent substrates that are included in the area A that is indicated in  FIG. 6  by the hatching are to be uniformly selected. Namely, with such a method, a transparent substrate whose reflection image diffusiveness is unfavorable may be included in the transparent substrates that are the candidates of the selection. Similarly, when a transparent substrate is to be selected by considering only the reflection image diffusiveness, transparent substrates that are included in the area B that is indicated in  FIG. 6  by the hatching are to be uniformly selected, and a transparent substrate whose resolution is unfavorable may be included in the transparent substrates that are the candidates of the selection. 
     Further, as described above, the resolution and the reflection image diffusiveness are in a tradeoff relationship. Thus, it is substantially impossible to obtain a transparent substrate having both the characteristics, namely, to obtain a transparent substrate that exists in the area that is indicated by the circle mark. Consequently, a proper transparent substrate may not be selected only by separately considering the resolution and the reflection image diffusiveness. 
     In contrast, when the correlation diagram of the resolution index value T and the reflection image diffusiveness index value R, such as shown in  FIG. 6 , is used, a proper transparent substrate can be selected by simultaneously considering both the optical characteristics. Namely, in such a selection method, a transparent substrate can be properly selected, depending on purpose and use. Namely, with respect to the resolution and the reflection image diffusiveness, a transparent substrate can be selected, so that the most favorable characteristics can be achieved. 
     Next,  FIG. 7  shows a diagram that is obtained by plotting an example of a relationship between the sparkle index value S (the horizontal axis) and the reflection image diffusiveness index value R (the vertical axis) that is obtained for various types of transparent substrates. In the figure, as the sparkle index value of the horizontal axis becomes smaller, the sparkle of the transparent substrate is suppressed, and as the reflection image diffusiveness index value R of the vertical axis becomes greater, the reflection image diffusiveness of the transparent substrate is enhanced. 
     Note that, in  FIG. 7 , for reference, an ideal area of the transparent substrate that has both a favorable anti-glare property and favorable reflection image diffusiveness is indicated by a circle mark that is indicated as “Ideal.” 
     For the case of the anti-glare property and the reflection image diffusiveness, when a candidate transparent substrate is to be selected among various types of transparent substrates by considering a single optical property, for example, by only considering the anti-glare property, as before, the transparent substrates that are included in the area C that is indicated in  FIG. 7  by the hatching are to be uniformly selected. Namely, with such a method, a transparent substrate having unfavorable reflection image diffusiveness is to be included in the transparent substrates that are the candidates of the selection. Similarly, when a transparent substrate is to be selected by only considering the reflection image diffusiveness, the transparent substrates that are included in the area D that is indicated in  FIG. 7  by the hatching are to be uniformly selected, and a transparent substrate having an unfavorable anti-glare property is to be included in the transparent substrates that are the candidates of the selection. 
     Further, for the anti-glare property and the reflection image diffusiveness, it is difficult to optimize both the characteristics, and it is substantially impossible to obtain a transparent substrate such that both the characteristics exist in the ideal area that is indicated by the circle mark. Consequently, a proper transparent substrate may not be selected only by separately considering the anti-glare property and the reflection image diffusiveness. 
     In contrast, when the correlation diagram of the sparkle index value S and the reflection image diffusiveness index value R, such as shown in  FIG. 7 , is used, a proper transparent substrate can be selected by simultaneously considering both the optical characteristics. Namely, in such a selection method, a transparent substrate can be properly selected, depending on purpose and use. Namely, with respect to the anti-glare property and the reflection image diffusiveness, a transparent substrate can be selected, so that the most favorable characteristics are achieved. 
     Next,  FIG. 8  shows a diagram that is obtained by plotting an example of a relationship between the resolution index value T (the horizontal axis) and the sparkle index value S (the vertical axis) that is obtained for various types of transparent substrates. In  FIG. 8 , as the resolution index value T of the horizontal axis becomes smaller, the resolution of the transparent substrate is enhanced, and as the sparkle index value S of the vertical axis becomes smaller, the sparkle of the transparent substrate is suppressed (i.e., the anti-glare property is enhanced). 
     Note that, in  FIG. 8 , for reference, an ideal area of the transparent substrate that has both a favorable resolution and a favorable anti-glare property is indicated by a circle mark that is indicated as “Ideal.” 
     For the case of the resolution and the anti-glare property, when a candidate transparent substrate is to be selected among various types of transparent substrates by considering a single optical property, for example, by only considering the resolution, as before, the transparent substrates that are included in the area E that is indicated in  FIG. 8  by the hatching are to be uniformly selected. Namely, with such a method, a transparent substrate having an unfavorable anti-glare property is to be included in the transparent substrates that are the candidates of the selection. Similarly, when a transparent substrate is to be selected by only considering the anti-glare property, the transparent substrates that are included in the area F that is indicated in  FIG. 8  by the hatching are to be uniformly selected, and a transparent substrate having an unfavorable resolution is to be included in the transparent substrates that are the candidates of the selection. 
     In contrast, when the correlation diagram of the resolution index value T and the sparkle index value S, such as shown in  FIG. 8 , is used, a proper transparent substrate can be selected by simultaneously considering both the optical characteristics. Namely, in such a selection method, a transparent substrate can be properly selected, depending on purpose and use. Namely, with respect to the resolution and the anti-glare property, a transparent substrate can be selected, so that the most favorable characteristics can be achieved. 
     (A Case of Evaluation by Three Indexes) 
     Next, a case is explained in which optical characteristics of a transparent substrate are evaluated by using three indexes that are a resolution index value T, a reflection image diffusiveness index value R, and a sparkle index value S. 
     In this case, it suffices, further, to add an axis of a third index value to a correlation diagram that is for two evaluation index values, such as shown in  FIGS. 6 to 8 , that are used in the above-described section (the case of evaluation by two indexes). Namely, optical characteristics of a transparent substrate are to be evaluated in a three-axis space that is spanned by a resolution index value T (e.g., an X-axis), a reflection image diffusiveness index value R (e.g., a Y-axis), and a sparkle index value S (e.g., a Z-axis). 
     For example, by adding the sparkle index value S as the Z-axis to the two-dimensional diagram of the resolution index value T and the reflection image diffusiveness index value R that is shown in  FIG. 6 , the three optical characteristics can be represented at the same time. 
       FIG. 9  is a schematic diagram showing a three-dimensional diagram such that its axes correspond to such three optical characteristics. In the figure, the X-axis is the resolution index value T, the Y-axis is the reflection image diffusiveness index value R, and the Z-axis is the sparkle index value S. 
     Note that, in  FIG. 9 , for reference, a region of an ideal transparent substrate that has a favorable (high) resolution, favorable reflection image diffusiveness, and a favorable anti-glare property is indicated as a region that is surrounded by dotted lines. 
     However, as described above, the relationship between the resolution and the reflection image diffusiveness and the relationship between the anti-glare property and the reflection image diffusiveness are tradeoff relationships. Unfortunately, it is substantially impossible to obtain a transparent substrate that has all the characteristics, namely, a transparent substrate that is included in the region that is surrounded by the dotted lines. 
     However, when a three-dimensional diagram, such as shown in  FIG. 9 , is used, a proper transparent substrate can be selected by comprehensively considering the three optical characteristics, depending on purpose and use. 
     For example, for a transparent substrate that is applied to an optical device that is to be installed inside a vehicle, such as a car navigation system, a high resolution and a high anti-glare property are considered to be particularly important factors. Thus, for such a case, it suffices if a transparent substrate having reflection image diffusiveness that is as favorable as possible (e.g., R≥0.2) is selected, among transparent substrates having small resolution index values T (e.g., T≤0.2) and small sparkle index values S (e.g., S≤60). The portion of  FIG. 9  that is indicated by the hatching shows a region of the candidates for the transparent substrate that is suitable for the optical device that is to be installed inside a vehicle. 
     In this manner, in the method according to the embodiment of the present invention, the three optical characteristics can be comprehensively and quantitatively considered. Thus, a transparent substrate can be more properly selected, depending on purpose and use. 
     Further, in the method according to the present invention, values that are expressed in numerical forms are used as the resolution index value, the reflection image diffusiveness index value, and the sparkle index value. Consequently, the optical characteristics of the resolution, the reflection image diffusiveness, and the sparkle can be objectively and quantitatively determined without depending on a subjective view and prejudice of an observer. 
     (Regarding an Optical Device According to an Embodiment of the Present Invention) 
     Next, there is explained an example of a configuration of an optical device according to an embodiment of the present invention by referring to  FIG. 10 . 
       FIG. 10  schematically shows a cross section of the optical device according to the embodiment of the present invention. 
     As shown in  FIG. 10 , the optical device  500  includes a display device  510  and a transparent substrate  560 . The optical device  500  is a display device with cover glass. 
     The transparent substrate  560  has a first surface  562  and a second surface  564 . The transparent substrate  560  is disposed on a side of a display surface of the display device  510  so that the second surface  564  is at the side of the display device  510 . 
     Note that, in the example of  FIG. 10 , the transparent substrate  560  is directly disposed on the side of the display surface of the display device  510 . However, the transparent substrate  560  may not be in contact with the display device  510 . Another transparent substrate, or a space may be disposed between them. 
     The transparent substrate  560  may be formed of glass (e.g., soda-lime glass or aluminosilicate glass) or plastic (e.g., polycarbonate), for example. Further, when the transparent substrate  560  is formed of glass, a chemically strengthening process may be applied to at least one of the first surface  562  and the second surface  564  of the transparent substrate  560 . By doing this, the strength of the transparent substrate  560  can be enhanced. 
     Further, an anti-glare process may be applied to the first surface  562  of the transparent substrate  560 . 
     The display device  510  includes a display surface (not shown). The transparent substrate  560  is disposed on the display device  510  so as to cover the display surface. Note that the display device  510  can be any device, provided that it has a function for displaying an image on the display surface. A cover may be provided to the display device  510  for a purpose of preventing damage or the like. 
     The display device  510  may be a LCD device, an OLED device, or a PDP device, for example. 
     Note that, in the optical device  500 , the side of the first surface  562  of the transparent substrate  560  is the side which is to be viewed. 
     In the optical device  500 , the transparent substrate  560  has the following optical characteristics: 
     the resolution index value T≤0.2, 
     the reflection image diffusiveness index value R≥0.2, and 
     the sparkle index value S≤60. 
     Note that the resolution index value T, the reflection image diffusiveness index value R, and the sparkle index value S are values that are obtained by the above-described methods, respectively. 
     Especially, the resolution index value T may preferably be less than or equal to 0.3 so as not to adversely affect visibility of a display body. In particular, the resolution index value T may preferably be less than or equal to 0.2 with which it is difficult to perceive a difference in visibility. The reflection image diffusiveness index value R may preferably be greater than or equal to 0.2 with which an outline of a reflection image that tends to be viewed is blurred and the visibility of the display body is enhanced. The sparkle index value may preferably be less than or equal to 60 with which almost no sparkle can be sensed. 
     The transparent substrate  560  that has such optical characteristics has a favorable resolution and a favorable anti-glare property. Thus, light (an image) from a side of the display device  510  can be relatively clearly viewed. 
     Therefore, the optical device  500  that includes the transparent substrate  560  having such optical characteristics is suitable as an optical device that is to be installed inside a vehicle, such as a car navigation system. 
     Working Examples 
     Next, there are explained results of evaluation of resolution, evaluation of reflection image diffusiveness, and evaluation of sparkle that were implemented by using an actual transparent substrate. 
     (Regarding the Evaluation of the Resolution) 
     Various types of transparent substrates were prepared, and these transparent substrates were evaluated by the following method. 
     First, transparent substrates were prepared whose first surfaces were anti-glare processed by corresponding various methods. Each of the transparent substrates was formed of glass. The thickness of the transparent substrates was selected in a range from 0.5 mm to 3.0 mm. 
     Further, a standard test chart that was formed of plastic (a high definition resolution chart I-type: produced by Dai Nippon Printing Co., Ltd.) was prepared. 
     Next, each transparent substrate was disposed above the standard test chart. At that time, each of the transparent substrates was arranged, so that a side of the first surface (i.e., the anti-glare processed surface) of the transparent substrate was at the opposite side of the standard test chart. Note that the space between each of the transparent substrates and the standard test chart was set to 1 cm. 
     Next, the standard test chart was visually observed through the transparent substrate, and the limit of the bars that could be viewed (the number of TV lines) was evaluated. In this manner, a resolution level by visual observation was evaluated for each of the transparent substrates. Note that the maximum value of the TV lines of this standard test chart was 2000. 
     Next, by executing the operations such as shown in the above-described step S 110  to step S 130  by using a goniophotometer (GC5000L: produced by NIPPON DENSHOKU INDUSTRIES CO., LTD), the resolution index value T for each of the transparent substrates was calculated from the expression (1). Note that, at step S 120 , the range of the reception angle of the measurement device was from −85° to +85°, due to the constraints on the configuration as a measurement device. Since the transmitted light values from −90° to −85° and from +85° to +90° were almost zero, this measurement range did not cause any significant impact for calculating the resolution index value T. 
       FIG. 11  shows an example of a relationship between a result of determination of a resolution level by visual observation (the vertical axis) and a resolution index value T (the horizontal axis) that was obtained for each of the transparent substrates. 
     From  FIG. 11 , it can be seen that there is a negative correlation between them. Note that, in the vicinity of the resolution index value T of 0.1, there were several transparent substrates for which the resolution levels by the visual observation were saturated at the maximum value of 2000. 
     This result indicates that the resolution index value T corresponds to a tendency of the determination, by the observer, of the resolution by the visual observation, and therefore suggests that the resolution of the transparent substrate can be determined by using the resolution index value T. In other words, it can be said that, by using the resolution index value T, the resolution of the transparent substrate can be objectively and quantitatively determined. 
     (Regarding the Evaluation of the Reflection Image Diffusiveness) 
     Next, by using the above-described various transparent substrates that were used for the evaluation of the resolution, the reflection image diffusiveness of each of the transparent substrates was evaluated by the following method. 
     First, each of the transparent substrates was visually observed from the side of the first surface (i.e., the anti-glare processed surface), and the reflection image diffusiveness was evaluated in twelve levels that were from level  1  to level  12 . Note that the direction of the observation was the direction of 45° with respect to the thickness direction of the transparent substrate. 
       FIG. 12  collectively shows, for reference, examples of the reflection image diffusiveness that correspond to level  1  to level  12 , respectively. Note that this figure is obtained by separately taking photographs of the transparent substrates that correspond to these levels, respectively. 
     From  FIG. 12 , it can be seen that, along with level  1  to level  12 , the reflection image on the transparent substrate gradually becomes insignificant, namely, the reflection image diffusiveness of the transparent substrate tends to be enhanced. Note that the state of level  1  was obtained in the transparent substrate such that no anti-glare process was applied to each of its surfaces. 
     Next, by executing the operations such as shown in the above-described step S 210  to step S 230  by using a goniophotometer (GC5000L: produced by NIPPON DENSHOKU INDUSTRIES CO., LTD), the reflection image diffusiveness index value R for each of the transparent substrates was calculated from the expression (2). Note that, at step S 220 , a range of the reception angle of the measurement device was from +5° to +85°, due to the constraints on the configuration as a measurement device. Since the reflected light values from 0° to +50 and from +85° to +90° were almost zero, the measurement range did not cause any significant impact for calculating the reflection image diffusiveness index value R. 
       FIG. 13  shows an example of a relationship between a level of the reflection image diffusiveness that was evaluated by visual observation (the vertical axis) and the reflection image diffusiveness index value R (the horizontal axis) that was obtained for each of the transparent substrates. 
     From  FIG. 13 , it can be seen that there is a positive correlation between them. 
     This result indicates that the reflection image diffusiveness index value R corresponds to a tendency of the level of the reflection image diffusiveness by the visual observation by the observer, and therefore suggests that the reflection image diffusiveness of the transparent substrate can be determined by using the reflection image diffusiveness index value R. In other words, it can be said that, by using the reflection image diffusiveness index value R, the reflection image diffusiveness of the transparent substrate can be objectively and quantitatively determined. 
     (Regarding the Evaluation of the Sparkle) 
     Next, by using the above-described various transparent substrates that were used for the evaluation of the resolution, the sparkle of each of the transparent substrates was evaluated by the following method. 
     First, each of the transparent substrates was directly disposed on a side of a display surface of a display device (iPhone 4S (registered trademark)). At that time, each of the transparent substrates was disposed on the side of the display surface of the display device, so that the first surface of each of the transparent substrates (i.e., the anti-glare processed surface) was at the side of the observer. Note that an image that was displayed on the display device was a single-color green image, and the size of the image was set to 7.5 cm×5.1 cm. 
     Next, in this state, each of the transparent substrates was visually observed from the side of the first surface, and the sparkle was evaluated in 11 levels that were from level  0  to level  10 . Level  0  indicates that almost no sparkle can be observed, and level  10  indicates that the sparkle is extremely significant. Further, the values of the levels between them have tendency such that, as the value become greater, the sparkle becomes more significant. 
       FIG. 14  shows, for reference, examples of the transparent substrates that indicate the sparkle of level  0  and the sparkle of level  7 , respectively. Note that the level  0  was obtained for an optical device that included a transparent substrate and a display device. The surfaces of the transparent substrate were not anti-glare processed. 
     Next, by executing the operations such as shown in the above-described step S 320  to step S 330 , an ISC-A value of each of the transparent substrates was obtained by using the software Eyescale-4W (produced by I-System Co., Ltd.), and the sparkle index value S was set to that value. 
       FIG. 15  shows an example of a relationship between the sparkle index value S (the vertical axis) and the level of the sparkle by the visual observation (the horizontal axis). 
     From  FIG. 15 , it can be seen that there is a positive correlation between them. 
     This result indicates that the sparkle index value S corresponds a result of the determination of the sparkle through the visual observation by the observer, and therefore suggests that the sparkle of the transparent substrate can be evaluated by using the sparkle index value S. In other words, it can be said that, by using the sparkle index value S, the sparkle of the transparent substrate can be objectively and quantitatively determined. 
     In this manner, it has been verified that the resolution index value T, the reflection image diffusiveness index value R, and the sparkle index value S can be used as quantitative indexes of the resolution, the reflection image diffusiveness, and the anti-glare property of the transparent substrate, respectively. 
     The present invention can be utilized, for example, for an optical device in which a transparent substrate is disposed on a side of a display surface of a display device, such as a LCD device, an OLED device, and a PDP device.