Patent Publication Number: US-2023152577-A1

Title: Image display apparatus

Description:
BACKGROUND 
     Technical Field 
     The disclosure relates to an image display apparatus suitable for a head mount display or the like for magnifying and observing an image on an image display element via an eyepiece optical system. 
     Description of the Related Art 
     In order to realize a thin and lightweight image display apparatus with a wide view angle, Japanese Patent Laid-Open No. (“JP”) 2021-71602 discloses a configuration that folds an optical path using polarization. 
     However, in the configuration disclosed in JP 2021-71602, an emission angle of light emitted from a peripheral portion of an image display element is so large that color drift (or displacement) occurs in the peripheral portion of the image display element from a central portion of the image display element, and the peripheral portion of the image cannot be observed in correct color. 
     SUMMARY 
     The disclosure provides an image display apparatus that is advantageous in reducing color drift of a peripheral portion of an image display element from a central portion of the image display element. 
     An image display apparatus according to one aspect of the disclosure includes an image display element, and an eyepiece optical system configured to guide light from the image display element to an exit pupil. The eyepiece optical system includes at least one correction element having different characteristics between a central area and a peripheral area different from the central area. When first light is light emitted from a central portion of the image display element, and second light is light emitted from an outermost peripheral portion of the image display element, the characteristics of the at least one correction element are set such that color drift of the second light after pass through the peripheral area from the first light after pass through the central area is less than color drift of the second light before pass through the peripheral area from the first light before pass through the central area. 
     An image display apparatus according to another aspect of the disclosure includes an image display element, and an eyepiece optical system configured to guide light from the image display element to an exit pupil. A white balance is different between a central portion of the image display element and an area different from the central portion of the image display element in the eyepiece optical system such that after first light emitted from the central portion of the image display element and second light emitted from an outermost peripheral portion of the image display element pass through the eyepiece optical system, color drift of the second light from the first light is less than color drift of the second light from the first light before the first light and the second light pass through the eyepiece optical system. 
     Further features of the disclosure will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    explains a head mount display as an example of an image display apparatus according to a first embodiment. 
         FIG.  2    explains a right-eye eyepiece optical system according to the first embodiment. 
         FIG.  3    is an external view of the head mount display according to the first embodiment. 
         FIG.  4    explains the right-eye eyepiece optical system according to the first embodiment. 
         FIG.  5    explains an emission angle of light emitted from an image display element according to the first embodiment. 
         FIG.  6    explains color drift of the image display element in the first embodiment. 
         FIGS.  7 A to  7 C  explain characteristics of a polarizing plate and polarizing beam splitter in the first embodiment. 
         FIG.  8    explains color drift of the eyepiece optical system according to the first embodiment. 
         FIG.  9    explains a head mount display as an example of an image display apparatus according to a second embodiment. 
         FIG.  10    explains a right-eye eyepiece optical system according to the second embodiment. 
         FIG.  11    explains an emission angle of light emitted from the image display element according to the second embodiment. 
         FIG.  12    explains color drift of the image display element according to the second embodiment. 
         FIGS.  13 A and  13 B  explain a characteristic of the polarizing plate according to the second embodiment. 
         FIG.  14    explains color drift of the eyepiece optical system according to the second embodiment. 
         FIGS.  15 A and  15 B  illustrate an example of a characteristic of the polarizing plate according to the second embodiment. 
         FIG.  16    explains a head mount display as an example of an image display apparatus according to a third embodiment. 
         FIG.  17    explains a right-eye eyepiece optical system according to the third embodiment. 
         FIG.  18    explains a characteristic of a polarizing plate according to the third embodiment. 
         FIG.  19    explains a head mount display as an example of an image display apparatus according to a fourth embodiment. 
         FIG.  20    explains a head mount display as another example of the image display apparatus according to the fourth embodiment. 
         FIG.  21    explains a head mount display as an example of an image display apparatus according to a fifth embodiment. 
         FIG.  22    illustrates the head mount display according to the fifth embodiment connected to a PC. 
         FIG.  23    explains a right-eye eyepiece optical system according to a sixth embodiment. 
         FIG.  24    explains a characteristic of a minus filter according to the sixth embodiment. 
         FIG.  25    explains the characteristic of the minus filter according to the sixth embodiment. 
         FIG.  26    explains an emission spectrum of an image display element according to the sixth embodiment. 
         FIG.  27    explains the emission spectrum of the image display element according to the sixth embodiment. 
         FIG.  28    explains the emission spectrum of the image display element according to the sixth embodiment. 
         FIG.  29    explains a right-eye eyepiece optical system according to a seventh embodiment. 
         FIG.  30    explains a characteristic of an infrared (IR) (cut) filter according to the seventh embodiment. 
         FIG.  31    explains the characteristic of the IR cut filter according to the seventh embodiment. 
         FIG.  32    explains an emission spectrum of an image display element according to the seventh embodiment. 
         FIG.  33    explains the emission spectrum of the image display element according to the seventh embodiment. 
         FIG.  34    explains the emission spectrum of the image display element according to the seventh embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Referring now to the accompanying drawings, a detailed description will be given of embodiments according to the disclosure. Corresponding elements in respective figures will be designated by the same reference numerals, and a duplicate description thereof will be omitted. 
     First Embodiment 
       FIG.  1    explains a head mount display (HMD hereinafter)  101  as an example of an image display apparatus according to this embodiment. The HMD  101  includes lenses  104 ,  105 ,  106 , and  107 , a right-eye image display element  108 , and a left-eye image display element  109 . The lenses  104  and  105  constitute a right-eye eyepiece optical system, and the lenses  106  and  107  constitute a left-eye eyepiece optical system. Each of the right-eye image display element  108  and the left-eye image display element  109  is an organic Electro Luminescence (“EL”) display. 
     The right-eye eyepiece optical system magnifies and projects the original image displayed on the right-eye image display element  108  as a virtual image and guides it to a right eye  102  of an observer. The left-eye eyepiece optical system magnifies and projects the original image displayed on the left-eye image display element  109  as a virtual image and guides it to a left eye  103  of the observer. Each of the right-eye eyepiece optical system and the left-eye eyepiece optical system has a focal length of 12 mm, a horizontal display view angle of 45°, a vertical display view angle of 34°, and a diagonal display view angle of 54°. A distance (eye relief) between the HMD  101  and each eyeball of the observer is 18 mm. 
     The eyepiece optical system according to this embodiment has a configuration that folds an optical path using polarization. The optical path of the eyepiece optical system will be described with reference to the right-eye eyepiece optical system.  FIG.  2    explains the right-eye eyepiece optical system according to this embodiment. The right-eye eyepiece optical system includes, in order from the side of the right-eye image display element  108 , a polarizing plate (first polarizing plate)  110 , a phase plate (first phase plate)  111 , the lenses  105  and  104 , a phase plate (second phase plate)  113 , and a polarizing beam splitter (referred to as PBS hereinafter)  114 . A half-mirror  112  is deposited on the surface of the lens  104  on the side of the lens  105 . The surface on which the half-mirror  112  is vapor-deposited acts as a half-transmissive reflective surface (half-transmissive reflective film). Each of the phase plate  113  and PBS  114  has a planar shape. Each of the phase plates  111  and  113  is a quarter waveplate having a phase difference of λ/4. 
     In this embodiment, a slow axis of the phase plate  111  is tilted by 45° to a polarization direction transmitting through the polarizing plate  110 , a slow axis of the phase plate  113  is tilted by −45° to the polarization direction transmitting through the polarizing plate  110 , and the polarization direction transmitting through the polarizing plate  110  and the polarization direction transmitting through the PBS  114  are orthogonal to each other. In such a configuration, the light emitted from the right-eye image display element  108  passes through the polarizing plate  110 , becomes linearly polarized light, passes through the phase plate  111 , and becomes circularly polarized light. Thereafter, the polarized light passes through the half-mirror  112 , passes through the phase plate  113 , and becomes linearly polarized light. Since the polarization direction of the linearly polarized light is orthogonal to the polarization direction transmitting through the PBS  114 , the linearly polarized light is reflected by the PBS  114 , passes through the phase plate  113 , and becomes circularly polarized light. Thereafter, this light is reflected by the half-mirror  112 , then transmits through the phase plate  113 , and becomes linearly polarized light. Since the polarization direction of the linearly polarized light coincides with the polarization direction transmitting through the PBS  114 , the linearly polarized light is guided to the right eye  102  of the observer through the PBS  114 . A similar optical path is formed in the left-eye eyepiece optical system. 
     The thus-configured eyepiece optical system that folds the optical path using polarization as in this embodiment can make the HMD  101  thinner, reduce the focal length of the eyepiece optical system, and thus enable an image to be observed at a wide view angle. 
       FIG.  3    is an external view of the HMD  101 . The HMD  101  is a head mount type image display apparatus and thus lightweight is demanded. Therefore, each lens in the eyepiece optical system may be made of a resin having a specific density smaller than that of glass. This embodiment improves the aberration correcting effect using the lenses  104  and  106  each made of a resin and configured as a plano-convex aspherical lens. Each of the lenses  105  and  107  is made of a resin and configured as a double-sided aspherical lens. 
       FIG.  4    explains the right-eye eyepiece optical system according to this embodiment. In the right-eye eyepiece optical system according to this embodiment, the position of the exit pupil is set to a position of 28 mm, which is made by adding the eye relief (=18 mm) and a rotation radius (=10 mm) of the eyeball, and an exit pupil diameter is set to 6 mm. Thereby, even in a case where the eyeball is rotated to observe an image up, down, left, and right, the light in that direction enters the eyeball. The eye relief may be 15 mm or longer so that the observer wearing glasses can wear the HMD  101 . As the eye relief becomes longer, the outer diameter of each lens becomes larger and the HMD  101  becomes larger. Therefore, the eye relief may be 25 mm or less. 
     Since the eyepiece optical system according to this embodiment has a short focal length and is thin, the emission angle of light emitted from the peripheral portion of the image display element to the exit pupil of the eyepiece optical system is large, as understood from  FIG.  4   . The peripheral portion is an area outside the central portion of the right-eye image display element  108 .  FIG.  5    explains the emission angle of the light emitted from the image display element according to this embodiment, and illustrates a relationship between the display view angle and the emission angle of the light emitted from the image display element to the exit pupil. The emission angle of the light emitted from the image display element is the emission angle of the principal ray guided to the center of the exit pupil. As illustrated in  FIG.  5   , as the display view angle increases, the emission angle increases, the emission angle of the light emitted from the horizontal end of the image display element is 37 degrees, and the emission angle of the light emitted from the vertical end is 29 degrees. In order to shorten the focal length of the eyepiece optical system and to make thinner the HMD  101 , the emission angle of the light emitted from the peripheral portion of the image display element to the center of the exit pupil may be 30 degrees or larger. 
     As the emission angle of the light emitted from the image display element becomes large, color drift occurs due to the interference of the light emitting layer of the organic EL and the influence of the color mixing of the color filter. In this embodiment, as the emission angle of the light emitted from the image display element increases, the chromaticity changes toward red and color drift occurs, as illustrated in  FIG.  6   . Therefore, in a case where the observer observes a white image, red is observed in the peripheral portion of the image and thus the image cannot be observed naturally. 
     Accordingly, this embodiment makes at least one of the polarizing plate  110  and the PBS  114  function as a correction element having different characteristics between the central portion and an area (peripheral portion) different from the central portion. In a case where the polarizing plate  110  functions as the correction element, the polarizing plate  110  has such a characteristic that the spectral transmittance is different between the central portion and the peripheral portion. In a case where the PBS  114  functions as the correction element, the PBS  114  has such a characteristic that at least one of the spectral transmittance and the spectral reflectance is different between the central portion and the peripheral portion. Thereby, after the first light emitted from the central portion of the right-eye image display element  108  and the second light emitted from the outermost peripheral portion pass through the right-eye eyepiece optical system, color drift of the second light from the first light can be made less than color drift of the second light from the first light before the first light and the second light pass through the right-eye eyepiece optical system. That is, the color drift in the peripheral portion of the right-eye image display element  108  can be canceled.  FIGS.  7 A to  7 C  explain the characteristics of the polarizing plate  110  and the PBS  114 .  FIG.  7 A  illustrates the transmittance (spectral transmittance) of the polarizing plate  110  to light at an incident angle of 0 degrees for each wavelength.  FIG.  7 B  illustrates the reflectance (spectral reflectance) of PBS  114  to the light at an incident angle of 0 degrees for each wavelength.  FIG.  7 C  illustrates the transmittance (spectral transmittance) of PBS  114  to the light at an incident angle of 0 degrees for each wavelength. The blue dominant wavelength of the image display element is 450 nm, the green dominant wavelength is 525 nm, and the red dominant wavelength is 610 nm.  FIGS.  7 A to  7 C  illustrate the transmittance and reflectance at each wavelength. The blue wavelength (first wavelength) may be included in a range from 430 nm to 480 nm. The green wavelength (second wavelength) may be included in a range from 520 nm to 570 nm. The red wavelength (third wavelength) may be included in a range from 600 nm to 650 nm. In a case where the spectral transmittance is made different between the central portion and the peripheral portion, the change may be made unnoticeable and gradual. 
     As illustrated in  FIG.  7 A , the transmittances to blue and green lights of the polarizing plate  110  little change even if the incident angle increases, but the transmittance to red light of the polarizing plate  110  decreases as the incident angle increases. The reflectance and transmittance of the PBS  114  show similar tendencies. The light emitted from the peripheral portion of the right-eye image display element  108  has a large emission angle and large incident angles on the polarizing plate  110  and the PBS  114 . Thus, as the red color-drifted light emitted from the peripheral portion of the right-eye image display element  108  passes through the polarizing plate  110  and is reflected and transmits through the PBS  114 , the intensity of its red color and its color drift can be reduced. 
       FIG.  8    explains the color drift of the eyepiece optical system, and illustrates a relationship between the emission angle and the chromaticity after the light passes through the eyepiece optical system. With the image display element alone, as the emission angle increases, the chromaticity changes toward red as illustrated in  FIG.  6   , but in a case where the light passes through the eyepiece optical system, the chromaticity change becomes smaller as illustrated in  FIG.  8    and the color drift can be reduced. In this embodiment, the chromaticity difference x is 0.1 and the chromaticity difference y is 0.055 on the chromaticity coordinate system (chromaticity diagram) as the emission angle increases in the image display element alone, but the chromaticity difference x can be reduced down to 0.04 and the chromaticity difference y can be reduced down to 0.015 after the light passes through the eyepiece optical system. In order for the observer to hardly notice color drift and naturally observe an image, the chromaticity difference x may be 0.05 or less and the chromaticity difference y may be 0.05 or less on the chromaticity coordinate system. 
     In this embodiment, a wavelength at which a ratio of the intensity of the light emitted from the peripheral portion of the image display element to the exit pupil to the intensity of the light emitted from the central portion of the image display element to the exit pupil becomes maximum is 610 nm that is the red dominant wavelength, and red color drift occurs as the emission angle increases. Accordingly, color drift of the image display element can be canceled by setting to red, among the red, green, and blue wavelengths, a wavelength at which the transmittance of the polarizing plate  110  and the transmittance and reflectance of the PBS  114  are minimum. Thus, the color drift of the peripheral portion of the right-eye image display element  108  from the central portion of the right-eye image display element  108  is reduced by such characteristics of the polarizing plate  110  and the PBS  114  as the color correction elements that the spectral transmittance is different between each central portion and each peripheral portion of the polarizing plate  110  and the PBS  114 , and thereby the observer can observe a more natural image. 
     The right-eye image display element  108  according to this embodiment has an emission angle of 37 degrees of the light emitted from its horizontal end and half a horizontal display view angle of 22.5 degrees, and an incident angle on the polarizing plate  110  is larger than that on the PBS  114 . Thus, the emission angle characteristic of the right-eye image display element  108  may be canceled by the characteristic of the polarizing plate  110  close to that of the right-eye image display element  108 . 
     In the optical path of the right-eye eyepiece optical system, the PBS  114  acts twice by reflection and transmission. Therefore, if the emission angle characteristic of the right-eye image display element  108  is canceled by the characteristic of the PBS  114 , the effect of reducing color drift is high. In a case where a color drift direction is different between the horizontal direction and the vertical direction due to the arrangement of the color filters of the right-eye image display element  108 , the characteristics of the polarizing plate  110  and the PBS  114  may be changed in the horizontal direction and the vertical direction. In a case where a large emission angle of the light emitted from the right-eye image display element  108  causes red color drift in the horizontal direction and blue color drift in the vertical direction, the characteristic in the horizontal direction of the polarizing plate  110  may be set such that the transmittance of red is low and the characteristic in the vertical direction of the polarizing plate  110  may be set such that the transmittance of blue is low. 
     This embodiment reduces the chromaticity drift in the case of the large emission angle of the light emitted from the right-eye image display element  108 , but may reduce drift of a color difference ΔE*. In that case, the color difference ΔE* may be 15 or less. 
     In this embodiment, the surface on which the half-mirror  112  of the lens  104  is deposited is a convex surface that is convex toward the right-eye image display element  108 . Depositing the half-mirror  112  on the convex surface can realize a wide view angle while thinning the HMD  101 . Forming the convex surface on which the half-mirror  112  is vapor-deposited into an aspherical shape can enhance the aberration correcting effect. 
     In this embodiment, the lenses  105  and  107  are resin lenses, but they may be glass lenses because the outer shape is small and the influence of the weight increase is small. The very little birefringence of the glass lens can provide high-quality image observation. 
     In order to reduce ghost light of the external light and to increase the contrast of the observed image, a polarizing plate may be disposed between the PBS  114  and each eyeball of the observer. 
     Since this embodiment compromises a longer eye relief and the thinner HMD  101 , the surface on the side of the right eye  102  of the observer on which the phase plate  113  and the PBS  114  of the lens  104  are disposed is set to be planar. In a case where the surface of the lens  104  on the side of the observer is concave toward the observer, the lens  104  becomes thick in order to secure the eye relief in the peripheral portion. In addition, the convex surface makes thicker the lens  104  in order to secure the thickness of the lens edge portion. 
     In this embodiment, each of the phase plates  111  and  113  is a quarter waveplate having a phase difference of λ/4, but the phase difference may shift from λ/4 so as to cancel the birefringences of the lenses  104  and  105 . In this case, a sum of the phase differences of the lens  104  and the phase plate  113  may be 3λ/20 or more and 7λ/20 or less. In addition, a sum of the phase differences of the lens  105  and the phase plate  111  may be 3λ/20 or more and 7λ/20 or less. If each value is out of the above range, the intensity of ghost light will increase and a more natural image cannot be observed. 
     In this embodiment, the right-eye image display element  108  emits unpolarized light as the organic EL, but may emit linearly polarized light as a liquid crystal display. In this case, the polarizing plate  110  becomes unnecessary, and the HMD  101  can be made thinner and the number of components can be reduced. 
     In this embodiment, the eyepiece optical system uses the PBS  114  that switches transmission and reflection in the direction of the linearly polarized light, but may use a PBS that switches transmission and reflection in the direction of the circularly polarized light. In this case, the phase plate  113  becomes unnecessary, and the HMD  101  can be made thinner and the number of components can be reduced. 
     Second Embodiment 
       FIG.  9    explains a head mount display (HMD hereinafter)  201  as an example of the image display apparatus according to this embodiment. The HMD  201  includes lenses  204 ,  205 ,  206 , and  207 , a right-eye image display element  208 , and a left-eye image display element  209 . The lenses  204  and  205  constitute a right-eye eyepiece optical system, and the lenses  206  and  207  constitute a left-eye eyepiece optical system. The lenses  204  and  205  and the lenses  206  and  207  are cemented lenses and can be easily held. Each of the right-eye image display element  208  and the left-eye image display element  209  is an organic EL display. 
     The right-eye eyepiece optical system magnifies and projects the original image displayed on the right-eye image display element  208  as a virtual image and guides it to a right eye  202  of the observer. The left-eye eyepiece optical system magnifies and projects the original image displayed on the left-eye image display element  209  as a virtual image and guides it to a left eye  203  of the observer. Each of the right-eye eyepiece optical system and the left-eye eyepiece optical system has a focal length of 13 mm, a horizontal display view angle of 60°, a vertical display view angle of 60°, and a diagonal display view angle of 78°. A distance (eye relief) between the HMD  201  and each eyeball of the observer is 20 mm. 
     The eyepiece optical system according to this embodiment has a configuration that folds an optical path using polarization. The optical path of the eyepiece optical system will be described with reference to the right-eye eyepiece optical system.  FIG.  10    explains the right-eye eyepiece optical system according to this embodiment. The right-eye eyepiece optical system includes, in order from the side of the right-eye image display element  208 , a polarizing plate (first polarizing plate)  210 , a phase plate (first phase plate)  211 , the lenses  205  and  204 , and a phase plate (second phase plate)  213 , a polarizing beam splitter (referred to as PBS hereinafter)  214 , and a polarizing plate (second polarizing plate)  215 . A half-mirror  212  is deposited on the surface on the side of the lens  205  of the lens  204 . The surface on which the half-mirror  212  is deposited acts as a half-transmissive reflective surface. Each of the phase plate  213 , the PBS 214 , and polarizing plate  215  has a planar shape. Each of the phase plates  211  and  213  is a quarter waveplate having a phase difference of λ/4. 
     In this embodiment, a slow axis of the phase plate  211  is tilted by 45° to the polarization direction transmitting through the polarizing plate  210 , a slow axis of the phase plate  213  is tilted by −45° to the polarization direction transmitting through the polarizing plate  210 , and the polarization direction transmitting through the polarizing plate  210  and the polarization direction transmitting through the PBS  214  are orthogonal to each other. The polarization direction transmitting through the PBS  214  and the polarization direction transmitting through the polarizing plate  215  are equal to each other. In such a configuration, the light emitted from the right-eye image display element  208  passes through the polarizing plate  210 , becomes linearly polarized light, passes through the phase plate  211 , and becomes circularly polarized light. Thereafter, the polarized light passes through the half-mirror  212 , then passes through the phase plate  213 , and becomes linearly polarized light. Since the polarization direction of the linearly polarized light is orthogonal to the polarization direction transmitting through the PBS  214 , the linearly polarized light is reflected by the PBS  214 , passes through the phase plate  213 , and becomes circularly polarized light. Thereafter, this light is reflected by the half-mirror  212 , then transmits through the phase plate  213 , and becomes linearly polarized light. Since the polarization direction coincides with the polarization direction transmitting through the PBS  214 , the linearly polarized light is guided to the right eye  202  of the observer through the PBS  214  and the polarizing plate  215 . A similar optical path is formed in the left-eye eyepiece optical system. 
     This embodiment disposes the polarizing plate  215  between the PBS  214  and the right eye  202  of the observer in order to reduce the ghost light of the external light and to increase the contrast of an observed image. 
     The thus-configured eyepiece optical system that folds the optical path using polarization as in this embodiment can make the HMD  201  thinner, reduce the focal length of the eyepiece optical system, and thus enable an image to be observed at a wide view angle. 
     The HMD 201  is a head mount type image display apparatus, and thus lightweight may be demanded. Therefore, each lens in the eyepiece optical system may be made of a resin having a specific density smaller than that of glass. This embodiment improves the aberration correcting effect using the lenses  204 ,  205 ,  206 , and  207  each made of a resin and configured as an aspherical lens. 
     In the eyepiece optical system according to this embodiment, the position of the exit pupil is set to a position of 30 mm, which is made by adding the eye relief (=20 mm) and a rotation radius (=10 mm) of the eyeball, and an exit pupil diameter is set to 6 mm. Thereby, even in a case where the eyeball is rotated to observe an image up, down, left, and right, the light in that direction enters the eyeball. The eye relief may be 15 mm or longer so that the observer wearing glasses can wear the HMD  201 . As the eye relief becomes longer, the outer diameter of each lens becomes larger and the HMD  201  becomes larger. Therefore, the eye relief may be 25 mm or less. 
     Since the eyepiece optical system according to this embodiment has a short focal length and is thin, the emission angle of light emitted from the peripheral portion of the image display element to the exit pupil of the eyepiece optical system is large.  FIG.  11    explains the emission angle of the light emitted from the image display element according to this embodiment, and illustrates a relationship between the display view angle and the emission angle of the light emitted from the image display element to the exit pupil. The emission angle of the light emitted from the image display element is the emission angle of the principal ray guided to the center of the exit pupil. As illustrated in  FIG.  11   , as the display view angle increases, the emission angle increases, and both emission angles of the light emitted from the horizontal end and the vertical end of the image display element are 47 degrees. In order to shorten the focal length of the eyepiece optical system and to make thinner the HMD  201 , the emission angle of the light emitted from the image display element to the center of the exit pupil may be 30 degrees or larger. 
     As the emission angle of the light emitted from the image display element increases, color drift occurs due to the interference of the light emitting layer of the organic EL and the influence of the color mixing of the color filter. In this embodiment, as the emission angle of the light emitted from the image display element increases, the chromaticity changes toward blue and color drift occurs, as illustrated in  FIG.  12   . Therefore, in a case where the observer observes a white image, blue is observed in the peripheral portion of the image and thus the image cannot be observed naturally. 
     Accordingly, this embodiment makes at least one of the polarizing plates  210  and  215  function as a correction element having different characteristics between the central portion and an area (peripheral portion) different from the central portion. More specifically, at least one of the polarizing plates  210  and  215  has such a characteristic that the spectral transmittance is different between the central portion and the peripheral portion. Thereby, after the first light emitted from the central portion of the right-eye image display element  208  and the second light emitted from the outermost peripheral portion of the right-eye image display element  208  pass through the right-eye eyepiece optical system, color drift of the second light from the first light can be made less than color drift of the second light from the first light before the first light and the second light pass through the right-eye eyepiece optical system. That is, the color drift in the peripheral portion of the right-eye image display element  208  can be canceled.  FIGS.  13 A and  13 B  explain the characteristics of the polarizing plates  210  and  215 .  FIGS.  13 A and  13 B  illustrate the transmittances (spectral transmittance) of the polarizing plates  210  and  215  to light at an incident angle of 0 degrees for each wavelength, respectively. The blue dominant wavelength of the image display element is 470 nm, the green dominant wavelength is 545 nm, and the red dominant wavelength is 605 nm.  FIGS.  13 A and  13 B  illustrate the transmittances at each wavelength. The blue wavelength (first wavelength) may be included in a range from 430 nm to 480 nm. The green wavelength (second wavelength) may be included in a range from 520 nm to 570 nm. The red wavelength (third wavelength) may be included in a range from 600 nm to 650 nm. 
     As illustrated in  FIG.  13 A , the transmittances to green and red lights of the polarizing plate  210  change little even if the incident angle increases, but the transmittance to blue light decreases as the incident angle increases. The transmittance of the polarizing plate  215  shows a similar tendency. The light emitted from the peripheral portion of the right-eye image display element  208  has a large emission angle and large incident angles on the polarizing plates  210  and  215 . Thus, as the blue color-drifted light emitted from the peripheral portion of the right-eye image display element  208  passes through the polarizing plates  210  and  215 , the intensity of the blue color and its color drift can be reduced. 
       FIG.  14    explains the color drift of the eyepiece optical system, and illustrates a relationship between the emission angle and the chromaticity after the light passes through the eyepiece optical system. With the image display element alone, as the emission angle increases, the chromaticity changes toward blue as illustrated in  FIG.  12   , but in a case where the light passes through the eyepiece optical system, the chromaticity change becomes smaller as illustrated in  FIG.  14    and the color drift can be reduced. In this embodiment, the chromaticity difference x is 0.055 and the chromaticity difference y is 0.07 on the chromaticity coordinate system as the emission angle increases in the image display element alone, but the chromaticity difference x can be reduced down to 0.015 and the chromaticity difference y can be reduced down to 0.02 after the light passes through the eyepiece optical system. In order for the observer to hardly notice color drift and naturally observe an image, the chromaticity difference x may be 0.05 or less and the chromaticity difference y may be 0.05 or less on the chromaticity coordinate system. 
     In this embodiment, a wavelength at which a ratio of the intensity of the light emitted from the peripheral portion of the image display element to the exit pupil to the intensity of the light emitted from the central portion of the image display element to the exit pupil becomes maximum is 470 nm that is a blue dominant wavelength, and blue color drift occurs as the emission angle increases. Accordingly, color drift of the image display element can be canceled by setting to blue, among the red, green, and blue wavelengths, a wavelength at which the transmittances of the polarizing plates  210  and  215  are minimum. Thus, the color drift of the peripheral portion of the image display element from the central portion of the image display element is reduced by such characteristics of the polarizing plates  210  and  215  as the color correction elements that the spectral transmittance is different between each central portion and each peripheral portion, and thereby the observer can observe a more natural image. 
     The right-eye eyepiece optical system according to this embodiment has an emission angle of 47 degrees of the light emitted from the horizontal end of the right-eye image display element  208  and half a horizontal display view angle of 30 degrees, and an incident angle on the polarizing plate  210  is larger than that on the polarizing plate  215 . Thus, the emission angle characteristic of the image display element may be canceled by the characteristic of the polarizing plate  210  close to the right-eye image display element  208 . The half display view angle is larger than the emission angle of the light emitted from the horizontal end of the right-eye image display element  208 , and the incident angle on the polarizing plate  215  may be larger than the incident angle on the polarizing plate  210 . In this case, the emission angle characteristic of the right-eye image display element  208  may be canceled by the characteristic of the polarizing plate  215 . 
     In this embodiment, the color drift caused by the emission angle of the light emitted from the image display element is canceled by the characteristic of the polarizing plate, but may be canceled by the characteristic of the phase plate. In that case, in order to change the transmittance and reflectance in a case where the polarizing plate or PBS is combined, a transmittance (spectral transmittance) for each wavelength of the phase plate may be changed, or a phase amount for each wavelength of the phase plate may be changed. 
     The color drift caused by the emission angle of the light emitted from the image display element may be canceled by the characteristic in which at least one of the spectral transmittance and the spectral reflectance is different between the central portion of the half-mirror and an area different from the central portion of the half-mirror. In that case, as illustrated in  FIG.  9   , an incident-angle difference during light transmission through the half-mirror between the light emitted from the central portion of the image display element and the light emitted from the peripheral portion of the image display element is large, but an incident-angle difference during light reflection on the half-mirror is small. Thus, the color drift may be canceled by the characteristic during the light transmission through the half-mirror. 
     In this embodiment, a large emission angle of the light emitted from the image display element causes color drifts to blue regardless of the emission direction of the light. Since the colors to be drifted are the same, it is unnecessary to change the characteristic of the polarizing plate according to the incident direction. Since the colors to be drifted are blue, which is a color having low human luminous efficiency, the color drift felt by the observer is small even if the intensity of the light emitted from the image display element becomes higher. 
     In a case where the polarizing plate  210  has characteristics that are different between the horizontal direction and the vertical direction, the human visual field in the horizontal direction is wider than that in the vertical direction and the observer is likely to notice color drift in the horizontal direction, so that the characteristic that makes smaller the color drift in the horizontal direction may be used. For example, the characteristic in the horizontal direction may be set as illustrated in  FIG.  15 A  having a high blue canceling effect, and the characteristic in the vertical direction may be set as illustrated in  FIG.  15 B . In a case where the polarizing plate  215  has characteristics that are different between the horizontal direction and the vertical direction, the characteristics may be set so as to make smaller the color drift in the horizontal direction. 
     This embodiment reduces the chromaticity drift in a case of the large emission angle of the light emitted from the right-eye image display element  208 , but may set the characteristics of the polarizing plates  210  and  215  so as to reduce the drift of the color difference ΔE*. In that case, the absolute values of the transmittance and the reflectance for each color may be changed. 
     In this embodiment, the surface on which the half-mirror  112  of the lens  204  is deposited is a convex surface that is convex toward the right-eye image display element  208 . Depositing the half-mirror  212  on the convex surface can realize a wide view angle while thinning the HMD  201 . Forming the convex surface on which the half-mirror  212  is vapor-deposited into an aspherical shape can enhance the aberration correcting effect. 
     In this embodiment, since the lenses  204  and  205  are cemented, the surface on which the half-mirror  212  is vapor-deposited may be the surface on the side of the right eye  202  of the observer of the lens  205 . Even in that case, the surface on which the half-mirror  212  is vapor-deposited is a convex surface that is convex toward the right-eye image display element  208 . 
     Since this embodiment compromises a longer eye relief and the thinner HMD  201 , the surface on the side of the right eye  202  of the observer on which the phase plate  213  and the PBS  214  of the lens  204  are formed is set to be planar. 
     In this embodiment, each of the phase plates  211  and  213  is a quarter waveplate having a phase difference of λ/4, but the phase difference may shift from λ/4 so as to cancel the birefringences of the lenses  204  and  205 . In this case, a sum of the phase differences of the lens  204  and the phase plate  213  may be 3λ/20 or more and 7λ/20 or less. In addition, a sum of the phase differences of the lens  205  and the phase plate  211  may be 3λ/20 or more and 7λ/20 or less. If each value is out of the above range, the intensity of ghost light will increase and a more natural image cannot be observed. 
     Third Embodiment 
       FIG.  16    explains a head mount display (HMD hereinafter)  301  as an example of the image display apparatus according to this embodiment. The HMD  301  includes lenses  304 ,  305 ,  306 , and  307 , a right-eye image display element  308 , and a left-eye image display element  309 . The lenses  304  and  305  constitute a right-eye eyepiece optical system, and the lenses  306  and  307  constitute a left-eye eyepiece optical system. The lenses  304  and  305  and the lenses  306  and  307  are cemented lenses and can be easily held. Each of the right-eye image display element  308  and the left-eye image display element  309  is an organic EL display. 
     The right-eye eyepiece optical system magnifies and projects the original image displayed on the right-eye image display element  308  as a virtual image and guides it to a right eye  302  of the observer. The left-eye eyepiece optical system magnifies and projects the original image displayed on the left-eye image display element  309  as a virtual image and guides it to a left eye  303  of the observer. Each of the right-eye eyepiece optical system and the left-eye eyepiece optical system has a focal length of 13 mm, a horizontal display view angle of 60°, a vertical display view angle of 60°, and a diagonal display view angle of 78°. A distance (eye relief) between the HMD  301  and the observer&#39;s eyeball is 20 mm. 
     The eyepiece optical system according to this embodiment has a configuration that folds an optical path using polarization. The optical path of the eyepiece optical system will be described with reference to the right-eye eyepiece optical system.  FIG.  17    explains the right-eye eyepiece optical system according to this embodiment. The right-eye eyepiece optical system includes, in this order from the side of the right-eye image display element  308 , a polarizing plate (first polarizing plate)  310 , a phase plate (first phase plate)  311 , the lenses  305  and  304 , a phase plate (second phase plate)  313 , a polarizing beam splitter (hereinafter referred to as PBS)  314 , and a polarizing plate (second polarizing plate)  315 . A half-mirror  312  is deposited on the surface on the side of the lens  305  of the lens  304 . The surface on which the half-mirror  312  is deposited acts as a half-transmissive reflective surface. Each of the phase plate  313 , PBS  314 , and polarizing plate  315  has a planar shape. Each of the phase plates  311  and  313  is a quarter waveplate having a phase difference of λ/4. Since the optical path of the eyepiece optical system is similar to that of the first or second embodiment, a description thereof will be omitted. 
     Since the eyepiece optical system according to this embodiment has a short focal length and is thin, the emission angle of light emitted from the peripheral portion of the image display element to the exit pupil of the eyepiece optical system is large and both emission angles of the light emitted from the horizontal end and the vertical end of the image display element are 47 degrees. In order to shorten the focal length of the eyepiece optical system and to make thinner the HMD  301 , the emission angle of the light emitted from the peripheral portion of the image display element to the center of the exit pupil may be 30 degrees or more. 
     As the emission angle of the light emitted from the image display element increases, color drift occurs due to the interference of the light emitting layer of the organic EL and the influence of the color mixing of the color filter. In this embodiment, as the emission angle of the light emitted from the image display element increases, blue color drift occurs at the right end of the displayed screen and red color drift occurs at the left end due to the color mixing of the color filters. Therefore, in a case where the observer observes a white image, red or blue is observed in the peripheral portion of the image and thus the image cannot be observed naturally. 
     Accordingly, this embodiment makes the polarizing plate  310  function as a correction element having different characteristics between the central portion and an area (peripheral portion) different than the central portion. More specifically, the polarizing plate  310  has such a characteristic that the spectral transmittance is different among the central portion, the right end of the peripheral portion, and the left end of the peripheral portion. Thereby, after the first light emitted from the central portion of the right-eye image display element  308  and the second light emitted from the outermost peripheral portion of the right-eye image display element  308  pass through the right-eye eyepiece optical system, color drift of the second light from the first light can be made less than color drift of the second light from the first light before the first light and the second light pass through the right-eye eyepiece optical system. That is, the blue color drift at the right end of the peripheral portion of the right-eye image display element  308  and the red color drift at the left end of the peripheral portion of the right-eye image display element  308  can be canceled.  FIG.  18    explains the characteristic of the polarizing plate  310 , and illustrates the spectral transmittance of the central portion, the spectral transmittance at the right end of the peripheral portion, and the spectral transmittance at the left end of the peripheral portion of the polarizing plate  310 . As illustrated in  FIG.  18   , the transmittance of the wavelength on the blue side is made lower at the right end, and the transmittance of the wavelength on the red side is lower at the left end than the transmittance at the center of the polarizing plate  310 . Therefore, as the blue color-drifted light emitted from the right end of the image display element passes through the right end of the polarizing plate  310 , the intensity of its blue color and its blue color drift can be reduced. As the red color-drifted light emitted from the left end of the right-eye image display element  308  passes through the left end of the polarizing plate  310 , the intensity of its red color and its red color drift can be reduced. The blue dominant wavelength of the right-eye image display element  308  is 470 nm, the green dominant wavelength is 545 nm, and the red dominant wavelength is 605 nm. The blue wavelength (first wavelength) may be included in a range from 430 nm to 480 nm. The green wavelength (second wavelength) may be included in a range from 520 nm to 570 nm. The red wavelength (third wavelength) may be included in a range from 600 nm to 650 nm. 
     As described above, the color drift of the peripheral portion from the central portion of the right-eye image display element  308  can be reduced by changing the spectral transmittance between the central portion and the peripheral portion of the polarizing plate  310  that is a color correction element and thus the observer can observe a more natural image. 
     In this embodiment, as illustrated in  FIG.  16   , a light beam width of the polarizing plate  310  close to the right-eye image display element  308  is so small that it is easy to separate each view angle, and thus the emission angle characteristic of the right-eye image display element  308  may be canceled by the characteristic of each location of the polarizing plate  310 . 
     In this embodiment, the color drift caused by the emission angle of the light emitted from the image display element is canceled by the characteristic of the polarizing plate, but may be canceled by the characteristic of the phase plate. In that case, in order to change the transmittance and reflectance in a case where the polarizing plate or PBS is combined, a transmittance (spectral transmittance) for each wavelength of the phase plate may be changed, or a phase amount for each wavelength of the phase plate may be changed. 
     The color drift caused by the emission angle of the light emitted from the image display element may be canceled by the characteristic in which at least one of the spectral transmittance and the spectral reflectance is different between the central portion of the half-mirror and an area different from the central portion of the half-mirror. In that case, as illustrated in  FIG.  16   , since a light beam width of the light from the image display element is so small that it is easy to separate each view angle, the color drift may be canceled by the characteristic of each location of the half-mirror. 
     Fourth Embodiment 
       FIG.  19    explains a head mount display (HMD hereinafter)  401  as an example of the image display apparatus according to this embodiment. The HMD  401  includes lenses  404 ,  405 ,  406 , and  407 , a right-eye image display element  408 , and a left-eye image display element  409 . The lenses  404  and  405  constitute a right-eye eyepiece optical system, and the lenses  406  and  407  constitute a left-eye eyepiece optical system. Each of the right-eye image display element  408  and the left-eye image display element  409  is an organic EL display. 
     The right-eye eyepiece optical system magnifies and projects the original image displayed on the right-eye image display element  408  as a virtual image and guides it to a right eye  402  of the observer. The left-eye eyepiece optical system magnifies and projects the original image displayed on the left-eye image display element  409  as a virtual image and guides it to a left eye  403  of the observer. Each of the right-eye eyepiece optical system and the left-eye eyepiece optical system has a focal length of 10 mm, a horizontal display view angle of 90°, a vertical display view angle of 90°, and a diagonal display view angle of 110°. A distance (eye relief) between the HMD  401  and the observer&#39;s eyeball is 12 mm. 
     The eyepiece optical system according to this embodiment has a configuration that folds an optical path by using polarization. Since the configuration is similar to that of the first embodiment, a detailed description thereof will be omitted. 
     Since the eyepiece optical system according to this embodiment has a short focal length and is thin, the emission angle of light emitted from the peripheral portion of the image display element to the exit pupil of the eyepiece optical system is large, and both emission angles of the light emitted from the horizontal end and the vertical end of the image display element are 60 degrees. In order to shorten the focal length of the eyepiece optical system and to make thinner the HMD  401 , the emission angle of the light emitted from the peripheral portion of the image display element to the center of the exit pupil may be 30 degrees or more. 
     As the emission angle of the light emitted from the image display element increases, color drift occurs due to the interference of the light emitting layer of the organic EL and the influence of the color mixing of the color filter. In this embodiment, as the emission angle of the light emitted from the image display element increases, blue color drift occurs in the peripheral portion of the display screen (image) due to the color mixing of the color filters. Therefore, in a case where the observer observes a white image, blue is observed in the peripheral portion of the image and thus the image cannot be observed naturally. 
     Accordingly, this embodiment makes at least one of the lenses  405  and  407  function as a correction element having different characteristics between the central portion and an area (peripheral portion) different from the central portion. More specifically, an antireflection film deposited on a curved surface on the side of the image display element of each of the lenses  405  and  407  has such a characteristic that the spectral transmittance is different between the central portion and the peripheral portion. Thereby, after the first light emitted from the central portion of the right-eye image display element and the second light emitted from the outermost peripheral portion of the right-eye image display element pass through the right-eye eyepiece optical system, color drift of the second light from the first light can be made less than color drift of the second light from the first light before the first light and the second light pass through the right-eye eyepiece optical system. That is, the color drift in the peripheral portion of the image display element can be canceled. 
     As described above, the color drift of the peripheral portion from the central portion of the image display element can be reduced by changing the spectral transmittance between the central portion and the peripheral portion of the antireflection film of each of the lenses  405  and  407  that are the color correction elements and thus the observer can observe the image more naturally. 
     Since the curved surface on the side of the image display element of each of the lenses  405  and  407  has a large difference between the incident angle of the light emitted from the central portion of the image display element and the incident angle of the light emitted from the peripheral portion of the image display element, color drift may be canceled by the characteristic of the antireflection film deposited on the curved surface on the side the image display element of each of the lenses  405  and  407 . An antireflection film having a similar characteristic may be deposited on both sides of each of the lenses  404  and  406  and on the surface on the observer side of each of the lenses  405  and  407 . In that case, the effect of reducing color drift can be enhanced. 
     In addition, since cover glass of the image display element has a large difference between the incident angle of light emitted from the central portion of the image display element and the incident angle of light emitted from the peripheral portion, an antireflection film may be deposited on the cover glass such that the transmittance of the blue wavelength may reduce as the incident angle increases. 
     As illustrated in  FIG.  20   , color correction elements  410  and  411  are disposed between the right-eye image display element  408  and the lens  405  and between the left-eye image display element  409  and the lens  407  such that the color drift in the peripheral portion may be canceled by the characteristic of the correction elements  410  and  411 . At that time, the color correction elements  410  and  411  have characteristics such that the transmittance of the blue wavelength decreases as the incident angle increases. 
     In a case where a color to be drifted is different depending on the location of the display screen (image) such that blur color drift occurs at the right end and red color drift occurs at the left end as in the third embodiment, the characteristic of the antireflection film of each of the lenses  405  and  407  may be changed among the center portion, the right end of the peripheral portion, and the left end of the peripheral portion. More specifically, the transmittance of the wavelength on the blue side is made lower at the right end, and the transmittance of the wavelength on the red side may be made lower at the left end than the transmittance at the center of the antireflection film. Thereby, as the blue color drifted light emitted from the right end of the image display element passes through the right end of each of the lenses  405  and  407 , the intensity of its blue color and the blue color drift can be reduced. As the red color drifted light emitted from the left end of the image display element passes through the left end of each of the lenses  405  and  407 , the intensity of its red color and the red color drift can be reduced. Even in this case, the color drift may be reduced by changing, between the right end and the left end with respect to the center, the spectral transmittance of the antireflection film of each of the lenses  404  and  406 , the transmittance of the antireflection film of the cover glass, or the transmittances of the color correction elements  410  and  411 . 
     Fifth Embodiment 
       FIG.  21    explains a head mount display (HMD hereinafter)  501  as an example of the image display apparatus according to this embodiment.  FIG.  22    illustrates the HMD  501  connected to a PC  510 . 
     The HMD  501  includes lenses  504 ,  505 ,  506 , and  507 , a right-eye image display element  508 , a left-eye image display element  509 , and an image processing unit  511 . The lenses  504  and  505  constitute a right-eye eyepiece optical system, and the lenses  506  and  507  constitute a left-eye eyepiece optical system. The lenses  504  and  505  and the lenses  506  and  507  are cemented lenses and can be easily held. Each of the right-eye image display element  508  and the left-eye image display element  509  is an organic EL display. 
     The right-eye eyepiece optical system magnifies and projects the original image displayed on the right-eye image display element  508  as a virtual image and guides it to a right eye  502  of the observer. The left-eye eyepiece optical system magnifies and projects the original image displayed on the left-eye image display element  509  as a virtual image and guides it to a left eye  503  of the observer. Each of the right-eye eyepiece optical system and the left-eye eyepiece optical system has a focal length of 13 mm, a horizontal display view angle of 60°, a vertical display view angle of 60°, and a diagonal display view angle of 78°. A distance (eye relief) between the HMD  501  and the observer&#39;s eyeball is 20 mm. 
     The eyepiece optical system according to this embodiment has a configuration that folds an optical path using polarization. Since the configuration is similar to that of the second embodiment, a detailed description thereof will be omitted. 
     Since the eyepiece optical system according to this embodiment has a short focal length and is thin, the emission angle of light emitted from the peripheral portion of the image display element to the exit pupil of the eyepiece optical system is large, and both emission angles of the light emitted from the horizontal end and the vertical end of the image display element are 47 degrees. In order to shorten the focal length of the eyepiece optical system and to make thinner the thinning of the HMD  501 , the emission angle of the light emitted from the peripheral portion of the image display element to the center of the exit pupil may be 30 degrees or more. 
     As the emission angle of the light emitted from the image display element increases, color drift occurs due to the interference of the light emitting layer of the organic EL and the influence of the color mixing of the color filter. In this embodiment, as the emission angle of the light emitted from the image display element increases, blue color drift occurs in the peripheral portion of the display screen (image) due to the color mixing of the color filters. Therefore, in a case where the observer observes a white image, blue is observed in the peripheral portion in the image and thus the image cannot be observed naturally. 
     Accordingly, this embodiment cancels the color drift of the peripheral portion by changing the characteristic between the central portion and the peripheral portion of the image displayed on the image display element. The image displayed on the image display element has a white balance based on the spectral transmittance of the eyepiece optical system. Usually, the white balance is the same on the entire surface of the image display element. In this embodiment, color drift occurs in the peripheral portion from the central portion, and thus the color drift in the peripheral portion is reduced by changing the white balance between the central portion and the peripheral portion by the image processing unit  511 . In consideration of a blue color drifted image in the peripheral portion, this embodiment adjusts the white balance in the peripheral portion so as to make larger the gains of red and green than the gain of blue and to make yellower the image in the peripheral portion than the image at the center. Thereby, the white balance of the image displayed in the peripheral portion of the image display element is yellower than the image displayed in the central portion, and the blue color drift is caused by the large emission angle, and thus the emitted image becomes an image in which color drift is reduced in the central portion. 
     Here, the blue dominant wavelength of the image display element is 470 nm, the green dominant wavelength is 545 nm, and the red dominant wavelength is 605 nm. The blue wavelength (first wavelength) may be included in a range from 430 nm to 480 nm. The green wavelength (second wavelength) may be included in a range from 520 nm to 570 nm. The red wavelength (third wavelength) may be included in a range from 600 nm to 650 nm. 
     In this embodiment, a wavelength at which a ratio of the intensity of the light emitted from the peripheral portion of the image display element to the exit pupil to the intensity of the light emitted from the central portion of the image display element to the exit pupil becomes maximum is 470 nm that is a blue dominant wavelength, and blue color drift occurs as the emission angle increases. Accordingly, color drift of the image display element can be canceled by setting to blue, among the red, green, and blue wavelengths, a wavelength at which the ratio of the peripheral gain to the central gain of the image display element becomes minimum. Thus, the color drift of the peripheral portion from the central portion of the image display element is reduced by changing the white balance between the central portion and the peripheral portion, and the observer can observe a more natural image. The white balance of the peripheral portion relative to the central portion may be changed gradually from the central portion to the peripheral portion or only in the outermost peripheral portion. In a case where the color drift of the peripheral portion is blue regardless of the location as in this embodiment, a gain value for each color to be changed from the central portion to the peripheral portion may be stored as a function of a distance from the central portion. In that case, the required memory capacity can be made smaller than a case of storing a data map. 
     In this embodiment, the image processing unit  511  changes the white balance, but the PC  510  that generates the display image of the HMD  501  may change the white balance. In this case, since the processing inside the HMD  501  is reduced, the power consumption of the image processing unit  511  can be reduced. 
     If the image display element is a liquid crystal display and color drift in the peripheral portion depends on the location such that blue color drift occurs at the right end and red color drift occurs at the left end, the color drift can be reduced by changing the characteristic between the right end and the left end. In this case, since the image at the right end is color drifted to blue, the white balance at the right end is adjusted so as to make larger the gains of red and green than the gain of blue and to make yellower the image at the right end than the image at the center. Since the image at the left end is color drifted to red, the white balance at the left end is adjusted so as to make larger the gains of green and blue than the gain of red and to make blue-greener (cyan) the image at the left end than the image at the center. Thereby, the white balance for the image displayed at the right end of the image display element is yellower than at the center, the blue color drift is caused by the large emission angle, and thus color drift at the right end in the emitted image is reduced in comparison with the center. The white balance for the image displayed at the left end of the image display element is more blue-green than at the center, the red color drift is caused by the large emission angle, and thus color drift at the left end in the emitted image is reduced in comparison with the center. 
     Sixth Embodiment 
       FIG.  23    explains a right-eye eyepiece optical system according to this embodiment. The right-eye eyepiece optical system according to this embodiment is different from the right-eye eyepiece optical system according to the first embodiment in that the right-eye eyepiece optical system according to this embodiment includes a dielectric multilayer film (minus filter)  601  as a correction element provided between the right-eye image display element  108  and the polarizing plate  110 . In this embodiment, a description of a configuration similar to that of the first embodiment will be omitted. In this embodiment, the above color drift is reduced by the minus filter  601 . A description will now be given of a method for reducing color drift using the dielectric multilayer film. 
     First, in general, a dielectric multilayer film is used as an optical filter that can reduce the transmittance in a specific wavelength range by alternately laminating a dielectric film having a high refractive index and a dielectric film having a low refractive index. In this specification, a wavelength range having transmittances lower than those of the surrounding wavelength range will be referred to as a stopband, and another high transmittance wavelength range will be referred to as a passband. The stopband has a relatively narrow width and a filter having such a characteristic that the transmittance is locally reduced as illustrated in  FIG.  24    is known as a minus filter. The minus filter of  FIG.  24    has a stopband in a low transmittance area near 500 nm, and a passband in the remaining high transmittance area. The structure, material, etc. of the multilayer film of the filter are properly designed according to a required characteristic. An optical filter using such a dielectric multilayer film is basically designed to acquire a desired transmission characteristic based on the interference effect and it is thus known as illustrated in  FIG.  25    that the transmission characteristic of the optical filter shifts to a short wavelength side during oblique incidence in comparison with the transmission characteristic during 0-degree incidence. This embodiment forms the minus filter on the surface on the organic EL side in  FIG.  23   , and Table 1 below illustrates a layered structure. A substrate is made of BK7 glass, Ta 2 O 5  is used for the high refractive index material, and SiO 2  is used for the low refractive index material, and they are laminated in 60 layers. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Film 
               
               
                 Number of 
                   
                 Thickness 
               
               
                 Layers 
                 Material 
                 (nm) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 60 
                 Ta 2 O 5   
                 125.0 
               
               
                 59 
                 SiO 2   
                 182.8 
               
               
                 58 
                 Ta 2 O 5   
                 126.6 
               
               
                 57 
                 SiO 2   
                 173.0 
               
               
                 56 
                 Ta 2 O 5   
                 121.6 
               
               
                 55 
                 SiO 2   
                 167.1 
               
               
                 54 
                 Ta 2 O 5   
                 123.8 
               
               
                 53 
                 SiO 2   
                 165.2 
               
               
                 52 
                 Ta 2 O 5   
                 124.0 
               
               
                 51 
                 SiO 2   
                 163.5 
               
               
                 50 
                 Ta 2 O 5   
                 125.7 
               
               
                 49 
                 SiO 2   
                 161.5 
               
               
                 48 
                 Ta 2 O 5   
                 126.1 
               
               
                 47 
                 SiO 2   
                 161.5 
               
               
                 46 
                 Ta 2 O 5   
                 127.3 
               
               
                 45 
                 SiO 2   
                 159.9 
               
               
                 44 
                 Ta 2 O 5   
                 127.6 
               
               
                 43 
                 SiO 2   
                 160.2 
               
               
                 42 
                 Ta 2 O 5   
                 128.0 
               
               
                 41 
                 SiO 2   
                 158.9 
               
               
                 40 
                 Ta 2 O 5   
                 128.0 
               
               
                 39 
                 SiO 2   
                 159.5 
               
               
                 38 
                 Ta 2 O 5   
                 128.3 
               
               
                 37 
                 SiO 2   
                 158.9 
               
               
                 36 
                 Ta 2 O 5   
                 128.3 
               
               
                 35 
                 SiO 2   
                 159.2 
               
               
                 34 
                 Ta 2 O 5   
                 128.5 
               
               
                 33 
                 SiO 2   
                 158.9 
               
               
                 32 
                 Ta 2 O 5   
                 128.5 
               
               
                 31 
                 SiO 2   
                 159.2 
               
               
                 30 
                 Ta 2 O 5   
                 128.5 
               
               
                 29 
                 SiO 2   
                 159.2 
               
               
                 28 
                 Ta 2 O 5   
                 128.3 
               
               
                 27 
                 SiO 2   
                 159.5 
               
               
                 26 
                 Ta 2 O 5   
                 128.0 
               
               
                 25 
                 SiO 2   
                 160.2 
               
               
                 24 
                 Ta 2 O 5   
                 127.6 
               
               
                 23 
                 SiO 2   
                 160.5 
               
               
                 22 
                 Ta 2 O 5   
                 127.1 
               
               
                 21 
                 SiO 2   
                 161.5 
               
               
                 20 
                 Ta 2 O 5   
                 126.6 
               
               
                 19 
                 SiO 2   
                 162.2 
               
               
                 18 
                 Ta 2 O 5   
                 125.9 
               
               
                 17 
                 SiO 2   
                 163.5 
               
               
                 16 
                 Ta 2 O 5   
                 125.2 
               
               
                 15 
                 SiO 2   
                 164.5 
               
               
                 14 
                 Ta 2 O 5   
                 124.0 
               
               
                 13 
                 SiO 2   
                 166.5 
               
               
                 12 
                 Ta 2 O 5   
                 122.8 
               
               
                 11 
                 SiO 2   
                 168.5 
               
               
                 10 
                 Ta 2 O 5   
                 121.2 
               
               
                 9 
                 SiO 2   
                 171.2 
               
               
                 8 
                 Ta 2 O 5   
                 119.0 
               
               
                 7 
                 SiO 2   
                 173.8 
               
               
                 6 
                 Ta 2 O 5   
                 119.5 
               
               
                 5 
                 SiO 2   
                 179.2 
               
               
                 4 
                 Ta 2 O 5   
                 120.5 
               
               
                 3 
                 SiO 2   
                 180.3 
               
               
                 2 
                 Ta 2 O 5   
                 118.8 
               
               
                 1 
                 SiO 2   
                 91.3 
               
            
           
           
               
            
               
                 Substrate 
               
               
                   
               
            
           
         
       
     
       FIG.  26    illustrates an emission spectrum distribution of the organic EL panel in a 0-degree emission direction and a transmittance distribution of the minus filter  601  in the 0-degree incident direction in this embodiment.  FIG.  27    illustrates an emission spectrum distribution of the organic EL panel in a 30-degree emission direction and a transmittance distribution of the minus filter  601  in the 30-degree incident direction.  FIG.  28    illustrates an emission spectrum distribution of the organic EL panel in the 50-degree emission direction and a transmittance distribution of the minus filter  601  in the 50-degree incident direction. First, a color difference ΔE*ab of the light in the 30-degree emission direction from the 0-degree emission direction without the minus filter  601  is 15.6, an absolute value of a chromaticity difference Δx is 0.0024, and an absolute value of a chromaticity difference Δy is 0.0227. A color difference ΔE*ab of the light in the 50-degree emission direction from the 0-degree emission direction without the minus filter  601  is 27.0, an absolute value of a chromaticity difference Δx is 0.0009, and an absolute value of a chromaticity difference Δy is 0.03608. 
     It is understood from the emission spectra of  FIGS.  26 ,  27 , and  28    that as the emission angle increases, the intensity of blue becomes higher and a color difference and a chromaticity difference become conspicuous. As illustrated in  FIG.  26   , the minus filter provided in this embodiment is designed to have a stopband near 500 nm during the 0-degree incidence. As illustrated in  FIGS.  27  and  28   , the minus filter functions to shift the stopband to a wavelength side where the stopband is short during the oblique incidence, and to gradually reduce the light transmittance in the blue wavelength range. As a result, when the light passes through the minus filter  601  in this embodiment, the color difference ΔE*ab of the light in the 30-degree emission direction from the 0-degree emission direction becomes 6.5, the absolute value of the chromaticity difference Δx becomes 0.007, and the absolute value of the chromaticity difference Δy becomes 0.004. The color difference ΔE*ab of the light in the 50-degree emission direction from the 0-degree emission direction becomes 10.2, the absolute value of the chromaticity difference Δx becomes 0.018, and the absolute value of the chromaticity difference Δy becomes 0.013. 
     Thus, the color drift characteristic, in which blue becomes stronger due to the oblique emission of the organic EL display, is reduced by shifting the stopband of the minus filter to the short wavelength side during oblique transmission. In a case where a blue component of the display tends to be stronger as in this embodiment, the stopband of the dielectric filter during the 0-degree incidence is designed to exist between the general blue wavelength band of 430 to 480 nm and the green wavelength band of 520 nm to 570 nm. Thereby, there is little color change during normal incidence and the blue component can be reduced during oblique incidence. Thus, if the stopband exists on a wavelength side longer than a color component to be reduced with respect to the emission angle, the color difference or chromaticity difference can be reduced by shifting the stopband in the visible light range during oblique incidence. This is similarly applied to the green component (520 to 570 nm) and the red component (600 to 650 nm). In correcting a plurality of color components, a filter having a plurality of stopbands is also applicable. 
     With the image display element alone according to this embodiment, the absolute value of the chromaticity difference x is 0.0009 and the absolute value of the chromaticity difference y is 0.03608 at an emission angle in the 50-degree direction. After the light passes through the eyepiece optical system, the absolute value of the chromaticity difference x increases up to 0.018, but the absolute value of the chromaticity difference y decreases down to 0.013. In order for the observer to observe a more natural image without noticing color drift, the absolute values of the chromaticity differences x and y may be 0.03 or less. In this embodiment, it is understood that the chromaticity difference is corrected to a level that enables the observer to observe a more natural image. 
     The color difference ΔE*ab can be reduced from 27.0 to 10.2. In order for the observer to observe a more natural image without noticing color drift, the color difference ΔE*ab on the chromaticity coordinate system may be 15 or less. In this embodiment, it is understood that the color difference is corrected to a level that enables the observer to observe a more natural image. 
     In this embodiment, the filter is disposed as a separate component, but may be provided on the surface of the cover glass of the organic EL panel for space saving and the reduced number of components. The filter may also be formed on the surface of any optical element. In these cases, a necessary multilayer film configuration may be properly designed according to the refractive index of the base material and the like. The configuration of the dielectric multilayer film can be properly designed according to the emission spectrum of the organic EL panel, the display optical system, the color drift characteristic at the emission angle in the oblique direction, and the like. 
     In the above description, the image observation apparatus according to this embodiment includes a dielectric multilayer film on one surface between the light emitting portion of the image display element and the exit pupil. Due to the incident angle characteristic of the dielectric multilayer film, a color difference or chromaticity difference of the light emitted from the peripheral portion of the image display element to the exit pupil from the light emitted from the central portion of the image display element to the exit pupil becomes smaller when the light passes through the dielectric multilayer film. 
     Seventh Embodiment 
       FIG.  29    explains a right-eye eyepiece optical system according to this embodiment. The right-eye eyepiece optical system according to this embodiment is different from the right-eye eyepiece optical system according to the second embodiment in that the right-eye eyepiece optical system according to this embodiment includes a dielectric multilayer film (IR cut filter)  602  as a correction element provided between the right-eye image display element  208  and the polarizing plate  210 . In this embodiment, a description of a configuration similar to that of the second embodiment will be omitted. In this embodiment, the IR cut filter  602  reduces the above color drift. A description will now be given of a method for reducing color drift using this dielectric multilayer film. 
     As illustrated in  FIG.  30   , the dielectric multilayer film filter having such a characteristic that the transmittance decreases in an infrared area is known as an IR cut filter. The IR cut filter of  FIG.  30    has a passband in the high transmittance range of less than 690 nm, and a stopband in the low transmittance range on a longer wavelength side of it. The structure, material, etc. of the multilayer film of the filter are properly designed according to a required characteristic. An optical filter using the dielectric multilayer film is basically designed to acquire a desired transmission characteristic based on the interference effect and it is known that the transmission characteristic shift to a short wavelength side during oblique incidence in comparison with the transmission characteristic during 0-degree incidence, as illustrated in  FIG.  31   . In this embodiment, an IR cut filter is formed on the surface on the organic EL (the right-eye image display element  208 ) side of the polarizing plate  210  in  FIG.  29   , and Table 2 below illustrates a layered structure. A substrate is made of BK7 glass, TiO 2  is used for the high refractive index material, and SiO 2  is used for the low refractive index material, and thy are laminated in 35 layers. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Number 
                   
                 Film 
               
               
                 of 
                   
                 Thickness 
               
               
                 Layers 
                 Material 
                 (nm) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 35 
                 SiO 2   
                 78.4 
               
               
                 34 
                 TiO 2   
                 86.2 
               
               
                 33 
                 SiO 2   
                 160.2 
               
               
                 32 
                 TiO 2   
                 82.9 
               
               
                 31 
                 SiO 2   
                 155.7 
               
               
                 30 
                 TiO 2   
                 81.8 
               
               
                 29 
                 SiO 2   
                 154.6 
               
               
                 28 
                 TiO 2   
                 80.6 
               
               
                 27 
                 SiO 2   
                 153.4 
               
               
                 26 
                 TiO 2   
                 80.6 
               
               
                 25 
                 SiO 2   
                 153.4 
               
               
                 24 
                 TiO 2   
                 80.6 
               
               
                 23 
                 SiO 2   
                 154.6 
               
               
                 22 
                 TiO 2   
                 82.9 
               
               
                 21 
                 SiO 2   
                 159.0 
               
               
                 20 
                 TiO 2   
                 88.5 
               
               
                 19 
                 SiO 2   
                 174.7 
               
               
                 18 
                 TiO 2   
                 109.8 
               
               
                 17 
                 SiO 2   
                 192.6 
               
               
                 16 
                 TiO 2   
                 103.0 
               
               
                 15 
                 SiO 2   
                 171.4 
               
               
                 14 
                 TiO 2   
                 103.0 
               
               
                 13 
                 SiO 2   
                 194.9 
               
               
                 12 
                 TiO 2   
                 116.5 
               
               
                 11 
                 SiO 2   
                 201.6 
               
               
                 10 
                 TiO 2   
                 117.6 
               
               
                 9 
                 SiO 2   
                 202.7 
               
               
                 8 
                 TiO 2   
                 116.5 
               
               
                 7 
                 SiO 2   
                 201.6 
               
               
                 6 
                 TiO 2   
                 115.4 
               
               
                 5 
                 SiO 2   
                 192.6 
               
               
                 4 
                 TiO 2   
                 110.9 
               
               
                 3 
                 SiO 2   
                 37.0 
               
               
                 2 
                 TiO 2   
                 9.0 
               
               
                 1 
                 SiO 2   
                 150.1 
               
            
           
           
               
            
               
                 Substrate 
               
               
                   
               
            
           
         
       
     
       FIG.  32    illustrates an emission spectrum distribution of the organic EL panel in a 0-degree emission direction and a transmittance distribution of the IR cut filter in the 0-degree incident direction in this embodiment.  FIG.  33    illustrates an emission spectrum distribution of the organic EL panel in a 30-degree emission direction and a transmittance distribution of the IR cut filter in the 30-degree incident direction.  FIG.  34    illustrates an emission spectrum distribution of the organic EL panel in the 50-degree emission direction and a transmittance distribution of the IR cut filter in the 50-degree incident direction. First, a color difference ΔE*ab of the light in the 30-degree emission direction from the 0-degree emission direction without the IR cut filter is 17.4, an absolute value of a chromaticity difference Δx is 0.0015, and an absolute value of a chromaticity difference Δy is 0.0212. A color difference ΔE*ab of the light in the 50-degree emission direction from the 0-degree emission direction is 21.7, an absolute value of a chromaticity difference Δx is 0.0083, and an absolute value of a chromaticity difference Δy is 0.0208. 
     It is understood from the emission spectra and chromaticity differences in  FIGS.  32 ,  33 , and  34    that as the emission angle increases, the intensity of red becomes higher and a color difference becomes conspicuous. As illustrated in  FIG.  32   , the IR cut filter provided in this embodiment is designed to have a stopband in a wavelength range of 730 nm or higher, which is an infrared wavelength range, during the 0-degree incidence. As illustrated in  FIGS.  33  and  34   , the IR cut filter functions to shift the stopband to a wavelength side where the stopband is short during the oblique incidence, and to gradually reduce the light transmittance in the red wavelength range. As a result, when the light passes through the IR cut filter  602  in this embodiment, the color difference ΔE*ab of the light in the 30-degree emission direction from the 0-degree emission direction becomes 13.1, the chromaticity difference Δx becomes 0.0023, and the chromaticity difference Δy becomes 0.0166. The color difference ΔE*ab of the light in the 50-degree emission direction from the 0-degree emission direction becomes 11.4, the chromaticity difference Δx becomes 0.0108, and the chromaticity difference Δy becomes 0.0173. Thus, the color drift characteristic, in which the red becomes stronger due to the oblique emission of the organic EL display, is reduced by shifting the stopband of the IR cut filter to the short wavelength side during oblique transmission. 
     In a case where a red component of the display tends to be strong as in this embodiment, the stopband of the dielectric filter during the 0-degree incidence is designed to exist at 640 nm or higher, which is a general red wavelength. Thereby, there is little color change during normal incidence and the red component can be reduced during oblique incidence. Thus, if a dielectric multilayer filter is used as in the first embodiment, in which the stopband exists on a wavelength side longer than a color component to be reduced with respect to the emission angle, the color difference or chromaticity difference can be reduced by shifting the stopband in the visible light range during oblique incidence. 
     In reducing the red component as in this embodiment, it is unnecessary to use a filter having a local stopband like a minus filter, and the embodiment can use the IR cut filter having a stopband in a wide range over the invisible range on a long wavelength side. 
     With the image display element alone according to this embodiment, the absolute value of the chromaticity difference x is 0.0083 and the absolute value of the chromaticity difference y is 0.0208 at an emission angle in the 50-degree direction. After the light passes through the eyepiece optical system, the absolute value of the chromaticity difference x increases up to 0.0108, but the absolute value of the chromaticity difference y decreases down to 0.0173. In order for the observer to observe a more natural image without noticing color drift, the absolute values of the chromaticity differences x and y on the chromaticity coordinate system may be 0.03 or less or 0.018 or less. In this embodiment, it is understood that the chromaticity difference is corrected to a level that enables the observer to observe a more natural image. 
     The color difference ΔE*ab can be reduced from 21.7 to 11.4. In order for the observer to observe a more natural image without noticing color drift, the color difference ΔE*ab on the chromaticity coordinate system may be 15 or less. In this embodiment, it is understood that the color difference is corrected to a level that enables the observer to observe a more natural image. 
     In this embodiment, the filter is disposed as a separate component, but may be provided on the surface of the cover glass of the organic EL panel for space saving and the reduced number of components. The filter may also be formed on the surface of any other optical element. In these cases, a necessary multilayer film configuration may be properly designed according to the refractive index of the base material and the like. The configuration of the dielectric multilayer film can be properly designed according to the emission spectrum of the organic EL panel, the display optical system, the color drift characteristic at the emission angle in the oblique direction, and the like. 
     Each embodiment can provide an image display apparatus that is advantageous in reducing color drift of a peripheral portion of an image display element from a central portion of the image display element. 
     While the disclosure has been described with reference to exemplary embodiments, it is to be understood that the disclosure is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2021-187505, filed on Nov. 18, 2021, and No. 2022-028542, filed on Feb. 25, 2022, each of which is hereby incorporated by reference herein in its entirety.