Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:16:25.
`
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`
`

`

`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:16:25.
`
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`
`

`

`OLED Displays
`Fundamentals and Applications
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:16:25.
`
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`
`

`

`Wiley-SID Series in Display Technology
`
`Series Editor:
`Anthony C. Lowe
`
`A complete list of the titles in this series appears at the end of this volume.
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:16:25.
`
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`
`

`

`4
`
`4
`
`OLED Display Module
`
`4.1 COMPARISON BETWEEN OLED
`AND LCD MODULES
`
`Figure 4.1 compares the components that are necessary for production of
`liquid crystal display (LCD) and OLED display modules.
`An LCD consists of many components because it must convert backlight
`emission to uniform area emission and switch on and off the light with a
`liquid crystal shutter, which is positioned between two polarizers.
`A typical LCD uses a cold-cathode fluorescent tube (CCFL) or multiple
`LEDs. Two types of LED are used: (1) that which emits short wavelength
`emission, which is converted to longer wavelengths by means of a fluores-
`cent material (downconversion) and (2) that which emits the three color
`primaries (red–green–blue [RGB]). Thus the light source for an LCD is
`either linear (fluorescent tube) or point, so the light must be converted to
`the area form to be used as a backlight unit. The light reflected by the
`reflector passes through the light guide and is diffused. A light guide is
`made of high refractive index material, such as an acrylic polymer, which
`delivers the light by total internal reflection due to the refractive index dif-
`ference between it and the surrounding air. The light guide structure is
`designed such that uniform luminance distribution across the area of the
`display can be achieved. Light exiting from the light guide is transmitted
`through the prism sheet and diffuser, and is then polarized by the rear polar-
`izer. Polarization of the light is changed by the field-induced orientation of
`
`OLED Displays: Fundamentals and Applications, First Edition. Takatoshi Tsujimura.
`© 2012 John Wiley & Sons, Inc. Published 2012 by John Wiley & Sons, Inc.
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:14:39.
`
`69
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`

`

`4 OLED Display Module
`
`Figure 4.1 Examples of display components used in LCD (a) and
`OLED (b) television sets.
`
`the LC molecules so that the designated light amount is exited when it
`passes exit polarizer, converted to R,G,B light by a color filter, and the exit
`light intensity is controlled by means of the exit polarizer and the light
`reaches the human eye.
`In OLED displays, light emission takes place in an OLED device fabricated
`on the surface of a glass substrate. When the display is observed in a bright
`environment, the reflection from the display surface causes deterioration of
`the contrast ratio, so a circular polarizer is often used. (see Section 4.4.3
`for further details), which reduces the brightness of the display by about
`50%. For example, a 45% transmittance polarizer is sometimes used. LCDs
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`70
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:14:39.
`
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`
`

`

`4.2 Basic Display Design and Related Characteristics
`
`also have an ambient light reflection problem. In an LCD, a black matrix
`is used to enhance the ambient contrast ratio. Generally, the reflection is
`more serious in the case of OLED, because reflective electrode is used for
`OLED when transmissive LCD uses transparent electrode for both side.
`In all cases, OLED displays require fewer components than LCDs. M.
`O’Regan et al. [1] reports the cost structure comparison between LCD and
`OLED. The paper ascribes the major reason of OLED’s high cost to the low
`material utilization, which is typically 5% by evaporation methods. Even
`assuming a significantly higher cost of TFT backplane and driver IC than
`LCDs and also assuming a lower yield of OLED than LCD by 10–20%,
`the total cost of OLED would become 30% cheaper if a high material uti-
`lization processing technique is used, such as a solution-based technique.
`
`4.2 BASIC DISPLAY DESIGN AND RELATED
`CHARACTERISTICS
`
`To design a high-quality display, many factors must be considered. Designers
`need to understand how intended specifications, such as luminance, color
`reproduction, and luminance uniformity, can be achieved through display
`design.
`
`4.2.1 Luminous Intensity, Luminance, and Illuminance
`
`The radiometric (energy) values taking spatial and temporal considerations
`into account are called radiant quantities, and radiometric values weighted
`by means of visual sensitivity as to the wavelength (spectrum luminous
`efficiency) are called photometric quantities. Luminous intensity, lumi-
`nance, and illuminance are the most well-known photometric quantity units.
`
`4.2.1.1 Luminous Intensity
`
`Here we assume that the luminous flux from a point light source traverses
`the areas between dm1 and dm2 as shown in Fig. 4.2. Let us assume that the
`luminous intensity values are P1 and P2. The luminous flux values Φ (in
`lumens [lm]) of areas dm1 and dm2 are the same:
`Φ =
`=
`2
`P dm P dm
`1
`1
`2
`
`(4.1)
`
`d
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:14:39.
`
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`

`

`4 OLED Display Module
`
`Figure 4.2 Relationship between
`luminous flux and luminous intensity.
`
`Figure 4.3 Relationship between
`luminous flux and luminance of an area
`light source.
`
`If the solid angle is dω, then the luminous flux per unit solid angle P is
`
`(4.2)
`
`
`
`Φω
`d d
`
`P
`
`=
`
`This parameter is called luminous intensity, measured in cd (candelas) = lm/
`sr (where sr is steradian, the unit for solid-angle measurement).
`
`4.2.1.2 Luminance
`
`Luminous intensity can be measured in terms of a point light source, but
`measurement of an area light source requires another metric. As an
`area light source can be quantified as a multipoint light source, the luminous
`flux of an area light source is much larger than that of a point light source.
`The luminous intensity of the area light source is called luminance, and the
`measurement unit is cd/m2 = lm/(sr·m2). As shown in Fig. 4.3, when the
`angle between the normal line of face A and the A–B line is θ, the area of
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`72
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:14:39.
`
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`
`

`

`4.2 Basic Display Design and Related Characteristics
`
`face A is regarded as dS cos θ from the location on line A–B, so the lumi-
`nance L can be expressed as follows:
`
`L
`
`=
`
`dS
`
`
`
`dP
`cosθ
`Here, using (4.2), we obtain
`Φ
`d
`cosθ ω
`d
`Luminance is used to express the display brightness.
`
`L
`
`=
`
`dS
`
`
`
`(4.3)
`
`(4.4)
`
`4.2.1.3
`
`Illuminance
`
`In Section 4.2.1.2 we noted that the luminous flux emitted per unit solid
`angle is called luminous intensity. Similarly, the luminous flux incident per
`unit area is called illuminance, and the measurement unit is lux (lx)
`(1 lx = 1 m/m2.). We can use the same equation (Eq. [4.4]) to calculate the
`luminous exitance, which is the luminous flux that exits from the source
`per unit area.
`Illuminance E (luminous exitance) can be expressed as
`Φ
`S
`
`
`
`d d
`
`E
`
`=
`
`(4.5)
`
`where, as shown in Fig. 4.4, dS is the area on the incident face of luminous
`flux dΦ.
`
`Figure 4.4 Relationship between
`luminous flux and illuminance of
`point light source.
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:14:39.
`
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`
`

`

`4 OLED Display Module
`
`Now let us consider how illuminance E of face B is defined. If the angle
`between face B and line A–B is ϕ, then the area of face B can be regarded
`as dS cos ϕ from line A–B. If the distance between points A and B is dAB,
`the solid angle dω that is occupied by this space can be expressed as
`follows:
`
`ω
`d
`
`=
`
`φ
`
`
`
`cos
`dS
`2
`dAB
`
`For the point light source (Fig. 4.4), using Eq. (4.2), we obtain
`cosφ
`PdS
`2
`dAB
`
`d
`
`Φ =
`
`
`
`(4.6)
`
`(4.7)
`
`
`
`P
`
`=
`
`d d
`
`E
`
`=
`
`(4.8)
`
`(4.9)
`
`Therefore, the illuminance E for a point light source is
`cosφ
`Φ
`2
`S
`dAB
`For the area light source (Fig. 4.5), the area denoted by ΔS, the following
`formula can be derived using ΔS instead of dS in (Eq. [4.4]):
`Φ
`θ ω
`cos
`d
`L S
`d
`θ
`cos
`cos
`L S
`dS
`2
`dAB
`
`φ
`
`
`
`∆ ∆
`
`= =
`
`Figure 4.5 Relationship between
`luminous flux and illuminance of
`area light source.
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`74
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:14:39.
`
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`
`

`

`4.2 Basic Display Design and Related Characteristics
`
`(4.10)
`
`Therefore the illuminance of an area light source is
`
`
`
`θ φ
`cos cos
`.
`2
`dAB
`
`Φ ∆
`d d
`
`S
`L S
`
`= =
`
`E
`
`Illuminance is the parameter used to quantify the brightness of an
`environment.
`
`4.2.1.4 Summary
`
`The four quantities discussed above can be summarized as follows:
`
`■ Luminous flux is the psychophysical (in terms of spectral sensitivity
`of observers’ eyes) visible light intensity per unit time, emitted by a
`light source.
`■ Luminous intensity is the luminous flux emitted per unit solid angle.
`■ Luminance is the luminous flux per unit solid angle emitted per unit
`area as projected on a plane normal to the line of sight. (This is also
`known as brightness.)
`Illuminance is the luminous flux incident per unit area on a surface.
`
`■
`
`The equations and units used to calculate these quantities are summarized
`in Table 4.1. The steradian (sr) is the unit for determining solid angle. (A
`steradian means the solid angle which occupies the area equal to the square
`of the spheric radius, on the sphere surface. The solid angle for a full
`
`TABLE 4.1 Summary of Photometric Parameters
`
`Unit
`
`lumen (lm)
`
`candela = lumen/steradian
`(cd = lm/sr)
`
`candela per square
`meter = lumen per
`steradian per square meter
`(cd/m2 = lm/[sr·m2])
`lux (lx = lm/m2)
`
`Quantification Equation
`Φ = dQ
`dt
`= φ
`ω
`
`d d
`
`P
`
`L
`
`=
`
`Φ
`d
`cosθ ω
`d
`
`dS
`
`Parameter
`
`Luminous flux
`
`Luminous intensity
`
`Luminance
`
`= Φ
`S
`
`d d
`
`E
`
`Illuminance
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:14:39.
`
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`

`4 OLED Display Module
`
`sphere is 4π.) When the surface reflection of a specific display is discussed,
`illuminance and luminance are often compared to calculate the contrast
`ratio in a lit environment as follows.
`If perfect diffusion is assumed for surface A, illuminance at point B can be
`expressed as
`θ
`⋅
`( ) cos
`dI
`θ
`⋅
`2
`cos )
`(
`r
`
`2
`
`θ
`2
`
`=
`
`dI
`4
`
`θ
`( )
`2
`r
`
`E
`
`B =
`
`As the illuminance is uniform at all points on the surface of the circle with
`radius r, total luminous flux can be calculated as
`π
`⋅
`4
`r
`2
`4
`r
`
`2
`
`π
`= ⋅
`
`I
`
`0
`
`I
`
`0
`
`Φ =
`
`As surface A is perfectly diffusive, luminance of reflective light on surface
`A can be described as
`
`L
`
`perfect =
`
`E
`π
`
`(E is the illuminance on surface A.)
`If reflectivity of surface A is K, this equation can be written as
`
`⋅π
`
`K E
`
`reflection =
`L
`
`The livingroom contrast ratio, discussed in Section 4.4.3.1, can be calcu-
`lated by
`
`⋅
`π
`
`L K
`
`=
`
`L L
`
`display
`⋅
`E
`(Normally, E = 200 (lux) is used for livingroom contrast ratio calculation.)
`See Fig. 4.6.
`
`CR
`livingroom
`
`=
`
`display
`
`reflection
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`4.2.2 OLED Current and Power Efficiencies
`
`When multiple OLED devices or OLED processes are compared or control-
`led, it is much easier to compare the efficiency metric than to compare
`actual measurement curves with many data points. The concepts of current
`efficiency and power efficiency are discussed below.
`Let us assume monochromatic light first. The luminous flux Φ(λ) (meas-
`ured in lm/m2) at wavelength λ can be expressed as
`
`76
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:14:39.
`
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`
`

`

`4.2 Basic Display Design and Related Characteristics
`
`Figure 4.6 (a) Perfect diffusion surface; (b) incident light from a
`perfect diffusion surface.
`
`= PK ym
`λ
`Φ( )λ
`
`( )
`
`(4.11)
`where P is emission intensity (W/m2), y(λ) is the relative luminosity at
`wavelength λ, and Km is the maximum luminosity factor 683 (lm/W). An
`actual emission has spectral width, so an integral that can be applied for all
`wavelengths is:
`
`
`
`(4.12)
`
`(4.13)
`
`λ λ
`F F
`
`(
`
`′
`λ
`( )
`d
`
`)
`
`∫
`Luminous flux Φ for all wavelengths can be calculated by integrating for
`all λ′ as follows:
`∫
`
`
`
`
`)
`
`′ =Φ(λ
`
`PK y
`m
`
`λ
`′
`(
`
`)
`
`Φ =
`
`PK
`
`m
`
`F
`
`λ λ λ
`( ) ( )
`d
`y
`
`F
`
`λ λ
`( )
`d
`
`∫
`
`
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`(4.14)
`
`Then the luminance L (cd/m2) can be expressed as
`
`
`
`Φπ
`
`L =
`
`The power efficiency (lm/W; also called luminous efficacy) can be expressed
`as
`
`(4.15)
`
`77
`
`
`
`λ
`( )
`
`⋅
`
`S
`IV
`
`=
`
`PK y
`m
`IV
`
`λ
`( )
`
`=
`
`PK y
`m
`S
`
`Φ W
`
`η
`e
`
`=
`
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:14:39.
`
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`
`

`

`4 OLED Display Module
`
`Figure 4.7 Example of a current–voltage curve of an OLED device.
`
`
`
`(4.16)
`
`On the other hand, current efficiency ηc (cd/A) is defined as the luminance
`divided by the current density, so it can be expressed as
`λ
`( )
`PK y
`m
`
`π⋅
`I
`An actual current–voltage (I-V) curve of an OLED device is illustrated in
`Fig. 4.7, which shows that the electric current flows when the voltage is
`positive but does not when it is negative, which means that the OLED
`device exhibits diode characteristics. For nonuniformity compensation of a
`display (described in Section 5.4.4), this diode feature can be utilized to
`impede the current flow of the driver TFT by applying negative voltage,
`which obstructs the detection of the driver TFT characteristic fluctuation to
`be compensated by a pixel circuitry.
`A luminance–voltage curve is shown in Fig. 4.8. This curve is also called
`the L-V curve. Similarly, luminance-current-voltage measurement is some-
`times called L-I-V measurement. (It should be noted that L means lumi-
`nance here, not inductance.)
`As shown in Fig. 4.9, the luminance is almost proportional to the electric
`current. The correlation between current efficiency and voltage is plotted
`
`=
`
`L J
`
`η
`c
`
`=
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`78
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:14:39.
`
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`
`

`

`4.2 Basic Display Design and Related Characteristics
`
`Figure 4.8 Example of a luminance–voltage curve of an
`OLED device.
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`Figure 4.9 Example plot of luminance versus current.
`
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:14:39.
`
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`

`

`4 OLED Display Module
`
`Figure 4.10 Graph showing dependence of voltage on current
`efficiency.
`
`in Fig. 4.10. Thus, the efficiency is subjected to the voltage (due to concen-
`tration quenching, described in Section 2.2.2.7; densely generated excitons
`by larger current cause more chance of losing energy by collision to the
`quencher or other excitons), which needs to be taken into account in the
`display design. In particular, in the case of a phosphorescent device, colli-
`sion between two triplet excitons causes quenching (as the high current
`generates many excitons by which many triplet excitons collide with each
`other to form a singlet state). This is called triplet–triplet (T–T) annihila-
`tion, which can significantly reduce efficiency in the high-current region.
`This is called the rolloff phenomenon.
`
`4.2.3 Color Reproduction
`
`Color reproduction is an important factor in ensuring color fidelity, espe-
`cially for photo images. Large-scale color reproduction can be achieved by
`applying display schemes containing the primary colors red, green, and
`blue, as discussed below. Emission spectra of red, green, and blue subpixels
`are shown in Fig. 4.11.
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`80
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:14:39.
`
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`
`

`

`4.2 Basic Display Design and Related Characteristics
`
`Figure 4.11 Example of emission spectrum of RGB (red + green + blue)
`subpixels.
`
`Here we assume that the emission spectrum is S (λ) and the color matching
`functions (sensitivity of the human eye to each wavelength; see the Appendix
`λ λ λ ; we can then express the tristimulus
`( ),
`( ),
`( )
`for actual values.) are x
`y
`z
`values (a parameter set, converted from human eye stimulus values as to
`three color primaries, designed so that only a positive value appears; stand-
`ardized as CIE1931XYZ color space) as follows, using an integral ranging
`from 380 to 780 nm.
`
`780
`
`S
`
`λ λ λ
`x( ) ( )
`
`d
`
`
`
`X K
`
`Y K
`
`Z K
`
`= ∫
`= ∫
`= ∫
`
`380
`
`780
`
`380
`
`780
`
`380
`
`S
`
`λ λ λ
`y( ) ( )
`
`d
`
`
`
`S
`
`λ λ λ
`z( ) ( )
`
`d
`
`
`
`(4.17)
`
`(4.18)
`
`(4.19)
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`Here, we define the K so that calculated Y is equal to the display luminance.
`Using the CIE1931-XYZ value, the (x, y) locus (generally used to indicate
`the display color reproduction capability) can be expressed as follows:
`
`x
`
`=
`
`X
`+ +
`X Y Z
`
`
`
`(4.20)
`
`81
`
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:14:39.
`
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`
`

`

`4 OLED Display Module
`
`Figure 4.12 Plot of the area reproducible by RGB color mixture.
`
`
`
`(4.21)
`
`y
`
`=
`
`Y
`+ +
`X Y Z
`This (x, y) value set can be calculated for red, green, and blue, respectively,
`as (xR, yR), (xG, yG), (xB, yB).
`The actual measurement results of an OLED emitter are shown in Fig. 4.12.
`The color range that can be shown by the combination of these three color
`primaries is found within the triangle that is made by the three color pri-
`maries in the (x, y) coordinate. This area is called the color gamut. To
`determine the color reproduction capability of a display, the metric NTSC%
`is often used (NTSC = National [US] Television System Committee). It is
`defined as the ratio of the area of the triangle achievable by the display
`divided by the area of the standard NTSC triangle, expressed as a
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`82
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:14:39.
`
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`
`

`

`4.2 Basic Display Design and Related Characteristics
`
`percentage. For example, XEL-1, the 11-in. OLED television commercial-
`ized in 2007, claimed a color gamut of 110%.
`Although NTSC was introduced as a cathode ray tube (CRT) television
`standard, eventually most CRT fluorescence materials were designed with
`emission color coordinates close to the European Broadcasting Union
`(EBU) standard or the Rec709 standard (the international standard for
`HDTV studios), which are much smaller in area than is the NTSC triangle
`(Fig. 4.13).
`
`Figure 4.13 Graphical representation of the three color standards applied in
`television manufacture.
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:14:39.
`
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`

`

`4 OLED Display Module
`
`On the other hand, digital cameras and computers often use the s-RGB
`standard, which is shown in Fig. 4.14 (it has the same color coordinates as
`EBU and Rec709). The Joint Photographic Experts Group (JPEG) standard
`also normally uses the s-RGB color coordinate standard; however, as the
`s-RGB triangle is not large enough to reproduce high fidelity colors, a new
`header standard EXIF2.2 has introduced the s-Ycc standard, which uses an
`extended dynamic range with grayscale values that extend from negative
`values to values greater than 255, while normal s-RGB uses 0 to 255 to
`express gray scales. The newer header standard EXIF2.21 can also handle
`the Adobe-RGB standard, which is popular in the publishing industry
`(Fig. 4.14).
`
`Figure 4.14 Graphical representation of the three color standards
`applied in digital camera manufacture.
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`84
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:14:39.
`
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`
`

`

`4.2 Basic Display Design and Related Characteristics
`
`TABLE 4.2 (x, y) Coordinate of Color Primaries for Each Standard
`
`Standard
`
`NTSC
`Rec-709(HDTV)
`EBU
`s-RGB
`Adobe-RGB
`
`Red
`
`(0.67,0.33)
`(0.64,0.33)
`(0.64,0.33)
`(0.64,0.33)
`(0.64,0.33)
`
`Green
`
`(0.21,0.71)
`(0.30,0.60)
`(0.29,0.60)
`(0.30,0.60)
`(0.21,0.71)
`
`Blue
`
`(0.14,0.08)
`(0.15,0.06)
`(0.15,0.06)
`(0.15,0.06)
`(0.15,0.06)
`
`The (x, y) color coordinates for four of these standards are listed in
`Table 4.2.
`To ensure color fidelity, s-RGB can reproduce most colors realistically
`because there are not many vivid colors outside the s-RGB triangle in the
`real world, as illustrated in Figs. 6.28 and 6.29. However, although highly
`saturated colors rarely occur, they have an intense impact on a viewer, so
`a color gamut wider than that of s-RGB is useful for showing such colors.
`In the television industry, the image boosting technique, which converts an
`original image into a more impressive image, such as one with more vivid
`colors, by means of graphic engine IC chips or a graphics processing unit
`(GPU), is often used to enhance the image impression. A wider color gamut
`is useful for such an application as well.
`As discussed earlier, the (x, y) coordinate is popular as a display color
`metric; however, there are problems with using the standard. Distance in
`the (x, y) coordinate is not proportional to differences in perception of the
`human visual system, so a larger area does not always mean better display
`capability. Figure 4.15 shows 10 times the perception limits of the human
`eye in different regions of the (x, y) color space reported by McAdam et al.
`[2]; this is known as the “McAdam ellipse.” As it is an ellipse, not a circle,
`the human eye does not have the same sensitivity for the x axis and the y
`axis. Also as shown in Fig. 4.15, the human eye is very sensitive to blue
`colors (left bottom of graph) but is less sensitive to red (right bottom) and
`is very insensitive to green colors (top). This illustrates that human eye
`sensitivity depends on the location on the (x, y) coordinate. Therefore it is
`not very meaningful to discuss the area to judge the display capability on
`the basis of an (x, y) coordinate system.
`The issues discussed above explain why many display companies and
`research institutes have used (u′, v′) color coordinates to express display
`capability. The CIE1976 color space coordinates (u′, v′) can be expressed as
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:14:39.
`
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`

`

`4 OLED Display Module
`
`Figure 4.15 Graphical representation of McAdam ellipse, showing the human
`eye’s perception limit (at 10× magnification).
`
`u
`
`′ =
`
`v
`
`′ =
`
`(
`
`−
`2
`
`x
`
`y
`
`+
`
`
`
`3)
`
`=
`
`=
`
`4
`X
`
`Y15
`
`(
`
`X
`
`+
`
`+
`
`3
`
`Z
`
`)
`
`
`
`.
`
`(4.22)
`
`(4.23)
`
`4
`x
`+
`12
`9
`Y9
`
`y
`+
`+
`+
`+
`−
`2
`12
`(
`
`3)
`
`Y15
`(
`3
`)
`x
`y
`X
`Z
`Performance comparisons using the (u′, v′) color coordinates indicate how
`a color display color can actually be perceived by the human eye.
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`86
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:14:39.
`
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`
`

`

`4.2 Basic Display Design and Related Characteristics
`
`Also, uniform color space is sometimes factored into any discussion of
`display capability or color boosting because the defective criteria (such as
`the luminance variation limit or the image burning limit according to life-
`time test discussed in Section 2.2.4, uniformity criteria discussed in Section
`5.4.4, or the metal bus design discussed in Section 5.4.3) should be deter-
`mined according to the limit of human perception.
`
`4.2.4 Uniform Color Space
`
`CIE-LAB and CIE-UV are two well-known uniform color spaces. On these
`uniform color spaces, lightness and color can be treated equally so that the
`length = 1 on the color space is almost equal to the human eye perception
`limit.
`In CIE-LAB uniform color space (Fig. 4.16), lightness L* is defined as
`
`−
`
`16
`
`Y Y
`
`n
`
`* =
`L
`
`116
`
`f
`
`
`
`Y Y
`
`n
`
`−
`
`f
`
`X X
`
`n
`
`
`Z Z
`
`n
`
`−
`
`Y Y
`
`n
`
`f
`
`f
`
`
`
`
`
`a
`
`* =
`
`500
`
`b
`
`* =
`
`200
`
`b*
`orange yellow
`yellow
`green yellow
`orange
`red orange
`
`yellow green
`
`green
`
`blue green
`
`chroma
`
`
`
`red
`
`hue
`
`violet red
`a*
`
`green blue
`
`red violet
`
`blue
`
`violet
`blue violet
`violet blue
`
`L* (lightness) isL* (lightness) is
`
`perpendicular to
`a*-b* plane
`
`Figure 4.16 CIE-LAB uniform color space.
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Applications : Fundamentals and Applications</i>, John Wiley &
` Sons, Incorporated, 2012. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/drexel-ebooks/detail.action?docID=817454.
`Created from drexel-ebooks on 2019-11-25 21:14:39.
`
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`

`4 OLED Display Module
`
`) +t
`(
`)6
`In the case of t > (
`3, f(t) = t1/3. In other cases, f(t) is 1
`29
`29
`3
`6
`9
`Plotting a*, b*, and L* on a three-dimensional graph, the angle of the line
`made by the plotted point and the original point in plane a*–b* shows the
`hue, and the distance between the plotted point and the original point in
`plane a*–b* is the chroma, as in the Munsell color system widely used in
`color science. In this color space, the color difference (including lightness
`difference) is defined as ΔEab by the following equation:
`
`.
`
`42
`
`2
`
`∆
`E
`
`ab =
`
`2
`
`∆
`*
`L
`
`+
`
`∆
`a
`
`*
`
`2
`
`+
`
`∆
`b
`
`*
`
`2
`
`For CIE-LUV uniform color space, the (u*,v*) coordinate is defined by the
`following equations:
`
`*
`
`u
`
`*
`
`v
`
`=
`=
`
`13
`
`− ′
`*(
`L u un
`− ′
`vn
`
`
`
`L v*(
`
`)
`
`)
`
`13
`
`Color difference can be expressed as follows for CIE-LUV uniform color
`space:
`
`2
`
`*
`
`2
`
`
`
`*2
`
`+
`+
`uv =
`∆
`∆
`∆
`∆
`*
`v
`u
`L
`E
`Color difference ΔE = 1 is known to be close to the human perception limit
`and is a useful criterion for determining parameters for display design,
`such as
`
`■ Acceptable display white point variation
`■ Acceptable lifetime and viewing angle
`■ Acceptable image sticking level.
`
`4.2.5 White Point Determination
`
`To display a white image, all red, green, and blue subpixels need to emit
`light. The white color changes the impression of a display quite a bit. The
`color should be selected according to the purpose of the display.
`When tristimulus values of red, green, and blue are (XR, YR, ZR), (XG, YG,
`ZG), (XB, YB, ZB) respectively, then
`
`G
`+
`+
`
`X
`
`white
`
`Y
`white
`
`Z
`
`white
`
`=
`=
`=
`
`Y
`R
`
`Z
`
`R
`
`R
`+
`+
`
`Copyright © 2012. John Wiley & Sons, Incorporated. All rights reserved.
`
`X
`
`+
`
`X
`
`+
`
`X
`
`B
`
`
`
`Y
`G
`
`Y
`B
`
`
`
`Z
`
`G
`
`Z
`
`B
`
`
`
`(4.24)
`
`(4.25)
`
`(4.26)
`
`88
`Tsujimura, Takatoshi. <i>OLED Display Fundamentals and Application