Conventional Films in the Backlight Module

Chapter 1 Introduction

1.2 Conventional Films in the Backlight Module

1.1 Preface

Diffusing film, which is an optically transmissive component, is provided to redistribute the propagating light. A film is regarded as a diffusing film whatever functions it possesses when light passes through, such as light collection, diffusion or beam shaping. In general, the micro-structures of diffusing film control the propagating direction of light for specified distribution. In this study, diffusing films with surface relief of lens array are applied to the backlight module (BLM). It is expected that diffusing films are able to replace the conventional components in the BLM in order to reduce the production cost.

1.2 Conventional Films in the Backlight Module

The liquid crystal display (LCD) industry develops quickly in the recent years.

The device supplying illumination of LCD is the backlight module. Fig. 1 shows the conventional BLM. Light extracting from the light source enters the wedge type light guide. After emitting from the light guide, light propagates through diffuser and is scattered into all directions of the hemi-spherical space. Then the scattered light is redirected by the light-collecting films and collected to the normal direction. These optical films above the light guide are employed to promote light uniformity and collimation in the BLM. The protective diffuser as shown in Fig. 1 preserves BEF from damage and reduces the periodic effect such as Moiré effect resulting from BEFs and the cells of liquid crystal.

Fig. 1 Conventional backlight module

In the backlight module, diffuser scatters light extracting from the light guide in order for light uniformity of the whole panel. Although the micro-structures of the light guide have the similar function of light diffusion, the light uniformity of the light guide is not sufficient. Diffuser is able to compensate the light guide for insufficient light uniformity. Conventionally, bead-coating and micro-structure embossing diffusers are applied in the BLM to increase the uniformity [1, 2]. Fig. 2 shows the particles on the surface of a bead-coating diffuser, whose diameter is about 50um. In addition, the conventional bead-coating diffuser can supply about 23% enhancement of brightness along the normal direction, as shown in Fig. 4, where the x-axis is the viewing angle and the y-axis is the relative radiant intensity. The dotted line represents the lambertian distribution light source as the reference, in which the maximum intensity is normalized to be 1. Diffuser with micro-sized bead functions both scattering and collection.

Fig. 2 The particles on the diffuser

Fig. 3 BEF whose pitch is 50um and grooves with prism angle fixed at 90°

Fig. 4 Light enhancement of conventional films in the BLM

Conventionally, two orthogonally-arranged brightness enhancement films (BEFs) with prismatic surface structures are treated as the light-collecting films to enhance the on-axis intensity along the normal direction [3, 4]. BEF has grooves with prism angle fixed at 90° and the pitch of the grooves is 50um, as shown in Fig 3. Fig. 4 indicates that one BEF enhances on-axis intensity achieved to 152%, and two orthogonally- arranged BEFs provide 220% intensity. Due to the periodic structures of BEFs, Moiré effect, which is the interference between BEFs and LC cells, is easily observed by the human eyes. Another diffuser above BEFs or the micro-structure film whose periodic structure is randomized are provided to reduce Moiré effect [5].

Chapter 2 Radiometry and Photometry

2.1 Solid Angle

Fig. 5 Solid angle

Solid angle (Ω ) whose vertex is located in the center of a sphere cuts off an area on the surface of the sphere, as shown in Fig. 5. Solid angle, whose unit is steradian (sr), is defined as below:

A

Ω =

r

, (1)

where A is the cut-off area of the sphere and r is the radius of the sphere. In addition, Fig. 6 indicates the solid angle in the spherical coordinates and its representation is as follows:

2 2 z axis. Then, the solid angle can be obtained by integrating equation (2).

sin d d

φ θ

θ φ θ

Ω =

∫∫

. (3)

Fig. 6 Solid angle in the spherical coordinates

2.2 Radiometry

In optics, Radiometry is the measurement of optical radiation, which is electromagnetic radiation within the wavelengths range between 0.01um and 1000um;

in other words, that includes the regions commonly called ultraviolet, visible and infrared. In general, the unit of radiant flux

φ

of measurement is watt. In the field of backlighting, radiometry is often regarded as an adequate way to characterize the optical behavior. For example, the distribution of the extracting light and the property

of the light source can be described by radiometry.

Irradiance E, which is the area density of radiant flux, describes light distribution on the received plane. The definition is the radiant flux per unit area of a specified surface which the propagating light is incident on or passing though, as shown in Fig.

7. The representation of irradiance is expressed as below:

( ) (watt/m )2

Radiant intensity I, which is the solid angle density of radiant flux, describes the characteristics of light source such as a point source. The definition is the radiant flux per unit solid angle of a specified surface which the propagating light is incident on or passing though, as shown in Fig. 8. Radiant intensity I is a useful concept for the point source rather than the plane source. The representation of radiant intensity is expressed as below:

( )

d

(watts/sr)

I d

θ

φ

Ω . (5)

Fig. 8 Radiant intensity

Fig. 9 Radiance

Radiance L, which is the area and solid angle density of radiant flux, describes the characteristics of light source such as a plane source. The definition is the radiant flux per unit area and per unit solid angle of a specified surface which the propagating light is incident on or passing though, as shown in Fig. 9. The representation of radiance is expressed as below:

2 2

( ) (watts/m sr)2

cos

d d

L x d dA d dA

φ φ

⊥

θ

= = ⋅

Ω Ω . (6)

Lambertian distribution is usually used to suppose the behavior of light source and the optical property of the rough surface, like the radiation of LED and the distribution of light reflection of ideal diffuser. Lambertian radiator having a constant radiance L is independent of directions.

( , ) constant

L θ φ

= (7)

Then, the radiant flux is as blow:

2 ( ) cos

d φ

L x d dA

θ

(8)

The radiant flux

φ

received from a Lambertian radiator is proportional only tocosθ , the angle of observation. In other words,

φ

is proportional to the projected area of the source. Fig. 10 indicates the angular distribution of the Lambertian radiator, in which the highest radiant intensity is normalized to be 1.

Fig. 10 the angular distribution of Lambertian radiator

2.3 Photometry

Light is additionally measured by the use of photometry, which is defined as electromagnetic radiation detectable by the human eyes. The wavelength range of the visible light is restricted from about 360nm to 830nm. Therefore, the difference between radiometry and photometry is that radiometry includes the entire optical radiation spectrum. The unit of luminous flux is lumen. The definitions of illuminance, luminous intensity, and luminance are the same as irradiance, radiant intensity, and radiance, respectively. Table 1 indicates the definitions and units of radiometry and photometry.

Table 1 Radiometry and photometry

Radiometry Photometry

Definitions

Quantities Units Quantities Units

( v)

φ

( v)

For the different wavelengths of the visible light spectrum, the responses of human eye are not the same; in other words, the sensitivities of light with different wavelengths we observe are not equal. At the wavelength 555um, which corresponds to the maximum spectral efficacy of human eyes, 1 watt can be transformed into 683 lumens. The conversion of visible light from watts to lumens at any other wavelength is calculated by following equation:

680

V

( ) ( )

d

λ λ λ

Φ =

∫

Φ (9)

Where Φ is the luminous flux, ( )_v Φ

λ

is the corresponding radiant spectrum of the radiation source, and

V

( )

λ

is the spectral luminous efficacy function, as shown in Fig. 11.

Fig. 11 Visible response of different wavelength

2.4 Bidirectional Reflection and Transmission Distribution Function

Bidirectional Reflection and Transmission Distribution Function (BRDF and BTDF), which are representations for light distribution, describe how much light is reflected and transmitted when light makes contact with a certain material. As shown in Fig. 12, BRDF taken for example is a function of light incoming direction and outgoing direction of viewer relative to a local orientation at the light interaction point.

BRDF is defined as follows:

Simply speaking, BRDF and BTDF are the distributions in the hemi-spherical space when light is incident on the material with several different incident angles. Fig.

13 indicates BTDF of conventional diffuser passed through by the propagating light with incident angle of 0°, 20°, 40° and 60°. In general, BRDF and BTDF are adopted to describe the scattering phenomenon. BRDF and BTDF are assumed to be position-invariant, which means that whatever positions the propagating light is incident on, BTDFs are the same.

Fig. 12 The definition of BRDF

Fig. 13 BTDF of conventional diffuser

Chapter 3 Beam Shaping Diffuser

3.1 Beam shaping Film with Lens Array

Beam shaping means that the propagating light is redistributed to form a specified shape in the spatial domain, such as the rectangular, circular and square shapes of light distributions. Beam shaping film, which is a transmissive component, designed for some special illumination purposes enable the efficient use of light by controlling light propagation and directing it to form a specified shape in the designated regions of space, as shown in Fig. 14.

Fig. 14 Beam shaping film which controls light propagation and directing it to form a specified shape

Engineered Diffuser^TM developed by RPC Photonics is a typical example to shape the spatial distribution [6, 7]. Engineered Diffuser is based on the refractive micro-structures, as shown in Fig. 15. The micro-lens elements are arranged with

100% fill factor. Unlike conventional random diffusers such as bead-coating and micro-structure embossing diffusers, Engineered Diffuser can diffuse the propagating light into any specified shape in the far-field. Fig. 16 shows the beam shaping patterns controlled by Engineered Diffusers^TM with different micro-structures. In this chapter, we adopt the characteristics of beam shaping film with lens array resulting in the square beam shaping patterns to apply to the scanning backlight module.

Fig. 15 The micro-structures of Engineered Diffuser

Fig. 16 Examples of LED light control with Engineered Diffusers

3.2 Scanning Backlight Module

Owing to the limitation of response time of liquid crystal, the motion blur is an issue in LCD. When watching the moving object, we can see the residue image behind the object like a tail, as shown in Fig. 17. In order to reduce the phenomenon, a method that a black frame is inserted between two frames can be chosen. Inserting black frame, however, sacrifices a mount of brightness as a result the contrast decreases. Furthermore, the scanning backlight is adopted to solve this problem [8].

As shown in Fig. 18, the backlight contains several rectangular light guides, which illuminate from top to under of the panel sequentially depending on the signal scanning way of the panel. In the ideal situation, each light guide performs high uniformity and efficiency and dose not influence each other. Fig. 19 shows the wedge type tandem light guides for large size panel [9]. Base on the technique of the scanning BLM, the researches, field sequential color LCD and high-dynamic-range (HDR) display, further develop to promote the image quality [10~12].

Fig. 17 Motion blur of a car moving

Fig. 18 The scanning way of the divided light guides

Fig. 19 The divided wedge light guide designed for large size panel

We proposed a film, beam shaping diffuser, whose structure is spherical micro-lens array, to produce the square beam shaping pattern applied to the division areas in the scanning BLM. Fig. 20 illustrates the direct type scanning BLM in the ideal situation, in which the collimated light source illuminates and then the propagating light passes through the beam shaping diffuser to result in the division areas. This direct type BLM only composed of beam shaping diffuser and LED light source reduces some traditional devices such as light guide, BEFs and diffusers. It is expected that the system we proposed can have the uniformity approaching 90% and the tolerance of incident divergent light source.

Fig. 20 Direct type scanning backlight module with beam shaping diffuser

3.3 Simulations

3.3.1 One-sided Lens Array

The shape of the beam shaping pattern primarily depends on the shape of the micro-structures on the film. In Fig. 21, the beam shaping patterns in the far field of the different beam shaping diffusers which the collimated light passes through were demonstrated. The relief structures of the periodic micro-lens array are arranged on the film surfaces, on which each lens surface is spherical and the lenses have different shapes. Fig. 21 shows the examples of (a) square, (b) triangular and (c) hexagonal lens shapes. According to these patterns in the far field, which are the same as those of the lens structures, we know the fact that the shape of the beam shaping pattern is resulted from the shape of each element. The structures of beam shaping diffuser we adopted are the square lens array. The far-field diffraction pattern was simulated by a square lens array subject to different sags.

Fig. 21 The periodic lens array which are (a) square, (b) triangular and (c) hexagonal result in the beam shaping patterns (d), (e) and (f), respectively.

In the following simulations, we chose the spherical lens sculptured into a square shape [Fig. 22(b)] while the square periodic lens is arranged as Fig. 21(a). The pitch p is fixed at 100um. The light source emits collimated light with circular shape with diameter of 0.5mm. Fig. 23 shows the patterns of beam shaping diffusers with different sags, which are observed on the plane located at one focal distance (f) and 100f. The patterns of the sags of 10um and 20um are square at 100f because the shape of the lens is square. However, the distortion of the square patterns is observed obviously while the sag is 30 and 40um. The reason is that spherical aberration occurs.

Fig. 22(a) The spherical surface lens (b) The lens sculptured into square shape

Fig. 23 The bean shaping patterns of the lens array diffusers with different sags observed on the plane located at focal distance of f and 100f

Fig. 24 Spherical aberration

Spherical aberration can be defined as the variation of focus. Fig. 24 is a somewhat exaggerated sketch of a simple lens forming an image of an object at a great distance away. The rays close to the optical axis come to the focus point which is very near the paraxial focus position. As the ray height at the lens increases, the position of the ray intersection with the optical axis moves farther and farther from the paraxial focus. Therefore, the larger the lens is, the more incident light deviates. As a result, the beam shaping patterns are not square and have distortion on the edge of the

aberration. The more obviously spherical aberration is shown, the larger light spot is.

Furthermore, we observe the uniformity and the profiles of the beam shaping patterns as the increase of the numbers of lens, as shown in Fig. 25. The sag of each lens is fixed at 10um, and the pitch is 100um. The light source emits collimated light, and the observed plane is located at 100f. As the numbers of lens increase, the higher uniformity can be obtained, and the hot spot in the center of the beam shaping pattern is more indistinct.

Fig. 25 The uniformities and the profiles of the beam shaping patterns of the diffusers with different numbers of lens

3.3.2 Both-sided Lens Array

Beam shaping diffuser with both-sided lens array shown as Fig. 26(b) is able to increase the tolerance of the imperfect collimated light which is assumed to emit with a divergent angle. The beam shaping diffusers with one-sided lens array mentioned above have low tolerance of the imperfect collimated light, and the beam shaping

pattern is never formed as square shape, as shown in Fig. 27. In order to determine a square shape, square shape ratio is defined as blow:

square shape ratio ^P

A

, (11)

where

A is the area of the beam shaping pattern, in which the boundary is the full

_P width at half maximum (FWHM). And

A is the area of the perfect square shape.

_S Both of the length,

l and

l , must be equal.

Fig. 26(a) One-sided lens array (b) both-sided lens array

Fig. 27 Imperfect beam shaping pattern

The beam shaping patterns in Fig. 28 indicate the comparison between the perfect and imperfect collimated light passing through the one-sided square lens array film.

The diameter of the light source is 2mm. The distance between the beam shaping diffuser and the light source is 0mm. The lens size is 100um, and the sag is 20um. The observed plane is located at 100f. In Fig. 28(c), the pattern with square shape ratio of 79% obviously is not controlled by the beam shaping diffuser. Therefore, the one-sided lens array film has low tolerance with incident divergent angle.

Fig. 28 The patterns resulting from the perfect and imperfect collimated light which passes through the one-sided square lens array film

In order to solve this problem, we proposed a method referring to the configuration in projector [13]. The flat plane on the opposite of the film is replaced with lens array in this case. Such two-sided lens array film is able to improve the drawback of low tolerance of one-sided film. The lenses on the two opposite lens array must coincide, and the distance between them is the focal length, f = 355um. Fig.

29 illustrates the lens array pair is able to redirect the incident light with an inclined

angle 10° shown as the red rays. The deviating light can be redirect to the area in which the perfect collimated light (white rays) produces the beam shaping pattern.

The beam shaping patterns in Fig. 30 indicate the comparison between the perfect and imperfect collimated light passing through the two-sided square lens array film. The difference of the square shape ratios is small than 1%. Consequently, the beam shaping diffuser arranged by two-sided lens array has better tolerance with incidence subject to one-sided lens array.

Fig. 29 The opposite lens correcting the red light with an inclined angle

Fig. 30 The patterns resulting from the perfect and imperfect collimated light which

3.3.3 Applying to the Backlight Module

Both-sided beam shaping diffuser is able to be adopted for the divided areas in the scanning BLM. The material of the film is PMMA, and the substrate thickness is 125um. We designed that the lens pitch is 100um and the sag is 24um. Then, two films are put together as a result the thickness of the film is the focal length of 250um, as shown in Fig. 31. The tolerance of light divergent angle of the diffuser we designed can be approaching 15°. Fig. 32 illustrates the direct type BLM within both-sided beam shaping diffuser. The distance between the detector and the both-sided micro-lens array diffuser is 120f, which is 30 mm. The light source close to the film illuminates imperfect collimated light with divergent angle 15° and the beam size is 10mm. One light source whose beam size is 10mm can produce a square beam shaping pattern whose length is 24mm. when combining 9 light sources, the length of the pattern is 54mm, as shown in Fig. 32(b), and we can get the uniformity achieved to 86%.

Fig. 31 Two one-sided lens array films put together

Fig. 32(a) The profile of the direct type BLM (b) the beam shaping pattern (c) 9 LED light sources

Chapter 4 High Gain Diffuser Film

4.1 Light Diffusion and Collection

Optical films are usually employed to promote light uniformity and collimation in the conventional backlight module (BLM). As shown in Fig. 33(a), after emitting from the light guide, light propagates through diffuser and is scattered into all directions of the hemi spherical space. Then the scattered light is redirected by two orthogonally arranged brightness enhancement films (BEFs) and collected to the normal direction. In order to reduce the production cost of the BLM, many novel

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