LCD Backlight Methodology and Applications Using Optical Enhancement Films
A good backlight unit is essential to the performance of any AMLCD, but it is often not enough to achieve optimal optical performance. Light must be properly managed regardless of the source. This article details the vital role played by backlight enhancement films in the performance of AMLCDs.
by Adi Abileah
ACTIVE-MATRIX liquid-crystal displays (AMLCDs) are transmissive devices. They do not produce luminance, but rather they act as light valves forming the image under an array of color filters and transparent regions of liquid-crystal material. Viewing an image on the front of an AMLCD panel requires a very bright and uniform source of light from behind the panel, which spreads over the entire surface area of the AMLCD. Uniformity in this case is generally defined as the same amount and color of luminance at any location in two dimensions (x,y) behind the LCD panel. This light source is commonly known as the backlight unit (BLU).
The BLU can consist of one or multiple light sources, ranging from stick fluorescent lamps and U-shaped lamps to flat lamps or light-emitting diodes (LEDs). Most BLUs in use today use cold-cathode fluorescent lamps (CCFLs), but these are increasingly being replaced with LEDs. Other sources such as powder electroluminescent (EL) devices, field-emission devices (FEDs), and organic light-emitting diodes (OLEDs) to be used as flat lamps have been proposed and in some cases demonstrated, but in general have rarely been used. From an optical point of view, these light sources can be divided into four categories:
1. Line sources, where the light emits continuously along one axis of the lamp.
2. Multi-line sources, where there are two or more continuous lines of emission.
3. Area sources, which can be envisioned as two-dimensional flat emitting surfaces.
4. Point sources.
However, achieving a two-dimensional uniformly lit area from these light sources is sometimes challenging. This article is concerned with methods that properly manage light from these various types of sources and specifically describes the latest technology in one of the most critical components of today's AMLCD panels: backlight enhancement films.
Basics of BLUs
The basic steps designers use to generate uniform light from BLUs are easy to appreciate and can be summarized as follows:
• Distribute the light over an area (by multiple light sources or a light guide).
• Extract the light forward.
• Improve the area uniformity.
• Shape the angular distribution of the light.
• Control or improve the polarization state of the light to match the LCD polarizer.
However, while these steps seem simple, accomplishing these within a very thin mechanical package and maintaining an acceptable level of light loss in the process can be a daunting challenge. BLU architectures can be generally summarized as follows:
1. Multiple stick lamps in a cavity surrounded by reflecting materials – this is commonly called a "direct" or "cavity" backlight (Fig. 1).
2. Multiple two-dimensional arrays of LEDs in a cavity – this approach has been used recently for high-brightness displays for outdoor applications and for high-dynamic-range (HDR) backlights.
3. A light guide that is illuminated with edge illumination of one or multiple stick lamps where the light guide transfers the illumination from the edge to the rear of the BLU (more details on this below).
4. The same light guide as in item 3 above, but with a strip of LEDs in place of the stick lamp.
The cavity backlight approach (items 1 and 2 above) demands the use of a good reflecting material in the sides and rear of the cavity to capture most of the light emitted from the lamps. Here, a simple white matte reflective paint can be used, but more often designers turn to engineered diffusing white reflector films that exhibit reflectivity values better than98% while being more stable over time and temperature than painted coatings. There are few films on the market that can do this job, one of the most common being Enhanced Specular Reflector (ESR) film produced by 3M.
The light reflected by the rear reflector includes not only the original emission from the lamps, but also any light that is reflected back into the cavity from higher surfaces. If that light is not reflected, it will be lost, thus affecting efficiency. Therefore, in many cases, the rear reflector also serves as a recycling function as well. Some optical models based on actual designs show that some light will be reflected back and forth as often as six times. Hence, it is very important that the reflectivity of the diffuse reflector will be as close to 100% as possible. For instance, reflectivity of a mere 98% will yield only 88% of the original light component remaining after the sixth reflection. For 95% reflectivity, this recycling efficiency will drop to 74%, a 26% loss in efficiency.
Some designs utilize mirror-reflecting surfaces (true specular reflectors). These surfaces do have a higher reflectivity than diffuse reflectors. Unfortunately, during recycling, they can cause light to be trapped bouncing back and to the edges, thus never allowing it to escape to the front of the system. Hence, the original reflectivity advantage is negated by the lack of scattering advantage, and little measurable recycling seems to occur. Specular reflecting materials are either shiny metal or PET coated with shiny organic material. Most BLU designs today incorporate diffuse reflectors.
Edge-Lit Light-Guide BLUs
In the edge-lit light-guide approach (Fig. 2), the major challenge is to capture most of the light from the source into the edge of the light guide. Collection efficiencies can range from as low as 25% to as high as 85%, which means a significant part of the optical design is affected in this first area. Higher collection efficiency can be achieved by matching the directed angles of the light source with the light-guide acceptance angles (the numerical aperture). A CCFL emits light in all directions along the long axis. In order to capture that light, a mirror reflector is typically put behind the lamp to direct all the light opposite the light-guide direction into the light guide. This reflector is usually a shiny, curved material and should have a parabolic shape with the lamp at its focal point. However, these lamps are relatively wide in diameter and cannot truly be considered as a point in the focus. In addition, because they are opaque, they block some of the light reflected from the back. Hence, most simple stick-light-based designs have only been able to achieve collection efficiencies of 40–60%.
Fig. 1: An example of a cavity backlight with a multiple "stick" CCFL.
Fig. 2: A cross section of an edge-lit backlight.
However, recently there have been improvements, some stimulated by the use of point-source LEDs that have been equally effective in improving the collection efficiency in all edge-lit configurations. When an array of LEDs is used as the edge-light source, such as that shown in Fig. 2, the light emitted takes the form of a linear array of point sources. Not only is it important to match the acceptance angle of the edge of the light guide, it is now also necessary to address the linear non-uniformity, sometimes referred to as the "headlight" effect (Fig. 3). This is particularly acute in systems that employ red, green, and blue LEDs rather than white LEDs.In this case, even small non-uniformities in the light mixing can produce colored headlight patterns in the resulting BLU profile. Two new generations of optical films are now being used to make this problem easier to solve:
• Film-based enhanced specular reflectors can be shaped around the LED or CCFL light sources and formed to produce the best match to the acceptance angle of the light guide without the need for expensive tooling and molding. These films are available from a number of suppliers worldwide and are achieving higher reflection efficiencies all the time.
• A new generation of light-distribution films is being used between the light source and the leading edge of the light guide to match the acceptance angle and, in the case of LEDs, to efficiently spread out the light in the linear axis to eliminate the hotspots. Candidates for this are the Brightness Enhancement Films (BEF) from 3M, which provide image-doubling capabilities in a variety of optical distances; a directional diffuser by Fusion Optix (Fig. 3), which spreads the light in a horizontal direction but concentrates it in the vertical direction; and similar materials from Luminit, which are holographic diffusers called Light Shaping Diffusers (LSDs) with variable parameters, depending on the specific design.
While it is hard to quote specific numbers for collection efficiencies in these cases, primarily because the results are so heavily design-dependent, one can expect a 20–50% improvement in light efficiency over a similar design not employing these materials.
Once the light is uniformly and efficiently delivered onto the edge of the light guide, the problem remains to effectively extract the light from the light guide uniformly in two dimensions. The most commonly used method is a dot pattern of white reflecting material that is painted through a screen mesh on the light-guide material. The density of the dots is inversely proportional to the light emitted without any pattern; therefore, achieving uniform extraction is a matter of simply calculating the light density needed and the remaining light available at each location. For a first approximation, the density of the dots should be linearly increasing going away from the light source, and uniform in the other dimension. A white reflector is usually placed behind the light guide to direct the light forward. This can be a similar film material as mentioned above (e.g., ESR film from 3M) or paint on the bottom of the housing. In some designs (e.g., those made by Global Lighting Technologies), the dot pattern is made of dot etching in the light-guide material, giving the effect of tiny lenslets. Regardless of the methodology, there is still a need to optimize the dot density for each system in order to obtain the desired uniformity.
Optimizing Uniformity and Efficiency
Now, the challenge becomes the management of the light to optimize its uniformity and to achieve the most efficient and uniform coupling possible into the LCD panel. There are usually three main steps required – diffusion, light shaping for angle correction, and pre-polarization – and they can be accomplished with as few as three or as many a six separate optical films.
First, diffusion is employed to randomize any spatial coherence in the light and make it as two-dimensionally uniform as possible, effectively re-directing the light to random directions in a distribution that is more or less Lambertian. Today, there are two main types of diffusers that both provide marked improvements over conventional frosted or "milky" glass diffusers that have been previously used.
• Volume diffusers are made from small pellets of plastic material that are compressed at high temperatures to generate a thin film. These films are semi-transparent and have several levels of haziness. An example is the Keiwa Shoko materials with choices of thickness and haze factors (higher and mild).
• Surface diffusers where the front surface of the plastic film is embossed to have multiple "hills and valleys," which generates by refraction random orientations of light rays. For example, GE makes surface-diffuser films. Their distribution is mostly Lambertian. However, they could be designed to have preferred orientation. One example of this is the holographic diffusers being sold by Luminit, which can be made with a wide variety of image profiles and customized to a particular BLU design by optical imaging techniques (Fig. 4).
Fig. 3: This LED array strip is diffused into a thin light guide using material from Fusion Optix. Note the headlight effect along the edge of the glass.
Once the light passes through the diffuser, it is presumably very uniform, but unfortunately it is now very randomly oriented. Angle correction is employed next to direct the light in the desired angles through the LCD panel. In many cases, the viewing cone of the LCD is matched by the light direction from the BLU to maximize luminance in the intended viewing directions. This can be accomplished in several different ways.
Brightness Enhancement Film (BEF). Brightness Enhancement Film (BEF) by 3M is the most commonly used film in backlights. Its purpose is to concentrate the light in one direction. For instance, people view displays mostly horizontally from the center and not to the vertical extremes of the screen. Therefore, concentrating the vertically directed light into a narrower cone of luminance increases the efficiency significantly. BEF films are prismatic in nature and generally cause steering of light in one linear direction at a time. They behave similar to a cylindrical Fresnel lens with a horizontal axis and focus at infinity.
One additional property of the BEF is its image-splitting property, which makes each point appear to be a doublet of points. This fills the gaps and makes the light more uniform. The BEF materials have mostly grooves that have a top angle of 90° (a right angle). The facets are smooth and generate refractions to smaller angles of light coming from below, causing a concentration of light. However, at very small angles, the facets act as retro-reflectors, reflecting the light backwards. Therefore, it is important to have good reflectors at the bottom of the structure to capture this light and re-cycle it forward.
A variety of films are available, but all are characterized by a top angle (90°) and pitch between the grooves (e.g., 50 μm). BEF II is a polyester substrate having an acrylic resin prismatic structure that is coated. It comes in top angle (¼) / pitch (μm) combinations of 90/50 or 90/24. The newest films (BEF III) come in variable (random) prism heights to reduce the Moiré effect caused by the pixel structure. The versions with notation M (Matte) indicate that the sharp edge of a prismatic structure is softened and the bottom surface is treated to give a softer fall-off in brightness as a function of the angles. The versions with notation T mainly indicate very thin film. The principle behind a BEF film is shown in Fig. 5.
Fig. 4: (a) An electron microscope image of the surface of a holographic diffuser by Luminit. (b) A holographic diffuser from Luminit over an array of white LEDs. (c) An electron-microscope image showing the molecular structure of the bulk material of a Fusion Optix diffuser. (d) A Fusion Optix directional diffuser (elliptical) spreading light in one direction.
Light-Shaping Devices (LSDs). Light-shaping devices (LSDs) made by Luminit (a division of POC) is a family of films that serves as a diffuser with an embossed surface. However, the "hills and valleys" are programmed to have an angular behavior with concentration along one axis that is different relative to the other, generating an elliptical cone of concentration. In some cases, it is circular. The master pattern to the embossing is made by a hologram using a laser as a diffused light source. For this reason, this element is referred to as an holographic optical element (HOE). One specific advantage of LSDs is that they can be tuned to a variety of two-dimensional distribution angles, reducingthe total number of intermediate films required.
Alternate Prism Films. Alternate prism films from companies such as Fusion Optix (Fig. 6) have recently entered the marketplace as a direct competitor to 3M's BEF. These combine two types of materials: volume diffusers and prism films. The volume diffusers, which in this case are the molecules of proprietary materials that behave non-symmetrically, are embedded within a resin that has a different index of refraction. With pre-defined recipes, they will achieve angular behavior as a concentration effect. The prism films are very similar to the BEF materials of 3M and behave as concentrators in one direction as explained above.
Often, designs employ two to as many as four intermediate light-steering films to tailor the angular distribution of the light to their needs. This is a relatively expensive process; in general, these materials cost many dollars per square foot. Many companies are actively working on further improvements that combine the properties of both diffusers and prisms. This is ongoing work that bears watching in the coming years.
As a final step, the light coming after the diffuser and BEF is mostly uniform over area and is concentrated in the preferred direction that it will be used after the display. However, the light has all directions of polarization. The bottom of an LCD has a polarizer that transmits in one direction and absorbs light from other orientations. To further improve the light efficiency, developers have been turning to reflective pre-polarizers that pass only the light with the correct polarization orientation and reflect all other orientations. 3M pioneered this material to reflect the unused polarization back to the light guide where it can hopefully be recycled. The material is called Double Brightness Enhancement Film (DBEF). This material needs to have a proper reflector underneath to recycle the reflected polarized light. When the light goes back into the light guide, it reflects against the rear surface and its polarization orientation is rotated. When it returns to the top of the stack, it is now in a suitable orientation to pass through the LCD. The recycling bounce can happen many times, but there are significant losses each time.
Fig. 5: The basic behavior of optical rays in brightness-enhancement films (BEFs).
Fig. 6: A structure of a side-illuminated LED backlight using a Fusion Optix light-control material embedded with a prismatic structure to enhance the illumination.
The BEF is a good reflector at small angles coming from the top because it has a retro-reflector characteristic (for small angles) and therefore the BEF helps to recycle light reflected by the DBEF. Polarizer-manufacturer Nitto Denko is making a product called Polarization Conversion Film (NIPOCS), which is a DBEF laminated to the bottom of a polarizer, applied directly to the AMLCD panel.
The amount of light efficiency improvement using the reflective polarizers is highly dependent on the overall BLU design. Theoretically, efficiency gains as high as 100% should be achieved. However, practically, the DBEF gain is between 30% and 60% (luminance increases to 1.3x–1.6x). The DBEF material is maintaining the angular behavior and neutral colors, but will work well only with proper reflections behind to recycle the reflected polarized component of the light.
It is easy to imagine that a significant development effort is ongoing in the optical-films sector. In the past 10 years, the improvements realized with new materials have generally doubled the light efficiency of BLUs. It is conceivable that the next wave will be homogenized films that perform several functions at once, resulting in an overall reduction of individual layers and thus further reducing cost. While this article has noted the offerings mainly from several U.S.-based companies, several Asian companies are also producing similar competitive products and pursuing similar paths of innovation. •