The demand on TFT-LCDs to create bright high-resolution full-color images in a thin packageand at low power levels has been unrelenting. This engineering challenge would not be possible without the use of optical films to collect, direct, filter, and otherwise "manage" the light through the display. Growing demand for "green" LCD panels, the use of LED backlights, and new power-consumption regulations are driving new backlight designs to add more optical films, creating opportunities for filmmakers who can provide improved technology.

by Paul Semenza

TFT-LCD PANELS are built from several components, including glass substrates, liquid-crystal mixtures, coatings, frames, and optical films. These films play an important role in the TFT-LCD module in aiding the control, diffusion, and management of light to improve viewing angle, contrast ratio, and other performance metrics.

Components of the Display Optical Stack

TFT-LCD optical films include polarizers, which linearly polarize the light into and out of the LCD panel, and backlight optical films, which are films or sheets that improve optical efficiency, recycle light in the backlight system, or diffuse brightness for uniformity. Backlight optical-film types include normal and multi-functional prism sheets as well as micro-lens, reflective polarizer, diffuser, and reflector films (Fig. 1). While two polarizers are always used, all of the other films can be used in varying quantities, or not at all, depending on the panel design, which, in turn, depends on screen size and application, cost, physical design, and power-consumption requirements.

Fig. 1: There are several different types of films used in TFT-LCD panels, including the ones used around the LC cell, within the backlight, and between the backlight and the cell.

Polarizers

TFT-LCD polarizers are themselves composed of different films (Fig. 2). Triacetyl cellulose (TAC) is an encapsulation film used to support and protect the (polyvinyl alcohol (PVA) layer that performs the polarization.

Fig. 2: Polarizers are multi-layered films combining structural, polarizing, and light-management functions.

The requirements for TAC are high light transmittance, low retardation, low color shift, high adhesion to PVA, and ease of manufacturing. Compensation films correct for the phase-retardation property of liquid crystals, which otherwise can result in contrast-ratio reduction, gray-scale inversion, and color shift at particular viewing angles. Surface treatment improves visibility and prevents loss of contrast due to reflection. Typical treatments include anti-glare, low reflection, anti-reflection, hard coating, and anti-static.

Prism Sheets – Multi-Function Prism Films

Prism film, also called lens film or prism sheet, contains micro-replicated prism structures on polyester (PET) or polycarbonate (PC) film. Prism sheets are used in the LCD backlight module to enhance luminance by directing off-axis light from the light source through the prism structure. A single sheet of prism film can direct light toward the viewer in the horizontal or vertical plane only; off-angle light in the other plane is not affected. However, by stacking two prism films – one for the horizontal off-axis light and one for the vertical off-axis light – on top of each other, atwo-dimensional optical system can be achieved.

Brightness-enhancement film (BEF) is the marketing name for prism film used by 3M. It is the dominant prism film, but is being challenged from several directions. First, some of the basic patent protection held by 3M on its BEF product is expiring, opening the door to competitors. Also, in notebook PCs, an alternative technology has emerged that marries a reverse prism film with a prism-functioning backlight to replace the two stacked prism sheets. Another emerging trend involves micro-lens or diffuser films, which combine diffuser and prism functions in one film (Fig. 3). As backlight brightness increases and cell transmittance improves, micro-lens film can replace conventional prism film or reflective polarizers, thereby reducing cost.

Fig. 3: Micro-lens film combines the functions of both prism-film and diffuser sheets, providing an optical gain in between that for these two sheets, and potentially reducing cost.

A further development is lenticular films, which have a lens arrays with rounded vertices and curved surface profiles and simultaneously exhibit light convergence and diffusion (Fig. 4). These films do not require protective films, do not suffer heat-induced waving in the luminance profile, and are easy to handle in the assembly process. Lenticular-type films offer 3–8% higher brightness (from dual collimation and higher lens density) than micro-lens film, as well as lower cost.

Fig. 4: Prism-sheet and multifunction-film technologies are compared.

Reflective Polarizers

Reflective polarizers recycle light from the backlight system that would be absorbed by the LCD and can increase brightness by 50–60%. These films select incident light with a specific polarization state to pass through and reflect the other polarization state back into the backlight where it can be recycled; they are used in TFT-LCD products requiring high brightness, such as TVs, high-end notebook PCs, and high-end monitors. There are different approaches to making reflective polarizers: the most common are multi-layered polarizers, which have hundreds of layers that are less than 100 nm thick, and constructions consisting of cholesteric liquid crystal and wire grids.

3M has strong IP and technical positions in multi-layered reflective polarizers, which it markets as dual-brightness-enhancement film (DBEF), but several panel makers are starting to seek alternative solutions. The TFT-LCD reflective-polarizer market grew rapidly over the past year, due to the need for green design, the use of LED backlights, and the trend toward reduced power consumption. However, increasing panel transmittance and LED luminance will impact the usage of DBEF.

Diffusers

Diffusers spread and randomize the light across the display to minimize brightness variations. In the most common type of diffuser plate, acrylic beads are spread in a resin layer, creating a medium whose refractive index varies with location; when light passes through this layer, the beads cause refractions, reflections, and scattering that lead to the overall optical-diffusion effect. Micro-lens arrays are also being used, and hologram and engineered approaches are in development (Fig. 5).

Fig. 5: Coated beads in resin are the most common approach to manufacturing diffusers, but other approaches that could result in improved performance are in production or development.

Reflectors

There are two types of reflector film: the most common are backlight reflectors (also called "white" or "silver" reflectors), which reflect and recycle light from the light-guide plate, and lamp reflectors, which reflect and recycle light inside the lamp cover. White reflectors are typically about 200 μm thick and have a reflection ratio of about 95%. The reflector film is made of PET. A UV coating is typically used on the surface of the reflector to limit color change. There is also a layer on the bottom of the reflector to increase reflection from the bottom surface. White PET, which is the base material for reflectors, is in shortage due to strong demand from photovoltaic modules. The global capacity for white PET is approximately 4k tons/month, and PV manufacturing is consuming about 1.5k tons/month. PET makers generally find that the PV business is more profit-able than displays. With the tight supply, some PET suppliers have raised prices, and some backlight units have adopted transparent PET.

Wide Variation in Film Stacks

Film stacks can vary widely, depending on requirements including cost, brightness, power consumption, thickness, and type and brightness of the backlight used. In order to maintain brightness with high energy efficiency for notebook panels, two cross-stacked prism sheets are necessary. Therefore, a stack of four films (upper diffuser plus cross prisms plus lower diffuser) is still the mainstream in notebook backlight units (BLUs) (Table 1). In order to reduce cost, the upper diffuser can be replaced by a prism sheet with a haze function (back coating or modified texture) and a conventional prism. Monitors typically use a three-film stack (upper diffuser plus prism plus lower diffuser). Korean panel makers have widely adopted two-film stacks (multi-function prism plus lower diffuser).

Film stacks in LCD-TV BLUs are highly dynamic and diverse, evolving every few quarters. Various optical films are available to LCD-TV BLU makers, and there is more design freedom to find the most cost-effective film stack that meets specifications (Tables 2 and3). Lenticular films, with performance between that of micro-lens and prism-sheet varieties, are used to reduce the number of cold-cathode fluorescent lamps (CCFLs) and are used with edge-lit LED BLUs to replace the prism sheets (Table 4).

As the number of LED chips used in edge-lit LED BLUs decreases, cross-type prism sheets (horizontal and vertical) are being used to maintain brightness. However, this typically results in a narrow viewing angle. The heat from the LEDs creates difficulties for reflectors and light-guide plates (LGPs), including warping. To provide stiffness and resist bending at high temperatures, thicker optical films are used.

Market Trends in Optical Films

Demand for all TFT-LCD optical films is expected to grow from 567 million square meters in 2009 to 700 million square meters in 2010. However, due to price declines, TFT-LCD optical-film revenues are expected to grow more slowly, from $9.3 to $10.5 billion (Table 5). Increased demand in total display area is leading polarizer and backlight films to grow by more than 25% in area in 2010. But while polarizer revenues will benefit from LCD TV, resulting in 16% growth in 2010, backlight films are facing price pressure and are expected to grow by less than 5%.

Source: DisplaySearch Quarterly Display Optical Film Report.

The supply of polarizers was tight in 2009 and early 2010; some makers had production issues and some makers closed fabs. However, the top five polarizer makers have been expanding capacity, which will ease the tight supply. The supply of polarizers can be limited by shortages of key materials such as PET films used as protection and release films. Unyielded TFT-LCD polarizer capacity is expected to increase by 23% to 463 million square meters in 2010. Nitto Denko and LG Chemical increased production line speed, and Sumitomo, Samsung Cheil, BMC, and CMI are expanding capacity. LG Chemical leads polarizer shipments in terms of both units and area, followed by Sumitomo in units due to strong notebook penetration and Nitto Denko in terms of area due to higher shipments of LCD TVs.

Prism-sheet revenue is expected to increase 4% in 2010 because LED models use more prism and lenticular film. However, the lenticular-film price is lower than that of prism film; therefore, the revenue growth rate is lower than the unit growth rate. Prism-film revenue is likely to drop after 2010.

After growing very rapidly in 2008, the trend toward using multi-function prism film for monitors, the dominant large-area application, slowed in 2009. Part of the reason is due to low-yield issues at BLU makers, and another is that LED models will use normal prism film for better brightness enhancement. Prism film with reflective polarizer functionality is widely used in portable applications, providing better brightness enhancement.

Micro-lens-film revenues are expected to fall in 2010 due to the reduction in the number of lamps used and LED models using a back prism and lenticular structure for increased optical performance. Large-sized TV panels (37 in. and larger) are the dominant application.

The reflective-polarizer market, where TV is the dominant application, grew in 2009 and 2010 due to the expansion in green designs, LED models, and low-power trends. Increasing panel transmittance and LED luminance will impact the usage of DBEF models after 2011.

Reflector-film demand is growing along with large-area demand, and increased performance requirements are keeping prices high. Also, the rapid growth of demand in the solar-cell industry has caused tight supply conditions.

Diffuser demand continues to grow with area, but multi-function prism film includes the top diffuser function and will reduce the usage of diffusers. In the meantime, some lenticular films used in LCD TVs do not require diffusers.

Looking Forward

Polarizers are essential for LCD panels, especially in wide-viewing-angle applications, while films that improve the luminance of the backlight and the LCD module are very important for cost reduction and green panel design; both grow with panel demand. However, cost pressure in the optical-film market is limiting revenue growth. As this trend is likely to continue for the next few years, making technical improvements to reduce production and materials cost is the most critical focus for polarizer and backlight optical-film suppliers. Especially for LCD TV, where power consumption is a major focus of set design, panel makers need to reduce the number of lamps and LEDs that use prism sheets to maintain brightness. Prism sheets are gradually becoming a requirement in LCD-TV panels, which will support increases in prism-film average price and help to reverse the revenue decline. •