Nanotechnology enhances film substrates for electronic displays

Paul Boughton

Devices with high resolution displays are now a part of everyday life; mobile phonesPDAsin-car entertainmentsatellite navigation equipmentindustrial instrumentation and control systems provide unmatched levels of performance and reliabilitywhile being smallerslimmer and more lightweight than ever.

Less visible though is the complexmulti-layer arrangement of materials and components behind each applicationresponsible for ensuring optimum light transmissionmechanical properties or protection.

Manufacturers of electronic displays have traditionally had to find a compromise between functionality and the characteristics of the materials usedoften limiting the quality of the display. In particularthe optical film materials used to produce light diffusion layers in LCDsand the exterior or upper layers in touch screens and OLED panelshave had a considerable impact on performanceaffecting factors such as light transmission and uniformitypanel flexibilityviewing anglesreflection and glareand resistance to chemicals and abrasions.

Superior light diffusion

Light diffusion films likefor examplethe MacDermid Autotype BLD rangeare designed to provide uniform backlit illumination for LCD displayswith as much light as possible reaching the viewer. Typicallythe construction of a backlight unit comprises a source of illuminationsuch as LEDs or a cold cathode fluorescent lamp (CCFL)plus a reflector and light guideabove which a prismatic layersandwiched between two light diffusion layersis mounted. The diffuser films are required to provide optimum levels of light transmissionhelping to minimise the power required to drive the backlightwhile effectively diffusing the light so that it is uniform across the LCD.

Traditional light diffusion films that use a PET base material with an acrylic or fixative surface coatingrequire diffusing particles and filler materials to provide acceptable properties. Some of these fillers can adversely affect the optical characteristics of the filmscattering or absorbing light and reducing levels of light transmission and uniformity.

As a solution to this problema new generation of films has been developed by MacDermid Autotypewhich eliminates the need for filler materials and features optically clear hard coatings that significantly improve the brightnessdiffusion and homogeneity of emitted lightwhile reducing light absorption losses through the diffuser to less than 1percent.

The new films use a mechanically stablehigh clarity PET basewith a micro-roughened anti-Newton back coatingto eliminate Moiré interferenceand a uniquely formulated front layer coatingthat is resistant to chemical and physical damageyet produces outstanding optical characteristics. In particularthe new films allow over 92percent of the light to be directly transmittedwithout affecting the colour balance or uniformity of the light emitted.

The robust mechanical properties of the new BLD Diffuser films make them easy to handle in production. There are no particles or fillers to become dislodged and therefore to contaminate the backlight production processleading to greater production yieldswith significantly less scrapand higher overall product quality. In additionthe new films are manufactured in the latest cleanroom production facilitiesensuring that the quality of the diffusion hardcoat layers are consistent and uniform across every sheet of film producedeliminating the problems of pin holes and scratches that were often associated with earlier generations of materials.

Developments in coating technology

In many electronics applicationsthe mechanical and chemical properties of the film substrates are also of importance. Touch screensflat panel displaysand lenses for mobile phones and PDAs require a durableperfectly clear and often flexible layerwhich is free from light reflections and glare. A number of new developments in coatings technology look set to improve the overall performance of these outer optical layers.

For instancean innovative film developed jointly by MacDermid Autotype and the Fraunhofer Institute for Solar Energy in Germanyreplicates the nano-structures found in the eyes of mothswhich have evolved to collect as much light as possible without reflectionin order to prevent moths being detected by night time predators.

The new filmcalled Autoflex MothEyeis now being manufactured in the UK using nano-replication techniques and is the first of its kind to combine both anti-reflective and anti-glare propertieswith a hardcoat finish that is resistant to scratcheschemicals and fingerprints and which has an exceptional level of optical clarity. The result is a toughdimensionally stable and formable filmusing PMMAPCPET and TAC substrateswhich reflects less than 1percent of visible lightregardless of viewing angleand eliminates the problems of iridescence and light glare that are often associated with conventional display materials.

The new film will be of particular benefit in small high resolution portable deviceswhere the combination of colour with mobility has traditionally had an adverse affect on battery lifeas it allows enhanced perceived brightness or reduced power consumption for any type of LCD panel without sacrificing reproduction characteristics or view angle.

Electro-magnetic interference

Electro-magnetic interference (EMI) shielding is needed to protect electronics equipmentby either filtering out conducted electromagnetic interference or shielding radiated electromagnetic wavesand must be incorporated into the design of the new film substrates as well as optical and mechanical demands.

The addition of display windows to equipment effectively creates a break in the shieldingrequiring a screen or mesh to be incorporated to attenuate the electromagnetic waves. Conventionallymanufacturers have used a screen with a regular square pattern. This severely restricts light transmission and can create moiré effects when placed over a computer display screen.

A company called Optical Filters Ltd has recently designed a solution to this problemusing a mesh pattern with rectangular openings formed in an extremely fine copper wire screenwhich is coated by means of a proprietary chemical blackening process and then integrated into a multi-layer construction between outer layers of a hardcoated polycarbonate optical film. This new design provides a superior level of light transmission of over 80 per cent and virtually eliminates the common problem of moiré fringing.

The specially coated optical film is crucial to the success of this designcreating a protective outer layer that is resistant to scratchesabrasions and a wide range of solventswhile exhibiting excellent optical clarity and mechanical characteristics.

These advanced optical film substrates are enabling manufacturers of a wide range of electronic products to improve the functionality and performance of LCD and other graphics displaysand giving product designers considerable potential to develop the capabilities of display screens even further. 

Dr Keith Parsons is Technology Manager Light Management FilmsMacDermid AutotypeWantageOxonUK.


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