Understanding the Function of Transmissive Film

At the 2008 Consumer Electronics Show (CES) in Las Vegas, Nevada, visitors marveled at concept cars showcasing the latest advancements in car audio systems, onboard technology, and passenger comfort. Among them was the sleek black BMW X5, presented by Visteon and 3M.

The black BMW showcased at CES served as a promotional prototype by Visteon and 3M, demonstrating the impressive results of their collaboration. The vehicle featured 50 innovative technologies, ranging from rear bumper distance sensors to an eye-catching 3D dashboard display.

One of the most fascinating, albeit understated, innovations was the integration of transmissive film, which significantly improved the clarity and brightness of the LCD displays on the car’s dashboard. This technology derives its name from transmissive LCD screens, which utilize a backlight to illuminate the individual pixels. Transmissive LCD screens are commonly found in computer monitors, premium LCD televisions, PDAs, cell phones, and even some digital watches.

3M has led the way in developing transmissive film, creating incredibly thin polymer sheets — as thin as 62µm (62 millionths of a meter) — which can be applied to either the inside or outside of an LCD screen to significantly enhance brightness, clarity, and image control.

An average LCD screen suffers from significant brightness loss between the backlight and the image that appears on the display. According to 3M, only 8 to 10 percent of the backlight is visible to the viewer [source: 3M]. The remaining light is absorbed by polarizers, color filters, and the liquid crystal in the LCD panel. By utilizing transmissive films, LCD makers can redirect, recycle, and amplify light, increasing energy efficiency by up to 120 percent [source: 3M].

LCD security can be a concern. For example, at an ATM, you wouldn’t want bystanders to see your bank balance. Or if you're writing a novel in a coffee shop, the person sitting next to you might get an unintended preview of your work. Transmissive films can prevent LCD images from being visible to anyone not directly in front of the screen.

So how do transmissive films function within an LCD device? And for that matter, how do LCD devices themselves work? Continue reading for an introduction to LCD technology and learn how various transmissive films can enhance the LCD experience, making it more efficient and user-friendly.

LCD Basics

The term LCD stands for liquid crystal display. Liquid crystal refers to a unique chemical compound that is the core of this advanced technology. But we’ll dive into that shortly.

Let’s begin with digital images. Every digital image, including those displayed on LCDs, is made up of millions of tiny pixels, which are the smallest visible units of the image. In a color LCD display, each pixel is divided into three subpixels, each representing a color: red, green, or blue. The colors we see on an LCD screen are created by adjusting the intensity of light shining through each red, green, and blue subpixel.

Here’s where it gets fascinating. LCD displays regulate the amount of light passing through each red, green, and blue subpixel using microscopic particles known as twisted nematic phase liquid crystals. Twisted nematics get their name because they naturally twist in their uncharged state. However, when an electric charge is applied, these crystals untwist.

What researchers discovered is that they could apply varying electric charges to the twisted nematics, causing them to untwist partially. In an LCD screen, each red, green, and blue subpixel has its own liquid crystal. For instance, if the crystal for the red pixel is fully untwisted while the blue and green remain closed, the pixel will display pure red. Untwist the blue crystal, and the pixel turns purple. By adjusting all three crystals, you can create every color imaginable across the spectrum.

Most premium LCD screens are termed transmissive because the light source is positioned behind the screen. This light is produced by a small fluorescent tube connected to a light guide, which directs the light towards the LCD panel.

When the light reaches the LCD panel, it must first pass through a polarizer that aligns the light with the rows of twisted liquid crystals. The crystals are then charged, allowing light to pass through only certain subpixels, which in turn defines the color of each pixel in the digital image.

To learn more about the technology behind LCD displays, check out our article on How LCDs Work.

So, how do transmissive films come into play? These films can be applied to either the backlight area or the LCD screen itself, significantly boosting the clarity, brightness, security, and energy efficiency of the system. Keep reading to discover the various types of transmissive films.

Types of Transmissive Film

3M develops and manufactures various types of transmissive films that can either be used individually or combined within a single product to improve the performance of the LCD display.

Let’s begin with brightness. 3M uses two types of brightness enhancement films in the LCD displays of the BMW concept car. The first is a single polymer sheet featuring a prismatic surface pattern. This pattern resembles the teeth of a gear or a series of outward-facing triangles on the flat surface of the film.

The prisms on the brightness enhancement film help redirect light from the backlight toward the viewer. Without these prisms, much of the backlight’s light would be lost—shining above or below the viewer’s line of sight. It’s similar to how light from a non-LCD television reflects off the ceiling or floor. With brightness enhancement film, all the light is directed straight toward the viewer, while any excess is absorbed by the back panel of the display.

The second transmissive film used to boost brightness is called dual brightness enhancement film. This film’s name comes from its design, which not only includes prisms but also an additional reflective sheet to recycle any light that doesn’t hit the prism at the right angle. In this setup, light continues to bounce around the back of the LCD until it strikes the brightness enhancement film at the perfect angle, allowing it to be directed out toward the viewer.

Antireflection transmissive films can be applied to an LCD display's surface to minimize glare and enhance clarity. 3M’s antireflective film consists of multiple layers. The first layer is an antifingerprint film that makes it easy to clean PDA or computer screens. The second layer is antiglare film, which diffuses light at various angles to reduce glare. The third layer protects PDA screens from scratches, while the fourth layer is a non-tacky adhesive that enables easy removal and replacement if the film is damaged or scratched [source: 3M].

For privacy and security purposes, 3M has created light control films featuring microlouvers that block the screen's display from anyone not directly in front of the LCD. Microlouvers function like microscopic Venetian blinds, directing light solely toward the viewer. Anyone positioned to the sides of the screen will only see a black display. 3M markets a special product for laptops and other LCD monitors called a Privacy Filter. This filter is a type of light control film that can be easily applied to a computer screen.

Another kind of transmissive film, known as enhanced specular reflector film, can be placed behind the backlight to optimize the light distribution from a single light source. This film consists of hundreds of extremely thin polymer layers with alternating high and low frequency reflectors. The outcome is a reflection rate exceeding 98 percent, which is even better than a typical silver bathroom mirror [source: 3M].

How Transmissive Film Works: Author's Note

God bless the good folks at 3M. If it were up to me to come up with the next technological breakthrough, we'd all still be using animal skins and flint tools. But somehow, researchers at companies like 3M manage to create a polymer sheet that’s 62 millionths of a meter thick. Not only that, but they embed this impossibly thin polymer with precisely crafted reflectors, refractors, and prisms that actually serve a functional purpose. Next time you marvel at the clarity and brightness of your iPad screen, remember to silently thank these innovators.

Sources

Transmissive Film: Cheat Sheet

Important Notes:

Test your knowledge with these fun quizzes!

Next, explore these related articles: