How To Create Sunlight Readable TFT LCD
There are more and more TFT displays used in outdoor applications, such as automobile display, digital signage and kiosks. High ambient light in outdoor environment often causes wash-out image and renders the screen not readable. Readability & sustainability of TFT display under direct sunlight is becoming vital. Topway Display has been developing sunlight readable LCD display solution for years. The company understands the ins and outs of sunlight readable TFT LCD.
Before going further into how to make an LCD sunlight readable, let’s explore what is visibility.
Determining Visibility
Visibility is the ease with which a viewer can detect an object, or in more scientific term: the relationship between luminance contrast and human eye’s threshold. Therefore, the higher an object’s contrast is, the better is its visibility.
What make LCD sunlight readable?
For an LCD to be readable in outdoor environment with very bright ambient light, the LCD screen’s brightness needs to exceed the intensity of light that is reflected from the display surface. To be comfortably viewed by human eyes, the LCD’s brightness needs to exceed its reflected light by a factor of 2.5 at minimum. Naturally, to make an LCD sunlight readable, we can work on two areas, increasing brightness or reducing reflectance.
Increase brightness to make sunlight readable LCD
Increase LED backlight power
On a clear day in direct sunlight, the ambient brightness is about 6000 cd/m2. And a typical TFT LCD with touch screen reflects about 14% of ambient light, which is around 840 cd/m2. These days, most LCD displays use LED backlight as light source. It is not too difficult to increase an LCD’s brightness to 800 ~ 1000 Nits, to overpower the bright reflected sunlight. Thus, you have a sunlight readable TFT LCD.
However, this method requires more backlight LEDs and/or higher driving current. The drawbacks are high power consumption, more heat dissipation, increased product size and shorter LED backlight lifespan. Apparently, increasing backlight to make TFT LCD sunlight-readable is not a very good solution.
Use transflective LCD
Transflective TFT LCD is a TFT LCD with both transmissive and reflective characteristics. A partially reflective mirror layer is added between LCD and backlight. This change turns part of the reflected ambient light into LCD’s light source, increasing the TFT display’s brightness. However, transflective TFT LCD is more expensive than transmissive one. At the same time, the partially reflective mirror layer will block some of the backlight, making it not ideal in indoor or low ambient light environment.
Reduce reflectance to make sunlight readable TFT Screen
What causes light reflection? When light traveling in one transparent medium encounters a boundary with another transparent medium, a portion of the light bounces off the border. Through the simplest version of Fresnel’s equation, we can calculate the amount of reflected light.
R=[(n2-n1)/(n2+n1)]^2 (n1 & n2 are indexes of refraction for 1st and 2nd material)
It is clear from the equation that the more difference between two materials, the more light will be reflected.
We know TFT LCD with touch panel has layers structure. There are several places light reflection happens.
The total reflectance on a TFT LCD with touch panel is the sum of reflected light on any interface where two materials meet. As an example, between polarizer and display glass, the difference in index of refractions for the two materials is very small, around 0.1. So the reflected light on this interface is only 0.1%. As Fresnel’s equation points out, we should focus reflection reduction on air interfaces. For air, its index of refraction is 1; for glass, it is 1.5. And that results in a reflectance of 4.5%. Therefore, the three air interfaces contribute majority of TFT LCD’s reflectance, at about 13%.
Reduce Top Surface Reflection
The quick and easiest thing we can do to reduce air-glass interface reflectance is to use an Anti-Reflection and Anti-Glare film or apply AR coating. An external film with AR properties not only reduces reflected light, but also brings other benefits.
For food industry application, shattered glass is a serious problem. An LCD screen with external film solves this issue nicely. As for automotive applications, in an accident, broken LCD with top AR film won’t produce sharp edge glass that could harms auto occupant. Nevertheless, a top film always reduces TFT LCD’s surface hardness. And it is susceptible to scratches. On the other hand, AR coating retains LCD’s hardness and touch performance. But it comes with a bigger price tag.
With above measurements, TFT LCD’s top layer reflectance can be reduced by 2~3%.
Reduce Ambient Light Getting Into LCD
Another quick and easy way to tackle reflectance is to affix a linear polarizer on the top of TFT screen. When ambient light gets to the top polarizer, only half of the light passes through. Which results in reflection light cutting to half. This is a very low cost way to increase TFT LCD’s contrast, such that making it more sunlight readable.
Again, putting any film on the top will reduce LCD’s surface hardness, and polarizer will cause a loss in transmission. Those are not very ideal.
Block Reflected Light
Laminating a circular polarizer in TFT LCD will get rid of a lot of reflectance. That is because when ambient light passes through circular polarizer it gets circularly polarized. And when it is reflected, the polarization direction flips by 180 degrees. So when reflected light comes back to the circular polarizer, nothing goes through to viewer’s eyes.
This method is very effective for an LCD display with resistive touch panel. We know resistive touch LCD has two air gaps: air gap between two ITO layers and air gap between touch panel and LCD display. Reflectance caused by the two air gaps is very high. Applying circular polarizer blocks off most of the reflected light, and makes the LCD display sunlight readable.
The disadvantage of such solution is its cost. Since we need not only a circular polarizer, but also a retarder film on the top of LCD display, making sure light originates from within LCD is not blocked by external circular polarizer.
Lessen Reflectance On Air Gap
Air gap reflection is the main culprit causing bad sunlight readability. We can improve this from two directions.
- Add AR films on both interfaces of internal air gap. The add-ons can reduce this area’s reflection from 8.5% to 2%. And since the AR films are not outside facing, they are much cheaper than the one used outside. Keeping the air gap also retains the ease of service, in case either touch panel or LCD display needs to be repaired.
- The most effective way is to eliminate air gap totally, by using optical bonding. In plain language, we fill air gap with special optical adhesive, to smooth out the area’s refraction index differences. Such that reflectance caused by internal air gap drops from 8.5% to 0.5%. Optical bonding is expensive but effective way to improve TFT LCD sunlight readability. It enhances durability and resistance to impact. Moreover, no air gap means no moisture condensation and fogging.
Summary
There are many ways to make TFT LCD sunlight readable. They all have their own pros and cons. With 20+ years' LCD design and manufacturing experience, Topway knows how to create the best sunlight readable TFT LCD for challenging environments. Leave us a message and let's start the conversation of creating suitable sunlight readable TFT LCD for your project.