Resistive touchscreen for Sunlight readability
By Jim Lee, firstname.lastname@example.org
There are many touchscreen technologies available in the market today. Truly sunlight readable touchscreens are not many. They are IR (infrared) touchscreen and SAW (surface acoustic wave) touchscreen. The principle operation of these two touchscreens are that IR touchscreen relies on infrared and SAW touchscreen relies on surface acoustic wave. Both touchscreens have no optical layer overlay on the display surface; therefore there is nothing between your eyes and the display elements. These two touchscreens, however, have drawbacks. See Table 1, page 3 for advantages and disadvantages. On the other hand, resistive touchscreen is most popular for indoor applications because of low-cost and high resolution and can use a pointed stylus for drawing fine lines. This write-up addresses the problems of resistive touchscreen for sunlight readability and methods to overcome these problems optically by examining its current technologies and means of improving its structure for sunlight readability.
Review Current Resistive Touchscreen Technology
Figure 1 is the glass-to-film, low-cost touchscreen. The two resistive layers of ITO (indium tin oxide) sandwiched together and separated by a ridged layer. This ridged layer reduces light transmission and optical clarity of the touchscreen. The film surface is prone to damage from scratches. Most current ITO layer design is around 15 angstrom thickness and has a resistance of 20 ohm/sq with an index of refraction of 2.0. In Figure 1, there are 5 interfaces marked asterisks. The upper most asterisk is the air to film interface, next is the ITO to ridged layer interface and so forth. These five interfaces create five layers of reflections. These reflections are undesirable for sunlight readability
Figure 2 is the glass-to-glass touchscreen and is an improvement of Figure 1. Notably is the replacement of the film by a glass layer. These provide two improvements. One, the top glass surface is more rugged for providing better scratches and chemicals resistant. As you can see, the improvements still have five interfaces marked with asterisks, which create five layers of undesirable reflections.
New Improved Resistive Touchscreen
Figure 3 is an improved glass-to-glass touchscreen of Figure 2. Figure 3 provides two important improvements. One is an anti-reflective and hydrophobic coating, which are deposited on the front surface, that facing the viewer. This layer provides an anti-reflective coating to minimize the reflection of the sun ray and the hydrophobic coating provides anti-finger print. Second improvement is to provide index matched ITO in two ITO layers. See Figure 3. The standard ITO has a refractive index of 2.0, same as those shown in Figures 1 and 2. The index matched ITO has the characteristics that the refractive index makes a gradual transition from 1.5 to 2.0. The 1.5 refractive index of the ITO interfaces with the refractive index of 1.5 of the glass. This eliminates the glass-to-ITO interface reflection. As a result, this arrangement provides minimum reflection except for the two interfaces between the index matched ITO and the air interface. These two interfaces have low reflection because of refractive index 1.5 of the is very close to the refractive index 2.0 of the ITO. This approach resulted in approximately 5 times lower reflection than the touchscreen of Figure 2.
This improved resistive touchscreen provides four important characteristics for sunlight readability, namely: low reflection, good optical clarity, high light transmission, which approaches 95%, and anti-finger print surface.
Transflective Film”: Testing shows it fails to create Sunlight Readable Displays Despite Industry Claims.
By Jim Lee
AMLCD displays are increasingly popular for billboards and other outdoor applications,
and manufacturers are focusing on developing sunlight-readable AMLCDs. Previously, a
display that provided 400 nits with a good reflective surface was acceptable – now the
demand has increased to 1,500 nits. To reduce backlight power consumption while
improving display readability, the display industry has tried various approaches. One
such approach, adding a “transflective film to an AMLCD display, sounds like an costeffective,
after-market solution. Some display manufacturers claim that the sun is so
bright, by using a transflective film to reflect sunlight, a standard display can output
1,500 nits without requiring any additional power.
But in reality, as testing by Insync Peripherals demonstrates, transflective film is
marketing hype: it sounds good, but doesn’t work.
The term “transflective” combines “transmissive” and “reflective.” The idea behind a
transflective film is that it is supposed to transmit light, letting the backlight shine
through; and also reflect sunlight, using ambient light to create a brighter display. In old
black-an-white LCDs (STN varieties), the passive matrix element itself is transflective:
The partial reflectivity of the passive LCD provides a good display image under direct
sunlight, while its partial transmissivity provides a good display image via a backlight
for day and nighttime viewing.
Low power using optics rather than increasing power usage is important because power
generates heat, which can cause the display to go black beyond its clearing temperature
and also reduces brightness when the lamp temperature goes above +80C.
To get true sunlight readability, the backlight continues to be the most important
component. In addition to an AMLCD with a high percentage of light transmission, the
other important contributing factor is an anti-reflective film (AR film) laminated on top
of the AMLCD or an anti-reflective glass (AR glass) bonded on top of the AMLCD.
Why Transflective Film is not Important
From our investigation, it turns out that the limiting factor in how much brightness can
come from direct sunlight is not the reflectivity of the transflective film. It is the
percentage of light transmission of the AMLCD, where the sunlight passes through the
AMLCD and then reflected back out though the AMLCD. This is the so-called light
transmission Square Factor. For example a 7% light transmission AMLCD, the sunlight
goes through the AMLCD and is then reflected back thought the AMLCD to the viewer.
The resulting light received by the viewer is 7% x 7% x 80% of the sunlight. Given a
transflective film with 80% reflectivity and 10,000 ft-C sunlight, the sunlight that reaches
the view is only 134 nits. On the other hand, an AMLCD with a higher light transmission
of 9% can yield 222 nits in direct sunlight.
Choose a Sunlight Readable Display, Article released in Electronics Product Magazine
The engineer should consider the tradeoffs between power and heat
Recent advances in liquidcrystal displays have created new outdoor applications for sunlight viewing,especially where CRTs are out
of the question because of their bulky size. LCDs are finding new applications daily in fast-food and retail-advertising
These applications demand not only sunlight readability, but larger displays. Previously, a display that provided 400 nits with a good reflective
surface was acceptable—now the demand has increased to 1,500 nits with a minimum display size of around 15 in. diagonal.
Among all the display technologies available for sunlight readability,liquid crystal holds the dominant place, offering lightweight, compact
size, and portability. However, because of the demand for brighter and larger displays, power requirements have also increased.
Displays larger than 10.4 in. pose thermal problems when used in a sunloading environment. To keep costs down, manufacturers have typically
used a brutal force method, increasing backlight power to increase display brightness.
This high-power backlight in turn generates tremendous heat that causes an active-matrix LCD (AMLCD) to go above its clearing temperature. Heat is
detrimental to AMLCD survivability. To overcome the heat produced by the backlight, a massive heatsink is needed to cool the system down in
an airtight display enclosure. In addition, expensive antireflective and infrared (AR/IR) face glass is laminated
onto the AMLCD to block heat generated by sunlight.
Display performance parameters:
There are several display parameters to be considered for sunlight readability. Likewise, there are many methods of implementing a
sunlight readable display as well. The following list of display performance parameters can be used to sort out the technical tradeoffs,
pros and cons:
- Brightness. The display must be bright enough to be legible under full sunlight. With good display reflectance,
the required brightness ranges between 400 and 1,500 nits.
- Power consumption. A 15- in. display can consume as much as 50 W for a brightness of 1,500 nits. A majority of this power translates into heat.
- Readability. The display image must be discernable by the naked eye under all viewing conditions from full sunlight
- Cost . Cost is tied to display implementation, which is described in more detail in the following section. Ideally an AMLCD will have a high percentage of light transmission.
Four other key components used to construct a display have an effect on the efficiency of the implementation. Different combinations of these
display components will affect the cost of the display and its viewing quality under various lighting conditions. The front glass uses an antireflective
coating on the side facing the viewer, while the other side is coated with an infrared coating.
The antireflective coating is used to minimize reflections caused by sunlight, and the infrared coating is used to reject heat due to sunloading. The rejection
spectrum is usually in the 700 to 1,100-nm region.
An antireflective film is laminated on front of the AMLCD to minimize polarized reflection between the air gap and the AMLCD surface.
In this example, a high-efficiency light guide channels the light from four cold cathode fluorescent lamps to the viewer.
An inverter board with dimming capability provides the brightness levels necessary to meet various ambient lighting conditions. In a variation
to the forgoing description, a transflective layer could be laminated onto the AMLCD to reflect sunlight, which itself would serve as the light source. The backlight uses two to
four cold-cathode fluorescent tubes (CCFLs) for nighttime viewing.
If the backlight is replaced with multiple CCFLs positioned behind the AMLCD, they would provide a brightness of 1,500 nits to the viewer.
However, they would also increase backlight power requirements and the backlight used in this way would require system-level heat
Recent innovations in backlight technologies have led to some implementations that provide a significant reduction in power consumption.
With a reduction in backlight power coupled with good thermal management, lamination of AR/IR glass onto the AMLCD are no longer necessary
for sunlight viewing under sunloading conditions.
The table summarizes the backlight power for various 15- in. display configurations— the benefits of proper thermal management are obvious.
For example, traditionally, a 15-in. display rendering 1,500 nits requires 50 W and a massive heat sink. Harnessing advances
in the technology, InSync powers the same 15-in. display running 1,500 nits with only 35 W, allowing a 15-W power cushion to
safely operate under heavy sunloading conditions.
|Display Configuration||Sunny Day||Cloudy Day||Night Time|
|Microprism Light Guide||15 W||15 W||8 W|
|Transflective Film||0 W||15 W||8 W|
|Multiple Lamp||35 W||15 W||8 W|