Page 6 - Science Focus (Issue 016)

Nowadays, touch screens
are ubiquitous in our lives. Everywhere we go, there’s always a
touch screen: be it at the restaurants or directories or even in your own pocket in
the form of a smartphone. It’s something we rely on every minute of our lives. It’s
amazing how a piece of glass responds to your touch so quickly and accurately,
but have you ever given much thought to why and how it works as it does? This
article will dive into the “how” and the “why” on the workings of the touch screen.
In fact, there are two commonly used touch screens. One works based on the
principle of resistance. The resistive touch screen actually contains two flexible
conductive layers separated by an air gap. The upper outward-facing layer is
coated with a thin, scratch-resistant material while the inner layer is braced against a
rigid layer, usually glass. When the touch screen is in operation, electrical currents run
through these two conductive layers. When you touch the screen, pressure is applied.
This pressure dents the upper conductive layer momentarily such that it comes into
contact with the lower conductive layer. This brief contact of the layers alters the
resistance in the area. Such a change in resistance is readily detected by sensors in
the touch screen and the precise location of the touch can be calculated. As such,
your touch is registered by the machine and the touch screen can be operated with
ease.

However, sometimes these touch screens refuse to budge even after constant
prodding. This is because older generations of this touch screen can only register one
touch at a time; if you press two different locations at the same time, the sensor will
be confused as to where you are actually touching, leading to non-responsiveness.
Thankfully, newer models of the resistive touch screen have been upgraded to be able
to sense more than one touch at the same time.
The other type of touch screen relies on the principle of capacitance. To start
off, capacitance is a property defined as the ability to hold electrical charges. In
capacitive touch screens, the topmost layer is made up of a large network of tiny fine
wires that are able to hold an electric charge. This network of wires holds electricity at
all times. It so happens that human skin is also a capacitor. So when your finger touches
the screen, a minute amount of electricity is transferred from the network to your finger.
This disturbs the original electrostatic field in the network, which is signaled by a drop
in voltage at that precise location. Again, this drop is sensed by the machine and the
location registers your touch to perform downstream tasks. As this type of technology
relies on the transfer of electricity from the screen to the human finger, it well explains
why capacitive touch screens cannot work when we touch it whilst wearing non-
conductive materials, such as gloves.

Comparatively, resistive touch screens are less costly to produce than capacitive
touch screens. As such, this has made the former more commonly used in society,
such as the touch screens at canteens or directories. However, as you may have
noticed, those touch screens are not as sensitive to your touch as your own
electronic devices. (You only need to compare the canteen ordering machine to
your own mobile phone.) This is because handheld devices usually utilize capacitive
touch screens, which have a noticeably higher touch sensitivity than resistive touch
screens. And besides, capacitive touch screens have a brighter and sharper display
than resistive touch screens as the former does not have air gaps in between
layers. Due to these pros, capacitive touch screens are becoming more and more
popular.

So there you have it, the working principles of touch screens. So next time you
play games on your phone or order food on a restaurant tablet, remember it was
the magic of touch screens that allowed us to perform such simple but pivotal
tasks.

[Editor’s remark: This article describes the two most common types of touch screens.
Nevertheless, additional, less widely used touch screen technologies exist.]