MaximumPC 2007 112

r & d BREAKING DOWN TECH —PRESENT AND FUTURE

68 MAXIMUMPC DECEMBER 2007

T

ouch screens may never replace our

clicky-clacky QWERTY keyboards—no,

we’ll have to wait for brain-stem probes for

that—but they are becoming more common.

In fact, devices using this technology have

been in use for more than 35 years and are

becoming ubiquitous—kiosks, tablet PCs,

desktop computers, and many handheld

devices all now rely on human touch.

While the end result is the same—a

display surface maps the coordinates of

an input—touch screens rely on different

phenomena to perform their functions, rang-

ing from electrical current to infrared light to

sound waves.

RESISTIVE VERSUS

CAPACITIVE

A resistive touch screen sandwiches several

thin, transparent layers of material over an

LCD or CRT. The bottom layer transmits

a small electrical current along an X and

Y path. Sensors monitor these voltage

streams, waiting for an interruption. When

the fl exible top layer is pressed, the two lay-

ers connect to form a new circuit. Sensors

measure the change in voltage, triangulating

the position to X and Y coordinates.

Resistive touch screens work with any

kind of input, including a stylus or fi nger, and

they’re usually very inexpensive to manufac-

ture. They’re less durable than other types

of touch screens, however, because the top-

most layer experiences a great deal of wear

from physical contact and constant fl exing.

Longevity isn’t a big problem for tablet PC

and PDA deployments—two of the most

common applications for resistive technol-

ogy—but it can be for public kiosks, which

are expected to endure more than 35 million

impacts over their lifetimes.

Capacitive screens move the electrical

layer to the top of the display. A minimal cur-

rent is broadcast and measured from the cor-

ners of the monitor. When a person touches

the screen, a small amount of the current is

drawn away by the body’s natural capaci-

tance. The sensors measure the relative loss

of the current and a microcontroller triangu-

lates the point where the fi nger made contact.

Capacitive screens are more durable

than resistive screens because their top lay-

ers are fabricated from rigid glass. They are

typically easier to read because thin layers

of material aren’t on top of the display sur-

face. The need for a live fi ngertip, however,

often makes them feel less accurate to the

end user than a stylus-driven interface.

Trackpads and handheld devices, such as

Apple’s iPod Touch and iPhone, commonly

use capacitive input.

SURFACE ACOUSTIC WAVE

Surface acoustic wave (SAW) screens use

beams of ultrasonic waves to form a grid

over the surface of a display. Sensors along

the X and Y axes monitor the waves; when

one is broken, the X and Y points are com-

bined to identify the touch coordinate.

SAW screens, like their capacitive coun-

terparts, are durable and provide a clear line

of sight to the display image, but the former

work with any kind of input, be it a fi ngertip,

a fi ngernail, or a stylus. On the other hand,

they’re more susceptible to interference from

dirt and other foreign objects that accumu-

late on the screen, registering surface con-

taminants as points of contact.

INFRARED AND

INFRARED IMAGING

Infrared touch screens are similar to SAW

screens in that they use a ring of sensors

and receivers to form an X/Y grid over a

display. But instead of sending electrical

current or sound waves across this grid,

infrared LEDs shoot invisible beams over the

surface of the display. The microcontroller

simply calculates which X and Y lines were

broken to determine the point of input.

These screens work with a stylus, fi nger,

or other pointer and give an unobstructed

view of the display. They’re also durable

because the point of input is registered

just above the glass screen; only incidental

contact is needed. Military applications

often use infrared screens because of the

product’s longevity.

Infrared imaging touch screens are vastly

different from touch screens that use tradi-

tional infrared input. IR imaging screens use

two or more embedded cameras to visually

monitor the screen’s surface. IR beams are

transmitted away from the cameras, illumi-

nating the outside layer of the display. When

the beams are disrupted by a fi ngertip or a

stylus, the cameras measure the angle of the

object’s shadow and its distance from the

camera to triangulate the disruption.

IR imaging allows a direct view of the

display. And since the input is registered

just above the glass, physical contact is not

required to initiate action. HP’s TouchSmart

IQ770, one of the fi rst mass-market touch-

screen computers designed for the home,

features this technology. HP markets the

TouchSmart as an in-home kiosk that fami-

lies can use for quick tasks without neces-

sarily having to rely on the mouse and key-

board for navigation.

White Paper: Touch-Screen Technology

A frame around the display houses LEDs and photoreceptors on opposite sides. The LEDs emit light, which is detected by

the photoreceptors. The display identifi es X and Y coordinates when the user’s fi ngertip blocks one or more of the beams.

How new displays put the

world at your fingertips.

BY ZACK STERN

HOW IT WORKS Infrared Touch-Screen Monitor

Edge of active display area

LEDs

create

a grid of

infrared

light

Opto-matrix frame inside bezel

Photoreceptors

Inside and outside edges of infrared transparent bezel

Edge of active display area