Getting in Touch with Touchscreens

What's the difference between a capacitive and a resistive touchscreen? Which came first? And are there other types of touchscreen technologies?

See omnystudio.com/listener for privacy information.

Episode Transcript

Speaker 1 (00:04):
Welcome to tech Stuff, a production from iHeartRadio. Hey there,
and welcome to tech Stuff. I'm your host, Jonathan Strickland.
I'm an executive producer with iHeartRadio. And how the tech
are you. Let's talk a bit about touch screens. So
in the grand scheme of things, they're a fairly recent invention.

(00:27):
If you look back at the original Star Trek series,
you can see that they are a recent invention because
they didn't think about touch screens when they were designing
the sets for Star Trek. The Enterprise, which is the
flagship of the Federation, used physical buttons and switches, not
touch screens. Now, that should not come as a surprise.

(00:50):
The set designers were taking their inspiration from electronic devices
and mainframe computers of the time and then just saying,
how can we make that look more futury? And you
can't blame them for failing to predict that in the
future people would interact with technologies through other means, including
voice and touch. By the time we get up to

(01:11):
Star Trek the next generation, things had changed quite a bit.
The controls on the new Enterprise were these sort of
touch sensitive panels. They had control surfaces that were built
directly into walls and consoles in such a way that
I bet it was someone's full time gig on the
set to just wipe down the surfaces to get rid
of all the smudges. They also had voice commands built

(01:34):
into their computer system at that point, so that was
pretty cool too. They kind of had both of those
blossoming technologies involved in Star Trek next generation. And there
are actually several different methods that you could follow to
create a touch screen or touch surface. So for example,

(01:54):
you could have a rear projection screen and you're projecting
image is from behind the screen onto the screen, and
also behind the screen, you could have a bunch of
near infrared cameras, and these near infrared cameras could detect
when a fingertip or some object makes contact with the

(02:16):
surface that's on the other side and then map that
to a program that creates the appropriate response. The original
Microsoft surface, which later would be called the Pixel Sense,
had something like this and used multiple near infrared cameras
I think five of them behind the screen to detect

(02:39):
and track objects that make contact with the screen. If
you don't recall, the pixel Sense had sort of a
table form factor. It was quite a large display, bigger
than what you would have with like a tablet. But
I wanted to talk about the differences between the two
most common touchscreen technologies that consumers typically encounter. So first

(03:02):
up is actually capacitive touch. This is really the type
of screen you're most likely to encounter these days. Most
touch screen technology falls back on this, and capacitive touch
predates the other technology that we'll talk about by about
five years or so. So back in nineteen sixty five,

(03:23):
there was a British engineer named E. A. Johnson who
developed capasitive touch technologies while working for the Royal Radar Establishment.
He wrote up his work in a paper he titled
Touch Displays a Programmed Man Machine Interface in nineteen sixty seven.
A capacitive screen consists of several layers, So we're going

(03:46):
to work from the bottom up, and by up, I
mean like at the top layer will be the surface
that you would interact with. So at the base you
have your actual display, right, this is what is generating
the image that you're going to see through the other layers.
So all the layers on top of this need to
be transparent, because otherwise you wouldn't be able to see

(04:08):
the stuff that's on the display, and you've kind of
eliminated the purposes of having a touchscreen device. Now, typically
you would have a thin glass substrate that would be
on top of the display, and then the next layer
up would be a conductive layer. So this is a
layer that creates an electrostatic field across it. On top

(04:30):
of that layer is a thin transparent layer, and this
is the layer that you could actually touch. So if
something conductive makes contact with this top layer, then some
of the electrostatic charge on the layer beneath the top

(04:50):
layer will transfer to that conductive material. So let's just
say it's your finger. Make it easy. So you touch
your finger to the surface of a screen. Your finger
is conductive, and once you touch the screen, some of
the charge on the surface underneath that top layer transfers

(05:11):
to your finger, and the charge decreases at the point
of contact. So you've got circuits that are built into
the edge of the screen, often at the corners, and
they detect where precisely that charge decrease in the capacitive
layer happens and registers this as a contact and then
that translates into an action based on whatever it is

(05:33):
you're doing so. Like if you're playing a game and
you move your finger across the screen, it says, all right, well,
the point of contact started at this position, it ended
at that position, and that means we need to reflect
that in moving a character from one point to another
or whatever it may be. Now, this is why if
you're wearing non conductive gloves, you can't interact with a

(05:56):
touch screen, a capacitive touch screen properly, unless you, you know,
carry around something like a hot dog around that would work.
I've actually seen people or pictures of people in Japan
doing that when the weather was really darn cold. Hot
dog phone. But also like anything that hasity, a conductive
rather a conductive surface would work. It's just that if

(06:19):
you're wearing gloves that insulate you, then that doesn't work.
That's why some gloves come with a little conductive mesh
at the fingertips so that you can still interact with
your capacitive touch screen devices while wearing the gloves. Now,
the version that Johnson invented way back in nineteen sixty

(06:39):
five was understandably limited. It could only detect the presence
of a touch. It couldn't tell the difference between one
finger or two fingers or anything like that. I don't
think it could even detect where on the screen the
touch happened, just that there was a touch. So in
other words, it was kind of an on off or

(07:00):
binary system. Either something conductive was in contact with the
screen or it wasn't. But this served as the foundation
for the capacitive touch screens we used today. The problem
is they were expensive, so while it was possible, it
didn't really proliferate because the use cases were fairly limited,

(07:23):
and it didn't make any sense to try and incorporate
that into consumer technology because whatever you made would be
way too expensive. The other common touch screen technology is
called resistive touch. In nineteen seventy, an inventor named G.
Samuel Hurst was trying to figure out a way to
more efficiently make use of a vandograph accelerator, and so

(07:46):
he came up with the idea of using electrically conductive paper. Essentially,
these papers would have like a grid along the X
and y axis of the paper, and you could detect
a change in voltage along those grids, so you could
you could plot a specific point of contact by the

(08:09):
way a vandograph generator, you know, a vandograph accelerator is
what Hearst was referring to, but that's because a vandograph
generator was used as a very primitive particle accelerator back
in the day. It is an electrostatic generator. You've probably
at least seen pictures of these, if not actually seen
one in use. So typically you're using a belt mounted

(08:34):
on some rollers that turn very quickly. This makes the
belt move very quickly, and the moving belt actually typically
makes contact with another surface, but it generates this electrostatic
charge and carries that charge to a hollow metal globe.
The globe itself is also mounted on top of a

(08:56):
column that's made of some sort of insulator material, so
this isolates the metal globe. Right, You're building up this
electrostatic charge in the metal globe and there's nowhere for
the charge to go because you've isolated the globe. And
then you can bring something conductive in you know, general
proximity of the globe, and as you get close enough,

(09:20):
the difference in electric potentials will cause a spark to form.
Like you essentially create a circuit very very briefly, and
then you get this zap of a spark and you've
probably seen, like I said, one of these, either in
video or maybe even in person. You're likely to find
it in like science classrooms to help demonstrate the principles

(09:43):
of electrostatics. But back in the day they were used
as particle accelerators in physics research. Yes, today it's a
toy and a science classroom, but back in the day,
it was a particle accelerator. Anyway, doctor Hurst used the
electrically conductive paper to plot charge on X and y axis,
and only a bit later did he realize that what

(10:04):
he was doing could potentially have other applications outside the lab.
I'll explain more, but first let's take a quick break.
So doctor Hurst and his team figured that they might

(10:25):
actually have some applications for this conductive paper beyond the
plotting of charges using a vandograph accelerator. And he thought
that he could make this into a touchscreen interface. So
this would be a resistive touch screen. They actually have

(10:46):
more layers than capacitive touch screens. That also means they
block a little more light than capacitive touch screens do,
so resistive screens tend to be dimmer than capacitive ones.
So let's go through those layers again and again, we're
going to start from the display side up to the
surface where you would make contact with the screen. So

(11:07):
at the very base you've still got your display, just
like with capacitive. On top of the display, you've got
a glass substrate. Above that you have a transparent conductive layer,
so again similar to what you would have with the
capacitive screen. But next you would have a layer of
what are called separator dots. So these are our little

(11:27):
supports that are non conductive. They are there to act
as a separator. They keep the first transparent conductive layer
separate from a second transparent conductive layer, so they're there
to keep space between those two layers. So again above

(11:50):
these separator dots is that second transparent conductive layer, and
then on the very top you have a flexible transparent
film on top. This is where you would make contact
with the screen. So when you push down on the screen,
whether it's with a conductive surface or not, what you're

(12:10):
doing is you're deforming the top most transparent layer to
push down and come into contact with the next transparent
conductive layer. That creates a circuit. So as long as
you're pushing down with enough force, you're creating the circuit
and it will detect that touch. So typically you've got

(12:35):
other circuits in the device that detect drops in voltage
or changes in voltage, and that's how they can detect
the precise location where the touch happened. So again, doesn't
matter if it's your finger, if you're wearing gloves, if
you're using a stylus, it doesn't really matter. What matters

(12:55):
is that that top transparent conductive layer comes into contact
with the bottom transparent conductive layer and creates a circuit.
So the capacitive screen actually came first, but the resistive
screen was more popular. It got more popular, and it
did so faster than capacitive. So why is that, Well,

(13:16):
mostly it comes down to cost. Also, like the fact
that you didn't have to have a conductive material to
work with it meant that you could actually use it
for lots of other stuff, including stuff where you might
have to do something like wear gloves, but you could
use a stylus like That's a useful part of that
technology is the fact that you can still work with

(13:37):
it even if you aren't able to, you know, use
your fingers directly on the screen. But it was much
cheaper and that was really the big thing, so capacitive
sort of took a back seat for a while, and
it would require a lot more innovation in the space
to make capacitive screens more attractive than resistive screens. However,
these days, most consumer devices you're going to come in

(14:00):
to contact with use capacitive touch screens, largely because, I mean,
they're still more expensive than resistive touch screens, but they
can display brighter images, so that's definitely a positive. They
tend to be more durable as well as you can
imagine if you've got a resistive touch screen, which is
it works based upon you pushing the screen hard enough

(14:22):
to make contact between two layers. I mean, you don't
have to push super hard, but it does have to
be enough pressure so that the system detects there's a
touch there. Well, as you might imagine, this eventually deforms
the upper transparent conductive layer, and that you can eventually

(14:43):
get to points where it's already close to or making
contact with the lower layer. Just kind of like having
a short circuit, right, and it makes it more difficult
to have an accurate experience. Using resistive touch screens. Doesn't
happen overnight, but over time it does happen, So that's
one of the other benefits capacitive touch screens have over resistive.

(15:07):
It's also easier to use capacitive touch screens for multi
touch functions in general, not that you couldn't do it
with resistive touch screens, but it's just it's easier when
you're not focusing on using pressure to make that point
of contact. You will still find resistive touch screens, however,
in devices that are aimed at lower price points, So

(15:30):
if you're looking at like a budget tablet, there are
a lot of industrial uses for resistive touch screens to
this day. And keep in mind, as I said at
the beginning of this episode, there are other types of
touch screen technologies besides these two. There's some that use acoustics,
there's some that use infrared lasers. Like I said with

(15:51):
the surface, there are the kinds that use you know,
cameras that are mounted behind the screen itself. It's not
like these two are the only two. There are lots
of other technologies. It's just those two are the ones
you're most likely to come into contact with, both figuratively
and literally. So I hope that this was interesting and informative.

(16:13):
A little tech Stuff Tidbits episode, and I'm trying to
do more of these because it's fun to do these
short ones. It's just a challenge because you know, I'm
a chatty Kathy. This episode probably could have been eight
minutes long and instead of going twice as long. So
but hey, I like your company, hope you like mine,
And if you have any suggestions for little things that

(16:34):
you would like explained in the tech space, even if
it's something like, hey, can you give a quick rundown
on logic gates and what those do or something along
those lines, let me know and I'll look into it.
And I hope you are all well, and I'll talk
to you again really soon. Tech Stuff is an iHeartRadio production.

(17:00):
For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts,
or wherever you listen to your favorite shows.