By Sevugan Nagappan, Marketing Manager,
Infrared Laser Products, Osram Opto
Optical solutions are increasingly popular for displays. Unlike traditional touchscreens,
optical screens can detect any type of pointer
or stylus and gloved fingers as they aren’t reliant on the
conductivity of these objects. Optical designs are also not
sensitive to scratches and can be used for any size screen.
In most cases, the optical components are mounted in a
frame around the display, so existing displays can be easily
upgraded. Optical touchscreens used to be considered too
expensive, too large, and too sensitive to ambient light.
However, IREDs now provide the basis for cost-effective,
low-profile touchscreens, lowering the price and erasing size
requirements. The sensitivity to ambient light can be over-
come by appropriate designs.
Commonly used technologies
for optical touchscreens all benefit
from highly efficient, thin-film chip
technology, which provides the
basis for compact IREDs with high
optical output. A wide range of
packages is available for all design
options, from narrow-angle emitters for light grids to high-power
emitters for illuminating large
displays. IREDS offer a wavelength
of 850 nm and light that is barely
visible to the naked eye, but easily
registered by the detectors.
The simplest solution for optical
touchscreens is a light grid created by rows of infrared
emitters and detectors placed opposite one another. The
components are mounted in a low-profile bezel around
the screen, just a few millimeters deep. A finger or stylus
blocks the light beams, causing the detector signal to atten-
uate at the appropriate point (Figure 1). This design can be
used as a multi-touch version if emitters and detectors are
switched sequentially and signals are evaluated.
Important factors for selecting an emitter are the size of
the component, its optical output, and its radiant intensity.
High radiant intensity is synonymous with an intense nar-
row-angle beam. High radiant intensities enable large screen
diagonals to be covered. Narrow beam angles, coupled with
narrow detection angles on the detectors, ensure that, even
on large displays, the beams from the individual emitters do
not hit more than one sensor. In some applications, particu-
larly ones with strong halogen lighting, daylight filters can
reduce the influence of ambient light on the detectors.
A light grid design can be easily scaled up to larger
screen diagonals. Compared with non-optical technologies,
this scaling involves less expense because the functional
components are mounted in the frame around the display.
The scaling factor in the case of optical touchscreens is
dependent on the circumference. In all other technologies,
the scaling factor is dependent on the square of the display.
Remember, light output reduces in proportion to the square
of the distance from the detector. This, in turn, leads to a
poorer signal-to-noise ratio for the touch signal, and it may
be necessary to adjust the emitter current accordingly.
A setup with line sensors needs far fewer components
than light grids. High-power IREDs flood the display with
infrared light from two corners, and detectors — optically
separated to prevent crosstalk — only receive a signal when
objects on the display reflect the infrared beams. In most
cases, the sensors are line scanners. The precise position and
size of the finger or stylus is calculated by evaluating both
signals using a procedure similar to triangulation. This design has the potential to produce touchscreens with a much
higher resolution than other technologies. It can be scaled
up to larger screens without the need for additional components, as long as the emitters produce enough light.
In a slightly modified version, light guides are mounted
around the display and fed with light by IREDs at the corners. Light exits the light guide at certain intervals along its
length, creating a curtain of light over the display. The line
sensors register the shadows cast by objects on the display.
While IREDs with a wide beam angle are suitable for
illuminating from the corners, the choice of emitters for the
light guide version depends on the design of the light guide.
IREDs with a narrow beam angle and flat surface are ideal
for injecting light into light guides. To free the touch signal
from ambient light influences, measure without infrared
illumination and then with infrared illumination, and calculate the difference between the two signals.
Large displays solutions
Large projection panels and consoles are mostly backlit
with infrared light. A finger or stylus on the display reflects
the light to one or more cameras. Alternatively, infrared
light can be injected into the glass of the display. The IRED
emits light into the glass so the beams are totally reflected
at the top and bottom surfaces. When an object touches the
surface, the light escapes and scatters so the signal at the detectors is changing. The simplest way to eliminate ambient
light effects in projection systems is to place bandpass filters
in front of the camera sensor. These two versions of optical
touchscreens do not require a bezel and therefore give designers greater flexibility.
Figure 1. Fingers or styluses create a shadow
over the detector. (Credit: Osram Opto)