object will appear at the sensor.
There are several system level physical
design elements that can be employed to
help mitigate the issues caused by the glass
cover and IR leakage. Optical barriers can
be placed around the IR LED and sensor to
prevent unwanted signal from reaching the
sensor. In theory, the optical barriers shown
in Figure 4 could extend the complete
distance from the substrate to the glass
and reduce the reflections off of the glass.
However, this is not mechanically practical
because the glass will bend during usage
and having the optical barriers touch the
glass will enhance the possibility that the
glass may break or crack. Fortunately, this
is another option to help solve the problem.
By using non-reflective (absorptive) ink on
both the top and the bottom sides of the
glass, the amount of reflected light is greatly
reduced and makes the issue manageable.
In smartphone designs, power consumption
of each system is critical and the same holds
true for the proximity detection system.
The radiated emission pattern of the IR LED
can be narrowed with the addition of a lens
which will reduce the amount of wasted light
energy. Lowering the amount of wasted light
energy means the IR LED can be operated at
a lower power level or for a shorter amount of
time for equivalent performance resulting in
The proximity sensor designs in
smartphones are very small. Some occupy
less than 10 cubic millimeters of space.
There are many dimensions that must
be tightly controlled. Some of these di-
mensions are shown in Figure 5 including
the XY spacing from the LED to the sensor,
the air gap between the optical barrier and
the glass, the Z height of the LED and the
sensor to the glass, the thickness of the glass and also the
distances the LED and sensor are from the optical barriers.
In Figure 5, the critical point must be tightly controlled to
ensure proper operation of the smartphone. The critical
point moves too far from the glass surface, it will not be
able to detect objects close to the glass and if the critical
point moves inside the glass, the crosstalk will increase
significantly. Even a movement of a few tens of microns can
completely break a system. Spatially placing components
reliably in a high volume environment is a significant
challenge for smartphone manufacturers.
Fig. 4: Inclusion of optical barriers
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