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12 / 192018-08-16 | Document No.: AN097
might feature a direct connection between the PCB where the SFH 4780S is
mounted on and the metal frame of the mobile housing or other metal structure
to support the heat transfer to the ambient. This is especially important as
otherwise an increased junction temperature might require to limit the maximum
drive current at higher ambient temperatures. In addition an increased junction
temperature automatically leads to a reduced optical output power vs. time (at a
given drive current).
Figure 11: Irradiance distribution of a target at different distances (10 cm, 20 cm,
30 cm, 40 cm). The simulation is based on an I
e
value of the SFH 4780S of 2.9 W/sr.
The calculated irradiance level is averaged over an area of 1 cm
2
(i.e. not presenting
the absolute peak irradiance). Additionally a typical eye diameter of 1 cm is indicated
within the graphs
Red glow
During high-power operation, dependent on the drive mode (pulse-on time, duty
cycle), the radiation of the IR-LED might be visible during operation. Depending
on the individual user this can be observed as a dark red glowing of the device.
During iris recognition this can be used as an indicator that the system works.
Additionally it gives the user some idea where to focus his eyes in order to
improve the overall system performance. Note that placing the IR-LED behind
cover glasses or special optical bandpasses (inks, filters) does not change this
user experience significantly.