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11 / 192018-08-16 | Document No.: AN097
At 65 ° C ambient temperature the optical power is reduced by ~ 12 %, thus the
overall required transmission at this temperature to still match 75 % overall
needs to be around 87 %. By using of Figure 8 and Figure 9 (FWHM ~ 35 nm at
T
a
= 65 °C) the upper cut-off wavelength can be estimated to be:
•λ
cut_off_upper
= 810 nm
+ 13 nm (centroid distribution)
+ 0.25 nm/K · 40 K (thermal shift)
+ 17 nm (cut-off to transmit > 87 %)
=~850nm
This results in a required filter transmission function according to Figure 10 (total
passband width ~ 70 nm). For a high R
th
connection or operation with higher
duty-cycle, the IR-LED’s junction might heat-up to higher degrees and
subsequently the upper cut-off wavelength might be shifted to longer
wavelengths by an additional ~ 10 nm (if heated up to the maximum junction
temperature of 145 °C).
A derived passband specification must additionally consider the filter shift due
to tilted rays hitting the filter (e.g. an angle of incidence (AOI) of ± 15° results
typical in a blue shift of the filter characteristics of 15 nm).
Eye / face illumination
The SFH 4780S features a high radiant intensity I
e
combined with a narrow
angular characteristic (half-angle typically ± 10°). This feature ensures a high
irradiance E
e
level at the users face / eye / target.
3
Note that the illumination of
the target is radial symmetric due to design.
Figure 11 presents graphs which indicate the irradiance distribution versus.
distance between the SFH 4780S operated with 1 A / 10 ms (nominal data sheet
conditions, I
e
= 2.9 W/sr) and a face / target. The graphs are done by averaging
the E
e
value over 1 cm² area.
Note that the influence of cover glasses or steering / beamshaping optics is not
considered here.
Thermal consideration
The SFH 4780S features a low R
th
package.
However, to utilize the full potential of the device, especially under high duty
cycle respectively DC operation, requires some good thermal management.
The typical heat transfer time from junction-to-solder-point is in the range of
between 1 ms to below 10 ms. Thus for pulses > 10 ms or shorter pulses with
high duty-cycle a good thermal heat buffer is recommended. Practical solutions
3. Please note that the testing procedure of the SFH 4780S is done to mimic the appli-
cation: For IE test a circular detector with diameter of 1.12 cm is located in 31.6 cm dis-
tance (0.001 sr measurement). This setup is in excellent agreement with the typical iris
scanning application.