Brochure

www.osram-os.com
15 / 192018-08-16 | Document No.: AN097
a
• Example 1: SFH 4780S biased with 500 mA (DC) for 9 s, assuming
I
e
= 2.2 W/sr (estimated maximum I
e
level at 500 mA). According to
equation 5 / Figure 13 E
e,lim(cornea)
= 3464 W/m². This level is reached at a
distance from the SFH 4780S of around 2.5 cm (considering the correction
factor k = 0.75, the actual E
e
level at 2.5 cm is 25 % below this limit,
resulting in an actual eye safe threshold of around 2.0 cm (not considering
any optics / cover glass which cause additional losses).
• Example 2: SFH 4780S biased with 500 mA, 50 ms pulse-on time
(I
e
= 2.2 W/sr), four pulses per second, and total operating time of 9 s. The
limit according to equation 5 / Figure 13 once again is 3464 W/m². At a
distance of 2.5 cm E
e,avg
is around 528 W/m² (with k ~ 0.75). At 0.6 cm
distance (k ~ 0.28) the E
e,avg
= 3409 W/m², thus reaching the limit
concerning eye safe operation.
Skin limit
Compared to the cornea limit this case can be neglected as the limits for skin are
more relaxed. However, it might be possible to get in direct contact with the light
source. In this case, the skin is only irritated at a small spot (typ.  ~3mm).
Additionally, heat transfer inside the skin due to perfusion and the cooling effect
from a cover glass might prevent the skin from reaching the critical temperature
(e.g. 47 °C for 10 s [2]) at all. For reference, the limit in equation 6 is valid for
exposure times ≤ 10 s only (as the impact of longer exposure times is sensed by
humans and lead to a turn-away reaction).
Retina limit
To get a better understanding on the limits concerning the retina some pre-
considerations need to be taken. To calculate the radiance of the SFH 4780S the
following equation 7 can be used:
with Z as the mean source extension (i.e. virtual emitter size). For the SFH 4780S
the virtual emitter size can be estimated to be around Z ~ 2.8 mm (by applying
the law of the conservation of the radiance, IEC 62471-2). An additional
parameter, α (in rad), the angular subtense, can be calculated by
E
e lim skin ,
20000 t
075,–
W
m
2
-------
= 6
L
e
I
e
Z
2
-----

I
e
28 mm ,
2
-------------------------
= 7

Z
d
---
28 mm ,
d
-----------------
= 8