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14 / 192018-08-16 | Document No.: AN097
With D as the duty cycle, t
pulse
as the pulse on time and t
period
the repetition time
(t
period,max
= 250 ms). To consider the nearest distance the following extract from
[4] might be helpful:
“ For analysis of the retinal exposure for small sources, such as a small diameter
optical fiber, the closest distance at which the human eye can sharply focus is
about 100 — 200 mm. A viewing distance of 100 mm requires extreme near-
point accommodation and really applies only to small children and to very
myopic individuals. Therefore, 100 mm viewing distance is generally only applied
for worst-case assessment of pointsource divergent beam lasers. For evaluation
of both the retinal thermal hazard and the blue-light photochemical hazard, a
closest viewing distance of 200 mm from the source can be assumed to
represent the worst-case exposure. At shorter distances, the image of a light
source would be out of focus and blurred. In most situations, such short viewing
conditions are unrealistic. A 20 cm worst case assessment distance is realistic
for conventional lamp sources (including LEDs). ”
Cornea limit
The IEC-62471 standard defines the irradiance limit E
e,lim(cornea)
for the cornea
exposure as (valid for exposure times t ≤ 1000 s).
Figure 13 presents the irradiance exposure limits for the cornea limit case.
Figure 13: Irradiance exposure limits as a function of exposure time (cornea, skin)
E
e lim cornea ,
18000 t
075,–
W
m
2
-------
 5
Time [s]
Irradiance [W/m²]
10
7
10
6
10
5
10
4
10
3
10
2
10
1
110
2
10
2
10
2
10
-1
10
-2
10
-3
10
Irradiance cornea
Irradiance skin