Page
31
A Appendix
-
Measurement Modes
A.1
I
-
Effective
In order to perceive a signal light or warning beacon, the human eye requires a
threshold of illuminance or luminance, which is higher for shorter presentation
times. The consequence of this is that for equal peak intensities, a narrow
pulse signal light (light
-
flash) can appear less intense to an observer compared
to a steady state signal light: the luminous range of rhythmic light is usually
smaller compared to that of a fixed light. Therefore the significant quantity is
not the peak intensity but the effective intensity.
The I
-
Effective measurement modes only apply to measurements of luminous
quantities with a photometer head. Generally, the quantity to be evaluated is
luminous intensity, which can be derived from an illuminance measurement.
Normally, a photometer head is delivered calibrated for illuminance, in order to
be universal. But on special order the photometer head can also be calibrated
in luminous intensity for a fixed distance. Please refer to the calibration certifi-
cate for details regarding the calibration of your specific detector head.
The relationship between both quantities, luminous intensity and illuminance,
is described by the inverse square law as shown in the equation below, which
only is valid for large measurement distances compared to the extension of the
luminous area. As a rule of thumb, one can estimate r to be 10 times the larg-
est dimension of the illuminating source area, except for narrow beam sources
(half peak angle < 5°).
where
I
luminous intensity
E
illuminance
r
distance from source to acceptance area of photometer head
A.1.1
I
-
Effective (Schmidt
-
Clausen, SC)
The effective (luminous) intensity of a signal light presented as a brief pulse
can be calculated by the
method
of Schmidt
-
Clausen
according to the follow-
ing equation
2
r
E
I
