Environmental Engineering Reference
In-Depth Information
be stated exactly which action spectrum was used. Typical examples of biological
action spectra are erythemal weighting
16
and the generalised plant action spectrum
17
.
The so called integrated UV-B and UV-A irradiances are examples of mathematical
weighting, i.e. the weighting function is a step function being equal one between 280
and 315nm for UV-B (315-400 nm for UV-A, respectively) and zero elsewhere.
10
0
10
0
s
ery
(
O
)
E
O
(
O
) sun
10
-1
10
-1
10
-2
erythemally weighted irradiance
E
ery
=
³
s
ery
(
O
) · E
O
(
O
)·d
O
10
-2
10
-3
10
-4
10
-3
10
-5
10
-4
10
-6
280
300
320
340
360
380
400
Wavelength,
O
(nm)
Figure 10.
Weighting of an outdoor spectral irradiance with the erythema action spectrum and
integrating gives the erythema-effective irradiance (here E
ery
= 0.154 W·m
-2
).
Figure 10 shows the example of an erythemally weighted terrestrial solar
spectrum.
If only weighted quantities are preferred broadband instruments are a good
choice. The requirements are
· a close match of the spectral response of the detector with the desired action spectrum
(see below)
· the capability to integrate over the appropriate wavelength range
· a linear response over the dynamical range which has to be considered.
Example: Robertson-Berger meter
. An instrument which is frequently used in
solar UV measurements is the Robertson-Berger meter (R-B meter)
18
. The basic idea is
to convert incoming UV radiation into green light by a phosphor. The green light is then
detected by a photodiode. Ambient visible radiation has to be blocked from the
photodiode. Therefore, an UG 11 UV filter is located in front of the phosphor (see
Figure 11). Unfortunately, this filter has a small transmission for red light. Therefore, a
green filter behind the phosphor is used to block the remaining red light. The system is
temperature controlled and the entrance optic with cosine response is protected by a
quartz dome. The wavelength dependency of the efficiency of the R-B meter quite
closely matches the erythema action spectrum. More details on the typical spectral and
angular responses of R-B meters can be found in
19,20
.
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