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The LP UVB 02 radiometer measures the global irradiance in the UVB spectral
region on a plane surface (Watt/m2). In particular, the instrument’s spectral sensitivity
is centered at 305nm with a 5nm band width (FWHM). The global irradiance
is the sum of the direct solar irradiance and the sky diffuse irradiance on a
surface parallel to the ground. In contrast to the visible spectrum where the direct
component prevails over the diffuse component, in the UVB spectral region light
is strongly diffused by atmosphere and thus the two components are equivalent.
Therefore it is of primary importance for the instrument to be capable of measure
both components accurately.
The LP UVB 02 probe is typically used in the following sectors:
• Monitoring the ozone layer. Indeed, the radiation around 295nm–315nm is
strongly absorbed by ozone located in the stratosphere, therefore each small
variation of the ozone layer corresponds to an increase or decrease of the UV
radiation reaching the ground.
• Effects of UVB radiation (the most harmful to human health) on living beings.
• UVB radiation measurement in work spaces.
The LP UVB 02 radiometer needs power to function. Power is required to amplify
the weak signal generated by the photodiode. Indeed, the radiometer is a current/
voltage amplifi er (transimpedance amplifi er). This choice measures sun-produced
UVB irradiance. Indeed, the need to use sophisticated fi lters (partially attenuating
the signal concerned) and the relatively weak sun-produced irradiation in this
spectral area, in the best case, make the photodiode-generated current in the
order of hundreds of pAmpere. So it is not possible to use cable meters or tens
of meters long as the noise might be greater than the signal itself. Therefore the
signal must be amplifi ed.
LP UVB 02 is robust and was manufactured to operate for long periods without
maintenance (if powered correctly). This characteristic makes it suitable for location
in meteorological stations.
A platinum-resistance thermometer (Pt100) is inserted inside the LP UVB 02 in
order to control its temperature. Internal temperature must remain within its functioning
range, otherwise measurements could be affected by higher systematic
errors than those asserted in the manual. Exposure to temperature higher than
+60°C can alter the interferential-fi lters spectral characteristics.
Working Principle
The LP UVB 02 radiometer is based on an innovative solid state photodiode, the
spectral response of which was adapted to that desired by using special interferential
fi lters. In particular, the used photodiode and fi lters have exceptional
stability characteristics, both for temperature and through time. This allowed
manufacturing of an instrument that does not need heating, thus reducing energy
consumption.
Particular attention has been given to fi lter design so as to make the instrument
completely blind to wavelengths outside the concerned pass-band. The solar
energy within the 302nm–308nm spectral band is only 0.01% of the total energy
from the sun reaching Earth’s surface. The relevant spectral response curve is
shown in Fig. 1A (in linear scale) and Fig. 1B (in logarithmic scale).
Installation and Mounting of the Radiometer for the Measurement of the
Global Radiation
Before installing the radiometer refi ll the cartridge containing the silica-gel crystals.
Silica gel absorbs humidity in the dome chamber; in case of particular climatic
conditions this humidity can cause condensation on the internal side of the
dome and then modify the measurement. Do not touch the silica gel crystals with
your hands and do not wet them while refi lling the cartridge. Carry out the following
instructions in an environment as drier as possible:
1- loosen the three screws that fi x the white shade disk
2- unscrew the silica gel cartridge using a coin
3- remove the cartridge perforated cap
4- open the sachet containing the silica gel (supplied with the radiometer)
5- fi ll the cartridge with the silica-gel crystals
6- close the cartridge with its own cap, paying attention that the sealing O-ring
be properly positioned and undamaged
7- screw the cartridge to the radiometer body using a coin
8- check that the cartridge is screwed tightly (if not, the silica-gel life will be
reduced)
9- position the shade disk and tighten it with the screws
10- the radiometer is ready for use
• The LP UVB 02 has to be mounted in a readily accessible location to be able
to provide for a periodic cleaning of the external dome and for the maintenance.
Check also that no building, construction, tree or obstruction exceeds horizontal
plane where the radiometer lays. If this is not possible, select a site where
obstructions do not exceed 5 degrees of elevation, in the path followed by the sun,
between earliest sunrise and latest sunset.
• The radiometer has to be located far from any kind of obstruction, which might
throw the solar radiation (or its shade) on the radiometer.
• The LP UVB 02 radiometer is provided with a spirit level for carrying an accurate
horizontal leveling. The adjustment is made by means of two leveling screws that
allow to adjust the radiometer inclination. Use the two 6mm-diameter and 65mminteraxial-
distance holes to mount the instrument on a plane. Remove the shade
disk to access the holes and reposition it after mounting
Maintenance
It is important to keep the outer domes clean to grant the best measurement
accuracy. Consequently, cleaning the dome more often will give more accurate
measurements. Cleaning can be carried out using water and standard papers
for lens, or, if not suffi cient, using pure ETHYL alcohol. After using alcohol, clean
again the dome with water only.
Because of the high rise/fall in temperature between day and night, some condensation
might appear on the radiometer dome. In this case the performed reading
is highly overestimated. To minimize the condensation growth, the radiometer is
provided with a cartridge containing desiccant material: Silica gel. The effi ciency
of the Silica gel crystals decreases in time with humidity absorption. Silica-gel
crystals are active when their colour is yellow, and they turn blue when they
loose their power. See the instructions of fi gure 3 on how to replace them. Silica
gel typical duration goes from 2 to 6 months depending on the environment where
the radiometer works.
Yearly calibration of the instrument is recommended. Calibration can be performed
by DeltaOhm Metrological Laboratories, or by connecting it to an identical
instrument calibrated with reference to a Primary Metrological Institute having a
known calibration factor.
Calibration and Measurements
The radiometer S sensitivity (or calibration factor) allows to determine the irradiance
by measuring a signal in Volts generated by the internal amplifi cation circuit.
It is possible that an offset be present on the output signal of some fractions of
millivolts (0.3-0.4mV), in which case it is also recommended that the data be
acquired at night and subtract the night-measurement offset from the performed
measurements. Once the difference of potential (VoutUV+) has been measured
at the ends of the resistance, the Ee irradiance is obtained applying the following
formula:
Ee= [VoutUV+] / S
where:
Ee: is the irradiance expressed in W/m2,
VoutUV+: is the difference of potential measured by the multimeter and
expressed in V,
S: is the calibration factor in V/(W/m2), shown on the radiometer label
(and mentioned on the calibration report).
In the presence of a possible offset of OF Volts, the previous calculations must
be modifi ed as follows:
Ee= ([VoutUV+] - OF)/S
Similarly, to know the instrument internal temperature once the “VoutTemp+� voltage
in volts is known, we get:
T=100Å [VoutTemp+] - 40 °C
Supposing a voltage VoutTemp+=0.532V is read, the previous formula gives the
radiometer internal temperature:
T=(100 • 0.532) - 40 °C =13.2 °C
Radiometers are factory calibrated one by one and they are marked by their own
calibration factor. Calibration is carried out by measuring the radiometer-produced
output signal when hit by a parallel and homogeneous light-beam on the diffuser
surface. The source used is a monochromatic source at 304nm. The source is
periodically calibrated by a Primary Metrological Institute.
Note: currently no international calibration standards for this type of radiometer
exist; therefore, the calibration coeffi cient only makes sense if the
procedure followed to obtain it has been specifi ed. Therefore the user has
to consider that the same radiometer calibrated with different procedures
can have different sensitivity factors, as explained in the article “Source of
Error in UV Radiation Measurements�, T. C. Larason, C. L. Cromer issued in
the “Journal of Research of the National Institute of Standards and Technology�
Vol. 106, Num. 4, 2001. (The article is available free of charge on the
NIST web site at the following address: http://www.nist.gov/jers)
Technical characteristics
UV MEASUREMENT
Typical sensitivity: H5V/(W/m2)
Response time: