ARM TR-024

> UV-B Radiometer (UVB) Handbook
>
> November 2004
>
> T. Stoffel
>
> Work supported by the U.S. Department of Energy,
>
> Office of Science, Office of Biological and Environmental Research

# Contents {#contents .Heading-Front}

[1. General Overview 1](#general-overview)

[2. Contacts 1](#contacts)

[3. Deployment Locations and History
1](#deployment-locations-and-history)

[4. Near-Real-Time Data Plots 1](#near-real-time-data-plots)

[5. Data Description and Examples 2](#data-description-and-examples)

[6. Data Quality 4](#data-quality)

[7. Instrument Details 5](#instrument-details)

# General Overview

A UV-B radiometer is used to measure ultraviolet solar irradiance on a
planar surface. Accurate measurements of UV are needed for erythemal
(sunburning) and other biological concerns, as well as to help
understand the primary source of energy input to the
earth-atmosphere-ocean system. The ARM Program uses UVB radiometers for
ground-based in situ UV radiation measurements collected by the
ARCS/SKYRAD instrument platforms in the TWP and NSA regions. UVB
instruments are not currently installed in the SGP region.

# Contacts

## Mentor

Tom Stoffel\
National Renewable Energy Laboratory\
1617 Cole Blvd.\
Golden, CO 80401-3393\
Phone: 303-384-6395\
Fax: 303-384-6391\
Email: [**tom_stoffel@nrel.gov**](mailto:Tom_Stoffel@nrel.gov)

## Instrument Developer

Solar Light Co., Inc.\
721 Oak Lane\
Philadelphia, PA 19126\
Phone: 215-927-4206\
Fax: 215-927-6347\
E-mail: [**info@solar.com**](mailto:info@solar.com)\
Web: [**http://www.solar.com**](http://www.solar.com/)

# Deployment Locations and History 

Solar Light 501A UV-Biometers are currently deployed in the Tropical
Western Pacific (TWP) and the North Slope of Alaska (NSA).

# Near-Real-Time Data Plots

[Barrow, Alaska Quicklooks](http://nanuna.gi.alaska.edu/nsadocs/)

[SHEBA Ice Camp Quicklooks](http://nanuna.gi.alaska.edu/shebadocs/)

[Atqasuk, Alaska Quicklooks](http://nanuna.gi.alaska.edu/atqdocs/)

TWP Data Quicklooks are available from [DQ HandS](http://dq.arm.gov/)
(Data Quality Health and Status).

# Data Description and Examples

See Near-Real-Time Data Plots, Section 4, and Data Quality Health and
Status, Section 6.1.

## Data File Contents

### Primary Variables and Expected Uncertainty

A UV-B Radiometer measures the amount of ultraviolet radiation incident
on a planar surface. Measurements are reported in milliwatts per square
meter (mW/m^2^).

#### Definition of Uncertainty

Data are returned in units of MED/HR. Specifications state the
instrument has a measurement range of 0 - 10 MED/HR, and a resolution
better than 0.01 MED/HR. 1 MED/HR is approximately equal to 5.83x10^-6^
watts/cm^2^ (0.0583 W/m^2)^. Using this conversion, the range is 0 to
583 mW/m^2^, and the resolution is approximately 0.58 mW/m^2^.

Data logger and sampling uncertainties may be considered separately.

The model 501A UV-Biometer is capable of providing 1-minute averages of
down welling total hemispheric ultraviolet-B irradiance measurements
within +/- 5% at the 95% confidence level. This estimate assumes the
instrument has been calibrated within the previous year, is properly
installed, and is adequately maintained.

There are a number of conditions under which UV-B measurements may be
incorrect. The data user should examine the data quality flags and data
quality reports (DQR) to determine whether significant malfunctions have
occurred due to problems with an instrument or data logger. The most
frequent sources of error are described briefly here.

Because the 501A is an active instrument, a failure in the site power or
in the DC power supply will cause faulty data.

Contaminated optical surfaces can reduce or enhance the apparent UV at
the detector. Reduction of the dome transmittance due to contamination
will decrease the signal from a radiometer. Under some circumstances,
the presence of dew, frost, snow or ice can enhance the amount of
radiation reaching the detector.

The downwelling hemispheric ultraviolet irradiance will be in error if
the UV radiometer detector is not horizontal. Tilting toward or away
from the sun by as little as one degree can bias the instrument\'s
signal.

Local obstructions to the sun such as trees, buildings, power lines, or
towers can shade or reflect sunlight naturally available to the
instrument. The locations of all measurement stations have been selected
to eliminate or greatly reduce the effects of such obstructions to
accurate irradiance measurements. Generally, deviations of the local
horizon within an elevation angle of 5° will not result in measurable
differences for a UV-Biometer.

### Secondary/Underlying Variables

This section is not applicable to this instrument.

### Diagnostic Variables

This section is not applicable to this instrument.

### Data Quality Flags

Data are clipped below 0 W/m^2^ and above 1.2 W/m2 (TWP), or 0.15 W/m^2^
(NSA).

### Dimension Variables

This section is not applicable to this instrument.

## Annotated Examples

This section is not applicable to this instrument.

## User Notes and Known Problems

This section is not applicable to this instrument.

## Frequently Asked Questions

**How often should the outer dome be cleaned?**

Research-quality irradiance measurements require frequent cleaning of
all optical surfaces. The frequency and amounts of contamination are
site dependent. ARM suggests cleaning all exposed optical surfaces every
day, preferably early in the morning. Domes should also be wiped clean
after precipitation events.

**What is the maximum signal cable length?**

From the instrument documentation: \"50 ft. (15m) standard, if longer
the voltage drop of the temperature controller power must be taken into
account.\"

**When should the desiccant be changed?**

When the indicating paper in the desiccator plug turns pink, the
desiccant needs to be recharged or replaced.

**What is the maximum irradiance expected from a UVB radiometer
measurement?**

Typical signal levels for the TWP region peak around 200 mW/m^2^. For
NSA, peak values are over 100 mW/m^2^.

**Does it matter where I install a UVB radiometer?**

Yes. The instrument should be installed in a location with a free
horizon \-- there should be no significant obstructions to the
hemispheric field of view of the sky. Obstructions to the sky view can
artificially lower the readings by shading the instrument. Some
obstructions can artificially increase the readings by reflecting direct
UV onto the sensor.

Some examples of objects to avoid when positioning an instrument include
poles, roof vents, elevator shafts, trees, light-colored buildings or
buildings with glass windows. Basically anything that extends
significantly above the height of the instrument sensor.

Special care should be taken to stay away from objects that may shade
the direct sunlight at any time during the year. A [solar
calculator](http://www.srrb.noaa.gov/highlights/sunrise/gen.html) can be
used to calculate the extreme positions of the sun around June 22 and
December 22. On these dates, the sun rises and sets farthest north and
south, respectively, of any time of year. Placing the instrument so that
no obstructions lie to the east or west between these extreme sunrise
and sunset positions will help avoid unwanted shading of the sensor.

# Data Quality

## Data Quality Health and Status

The following links go to current data quality health and status
results.

-   [DQ HandS](http://dq.arm.gov/) (Data Quality Health and Status)

-   [NCVweb](http://dq.arm.gov/ncvweb/ncvweb.cgi) for interactive data
    plotting using.

The tables and graphs shown contain the techniques used by ARM\'s data
quality analysts, instrument mentors, and site scientists to monitor and
diagnose data quality.

## Data Reviews by Instrument Mentor

Data are routinely compared with total solar for qualitative similarity.

## Data Assessments by Site Scientist/Data Quality Office

All DQ Office and most Site Scientist techniques for checking have been
incorporated within [DQ HandS](http://dq.arm.gov/) and can be viewed
there.

## Value-Added Procedures and Quality Measurement Experiments

No VAPs or QMEs are anticipated for the UV-B Radiometer. However, many
of the scientific needs of the ARM Program are met through the analysis
and processing of existing data products into \"value-added\" products
or VAPs. Despite extensive instrumentation deployed at the ARM CART
sites, there will always be quantities of interest that are either
impractical or impossible to measure directly or routinely. Physical
models using ARM instrument data as inputs are implemented as VAPs and
can help fill some of the unmet measurement needs of the program.
Conversely, ARM produces some VAPs not in order to fill unmet
measurement needs, but instead to improve the quality of existing
measurements. In addition, when more than one measurement is available,
ARM also produces \"best estimate\" VAPs. A special class of VAP called
a Quality Measurement Experiment (QME) does not output geophysical
parameters of scientific interest. Rather, a QME adds value to the input
datastreams by providing for continuous assessment of the quality of the
input data based on internal consistency checks, comparisons between
independent similar measurements, or comparisons between measurement
with modeled results, and so forth. For more information, see the [VAPs
and QMEs web page](http://www.arm.gov/data/vaps.stm).

# Instrument Details

## Detailed Description

### List of Components

A UV-B radiometer is the lowest component-level device used by an ARM
instrument platform. Data are collected by the ARCS SKYRAD data logger.
Please refer to ARCS/SKYRAD for a description of data loggers used in
these systems.

###  {#section .unnumbered}

### System Configuration and Measurement Methods

The ARM Program uses Solar Light Co., Inc. Model 501A UV-Biometers for
UVB irradiance measurements. The 501A is a Robertson-Berger type meter,
using a GaAs diode under a phosphor-coated glass detector as its primary
means of sensing UV radiation. A quartz dome protects the sensor from
the elements. The instrument has a metal body with a white powder-coated
exterior, a spirit level for accurately determining the horizontal
orientation, adjustable leveling screws and an indicating desiccator
which can be readily inspected.

This active device requires DC power to operate the sensor circuitry
(5.5 to 25 V, 5mA max) and temperature stabilization (11 to 15 V, 1 A
max). The output signals include a voltage proportional to the UV
irradiance reaching the detector, and a separate voltage output
proportional to the sensor temperature. The latter is used to correct
the UV signal for temperature dependence. A shielded signal cable
connects the 501A to a data logger. The data logger is programmable and
provides 1-minute statistics of analog to digital conversions made every
second (see SKYRAD information for details).

UV-B 501As are located to provide maximum solar access (i.e., the local
horizon is unobstructed to the extent possible). The radiometer is
carefully leveled to provide accurate downwelling UV irradiance.

The UVB radiometer is located on the top platform of the SKYRAD stand of
the ARCS installations in TWP and NSA. The instrument sits on a raised
mount which brings the sensor surface to the same level as the other
SKYRAD sensors. This is done to minimize shading and reflections from
the other instruments.

Data are collected by the SKYRAD data logger. Sampling occurs every 1
second, with data reported as 1-minute averages of 1-second data.
Minimum, maximum and standard deviation values are also reported for
each minute. Three instantaneous data readings are recorded in the
a-level data streams for 10, 30 and 50 seconds after the minute.
Calibration factors are multiplied in by the data logger; data are
reported in mW/m^2^.

### Specifications

The field of view of the UVB radiometer is the full sky hemisphere (2-pi
steradian). The instrument is operated in a level position. The output
range is 0 to 583 mW/m^2^, and the resolution is approximately 0.58
mW/m^2^.

The nominal sensor temperature is 25° C. A Peltier element is used to
heat or cool the sensor to the nominal temperature. Temperature
stabilization is ±1° C, with a settling time less than 10 minutes for a
20° C change. The heater is capable of ranges up to 50° C above ambient;
cooling is to 20° C below ambient.

## Theory of Operation

The theory of operation for the 501A UV-Biometer is the same as that of
the Robertson-Berger meter. Solar light enters the detector through the
quartz dome and input filter. The partially filtered light, containing
the full UV spectrum, excites the phosphor. The visible light emitted by
the phosphor is detected by the GaAsP diode, which produces a current
proportional to the amount of UV. This current is converted to a voltage
and amplified to an output between 0 and 2.5V. The detector temperature
is converted to a linear output voltage in the range of ±2.5V, with 1.0V
equal to 25° C, and a slope of 20 mV/°C. The temperature of the diode
and phosphor are controlled to 25° C (±1°C) by a Peltier element.

## Calibration

### Theory

Instrument calibration should be performed by the manufacturer on a
yearly cycle.

### Procedures

This section is not applicable to this instrument.

### History

Calibration history information for specific instruments is available
from the Instrument Mentor.

## Operation and Maintenance

This section is not applicable to this instrument.

### User Manual

This section is not applicable to this instrument.

### Routine and Corrective Maintenance Documentation

Desiccators are changed when necessary. Desiccator indicator changes
color from blue to pink when saturated.

Instrument dome is cleaned with distilled water and lint-free cloth each
morning, and after precipitation events.

Instrument level is checked \-- and corrected if necessary \-- at least
monthly.

### Software Documentation

This section is not applicable to this instrument.

### Additional Documentation

This section is not applicable to this instrument.

## Glossary

Erythema - an approximation of the relative biological effectiveness
(ability to cause sunburn) of different wavelengths in the UVA and UVB
regions of the spectrum.

Irradiance - the amount of energy (light, heat, UV) per unit time per
unit area.

MED/HR - Minimum Erythema Dose per Hour. A measure of biological
effectiveness of UV irradiation. One MED/HR is approximately equal to
5.83 \* 10^-6^ watts/cm^2^.

Radiance - the amount of energy per unit time per unit solid angle per
unit area in the direction of travel.

Ultraviolet (UV) - the region of the electromagnetic spectrum that falls
outside of the visible, just beyond violet. Defined as wavelengths
between 200 and 390 nanometers.

UVA - the region of the electromagnetic spectrum between 320 and 390
nanometers.

UVB - the region of the electromagnetic spectrum between 290 and 320
nanometers.

wavelength - simply, the length between consecutive peaks or troughs in
any wave. Expressed in units of length, i.e., nanometers (nm). In
electromagnetic radiation, wavelength defines the color of light if it
is visible, or the type of radiation (infra-red, ultraviolet, etc.) if
it is outside of the visible region of the electromagnetic spectrum.
Mathematically, the wavelength times the frequency of the radiation
measured in cycles-per-second, or Hertz) is equal to the speed of light.

Also see the [ARM Glossary](http://www.arm.gov/about/glossary.stm).

## Acronyms

ARCS: Atmospheric Radiation and Cloud System

DC: Direct Current

DQR: Data Quality Report

MED/HR: Minimum Erythema Dose per Hour

NSA: North Slope of Alaska

SGP: Southern Great Plains

SKYRAD: SKY RADiation Instrument Systems. Measures down welling
radiation

TWP: Tropical Western Pacific

UV: Ultraviolet radiation

Also see the [ARM Acronyms and
Abbreviations](http://www.arm.gov/about/acronyms.stm).

## Citable References

Berger, D. 1976. The sunburning ultraviolet meter: design and
performance. *Photochem. Photobio*l. 24, 587-593.

Blumthaler, M., and W. Ambach. 1986. Messungen der
Temperaturkoeffizienten des Robertson-Berger Sunburn Meters und des
Eppley UV-radiometers. *Arch. Met. Geophys. Biocl*., Ser. B, 36,
357-363.

Blumthaler, M., W. Ambach, M. Morys, and J. Slomka. 1989. Comparison of
Robertson-Berger UV Meters From Innsbruck and Belsk. *Publs. Inst.
Geophys. Pol. Acad. Sc*., D-32 (230), Warsaw.

McKinlay, A., and B.L. Diffey. 1987. A reference action spectrum for
ultra-violet induced erythema in human skin. In *Human Exposure to
Ultraviolet Radiation: Risks and Regulations* (Edited by W.F. Passchier
and B.F.M. Bosnajakovic), pp83-87, Elsevier, Amsterdam.

Solar Light Co., Inc. 1991. *UV-Biometer Model 501A Version 3 User\'s
Manual*.