ARM TR-015

> Infrared Thermometer (IRT) Handbook
>
> October 2006
>
> V. R. Morris
>
> 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 2](#near-real-time-data-plots)

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

[6. Data Quality 7](#data-quality)

[7. Instrument Details 8](#instrument-details)

# Tables {#tables .Heading-Front}

[1. Current Status and Locations 1](#__RefHeading___Toc107738175)

[2. Primary Variables 2](#__RefHeading___Toc107738176)

[3. Secondary Variables 3](#__RefHeading___Toc107738177)

[4. Diagnostic Variables 3](#__RefHeading___Toc107738178)

[5. Data Quality Flags 3](#__RefHeading___Toc107738179)

[6. Thresholds for SKYRAD 4](#__RefHeading___Toc107738180)

[7. Thresholds for GNDRAD 4](#__RefHeading___Toc107738181)

[8. Time Quality Flags 4](#__RefHeading___Toc107738182)

[9. Limits for Time 5](#__RefHeading___Toc107738183)

[10. Dimension Variables 5](#__RefHeading___Toc107738184)

[11. Spectral Response Function 6](#__RefHeading___Toc107738185)

[12. Configuration 8](#__RefHeading___Toc107738186)

[13. Calibration History 12](#__RefHeading___Toc107738187)

# General Overview

The Infrared Thermometer (IRT) is a ground-based radiation pyrometer
that provides measurements of the equivalent blackbody brightness
temperature of the scene in its field of view. The downwelling version
has a narrow field of view for measuring sky temperature and for
detecting clouds. The upwelling version has a wide field of view for
measuring the narrowband radiating temperature of the ground surface.

# Contacts

## Mentor

Victor Morris\
Pacific Northwest National Laboratory\
P.O. Box 999, MS K9-38\
Richland, WA 99352\
Phone: 509-372-6144\
Fax: 509-372-6247\
E-mail: <vic.morris@arm.gov>

## Instrument Developer

Wintronics, Inc.\
P.O. Box 337\
Millington, NJ 07946\
Phone: 908-647-0144\
Fax: 908-647-8379\
E-mail: [www.wintron.com](http://www.wintron.com/)

# Deployment Locations and History 

[]{#__RefHeading___Toc107738175 .anchor}Table 1. Current Status and
Locations

  ----------- ------------- ------------------ -------------- ----------------- -------------
  **Serial\   **Property\   **Function**       **Location**   **Installation\   **Status**
  Number**    Number**                                        Date**            

  517         WD11383       downwelling        Wintronics     \--               damaged

  863         WD17512       GNDRAD upwelling   TWP/CF2        1998/11/18        operational

  864         WD17513       GNDRAD upwelling   TWP/CF3        2002/03/08        operational

  865         WD17514       upwelling          TWP/CF3        \--               spare

  867         WD17516       SKYRAD downwelling TWP/CF3        2002/02/25        operational

  868         WD17517       SKYRAD downwelling TWP/CF2        1998/11/18        operational

  1026        WD18682       10M upwelling      SGP/CF1        1995/10/18        operational

  1029        WD18683       25M upwelling      SGP/CF1        1995/10/18        operational

  1250        WD20062       GNDRAD upwelling   TWP/CF1        2003/06/29        operational

  1251        WD20064       GNDRAD upwelling   NSA/CF1        1999/03/04        operational

  1252        WD20063       GNDRAD upwelling   NSA/CF2        1998/04/06        operational

  1253        WD20065       SKYRAD downwelling NSA/CF2        1998/02/14        operational
  ----------- ------------- ------------------ -------------- ----------------- -------------

**Table 1**. Current Status and Locations (cont'd)

  ----------- ------------- ------------------ -------------- ----------------- -------------
  **Serial\   **Property\   **Function**       **Location**   **Installation\   **Status**
  Number**    Number**                                        Date**            

  1254        WD20066       downwelling        SGP/CF1        2001/01/25        operational

  1255        WD20067       SKYRAD downwelling NSA/CF1        1998/10/15        operational

  1553        WD23001       downwelling        PNNL           \--               spare

  1628        WD23029       upwelling          PNNL           \--               spare

  1938        WD33301       downwelling        SGP            \--               spare

  2301        WD45256       GNDRAD             AMF            2004/10/20        operational

  2302        WD45257       SKYRAD             AMF            2004/10/20        operational
  ----------- ------------- ------------------ -------------- ----------------- -------------

# Near-Real-Time Data Plots

Available [data plots and other data
products](http://www.arm.gov/data/data_plots.stm).

# Data Description and Examples

Available [data plots and other data
products](http://www.arm.gov/data/data_plots.stm).

## Data File Contents

Datastreams produced by the IRT that are available from the [ARM
Archive](http://www.archive.arm.gov/) are as follows:

-   **irt** - 1-min averaged sky temperature

-   **irt2s** - 2-sec instantaneous sky temperature

-   **irt10m** - 1-min averaged surface temperature from 10 m

-   **irt25m** - 1-min averaged upwelling irradiance and surface
    temperature from 25 m

-   **skyrad60s** - 1-min averaged downwelling irradiance and sky
    temperature from SKYRAD

-   **gndrad60s** - 1-min averaged upwelling irradiance and surface
    temperature from GNDRAD.

### Primary Variables and Expected Uncertainty

The downwelling IRT measures the infrared radiation emitted by the sky
or cloud-base and transforms it into a standardized output signal that
is proportional to temperature in Kelvins. The upwelling IRT measures
the ground-surface temperature in Kelvins.

[]{#__RefHeading___Toc107738176 .anchor}Table 2. Primary Variables

  ---------------------- ---------------------------------------- ----------
  **Variable Name**      **Quantity Measured**                    **Unit**

  sky_ir_temp            Sky Infrared Temperature                 K

  sfc_ir_temp            Surface Infrared Temperature             K
  ---------------------- ---------------------------------------- ----------

#### Definition of Uncertainty

For a 0-1 volt output range and 100 K span, the temperature uncertainty
is 0.0244 K.

### Secondary/Underlying Variables

[]{#__RefHeading___Toc107738177 .anchor}Table 3. Secondary Variables

  ----------------- --------------------------------------------- ----------
  **Variable Name** **Quantity Measured**                         **Unit**

  time              Time offset from midnight                     seconds

  sky_ir_temp_std   Sky Infrared Temperature, Standard Deviation  K

  sky_ir_temp_max   Sky Infrared Temperature Maxima               K

  sky_ir_temp_min   Sky Infrared Temperature Minima               K

  sfc_ir_temp_std   Surface Infrared Temperature, Standard        K
                    Deviation                                     

  sfc_ir_temp_max   Surface Infrared Temperature Maxima           K

  sfc_ir_temp_min   Surface Infrared Temperature Minima           K
  ----------------- --------------------------------------------- ----------

### Diagnostic Variables

[]{#__RefHeading___Toc107738178 .anchor}Table 4. Diagnostic Variables

  ------------------ -------------------------- ----------------------------
  **Variable Name**  **Quantity Measured**      **Unit**

  time_offset        Time offset from base_time seconds

  logger_temp        Logger Temperature         C

  internal_voltage   Battery Voltage            V
  ------------------ -------------------------- ----------------------------

### Data Quality Flags

Most fields contain a corresponding, sample by sample, automated quality
check field in the b1 level datastreams. These flags are named
**qc\_\<fieldname\>**. For example, the **sky_ir_temp** field also has a
companion **qc_sky_ir_temp** field. Possible values for each sample of
the **qc\_\<fieldname\>** are shown in the table below.

[]{#__RefHeading___Toc107738179 .anchor}Table 5. Data Quality Flags

  ----------- ------------------------------------------------------------
  **Value**   **Definition**

  0           All QC checks passed

  1           Sample contained \'missing data\' value

  2           Sample was less than prescribed minimum value

  3           Sample failed both \'missing data\' and minimum value checks

  4           Sample greater than prescribed maximum value

  5           Sample failed both minimum and maximum value checks (highly
              unlikely)

  7           Sample failed minimum, maximum and missing value checks
              (highly unlikely)
  ----------- ------------------------------------------------------------

**Table 5**. Data Quality Flags (cont'd)

  ----------- ------------------------------------------------------------
  **Value**   **Definition**

  8           Sample failed delta check (change between this sample and
              and previous sample exceeds a prescribed value

  9           Sample failed delta and missing data checks

  10          Sample failed minimum and delta checks

  11          Sample failed minimum, delta and missing value checks

  12          Sample failed maximum and delta checks

  14          Sample failed minimum, maximum and delta checks

  15          Sample failed minimum, maximum, delta and missing value
              checks.
  ----------- ------------------------------------------------------------

The minimum and maximum thresholds are currently defined as follows for
datastreams **irt**, **irt2s**, and **skyrad60s**:

[]{#__RefHeading___Toc107738180 .anchor}Table 6. Thresholds for SKYRAD

  ------------------------ ----------- ---------- ----------- -----------
  **Field Name**           **Units**   **Min**    **Max**     **Delta**

  sky_ir_temp              K           213        313         50

  sky_ir_temp_max          K           213        313         50

  sky_ir_temp_min          K           213        313         50

  sky_ir_temp_std          K           0          20          \--
  ------------------------ ----------- ---------- ----------- -----------

The minimum and maximum thresholds are currently defined as follows for
datastreams **irt10m**, **irt25m**, and **gndrad60s**:

[]{#__RefHeading___Toc107738181 .anchor}Table 7. Thresholds for GNDRAD

  ------------------------ ------------ ----------- ----------- -----------
  **Field Name**           **Units**    **Min**     **Max**     **Delta**

  sfc_ir_temp              K            223         323         50

  sfc_ir_temp_max          K            223         323         50

  sfc_ir_temp_min          K            223         323         50

  sfc_ir_temp_std          K            0           20          \--
  ------------------------ ------------ ----------- ----------- -----------

In addition to the above data quality checks, the **qc_time** field is
also supplied. The purpose of the qc_time field is to help detect
duplicate samples, missing samples or other sample time problems. The
qc_time field contains a value for each sample time. Refer to the table
below for details.

[]{#__RefHeading___Toc107738182 .anchor}Table 8. Time Quality Flags

  -------------- --------------------------------------------------------
  **Value**      **Description**

  0              Dt is within specified range

  1              Dt is 0, duplicate sample

  2              Dt is less than specified lower limit

  4              Dt is greater than specified upper limit
  -------------- --------------------------------------------------------

Finally, the table below specifies the **qc_time limits** used for each
of the irt datastreams.

[]{#__RefHeading___Toc107738183 .anchor}Table 9. Limits for Time

  ---------------------------- ---------------------- --------------------
  **Datastream**               **Lower\               **Upper\
                               Limit**                Limit**

  irt                          60                     60

  irt2s                        2                      2

  irt10m                       60                     60

  irt25m                       60                     60

  skyrad60s                    60                     60

  gndrad60s                    60                     60
  ---------------------------- ---------------------- --------------------

Additional information may be found at
[GNDRAD](http://science.arm.gov/tool/dod/showdod.php?Inst=gndrad) and
[SKYRAD](../%20http://science.arm.gov/tool/dod/showdod.php%3FInst=skyrad)
Data Object Design Changes for ARM netCDF file header descriptions.

### Dimension Variables

[]{#__RefHeading___Toc107738184 .anchor}Table 10. Dimension Variables

  ---------------- -------------------------- ----------------------------
  **Variable       **Quantity Measured**      **Unit**
  Name**                                      

  base_time        Base time in Epoch         seconds

  lat              north latitude             degrees

  lon              east longitude             degrees

  alt              Altitude                   meters above Mean Sea Level
  ---------------- -------------------------- ----------------------------

## Annotated Examples

This section is not applicable to this instrument.

## User Notes and Known Problems

Positive "spikes" are produced in the sky temperature measurements
during daily preventative maintenance due to water and/or alcohol used
to clean the gold mirror.

An apparent warm bias of the surface temperature is exhibited by the
IRT, compared to the PIR, at sites where the instrument is mounted on a
tower 10 m above the ground, especially in summer. The PIR responds
strongly to air temperature while the IRT responds strongly to ground
temperature. The bias goes away or becomes very small at night because
the ground temperature approaches equilibrium with the air. The bias is
also small when it is raining or very overcast (as indicated by low
solar values). The bias becomes large during the day when the sun is
out. For example, on sunny days at TWP/CF3, the ground temperature as
seen by the IRT is about 51C while the PIR sees a temperature about 10C
cooler. The actual air temperature at Darwin in the afternoon is about
32C. So it appears that the PIR is reporting a temperature approximately
mid-way between the ground temperature and the air temperature. This
effect is also experienced at Manus and Nauru but it is more pronounced
at Darwin because the radiometers are further from the ground.

The SGP downwelling IRT was removed from the microwave radiometer (MWR)
and associated datastreams on 11/5/2002 (ECO-267, BCR-581).

The SGP upwelling 10m and 25m IRTs were removed from the MFR dataloggers
and associated datastreams on 9/26/2001 and 9/27/2001, respectively
(ECO-149, BCR-306).

The Tropical Western Pacific (TWP) SKYRAD IRT rain
detector/shutter/enclosure assemblies were replaced with a gold
mirror/solar cover assembly at ARCS1 on 10/19/2001 and at ARCS2 on
10/29/2001 (P000610.1, ECO-170).

## Frequently Asked Questions

**How is the IRT compared with AERI?\
**\
See Calibration Theory, Section 7.3.1.

**What is the difference between the skin temperature, the effective
ground radiating temperature measured with the upwelling IRTs, and the
radiative temperature determined with the upwelling PIRs?\
**\
The skin temperature is the actual temperature of the ground surface, as
measured with a thermometry device. The effective ground radiating
temperature measured with IRTs is the temperature equivalent, using the
Stefan-Boltzmann law, of the infrared radiant energy from the ground,
assuming it acts as a blackbody with an emissivity of 1. The radiative
temperature measured with the PIRs is longwave irradiance emitted and
reflected by the ground surface.

**What is the spectral response function of the IRT?**\
**\
**See table below or download this [space-delimited
file](http://www.arm.gov/instruments/publications/irt_srf.txt) for use.

[]{#__RefHeading___Toc107738185 .anchor}Table 11. Spectral Response
Function

  ------------------------------- ---------------------------------------
  **Wave\                         **Spectral\
  Length µm**                     Response %**

  9.40                            0.000000

  9.46                            4.580252

  9.58                            12.273370

  9.70                            29.645053

  9.82                            48.180234

  9.94                            56.015325

  10.06                           59.592843

  10.18                           59.582579

  10.30                           61.141282

  10.42                           63.120466

  10.54                           64.794606

  10.66                           66.224009

  10.78                           67.286336

  10.90                           66.575086

  11.02                           65.575695
  ------------------------------- ---------------------------------------

**Table 11**. Spectral Response Function (cont'd)

  ------------------------------- ---------------------------------------
  **Wave\                         **Spectral\
  Length µm**                     Response %**

  11.14                           65.601176

  11.26                           66.965032

  11.38                           67.696559

  11.50                           42.629668

  11.62                           14.432280

  11.74                           4.815956

  11.80                           0.000000
  ------------------------------- ---------------------------------------

**In what datastream are the surface temperature measurements at the SGP
central facility?**

The \"sfc_ir_temp\" data are found in datastreams sgpmfr10mC1.a1 and
sgpmfr25mC1.a1 from 5/19/1997 to 3/31/2001, sgpmfrirt10mC1.a1 and
sgpmfrirt25mC1.a1 from 4/1/2001 to 9/26/2001, sgpirt10mC1.a1 and
sgpirt25mC1.a1 from 9/26/2001 to 12/18/2003, and sgpirt10mC1.b1 and
sgpirt25mC1.b1 from 12/18/2003 to present.

# 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

On a weekly basis, the instrument mentor produces and inspects plots of
the data from the downwelling and upwelling IRTs at the SGP central
facility and the TWP, North Slope of Alaska (NSA), and AMF SKYRAD and
GNDRAD IRTs. Time series and scatter plots are produced and inspected to
compare sky temperature measured by the IRT and atmospheric emitted
radiance interferometer (AERI) and the surface temperature measured by
the IRT and precision infrared radiometer (PIR). Data Quality Reports
(DQRs) are submitted when needed, and a summary report of data quality
is sent monthly to the DQ Office.

## 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

Many of the scientific needs of the ARM Program are met through the
analysis and processing of existing data products into "value-added"
products (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](http://www.arm.gov/data/vaps_all.php) web page and
specifically, the [AERI LBL
CLOUDS](http://science.arm.gov/vaps/qmealbc.stm) VAP.

# Instrument Details

## Detailed Description

### List of Components

Heitronics KT19.85 Infrared Radiation Pyrometer

Wintronics PSJB300.24 Power Supply and Junction Box

Radiometrics MP3965 IRT Case (downwelling only)

Radiometrics MP3964 IRT Saddle (downwelling only)

Edmund NT32-089 gold mirror (downwelling only)

DC power/analog signal cable

### System Configuration and Measurement Methods

The downwelling IRT is mounted at a height of 1-2 m above the ground,
oriented so the zenith view of the sky is reflected into the lens by a
gold mirror. The upwelling IRT is mounted at a height of 2-10 m above
the ground, oriented so the mounting platform is not in the field of
view and to ensure that the ground and vegetation cover are
representative of the local area.

The following internal configuration is presently in use:

[]{#__RefHeading___Toc107738186 .anchor}Table 12. Configuration

  -------------------------------- ------------------ -------------------
  **Parameter**                    **Downwelling**    **Upwelling**

  Target emissivity                1.0                1.0

  Temperature unit                 Kelvins            Kelvins

  Temperature span (TWP&NSA)       213-463 K          223-473 K

  Temperature span (SGP)           213-313 K          223-323 K
  -------------------------------- ------------------ -------------------

**Table 12**. Configuration (cont'd)

  -------------------------------- ------------------ -------------------
  **Parameter**                    **Downwelling**    **Upwelling**

  Response time (90%)              1 second           3 seconds

  Analog output (TWP&NSA)          0-10 volts         0-10 volts

  Analog output (SGP)              0-1 volt           0-1 volt

  Digital output                   9.6Kb/8NP/1S/LF    9.6Kb/8NP/1S/LF
  -------------------------------- ------------------ -------------------

The radiation pyrometer is a measuring transducer, which receives the
infrared radiation emitted by the measuring object and transforms it
into a standardized output signal. If the emissivity is known, the
temperature of the object can be determined. The ARM IRTs use a constant
emissivity of 1.0 for sky, clouds, and ground. The radiation pyrometer
KT19.85 operates within the spectral range of 9.6 to 11.5 µm where the
transmission of the atmosphere is very high. There is very little
weakening of the infrared radiation due to CO~2~ or to water vapor
contained in the air.

The working principle of infrared radiation pyrometers uses optical
modulation of thermal radiation intercepted by an infrared detector. In
general, this is accomplished by an optical chopper (mechanical blades
driven by an electric motor), which periodically interrupts the incident
radiation from the measured target to the detector. During each
interruption the detector is exposed to an internal blackbody reference
source having a defined temperature. Infrared detectors of the
pyroelectric type must be operated with the "chopped radiation" method
because they respond to radiation differences and not to absolute
radiation intensities. The detector intercepts infrared radiation
emitted by the measured target and, at the same time, radiation emitted
by the detector enclosure. During the short chopping cycles, in the
millisecond range, the temperature of the pyrometer\'s housing does not
change. The bias is thus eliminated and substituted by the reference
signal, which can be easily measured or controlled within the specified
reference accuracy over the operational ambient temperature range. Thus,
the "chopped radiation" method eliminates thermal drift and
automatically provides a modulated signal with a precisely defined
frequency.

Each IRT has its own characteristics because of small deviation in
filters, detector sensitivity, and lenses. These individual
characteristics are compensated by comparing the temperature reading of
the instrument with the blackbody radiation at several temperatures and
adapting the linearization. The radiance can be calculated from the
measured temperature by integrating Plank\'s law over the spectral
range.

The downwelling IRT reports the effective blackbody temperature of the
sky in the portion of the infrared spectrum sensed by the instrument.
When there is no cloud within the field of view of the instrument, this
temperature depends almost entirely on the amount of water vapor in the
atmosphere above the instrument. When there is a lot of water vapor
present (e.g., during summer), the sky temperatures may exceed 250 K.
When there is very little water vapor present (e.g., during winter at
the Southern Great Plains \[SGP\] site), the sky temperatures may fall
below 180 K. Note that the lower limit of the IRT range is 213 K
(although its calibration is only good down to 223 K). When the sky
temperature falls below 213 K, the IRT will signal an error condition.
When a cloud enters the field of view of the IRT, an increase in the
reported temperature should be observed. How much the temperature will
increase depends on how high the cloud is, how thick the cloud is, and
how much water vapor is in the atmosphere between the instrument and the
cloud. Low, heavy clouds will generally produce the largest increase in
temperature, especially in winter when the clear-sky temperature is low;
high, wispy clouds will produce a lesser increase. During the summer
when the clear sky temperature is relatively large, high clouds may
produce only a slight increase.

The narrowband radiating temperature reported by the upwelling IRT will
be very close to the physical temperature of the ground/vegetation in
its field of view. To the extent that the physical temperature varies in
the course of a diurnal cycle so too will the IR temperature reported by
the instrument.

### Specifications

**Spectral Sensitivity**: 9.6 to 11.5 µm.

**Temperature Measuring Range**: 213 to 673 K.

**Temperature Resolution in ±K (emissivity = 1, response time = 1 s)**:
1.10 K at 223 K; 0.45 K at 293 K; 0.25 K at 373 K.

**Accuracy**: ±0.5 K.

**Operational Ambient Temperature**: 0° to 60°C.

**Storage Temperature**: -20° to +70°C.

**Weight**: 1.5 kg.

**Analog Output Resolution**: 12 bit.

**Optical Field of View (at 3 m)**: downwelling (S921 lens, f = 120
mm) - 2.64°; upwelling (M6 lens, f = 20 mm) - 30.51°.

**Operating Voltages**: 24 V AC (±10%) at 48 to 400 Hz or 26 V DC
(±15%).

**Current Consumption**: 80 mA.

## Theory of Operation

It is well known that radiation emitted by objects can be felt by the
thermal sensors of our skin. "Temperature radiation" is emitted by all
material objects at temperatures above the absolute zero point.
Temperature radiation is primarily emitted at wavelengths in the
invisible infrared region, but at high temperatures also in the visible
spectrum. Instruments capable of measuring this radiation and providing
an output signal calibrated in temperature units are called radiation
thermometers, radiation pyrometers, or simply pyrometers. The scientific
study of the determination of temperatures by the noncontact measurement
of self-emitted surface radiation is called radiation pyrometry.

A radiation thermometer measures the radiance power from the target
area. The self-emitted radiant power is smaller than the radiant power
from a blackbody surface at the same temperature. The reflected and
transmitted radiant powers are emitted by the foreground and the
background, respectively. The sum of emissivity, reflectance, and
transmittance is always 1. In accordance with Planck\'s law, the
radiances are clearly related to the temperatures, provided the relative
spectral response (determined by the optical system and the radiation
detector) is known. The output signal of a linear detector is
proportional to the measured radiance. It is thus permissible to read
the measured radiance in terms of a "measured temperature" and to
calibrate the indicating meter for the output signal in temperature
units (i.e., in Kelvins).

## Calibration

### Theory

The calibration and testing of Heimann Radiation Pyrometers are done
according to the *Standard Test Methods for Radiation Thermometers
(Single Waveband Type) E1256-88* of the American Society for Testing and
Materials (ASTM).

### Procedures

**Factory Calibration Procedures**

By means of a blackbody calibration source, the display of the radiation
pyrometer can be checked and recalibrated, if required. It is advisable
to carry out the test under high temperature (80°-100°C). For this
purpose, the radiation pyrometer is placed in front of a blackbody
calibration source so that the IRT is focused on the radiator. The
temperature of the blackbody radiator must be measured by means of a
calibrated probe. On the indicator at the back of the device, select
operating mode "CAL" via the "MODE" key after entering the code. After
pressing key "ENTER," the calibration factor and measure temperature can
be read from the display. Using the "UP/DOWN" keys, the factor can be
changed so the temperature matches the blackbody temperature. By
actuating key "ENTER" again, the new calibration factor is stored and
the device will automatically be adjusted to this temperature.

**Mentor Calibration Procedures**

The ASTM method utilizes a blackbody radiator heated to 350°C (623 K).
This is 350-400 degrees above the range of sky temperatures to be
measured. For this reason, the mentor recommends calibrating the
upward-looking instruments against a cyrogenically-cooled spectrometer
such as the AERI spectrometer built by the University of Wisconsin. This
is accomplished as follows:

Collocate the IRT with the AERI to observe the same sky conditions. AERI
provides a spectrally resolved measurement over the range of the IR
thermometer\'s detector response. For each AERI spectrum, compute the
narrowband radiance observed by the IRT:

L~IRT~ = ∫L(l,T)S(l)dl

where l is the wavelength or wavenumber, L(l,T) is the AERI spectral
radiance, and S(l) is the IRT spectral response function. Then
iteratively solve the following expression for the blackbody radiating
temperature T~bb~ until it agrees with the narrowband radiance computed
from the AERI spectrum:

L~IRT~ = ∫B(T~bb~,l)S(l)dl

where B(T~bb~,l) is the Planck function.

Plotting the temperature reported by the IRT, T~obs~, against the
blackbody temperature computed from the AERI spectra, T~bb~, for a wide
range of sky temperatures (i.e., clear and cloudy skies, low to high
precipitable water vapor) will permit the construction of a regression:
T~bb~ = a + b T~obs~. The IRT calibration can then be adjusted (see
above) to match the AERI-derived values.

Although this is not an absolute calibration, it will reference the IRT
to AERI and will permit the calibration to be tuned to the range of sky
temperatures actually encountered. Additionally, one could use the AERI
data to determine corrections for water vapor contribution (since what
we hope to measure with the IRT is cloud base temperature). This will be
significant during the summer at SGP and year round at TWP.

### History

[]{#__RefHeading___Toc107738187 .anchor}Table 13. Calibration History

  ---------- --------------- --------------- ----------------------------------------
  **Serial   **Calibration   **Calibration   **Comments**
  Number**   Date**          Factor**        

  517        91/90/12        2263            Initial calibration by Heimann

             96/10/28        2263            Lens replaced and compared with AERI
                                             (PIF 960809.2)

             96/12/10        2263            Compared with SN 1254 by NREL (PIF
                                             961203.2)

             97/09/10        2263            Compared with AERI (PIF 970428.1)

  863        93/09/19        2440            Initial calibration by Pyrometrics

  864        93/09/19        2300            Initial calibration by Pyrometrics

             99/04/07        2300            Compared with SN 517 and AERI

  865        93/09/19        2640            Initial calibration by Pyrometrics

             03/11/04        2628            Repaired and calibrated by Wintronics

  866        93/09/19        1649            Initial calibration by Pyrometrics

  867        93/09/19        1847            Initial calibration by Pyrometrics

             98/01/10        2358            Repaired and calibrated by Wintronics

             99/04/07        2358            Compared with SN 517 and AERI

  868        93/09/19        2221            Initial calibration by Pyrometrics

             97/09/19        2239            Repaired and calibrated by Wintronics

  1026       95/08/16        2276            Initial calibration by Pyrometrics

  1029       96/08/16        2114            Initial calibration by Pyrometrics

  1250       96/10/01        2214            Initial calibration by Wintronics

             97/09/10        2214            Compared with SN 517 and AERI (PIF
                                             970428.1)

             99/04/07        2214            Compared with SN 517 and AERI

  1251       96/10/01        2106            Initial calibration by Wintronics

  1252       96/10/01        2116            Initial calibration by Wintronics

             99/04/07        2116            Compared with SN 517 and AERI

             01/03/22        2138            Repaired and calibrated by Wintronics

  1253       96/10/01        1300            Initial calibration by Wintronics

             98/01/10        4600            Repaired and calibrated by Wintronics
                                             (PIF 980918.3)

             99/04/07        4600            Compared with SN 517 and AERI

  1254       96/10/01        1307            Initial calibration by Wintronics

             97/09/10        1307            Compared with SN 517 and AERI (PIF
                                             970428.1)

             97/12/19        1281            Calibrated by Wintronics

             99/04/07        1281            Compared with SN 517 and AERI

  1255       96/10/01        1255            Initial calibration by Wintronics

             98/07/13        1207            Repaired and calibrated by Wintronics
  ---------- --------------- --------------- ----------------------------------------

**Table 13**. Calibration History (cont'd)

  ---------- --------------- --------------- ----------------------------------------
  **Serial   **Calibration   **Calibration   **Comments**
  Number**   Date**          Factor**        

             02/10/17        1145            Repaired and calibrated by Wintronics

  1553       98/01/10        1260            Initial calibration by Wintronics

             99/04/07        1260            Compared with SN 517 and AERI

  1628       98/03/04        4226            Initial calibration by Wintronics

             99/04/07        4226            Compared with SN 517 and AERI

  1938       01/09/01        5086            Initial calibration by Wintronics
  ---------- --------------- --------------- ----------------------------------------

## Operation and Maintenance

### User Manual

This section is not applicable to this instrument.

### Routine and Corrective Maintenance Documentation

See the following links:

-   [SGP IRT Preventative Maintenance
    Procedure](http://www.ops.sgp.arm.gov/pm_proc/irtpm.htm)

-   [TWP Operating
    Procedure](http://www.twppo.lanl.gov/internal/pages/man_operations.html#SectionA17)

-   [NSA Preventative Maintenance Procedure
    Manual.](http://agvik.gi.alaska.edu/nsa_manuals/PMmanualBRW.pdf)

### Software Documentation

ARM netCDF file header descriptions may be found at
[GNDRAD](../%20http://science.arm.gov/tool/dod/showdod.php%3FInst=gndrad)
and
[SKYRAD](../%20http://science.arm.gov/tool/dod/showdod.php%3FInst=skyrad)
Data Object Design Changes.

### Additional Documentation

See Routine and Corrective Maintenance Documentation, Section 7.4.2.

## Glossary

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

## Acronyms

See the [ARM Acronyms](http://www.arm.gov/about/acronyms.stm).

## Citable References

Morris, V, C Long, and D Nelson. 2006. "Deployment of an Infrared
Thermometer Network at the Atmospheric Radiation Measurement Program
Southern Great Plains Climate Research Facility." In *Proceedings of the
Sixteenth Atmospheric Radiation (ARM ) Science Team Meeting*, Ed. by D
Carrothers, U.S. Department of Energy, Richland, WA.