temp mesurement eng a_w

Content
CONTENT ..................................................................................3
1 INTRODUCTION.........................................................................5
2 DISCOVERY OF INFRARED RADIATION.......................................5
3 ADVANTAGES OF USING INFRARED THERMOMETERS...............5
4 THE INFRARED SYSTEM .............................................................6
4.1 The Target ..................................................................................6
4.1.1 Determining Emissivity............................................................8
4.1.2 Measuring Metals ...................................................................9
4.1.3 Measuring Plastics ..................................................................9
4.1.4 Measuring Glass......................................................................9
4.2 Ambient Conditions .................................................................10
4.3 Optics and Window..................................................................12
4.4 Sighting Devices .......................................................................14
4.5 Detectors..................................................................................15
4.6 Display and Interfaces..............................................................15
4.7 Technical Parameters of IR Thermometers..............................16
4.8 Calibration................................................................................17
5 SPECIAL PYROMETERS............................................................. 18
5.1 Fiber-optic Pyrometers ............................................................18
5.2 Ratio Pyrometers......................................................................18
5.3 Imaging Systems.......................................................................20
5.3.1 IR Linescanners......................................................................20
5.3.2 Matrix Cameras.....................................................................21
6 SUMMARY............................................................................... 22
7 BIBLIOGRAPHY ........................................................................ 23

Fluke Process Instruments  Principles of Noncontact Temperature Measurement 5
1 Introduction
This booklet is written for people who are unfamiliar
with noncontact infrared temperature measurement.
A conscious attempt has been made to present the
subject matter as briefly and as simply as possible.
Readers who wish to gain more in-depth knowledge
can follow the suggestions for further reading in the
bibliography. This manual focuses on the practical op-
erations of noncontact temperature measurement de-
vices and IR thermometry, and answers important
questions that may arise. If you plan to use a noncon-
tact temperature measurement device and require
further advice, send us the completed form (see ap-
pendix) prior to use.
2 Discovery of Infrared Radiation
Fire and ice, hot and cold – elemental extremes have
always fascinated and challenged people. Various
techniques and devices have been used throughout
time to accurately measure and compare temperature
conditions. For example, in the early days of ceramics
manufacture, meltable materials were used, which in-
dicated through deformation that certain higher tem-
peratures were reached. A baker on the other hand,
used a piece of paper – the quicker it became brown
in the oven, the hotter the oven was. The disad-
vantage of both techniques was that they were not re-
versible – cooling could not be determined. Also, the
accuracy of the results was very dependent on the
user and his or her experience. It was not until the in-
vention of the first thermoscope in the first half of the
17th Century that temperatures could begin to be
measured. An evolution of the thermoscope (which
had no scale) the thermometer had various scales
proposed. Between 1724 and 1742 Daniel Gabriel
Fahrenheit and by Anders Celsius defined what we
probably consider as the 2 most common.
Fig. 1: William Herschel (1738 – 1822) discovers IR radiation
The discovery of infrared radiation by the physicist
Wilhelm Herschel at the beginning of the 19th Century
opened new possibilities for measuring temperature –
without contact and thus without affecting the object
being measured and the measurement device itself.
Compared to early infrared temperature measure-
ment devices, which were heavy, awkward and com-
plicated to operate, the image of such devices today
has completely changed. Modern infrared thermome-
ters are small, ergonomic, easy to operate and can
even be installed into machinery. From versatile
handheld devices to special sensors for integration
into existing process systems, the spectrum of prod-
uct offerings is vast. A variety of accessories and soft-
ware for the collection and analysis of measurement
data are provided with most of infrared temperature
sensors.
3 Advantages of Using Infrared
Thermometers
Temperature is the most frequently measured physi-
cal parameter, second only to time. Temperature
plays an important role as an indicator of the condition
of a product or piece of machinery, both in manufac-
turing and in quality control. Accurate temperature
monitoring improves product quality and increases
productivity. Downtimes are decreased since the
manufacturing processes can proceed without inter-
ruption and under optimal conditions. Infrared tech-
nology is not a new phenomenon. It has been utilized
successfully in industry and research for decades. But
new developments have reduced costs, increased re-
liability, and resulted in smaller noncontact infrared
measurement devices. These factors have led to in-
frared technology becoming an area of interest for
new kinds of applications and users.
Fig. 2: Modern Digital Infrared Pyrometer in miniature size
(Fluke Process Instruments: Endurance Series)

The Infrared System
6 Fluke Process Instruments  Principles of Noncontact Temperature Measurement
What are the advantages offered by noncontact tem-
perature measurement?
1. It is fast (in the ms range) – time is saved, allowing
for more measurements and accumulation of more
data (temperature areas can be determined).
2. It facilitates measurement of moving targets (con-
veyor processes).
3. Measurements can be taken of hazardous or phys-
ically inaccessible objects (high-voltage parts,
large measurement distances).
4. Measurements of high temperatures (above
1300°C) present no problems. Contact thermome-
ters often cannot be used in such conditions, or
they have a limited lifetime.
5. There is no interference as no energy is lost from
the target. For example, in the case of a poor heat
conductor such as plastic or wood, measurements
are extremely accurate with no distortion of meas-
ured values, as compared to measurements with
contact thermometers.
6. Noncontact temperature measurement is wear-
free – there is no risk of contamination and no me-
chanical effect on the surface of the object. Lac-
quered or coated surfaces, for example, are not
scratched and soft surfaces can be measured.
Having enumerated the advantages, there remains
the question of what to keep in mind when using an
IR thermometer:
1. The target must be optically (infrared-optically) vis-
ible to the IR thermometer. High levels of dust or
smoke make measurement less accurate. Solid
obstacles, such as a closed metallic reaction ves-
sel, do not allow internal measurements.
2. The optics of the sensor must be protected from
dust and condensing liquids. (Manufacturers sup-
ply the necessary equipment for this.)
3. Normally, only surface temperatures can be meas-
ured, with the differing emissivities of different ma-
terial surfaces taken into account.
4 The Infrared System
An IR thermometer can be compared to the human
eye. The lens of the eye represents the optics through
which the radiation (flow of photons) from the object
reaches the photosensitive layer (retina) via the at-
mosphere. This is converted into a signal that is sent
to the brain. Fig. 3 shows how an infrared measuring
system works.
Fig. 3: Infrared Measuring System
4.1 The Target
Every form of matter with a temperature above abso-
lute zero (-273.15°C / -459.8°F) emits infrared radia-
tion according to its temperature. This is called char-
acteristic radiation. The cause of this is the internal
mechanical movement of molecules. The intensity of
this movement depends on the temperature of the ob-
ject. Since the molecule movement represents charge
displacement, electromagnetic radiation (photon par-
ticles) is emitted. These photons move at the speed
of light and behave according to the known optical
principles. They can be deflected, focused with a lens,
or reflected by reflective surfaces. The spectrum of
this radiation ranges from 0.7 to 1000 µm wavelength.
For this reason, this radiation cannot normally be seen
with the naked eye. This area lies within the red area
of visible light and has therefore been called "infra"-
red after the Latin, see Fig. 4.
Fig. 5 shows the typical radiation of a body at different
temperatures. As indicated, bodies at high tempera-
tures still emit a small amount of visible radiation.
Therefore, everyone can see objects at very high tem-
peratures (above 600°C) glowing somewhere from
red to white. Experienced steelworkers can even esti-
mate temperature quite accurately from the color. The
classic disappearing filament pyrometer was used in
the steel and iron industries from 1930 on.
Summary
The main advantages of noncontact IR thermom-
etry are speed, lack of interference, and the ability
to measure in high temperature ranges up to
3000°C. Keep in mind that generally only the sur-
face temperature can be measured.
Target
Atmosphere
Sensor
with Optics
Display and
Interfaces

Fig. 4: The electromagnetic spectrum, with range from around 1 to
20 µm useful for measuring purposes
The invisible part of the spectrum, however, contains
up to 100,000 times more energy. Infrared measuring
technology builds on this. It can likewise be seen in
Fig. 5 that the radiation maximum move toward ever-
shorter wavelengths as the target temperature rises,
and that the curves of a body do not overlap at differ-
ent temperatures. The radiant energy in the entire
wavelength range (area beneath each curve) in-
creases to the power of 4 of the temperature. These
relationships were recognized by Stefan and Boltz-
mann in 1879 and illustrate that an unambiguous tem-
perature can be measured from the radiation signal.
/1/ /3/ /4/ /5/
Fig. 5: Radiation characteristics of a blackbody in relation to its tem-
perature /3/
Looking at Fig. 5, then, the goal should be to set up
the IR thermometer for the widest range possible in
order to gain the most energy (corresponding to the
area below a curve) or signal from the target. There
are, however, some instances in which this is not al-
ways advantageous. For instance, in Fig. 5, the inten-
sity of radiation increases at 2 µm – much more when
the temperature increases than at 10 µm. The greater
the radiance difference per temperature difference,
the more accurately the IR thermometer works. In ac-
cordance with the displacement of the radiation ma-
ximum to smaller wavelengths with increasing tem-
perature (Wien's Displacement Law), the wavelength
range behaves in accordance with the measuring
temperature range of the pyrometer. At low tempera-
tures, an IR thermometer working at 2 µm would stop
at temperatures below 600°C, seeing little to nothing
since there is too little radiation energy. A further rea-
son for having devices for different wavelength ranges
is the emissivity pattern of some materials known as
non-gray bodies (glass, metals, and plastic films). Fig.
5 shows the ideal—the so-called "blackbody". Many
bodies, however, emit less radiation at the same tem-
perature. The relation between the real emissive
power and that of a blackbody is known as emissivity
 (epsilon) and can be a maximum of 1 (body corre-
sponds to the ideal blackbody) and a minimum of 0.
Bodies with emissivity less than 1 are called gray bod-
ies. Bodies where emissivity is also dependent on
temperature and wavelength are called non-gray bod-
ies.
Furthermore, the sum of emission is composed of ab-
sorption (A), reflection (R) and transmission (T) and is
equal to one. (See Equation 1 and Fig. 6)
A + R + T = 1 (1)
Solid bodies have no transmission in the infrared
range (T = 0). In accordance with Kirchhof’s Law, it is
assumed that all the radiation absorbed by a body,
and which has led to an increase in temperature, is
then also emitted by this body. The result, then, for
absorption and emission is:
A  E = 1 – R (2)
Fig. 6: In addition to the radiation emitted from the target, the sensor
also receives reflected radiation and can also let radiation through.
Infrared range
used
A Ambient
B Reflection
C Emission
D Transmission
Sensor
Heat Source
Target
A
B
C
D

The Infrared System
The ideal blackbody also has no reflectance (R = 0),
so that E = 1.
Many non-metallic materials such as wood, plastic,
rubber, organic materials, rock, or concrete have sur-
faces that reflect very little, and therefore have high
emissivities between 0.8 and 0.95. By contrast, met-
als - especially those with polished or shiny surfaces
- have emissivities at around 0.1. IR thermometers
compensate for this by offering variable options for
setting the emissivity factor, see also Fig. 7.
Fig. 7: Specific emission at different emissivities
4.1.1 Determining Emissivity
There are various methods for determining the emis-
sivity of an object. So, you can find the emissivity of
many frequently used materials in a table. Emissivity
tables also help you find the right wavelength range
for a given material, and, so, the right measuring de-
vice. Particularly in the case of metals, the values in
such tables should only be used for orientation pur-
poses since the condition of the surface (e.g. pol-
ished, oxidized or scaled) can influence emissivity
more than the various materials themselves. It is also
possible to determine the emissivity of a material
yourself using different methods. To do so, you need
a pyrometer with emissivity setting capability.
1. Heat up a sample of the material to a known tem-
perature that you can determine very accurately
using a contact thermometer (e.g. thermocouple).
Then measure the target temperature with the IR
thermometer. Change the emissivity until the tem-
perature corresponds to that of the contact ther-
mometer. Now keep this emissivity for all future
measurements of targets on this material.
2. At a relatively low temperature (up to 260°C), at-
tach a special plastic sticker with known emissivity
to the target. Use the infrared measuring device to
determine the temperature of the sticker and the
corresponding emissivity. Then measure the sur-
face temperature of the target without the sticker
and re-set the emissivity until the correct tempera-
ture value is shown. Now, use the emissivity deter-
mined by this method for all measurements on tar-
gets of this material.
3. Create a blackbody using a sample body from the
material to be measured. Bore a hole into the ob-
ject. The depth of the borehole should be at least
five times its diameter. The diameter must corre-
spond to the size of the spot to be measured with
your measuring device. If the emissivity of the in-
ner walls is greater than 0.5, the emissivity of the
cavity body is now around 1, and the temperature
measured in the hole is the correct temperature of
the target /4/. If you now direct the IR thermometer
to the surface of the target, change the emissivity
until the temperature display corresponds with the
value given previously from the blackbody. The
emissivity found by this method can be used for all
measurements on the same material.
4. If the target can be coated, coat it with a matte
black paint ("3-M Black" from the company 3M or
"Senotherm" from Weilburger Lackfabrik (Grebe
Group)/2/, either which have an emissivity of
around 0.95). Measure the temperature of this
blackbody and set the emissivity as described pre-
viously.
SpecificEmission
ε = 1.0 (black body)
ε = 0.9 (gray body)
ε changes with wavelength
(non-gray body)
Wavelength in µm

4.1.2 Measuring Metals
The emissivity of a metal depends on wavelength and
temperature. Since metals often reflect, they tend to
have a low emissivity which can produce differing and
unreliable results. In such a case it is important to se-
lect an instrument which measures the infrared radia-
tion at a particular wavelength and within a particular
temperature range at which the metals have the high-
est possible emissivity. With many metals, the meas-
urement error becomes greater with the wavelength,
meaning that the shortest wavelength possible for the
measurement should be used, see Fig. 8.
Fig. 8: Measurement error in the case of 10% error in setting emis-
sivity dependent on wavelength and target temperature.
The optimal wavelength for high temperatures in the
case of metals is, at around 0.8 to 1.0 µm, at the limit
to the visible range. Wavelengths of 1.6, 2.2, and
3.9 µm are also possible. Good results can be
achieved using ratio pyrometers in cases (e.g. heating
processes) where measurement is to take place
across a relatively wide temperature range and the
emissivity changes with the temperature.
Fig. 9: Accurate temperature measurement of slabs, billets, or
blooms ensures product uniformity
4.1.3 Measuring Plastics
The transmittance of a plastic varies with the wave-
length and is proportional to its thickness. Thin mate-
rials are more transmissive than thick plastics. In or-
der to achieve optimal temperature measurement, it
is important to select a wavelength at which transmit-
tance is nearly zero. Some plastics (polyethylene, pol-
ypropylene, nylon, and polystyrol) are not transmis-
sive at 3.43 µm; others (polyester, polyurethane,
Teflon FEP, and polyamide) at 7.9 µm. With thicker (>
0.4 mm), strongly-colored films, you should choose a
wavelength between 8 and 14 µm.
Fig. 10: Spectral transmittance of plastic films. Independent of thick-
ness, Polyethylene is almost opaque at 3.43 µm.
If you are still uncertain, send a sample of the plastic
to the manufacturer of the infrared device to deter-
mine the optimal spectral bandwidth for measure-
ment. A lot of plastic films have reflectance of about
5%.
Fig. 11: Non-contact infrared temperature measurement of film ex-
trusion, extrusion coating, and laminating
4.1.4 Measuring Glass
When measuring the temperature of glass with an in-
frared thermometer, both reflectance and transmit-
tance must be considered. By carefully selecting the
wavelength, it is possible to measure temperature of
both the surface and at a depth.
8 – 14 µm
5 µm
3.9 µm
2.2 µm
1 µm
500 1000 1500 2000 2500 3000
0
2%
4%
6%
8%
10%
Temperature in °C
2 3 4 5 6 7 8 9 10 11 12 13 14
Wavelength in µm
0
0.2
0.4
0.6
0.8
1 0.3 mm thick
0.13 mm thick
Polyethylene

The Infrared System
Fig. 12: Spectral transmittance of glass depending on thickness
When taking measurements below the surface, a sen-
sor for 1.0, 2.2, or 3.9 µm wavelength should be used.
We recommend you use a sensor for 5 µm for surface
temperatures or 7.9 µm for surface temperatures for
very thin sheets or low temperatures. Since glass is a
poor conductor of heat, and can change surface tem-
perature rapidly, a measuring device with a short re-
sponse time is recommended.
Fig. 13: From the molten state through to the cooling process, con-
tinuous temperature monitoring ensures that glass retains its prop-
erties as it travels through the manufacturing process, here the tem-
pering of glass sheets
4.2 Ambient Conditions
Another reason for setting up an IR thermometer for a
particular spectral range only (spectral radiation py-
rometer), is the transmission behavior of the transmis-
sion path, usually the ambient air. Certain compo-
nents of the atmosphere, such as vapor and carbon
dioxide, absorb infrared radiation at particular wave-
lengths which result in transmission loss. If absorption
media is not taken into account, it can lead to a tem-
perature displayed below that of the actual target tem-
perature. Fortunately, there are "windows" in the in-
frared spectrum which do not contain these
absorption bands. In Fig. 14 the transmission curve of
a 1 m long air distance is represented. Typical meas-
uring windows are 1.1–1.7 µm, 2–2.5 µm, 3–5 µm
and 8–14 µm. Since the manufacturers have already
furnished infrared measuring devices with atmos-
pheric correction filters, the user is spared such wor-
ries.
Fig. 14: Transmittance of a 1 m long air distance at 32°C and rela-
tive 75% humidity./3/
Thermal radiation in the environment surrounding the
target should likewise be taken into account. The
higher temperatures of the furnace walls could lead to
errors in temperature measurement on metal pieces
in an industrial furnace. The possible effect of the am-
bient temperature has been taken into consideration
by many infrared measuring devices, with compensa-
tion built in. The other possibility is a too-high temper-
ature being displayed for the target. A correctly set
emissivity, along with automatic background temper-
ature compensation from a second temperature sen-
sor ensures extremely accurate results.
Summary
Every body emits infrared radiation. This radiation
is only visible to the naked eye at temperatures
above 600°C (e.g. glowing-hot iron). The wave-
length range is from 0.7 µm to 1000 µm. Black-
bodies absorb and emit 100% of the radiation that
corresponds to their characteristic temperature.
All other bodies are placed in relation to this when
evaluating their radiation emission. This is called
emissivity.
Wavelength in µm
0
2 3 4 5 6 7 8 910 12 14 20
100
50
1
Transmissionin%

Fig. 15: Background temperature compensation is important where
targets are cooler than the surrounding environment.
Dust, smoke, and suspended matter in the atmos-
phere can result in contamination of the optics and,
therefore, in false measured values. In order to pre-
vent deposition of suspended matter, optional air-
blowing attachments are offered. These are usually
screw-on pipe connections with a compressed air
supply. The air ensures overpressure in front of the
optics, thus keeping contaminating particles at bay. If
a great amount of dust or smoke is created during the
measurement procedure and affect the result, then ra-
tio pyrometers should be used.
IR sensors are electronic devices and can only work
within certain operating temperature ranges. Some
sensors allow an upper limit of 85°C. Above the per-
mitted operating temperature, air or water-cooling ac-
cessories must be used and there must be special
connection cables for the application of high temper-
ature. When using water-cooling it is often useful to
use it in conjunction with the air-blowing attachment
to prevent formation of condensation on the optics.
Fig. 16: Thermalert 4.0 Series pyrometer (Fluke Process Instru-
ments) withstand ambient temperatures up to 85°C (185°F) without
any additional cooling
Summary
Factors Solution
• Ambient radiation
is hotter than target
• Sensor with background
radiation compensation
• Shielding of target back-
ground
• Dust, vapor,
particles in the
atmosphere
• Air-blowing unit for lens
• Ratio pyrometer
• High operating tem-
perature
• Thermally insulated as-
sembly
• Water or air-cooling
• Air-blowing unit for lens
• Heat shield
Target, 900°C
Sensor
Oven, 1100°C

The Infrared System
4.3 Optics and Window
The optical system of an infrared thermometer picks
up the infrared energy emitted from a circular meas-
urement spot and focuses it on a detector. The target
must completely fill this spot, otherwise the IR ther-
mometer will "see" other temperature radiation from
the background making the measured value inaccu-
rate, see Fig. 17.
Fig. 17: The target must completely fill the spot to be measured,
otherwise the measured value will be incorrect (exception: ratio py-
rometer).
The optical resolution is defined as the relationship
between the distance of the measuring device from
the target, and the diameter of the spot (D:S). The
greater this value, the better the optical resolution of
the measuring device, and the smaller the target can
be at a given distance, see Fig. 18.
Fig. 18: Optical diagram of an infrared sensor. At a distance of
130 mm the spot measured is 33 mm, giving a ratio of around 4:1.
The optics themselves can be mirror optics or lens op-
tics. Lenses can only be used for particular wave-
length ranges due to their material wavelength
ranges. They are, however the preferred solution for
reasons of design. As a rule the optics is a so-called
fixed focus optics, i.e. the focal point is at a vendor-
defined measurement distance and only there the D:S
ratio indicated in the data sheet applies. Of course,
the pyrometer measures correctly at each other
measuring distance, however, the D:S ratio will be
slightly impaired. Here the tables and/or charts indi-
cated in the instruction manual of the device should
be carefully consulted. In terms of technology optics
offering a variable distance setting are the better so-
lution, since here the user can always choose the
maximum D:S value.
Fig. 19 shows a device with manual distance setting.
Via a button on the device or via remote control using
the digital interface a servomotor receives the respec-
tive commands.
Fig. 19: Pyrometer featuring a variable distance setting – Endur-
ance Series with variable focus (Fluke Process Instruments). The
variable focus can be controlled manually on-site. Among others a
through-the-lens sighting is used as an aiming device precisely
marking the spot also when the measurement distance is changed.
Table 1 shows some typical lenses and window ma-
terials for IR thermometers, along with their wave-
length ranges. /3/
For measurement in a closed reaction vessel, fur-
nace, or vacuum chamber, it is usually necessary to
measure through a suitable measuring window. When
selecting a material for the window, keep in mind that
the transmission values of the window are tuned to the
spectral sensitivity of the sensor. At high tempera-
tures, the material most often used is quartz glass. At
low temperatures (in the range 8–14 µm), it is neces-
sary to use a special IR-transmissive material such as
Germanium, Amtir, or Zinc Selenide. When choosing
the window, consider the spectral sensitivity parame-
ters, diameter of the window, temperature require-
ments, maximum window pressure difference, and
ambient conditions as well as the possibility of keep-
ing the window free from contamination on both sides.
It is also important to have transparency in the visible
range in order to be able to align the device better with
the target (e.g. in a vacuum container).
The transmittance of the window greatly depends
upon its thickness. For a window with a diameter of
25 mm, (which should be able to withstand the pres-
sure difference of one atmosphere), a thickness of
1.7 mm is adequate.
Windows with an anti-reflecting layer exhibit much
higher transmittance (up to 95%). If the manufacturer
states the transmittance for the corresponding wave-
length range, the transmission loss can be corrected
Very good critical incorrect
Sensor
Target larger
than spot
Target and
spot same
size
Target smaller
than spot
Spot
diameter
Distance

along with the emissivity setting. For example, an Am-
tir window with 68% transmittance is used to measure
a target with emissivity of 0.9. Then 0.9 is multiplied
by 0.68, resulting in 0.61. This is the emissivity value
to be set on the measuring device.
Recom-
mended IR
wave-
length
range
Maxi-
mum
window
temp
Trans-
mission
in visi-
ble
range
Resistance
to damp, ac-
ids, ammo-
nia com-
pounds
Suitable
for UHV
Sap-
phire
Al2O3
1...4 µm 1800°C yes very good yes
Fused
silica
SiO2
1...2.5 µm 900°C yes very good yes
CaF2 2...8 µm 600°C yes poor yes
BaF2 2...8 µm 500°C yes poor yes
AMTIR 3...14 µm 300°C no good -
ZnS 2...14 µm 250°C yes good yes
ZnSe 2...14 µm 250°C yes good yes
KRS5 1...14 µm - yes good yes
Table 1: Overview of various window materials
1 Optical glass 4 KRS5 7 Silicon
2 Calcium fluoride (CaF2) 5 Quartz glass 8 Lithium fluoride
3 Zinc Selenide (ZnSe) 6 Germanium 9 Chalcogenide glass IG-2 (Ge-As-Se)
Fig. 20: Transmittance of typical IR materials (1 mm thick)
Wavelength in µm
1 2 3 4 5 6 7 8 9 10 11 12 13 14
0
20
40
60
80
100

The Infrared System
4.4 Sighting Devices
Pyrometers are often fitted with an integrated aligning
telescope for directly optically aiming at the spot.
Sighting devices with video cameras and connected
displays simplify this task for the user and/or allow the
regular control of the pyrometer position also from a
control station. Moreover, pyrometers can be fitted
with lasers that are either built-in or screwed in front
of the device. The laser beam enables the user to aim
at the measuring spot even more quickly and pre-
cisely, which considerably simplifies the handling, in
particular of portable IR measuring devices. It is very
useful to sight on the measuring spot with a laser for
the measurement of moving objects and in poor light
conditions.
One can distinguish between the following laser sight-
ing setups:
A Laser beam
… with an offset from the optical axis. This is the sim-
plest model, especially for devices with low optical
resolution (for big measuring objects). The laser spot
aims approximately at the center of the measuring ob-
ject, but there is a noticeable error at close range.
B Coaxial laser beam
This laser beam comes out of the center of the optics
and remains along the optical axis. The center of the
measuring spot is precisely marked at any measuring
distance.
C Dual laser
Twin laser with two aiming points can be used to show
the diameter of the measuring spot over a long dis-
tance. With this, the user does not need to guess the
size of the diameter or calculate it beforehand. Fur-
thermore, it prevents the user from making mistakes
during the measurement. The IR and laser spot diam-
eters are not the same at close distances. The dis-
tance between the laser beams is slightly greater than
the spot being measured. This prevents the user from
making full use of the geometrical resolution stated for
this device.
D Crossed laser
The crossed laser is special version of the dual laser
and is used for sensors with dedicated focal point. The
distance at which the two laser dots overlap is the
point where the smallest area is measured (Focus
Point).
Fig. 21: Laser sighting
The use of the laser measuring spot proves to be an
effective visual help in guiding the infrared measuring
device precisely to the measuring object. The applica-
tion of an aligning telescope is useful only for the de-
termination of the measuring area when optically
aimed at bright objects (at high temperatures) or to
make measurements in strong daylight or at long dis-
tances.
Fig. 22: Devices with laser and optical sighting allow a precise spot
measurement even of small objects (Raynger 3i Series, Fluke Pro-
cess Instruments).
A
B
C
D

4.5 Detectors
The detector forms the core of the IR thermometer. It
converts the infrared radiation received into electrical
signals, which are then emitted as temperature values
by the electronic system. In addition to reducing the
cost of IR thermometers, the most recent develop-
ments in processor technology have meant increases
in system stability, reliability, resolution, and measure-
ment speed.
Infrared detectors fall into two main groups: quantum
detectors and thermal detectors. Quantum detectors
(photodiodes) interact directly with the impacting pho-
tons, resulting in electron pairs and therefore an elec-
trical signal. Thermal detectors (e. g. thermopiles or
bolometers) change their temperature depending
upon the impacting radiation. The temperature
change creates a voltage change in the thermopile
and a change in resistance in the bolometer. Thermal
detectors are much slower than quantum detectors
due to the self-heating required. (Here, much slower
means ms in relation to ns or s of the latter detec-
tors.) Quantum detectors are used above all for very
fast imaging systems and line scanners.
4.6 Display and Interfaces
The interfaces and types of measured value displays
available are important to the user. Some devices, es-
pecially hand-held ones, have a directly accessible
display and control panel combination which can be
considered the primary output of the measuring de-
vice. Analog or digital outputs control the additional
displays in the measuring station or can be used for
regulating purposes. It is also possible to connect data
loggers, printers, and computers directly.
Fig. 23: The data outputs of the IR thermometer can be connected
directly to printer or programmable logic controllers (PLC). Cus-
tomer-specific graphics and tables can be created using PC soft-
ware.
Industrial field bus systems are becoming ever more
significant and afford the user greater flexibility. For
example, the user can set the sensors from a control
station without having to interrupt the manufacturing
process. It is also possible to change parameters
when different products are running on the same pro-
duction line. Without such remote setting options, any
change to the sensor parameters - emissivity, meas-
uring range, or limit values - would have to be made
manually at the sensor itself. Since the sensors are
often mounted at difficult-to-access points, the intelli-
gent sensor ensures continuous monitoring and con-
trol of the process with minimal input from personnel.
If a malfunction occurs - ambient temperature too
high, interrupted supply, component failure - an error
message will appear automatically.
Summary
Just as with a camera, the performance of the op-
tics (e.g. telephoto lens), determines what size
target can be viewed or measured. The distance
ratio (distance from object: diameter of spot)
characterizes the performance of the optics in an
IR measuring device. The projected spot must be
completely filled for an exact measurement of the
target to result. For easier alignment, the optics
are equipped with a through-the-lens or laser
sighting device. A through-the-lens sighting de-
vice can be complemented by a built-in video
camera thus facilitating remote monitoring. If pro-
tective windows between the measuring device
and the target are necessary, the right window
material must be chosen. In this case, wave-
length range and operating conditions play a sig-
nificant role.

The Infrared System
Fig. 24: Examples of interfaces in current infrared measuring de-
vices.
The addressability of pyrometers facilitates operation
of a number devices on one network (multi-drop oper-
ation), resulting in lower installation costs. With the
multiplicity of bus protocols and types of field bus now
available, there are different converters (gateways) on
the market which perform the task of converting
(translating) device-specific commands into the ap-
propriate protocol (e.g. Profibus PD). The RS485 is
the common used hardware platform in this respect.
Also used are devices based on an Ethernet interface
that have their own IP address and thus can be di-
rectly accessed via a standard Web browser in an in-
tranet/internet. Here, applications for fast online
measurements at defined intervals are problematic in
network setups.
A further advantage of the pyrometer with a digital in-
terface is that it allows field calibration using calibra-
tion software available from the device manufacturer.
4.7 Technical Parameters of IR Thermometers
A complete summary including the related notes on
maintenance and validation measurement methods
can be found in /6/, /7/ and /8/.
The following important technical parameters charac-
terize radiation thermometers and should be taken
into account in the selection of the appropriate pyrom-
eter:
Measurement temperature range
The temperature range defined by the manufacturer
of the device where the measurement drift will not ex-
ceed defined limits.
Measurement uncertainty
Tolerance interval in which the true measurement
value lies at a specified probability, related to a given
measurement and ambient temperature.
Temperature drift
The temperature drift is the additional measurement
error caused by a deviation of the ambient tempera-
ture from the measurement uncertainty reference
temperature, e.g. 0.01 K/K for an ambient tempera-
ture of >23 °C.
Temperature resolution
(Noise-equivalent temperature difference)
Share of the measurement uncertainty caused by de-
vice-inherent noise. This parameter is expressed us-
ing the defined response time and the measurement
temperature, e. g. 0.1 K (at 100°C measurement tem-
perature and 150 ms response time).
Repeatability
Share of the measurement uncertainty of measure-
ments which are repeated within a short period of time
under the same conditions.
Long-term stability
Is expressed in the same way as the measurement
uncertainty, but relates to a longer period of time (sev-
eral months).
Spectral range
For broad-band spectral pyrometers the upper and
lower limits are indicated in µm; for narrow-band
spectral pyrometers the mean wavelength and a half-
width, e. g. 5 µm ± 0.5 µm, are indicated.
Size of the measuring area
(depending on measurement distance)
Usually the size of the measurement area is indicated
at which the signal has dropped to a certain value, e.g.
90%. This includes the indication of the measurement
Interface &
Outputs
Analogue
linear/
non-linear
2-wire
4 - 20 mA
current loop
4-wire
0/4 - 20 mA
0 - 10 V
thermocouple
Digital
serial
RS232,
RS485
Fieldbus
Profibus,
Ethernet,
Profinet,
Modbus,
HART

distance. Alternatively the distance ratio (distance
versus spot size, D:S) can be indicated.
Response time
Period of time elapsed between a change in temper-
ature of the target and the related display of the meas-
urement value. Complete details include the size of
the sudden temperature change as well as the limit at
which the measurement is made.
Example: t = 10 ms (25°C, 800°C, 95%)
Acquisition time
Minimum period of time during which a target needs
to be visible to the measuring device so that the re-
turned value can follow the measured value. A de-
layed display of the measurement value is possible.
As a rule the acquisition time is shorter than the re-
sponse time. The same details are indicated as for the
response time.
Example: t = 1 ms (25°C, 800°C, 95 %)
Operating and storage temperatures
The admissible ambient temperature at which the de-
vice may be operated or stored.
In addition, mechanical and electrical operating con-
ditions of the devices need to be observed (type of
protection, vibration resistance, etc).
4.8 Calibration
Pyrometers should be regularly checked and, in case
of deviations, newly calibrated, in order to guarantee
their long-term accuracy. To do so, the respective in-
stitution (e.g. the accredited laboratory) needs to
know the manufacturer’s calibration geometry, or the
application geometry of the device will be used. The
most important parameters are the measurement dis-
tance and the measurement area of the calibration
body and/or the size of the target. If a readjustment is
required the device should be returned to the manu-
facturer or the user may use the field calibration soft-
ware, if available, supplied by some manufacturers
with the device.
Connection of the calibration bodies to the ITS90 is
made, depending on the design, via reference pyrom-
eters (transfer standard) or contact thermometers,
which need to be calibrated at regular intervals at the
competent accredited laboratories. The methods are
described in detail in /9/.
Fig. 25: Calibration of a black body using a transfer standard py-
rometer (Trirat LT, Fluke Process Instruments), Raytek TRIRAT LT
Raytek TRIRAT LT 1
Temperature
Measurement
uncertainty 2σ
-49,9°C 0,11 K
-20,0°C 0,08 K
0,0°C 0,07 K
25,1°C 0,07 K
50,1°C 0,07 K
100,0°C 0,08 K
150,0°C 0,17 K
200,0°C 0,18 K
250,0°C 0,20 K
270,0°C 0,21 K
1
Calibration certificate No.: 2034 PTB 02, opening diameter of the radi-
ation source: 60 mm, calibration in the focal point, ambient tempera-
ture of 23°C ± 1°C
Table 2: Indicating the temperature values and the related meas-
urement uncertainties of the transfer standard

Special Pyrometers
5 Special Pyrometers
5.1 Fiber-optic Pyrometers
Pyrometers with fiber optics are used for applications
involving strong electrical or magnetic interference
fields for measurements at high ambient tempera-
tures, under vacuum conditions or where only little
space is available. This makes it possible to place the
sensitive electronic system outside the danger zone.
Typical of these applications are induction heating
and induction welding. Since the fiber optics them-
selves contain no electronic components, the operat-
ing temperature can be raised significantly without the
need for cooling (up to 300°C). Installation and con-
tinuous operating costs per measuring point are low
since no water cooling is required.
Single fibers or multifiber bundles are used. Multifiber
bundles have the advantage of allowing a smaller
bending radius.
With modern devices, it is possible to replace the fi-
ber-optic cable and optics without recalibration.
Simply input a multi-digit factory calibration number.
Fiber-optics are available for wavelengths of 1µm and
1.6 µm. Targets from 250°C can be measured with
these.
Fig. 26: Modern digital fiber-optic pyrometer
(Endurance Series, Fluke Process Instruments)
5.2 Ratio Pyrometers
Special pyrometers (also called two-color or dual
wavelength pyrometers) have two optical and electri-
cal measuring channels identical in structure. Both
wavelength ranges are placed as close as possible to
each other and set very narrow-banded, so that the
effect of material-specific peculiarities (reflectance,
emissivity) from the target is near-identical to both
wavelengths. By means of a mathematical calculation
of ratio, certain influences on measurement can be
eliminated. The following procedures have proved
successful:
1. Splitting the measured radiation using two filters
which revolve in front of a radiation detector (filter
wheel). Measurement in both channels takes place
alternately which, in the case of fast-moving tar-
gets, can result in errors in ratio calculation (chan-
nel 1 sees a different point on the target than chan-
nel 2).
2. Splitting of the measured radiation using beam
splitters and two radiation detectors fitted with fil-
ters.
3. The measured radiation reaches - without the
beam-splitter - a double detector (sandwich de-
sign) fitted with filters. Here, the front detector rep-
resents the filter for the second detector behind it.
Using the pyrometer equations /5/ for channel 1 with
wavelength 1 and channel 2 with 2 The result for the
measured temperature Tmeas :
1/Tmeas = 1/Ttarget + (1 2)/(c2 (2 -1)) ln (2/1) (3)
If the emissivity in both channels is the same, then the
term after the plus sign becomes zero and the meas-
ured temperature corresponds to the target tempera-
ture Ttarget. (c2: second radiation constant in µm∙K).
The same can be applied to the target surface A,
which as A2 and A1 is of course identical in the case of
both channels, meaning that here too the term after
the plus sign is dispensed with.
1/Tmeas = 1/Ttarget + (1 2)/(c2 (2 -1)) ln (A2/A1) (4)
Thus, the measurement is independent of the size of
the target. Moreover, the object radiation being sent
to the pyrometer becomes reduced proportionally, not
only when there is a smaller measuring surface, but
also when the pyrometer "gets to see" the target for a
shorter time span. By this means, targets that are in
the line of sight for a shorter period than the response
time of the pyrometer can also be measured.
Changing transmittance characteristics in the
measurement path are eliminated in the same way.
The devices can be used where there is dust or
smoke present, or any other interfering factor that
reduces radiation from the target. Modern devices can
apply this effect (attenuation) to their own optics, and
send out an alarm signal at the appropriate level of
contamination (e.g. air purge failure with the air-
blowing attachment).
In some applications where the nature of the technol-
ogy means a certain particle density around the tar-
get, a ratio pyrometer with attenuation factor read-out
can provide additional information. Fig. 27 shows the

information given by a ratio pyrometer using PC soft-
ware. In addition to the temperature calculated from
the ratio, the measured temperatures from both indi-
vidual channels are given. Moreover, attenuation that
is calculated by comparing the two is displayed in per-
cent.
Fig. 27: Measuring data issued by PC software of a ratio pyrometer,
e.g. target temperature in measuring channel 1 (WBT), target tem-
perature in measuring channel 2 (NBT), and the target temperature
calculated from the ratio (2CT). The measured attenuation is also
displayed in percent (ATN) along with further information.
The following materials that have an oxidized surface
behave as gray bodies and can be measured with a
slope (relative emissivity) of 1.00:
Iron, Cobalt, Nickel, Steel, Stainless steel
The following materials that have a smooth, non-oxi-
dized surface behave as non-gray bodies and are
measured with a slope or relative emissivity of 1.06:
Iron, Cast iron, Cobalt, Nickel, Tungsten, Molyb-
denum, Steel, Stainless steel, Tantalum, Rhodium,
Platinum
Summary
Ratio pyrometers can measure temperature
when:
1. The target is smaller than the spot or is con-
stantly changing in size (background cooler
than target).
2. The target moves through the spot within the
response time.
3. The line of sight to the target is restricted (dust
or other particles, vapor or smoke).
4. Emissivity changes during measurement.
The attenuation factor provides additional infor-
mation about the technological process or can be
used as an alarm in the case of over-contamina-
tion of lenses or windows.

Special Pyrometers
5.3 Imaging Systems
In contrast to the recording of temperature spots the
temperature distribution on the target is of interest
here. Local temperature differences as well as the de-
tection of hot or cold spots often play a more important
role than absolute temperature values. Fig. 28 shows
the temperature differences of a plastic foil including
a material defect on the right edge.
Fig. 28: Thermal image of a plastic foil with material defect on the
right edge
The technical specifications of IR linescanners differ
from those of pyrometers, because here often the
whole angle of vision in degrees (e. g. 30°) and the
angle relating to the measurement point (pixel) in
mrad (e. g. 3 mrad) are indicated instead of the dis-
tance ratio (D:S). For a comparison with a single-spot
pyrometer a conversion can easily be made using a
measurement distance of one meter, since in this
case the mrad indication of a measurement pixel is
equivalent to the spot diameter in mm.
In addition, the response time is replaced by the line/
frame frequency.
5.3.1 IR Linescanners
IR linescanners are used for measuring moving tar-
gets, e. g. for conveyor or “web” processes. They dis-
play the temperature distribution diagonally to the
moving direction. The movement of the process itself
supplies the second coordinate for a complete thermal
image. Fig. 29 demonstrates the measuring principle
for a web process. Fig. 30 shows the temperature dis-
tribution across the foil and simultaneously the color
presentation of the temperature values as a thermal
image by joining several temperature profiles. The
drop-in temperature at the edges can clearly be seen.
Fig. 29: Measuring principle of a line linescanner
Fig. 30: Presentation of the measurement values of a linescanner
while measuring foil web processes: thermal image (left) and ther-
mal profile (right).
Opto-mechanical Systems
These systems use a spot sensor scanning the field
of view using a moving mirror. This enables the gen-
eration of very accurate profiles since every point of
the target is measured using the same sensor. As a
rule, the opto-mechanical assembly defines the MTBF
(Mean Time between Failures) of the measuring de-
vice. However, in view of today’s technology this value
may be several years. Line frequencies amounting to
several 100 Hz can be achieved and the number of
measurement points can reach a maximum of 1000.

Fig. 31: Principle of an opto-mechanical assembly with rotational
mirror
Since only one measurement point needs to be repro-
duced from the optical side, the optics can have a very
simple design, in contrast to line sensor systems. This
allows the implementation of low-cost systems. An-
other great advantage over line and matrix cameras
is the broad visual angle which is formed by the en-
trance window in combination with the reflecting mir-
ror unit. A visual angle of 90 degrees is no problem
and thus allows practicable measurement distances
even for broad web processes.
Line Sensor Systems
The number of measurement points is defined by the
number of pixels of a line sensor. Thus, no moving
mirror is used. Since, as a rule, pyro-electrical sensor
lines are used, and since this sensor only processes
alternating light signals, the measurement signal has
to be chopped by means of a special mechanical as-
sembly. Thus, the MTBF of this measurement princi-
ple is defined by the opto-mechanical design, too. Cal-
ibration requires some additional work in order to
compensate for the different pixel sensitivities, so that
the so-called pattern noise, which can be seen e.g.
when measuring a surface of homogenous tempera-
ture, will be as little as possible. This effect does not
occur with the measuring system described in the pre-
vious chapter. Interchangeable optics are available
which provide visual angles of a few degrees (tele-
photo lens) to a maximum of 60°.
5.3.2 Matrix Cameras
Matrix cameras can be designed completely without
mechanically moving parts and provide a complete
thermal image also from motionless targets. As a rule,
cooled CMT matrixes from military research are used
as matrixes (FPAs) for ultra-high-speed cameras. Ex-
periments using pyro-sensor matrixes have been
made for lower priced systems supplying video fre-
quencies. However, today bolometer matrixes are
widely used.
Bolometer FPAs
In recent years much progress has been achieved
with semiconductor-based bolometers. Noise-limited
temperature resolution may be better than 0.1 K and
frame frequencies achieve more than double of to-
day’s video standards. Today’s standard systems of-
fer a pixel resolution of 320x240 or full VGA resolution
of 640x480 measurement points.
Fig. 32: Modern IR imaging camera with a pixel resolution of
320x240 or 640x480 (ThermoView TV40, Fluke Process Instru-
ments)

Summary
6 Summary
Infrared thermometry measures the energy that is nat-
urally emitted from all objects, without actually touch-
ing them. This allows quick, safe measurement of the
temperature of objects that are moving, extremely hot,
or difficult to reach. Where a contact instrument could
alter the temperature, damage, or contaminate the
product, a noncontact thermometer allows accurate
product temperature measurement.
Compared to early infrared temperature measure-
ment devices, which were heavy, awkward, and com-
plicated to operate, the image of such devices today
has completely changed. Modern infrared thermome-
ters are small, ergonomic, easy to operate, and can
even be installed into machinery. From versatile
handheld devices to special sensors for integration
into existing process systems, the spectrum of prod-
uct offerings is vast.

7 Bibliography
/1/ Klaus Herrmann, Ludwig Walther:
Wissensspeicher Infrarottechnik, 1990,
Fachbuchverlag Leipzig
/2/ Stahl, Miosga: Grundlagen Infrarottechnik,
1980, Dr. Alfred Hütthig Verlag Heidelberg
/3/ VDI/VDE Richtlinie, Technische Temperatur-
messungen – Strahlungsthermometrie,
January 1995, VDI/VDE 3511 page 4
/4/ De Witt, Nutter: Theory and Practice of Radia-
tion Thermometry, 1988, John Wiley&Son,
New York, ISBN 0-471-61018-6
/5/ Wolfe, Zissis: The Infrared Handbook, 1978, Of-
fice of Naval Research,
Department of the Navy, Washington DC.
messungen – Spezifikation von Strahlungsther-
mometern, June 2001,
VDI/VDE 3511 page 4.1
messungen – Erhaltung der Spezifikation von
Strahlungsthermometern, January 2002,
VDI/VDE 3511 page 4.2
messungen – Standard-Test-Methoden für
Strahlungsthermometern mit einem Wellenlän-
genbereich, July 2005, VDI/VDE 3511 page 4.3
messungen – Kalibrierung von Strahlungsther-
mometern, July 2005, VDI/VDE 3511 page 4.4

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