Radiation Thermometry
(輻射測温學)
Presented by :
Standards and Calibration Laboratory (SCL)
Innovation and Technology Commission
The Government of the HKSAR
標準及校正實驗所
創新科技署
香港特別行政區政府
Contents
(I)
Blackbody Radiation
(II)
Emissivity
(III) Radiation Thermometers
(IV) Infrared Ear Thermometers
(V)
Errors in Radiation Thermometry
(VI) Calibration of Radiation Thermometers
(VII) Temperature Calibration Services offered by SCL
for Radiation Thermometers and Infrared Ear
Thermometers
Radiation Thermometry
in a Nutshell
 All materials emit thermal radiation (熱輻射) when
their temperature is above absolute zero.
 Amount of
temperature.
thermal
radiation
depends
 Therefore, we can measure temperature
determining the amount of thermal radiation.
on
by
Characteristics of
Radiation Thermometry
 Non contact
 Radiation thermometers do not need to be in
contact with the target.
 Use the surface of the target as sensor
 The amount of infrared emitted by the target
depends on the characteristic of the target surface
 Measure thermodynamic temperature (熱力學温度)
 Based on a universal physical law
(I)
Blackbody Radiation
(黑体輻射)
Electromagnetic Radiation
(電磁輻射)
 Radio waves, microwaves, infrared radiation, visible light,
ultraviolet radiation, X-rays and gamma rays are all
electromagnetic radiation.
 They are self-propagating wave with electric and
magnetic components.
 They differ in wavelength 波長 (frequency頻率).
 Wavelength of visible light (可見光): 0.4 µm to 0.7 µm
 Wavelength of infrared (紅外線): 0.7 µm to 1 mm
 Wavelength of ultraviolet (紫外線): 0.4 µm to 0.01 µm
Thermal Radiation
(熱輻射)  All materials at temperature above absolute zero (0 K)
emit electromagnetic radiation.
 Amount of radiation increases with temperature.
 Wavelength of radiation decreases with temperature.
 e.g.

cosmic microwave background radiation
(temperature ~2.725 K, peak wavelength ~1.9 mm)

Surface of the sun (temperature ~5500 °C, peak
wavelength ~500 µm )
Visible light
Ultraviolet
Infrared
6000 K
3000 K
1000 K
500 K
0.01
0.1
1
10
Wavelength (um)
Thermal Radiation at 500 K, 1000 K, 3000 K, 6000 K
100
Blackbody Radiation
(黑体輻射)  Besides temperature, amount of
thermal radiation also depends on
the characteristics of the emitting
surface.
 As rough examples, a white
surface emits less thermal
radiation whilst a black surface
emits more.
 Blackbody radiates the maximum
thermal radiation for a given
temperature when compared to
other surfaces.
Spectral Radiance
(光譜輻射亮度)  Spectral radiance is the rate of energy emitted by a
surface per unit area per unit wavelength per unit solid
angle.
 The unit for spectral radiance is W m-2 µm-1 sr-1
Planck’s Radiation Law
(普朗克輻射定律)
 Planck’s radiation law gives the spectral radiance L(λ,T)
emitted by a blackbody for a given temperature T (in
kelvin).
C1
L(λ , T) =
λ (e
5
 C1 = 1.191044x 10
–16
C2
λT
−1)
W m-2
 C2 = 0.014388 m K
 C1 and C2 are the first and second radiation
constants
Stefan-Boltzmann’s Law
(斯特藩-玻耳茲曼定律)
 Stefan-Boltzmann’s law gives the total rate of emission
of thermal energy from a unit area of a blackbody for a
given temperature T (in kelvin).
P = σT
 σ = 5.67 x 10
–8
W m-2 K-4
4
Stefan-Boltzmann’s Law
 Using the Stefan-Boltzmann’s law, some examples of
blackbody radiation is calculated as follows :
Temperature
Rate of Emission
23 °C
436 W m-2
500 °C
20 000 W m-2
1000 °C
150 000 W m-2
2500 °C
3 400 000 W m-2
5500 °C
77 000 000 W m-2
Wein’s Displacement Law
(維恩位移定律)
 Wein’s law gives the wavelength where the spectral
radiance is a maximum.
λ max T ≈ 2898µmK
6000 K
3000 K
1000 K
500 K
0.01
0.1
1
10
Wavelength (um)
Thermal Radiation at 500 K, 1000 K, 3000 K, 6000 K
100
Example 1
The surface temperature of sun is about 5800 K. Estimate
the peak of the solar radiation.
Solution :
λ max
2898
≈
K = 0.5µm
5800
Practical Blackbodies
 Practical Blackbodies are cavities, not surfaces.
 Light entering a cavity will be reflected and absorbed
many times before it exits the aperture of the cavity.
Hence a cavity can achieve a much higher emissivity
than a surface.
aperture of
blackbody
(II)
Emissivity
(發射率)
Emissivity
(發射率)  A real surface will emit less thermal radiation than a
blackbody at the same temperature.
 The emissivity ε is the ratio of the amount of radiation
actually emitted from a surface to that emitted by a
blackbody
 For blackbodies, the emissivity is very close to 1. For
real surface, it is less than 1.
Emissivity
 Emissivity for a material is a parameter. It is NOT a
constant.
 Emissivity is a function of the following :

temperature of surface

wavelength

angle of emission
Emissivities of Common Materials  For some common materials, the emissivities at
wavelength of 0.65 µm and over the band 8 to 13 µm are
as follows :
Material
ε0.65µm
ε8-13µm
Aluminium
0.11- 0.19
< 0.1
Carpets
-
0.85 – 0.95
Fire brick
0.75
-
Paper
-
0.85 – 1.00
Plastics
-
0.95
Stainless steel
0.40 – 0.60
0.10 - 0.25
Tungsten
0.35 - 0.50
-
Wood
-
0.85 - 0.95
Thermal Radiation from
Real Surfaces
 For real surfaces, Planck’s radiation law becomes
the following
L(λ , T) =
ε(λ, T)C1
λ (e
5
 C1 = 1.191044x 10
–16
C2
λT
−1)
W m-2
 C2 = 0.014388 m K
 ε(λ,T) = emissivity of surface
Stefan-Boltzmann’s Law for
Real Surfaces
 Stefan-Boltzmann’s law for real surfaces is as
follows
P = ε total σT
 σ = 5.67 x 10
–8
4
W m-2 K-4
 εtotal = total hemispherical emissivity of surface
(III)
Radiation Thermometers
(輻射温度計)
Types of radiation thermometer
 Total radiation thermometers (全輻射温度計)

Measure the total radiance of a surface. Sensitive to
variation in sensor temperature and emissivity of target
 Spectral band radiation thermometers (帶域輻射温度計)

Use a filter to measure only thermal radiation in a
narrow wavelength band (e.g. 0.65 μm, 0.85 μm)

The most common type of radiation thermometers

The choice of wavelength depends on the temperature
range, the environment and the target surface

The emissivity of the target must be either measured or
estimated.
A Radiation Thermometer (-50 °C to 1000 °C)
Operating Wavelength : 8 to 13 µm
A Radiation Thermometer (600 °C to 3000 °C)
Operating Wavelength : 0.96 to 1.05 µm
(IV)
Infrared Ear
Thermometer
(紅外線耳温計)
Detection of Human Body
Temperature by
Infrared Ear Thermometers

The human body temperature is controlled by the
hypothalamus in the brain.

The eardrum, also called the tympanic membrane, is
close to and shares blood with the hypothalamus.

The temperature of the eardrum reflects the core body
temperature.

Infrared Ear Thermometer measures the thermal energy
radiated from the surface of the eardrum. It includes a
detector that measures infrared radiation at
wavelengths around 10 μm.

Infrared Ear Thermometer
is actually a special type of
radiation thermometer
designed to measure the
temperature of the
eardrum.

The auditory canal is close
to a blackbody with
emissivity approaching 1.
Infrared Radiation Emitted by Human Body
-2
-1
Thermal Radiation (Wm um )
Thermal Radiation at 35 °C, 37 °C and 39 °C
40
35
30
25
20
15
35 °C
10
37 °C
5
39 °C
0
0
2
4
6
8
10
Wavelength (um)
12
14
16
Standards for Infrared Ear
Thermometers 
ASTM E1965-98, "Standard Specification for Infrared
Thermometers for Intermittent Determination of Patient
Temperature"

EN 12470-5:2003, "Clinical Thermometers - Part 5 :
Performance of Infra-red Ear Thermometers (with Maximum
Device)"
Requirements of Infrared Ear
Thermometers (1)


Measurement Range
ASTM E1965-98
at least 34.4 °C to 42.2 °C
EN 12470-5:2003
35.5 °C to 42.0 °C
Accuracy
ASTM E1965-98
Maximum Permissible Laboratory
Error
< ± 0.2 °C (for 36 °C to 39 °C)
< ± 0.3 °C (<36 °C or >39 °C)
EN 12470-5:2003
Maximum Permissible Error
< ± 0.2 °C (for 35.5 °C to 42.0 °C)
< ± 0.3 °C (<35.5 °C or >42.0 °C)
Requirements of Infrared Ear
Thermometers (2)


Display Resolution
ASTM E1965-98
0.1 °C
EN 12470-5:2003
0.1 °C
Operating Range
ASTM E1965-98
16 °C to 40 °C
EN 12470-5:2003
16 °C to 35 °C
(V)
Errors in using Radiation
Thermometers
Errors in using radiation thermometers  3 main categories of errors :
 Errors arising from target area : e.g. emissivity,
reflection
 Errors arising from tranmission path : e.g. absorption,
scattering, size of source effect.
 Errors arising from signal processing : e.g. variation in
ambient temperature, linearisation
Estimation of Temperature Error
 The major errors in using a spectral band radiation
thermometer arise from either the measured radiance or
the estimation of target emissivity.
 The following equation estimates the uncertainty of the
measured temperature
σT
measured
=
λT 2
c2
σL
σ ε (λ ) 2
(
) +(
)
Lmeasured
ε (λ )
measured
2
Estimation of Temperature Error
 From the equation, we can see that
 The errors and uncertainties increase with operation
wavelength. Hence, generally a thermometer with a
short operating wavelength is more accurate.
 The errors and uncertainties increase with the
square of temperature.
 The errors and uncertainties increase with 1/ε(λ).
Therefore, the error is larger when measuring low­
emissivity substances, such as metals.
Errors Arising from Emissivity
(發射率引起的誤差)
 The difficulty in estimating the emissivity of the target
surface is the largest source of errors in radiation
thermometry.
 For example, the emissivity of a nickel alloy varies
between 0.1 and 0.95 depends on the state of oxidation,
roughness and wavelength.
 To make a good estimate, it is important to know the
operating wavelength of the thermometer, the material
and surface properties (e.g. roughness, oxidation) of
the target.
 In general, manufacturers of radiation thermometers will
provide a list of emissivities. If followed properly, it is
possible to make estimate to about ± 0.05.
Example 2
A radiation thermometer with operating wavelength of 0.65
µm is used to measure the temperature of a piece of highly
oxidised steel at around 1200 °C. The emissivity is about
0.80 ± 0.10. Calculate the uncertainty of the measured
temperature due to the uncertainty of emissivity.
Solution :
U Tmeasured
0.65×10 −6 × (1200 + 273.15) 2
=
0.014388
= 12.3°C
0.1 2
( )
0.8
Errors Arising from Emissivity
 It is difficult to estimate the emissivity based on visual
assessment. Surface that are black in the visible
spectrum may NOT be black in the infrared. On the other
hand, white paint can be very black in the infrared.
 For most surfaces the emissivity also depend on the
viewing angle. Normally the published values for
emissivity are for normal incidence. Hence radiation
should be used at normal incidence to the target
surface.
Errors Arising from Reflection
(反射引起的誤差)
 A radiation thermometer assumes all radiation is emitted
from the target surface. Anything added to the emitted
radiation, such as reflection, will become error in
measured temperature.
 Reflection problem is more serious when measuring low
temperatures since the whole environment is emitting
radiation at ambient temperature of about 300K.
 The best way to reduce reflection error is to eliminate the
unwanted radiation sources. Some examples are the
sun, the lamps, etc.
 The target surface should be shaded from unwanted
radiation sources.
Absorption Errors
(吸收引起的誤差)
 Anything in the path between the target and the radiation
thermometer can absorb thermal radiation.
 Common absorption sources are : atmospheric gases
(e.g. carbon dioxide, water vapour), windows and
suspended material
 The absorption error increases with distance. The
magnitude of the error can be estimated by varying the
distance between the radiation thermometer and the
target.
Scattering Errors
(散射引起的誤差)
 Caused by dust in the transmission path between the
target and the radiation thermometer
 Dust has 3 effects :

It scatters away radiation from target

It scatters radiation from other source to the
thermometer

It may emit blackbody radiation.
Size-of-source Effects
(輻射源尺寸效應)
 A radiation thermometer collects radiation from a zone
on the target.
 The size of the zone varies with the distance between
the target and the thermometer and is called field of
view.
 The target must fill the field of view completely.
Otherwise error will occur.
 The variation of thermometer readings with the size of
the target zone is called the size-of-source effects.
 To reduce this effect, always overfill the field of view as
much as possible.
Some radiation
thermometers
use laser to
mark the field of
view
Ambient Temperature Effects
(室温效應)
 As sensitivity of the detector and the wavelength
response of the filter are temperature dependent,
radiation thermometers are susceptible to change in
ambient temperature.
 The ambient temperature effect is more serious for low
temperature radiation thermometer.
 Avoid rapid change in ambient temperature.
 When bringing a radiation thermometer to a new
environment, allow sufficient time, say 1 hours, for it to
stabilise before taking readings.
(VI)
Calibration of Radiation
Thermometers and
Infrared Ear
Thermometers
Calibration Methods
 Transfer from a calibrated blackbody
 Transfer from a calibrated radiation thermometer
 Transfer from a calibrated contact type thermometer
such as thermocouple or platinum resistance
thermometer (PRT)
 Use fixed-point blackbody
Transfer from a calibrated
radiation thermometer
 Step 1 : Establish the radiance temperature of a
blackbody using a calibrated reference radiation
thermometer.
reference
radiation
thermometer
Transfer from a calibrated
radiation thermometer
 Step 2 : Calibrate the radiation thermometer under test
with the blackbody.
radiation
thermometer
under test
Transfer from a calibrated
contact type thermometer
 The temperature of the blackbody is measured by a
contact type thermometer such as thermocouple or PRT.
PRT or
thermocouple
radiation
thermometer
under test
A Blackbody
Calibration Source
Calibration of Infrared Ear
Thermometers Infrared Ear
Thermometer under Test
Aperture of blackbody
Detector
Blackbody radiator maintained at a constant
temperature between
34 °C and 43 °C in a constant
temperature bath
Blackbody Radiator for Calibration
of Infrared Ear Thermometers
(Based on EN 12470-5:2003 )
Infrared Ear Thermometer
Calibration System
(VII)
Temperature Calibration
Services offered by SCL for
Radiation Thermometers
Radiation Thermometers
Specific tests or properties
measured
Best measurement capability
Calibration over the following
ranges :­
-40 °C to 0 °C
0.5 K
0 °C to 200 °C
(0.33 + 0.003 t) K
200 °C to 1000 °C
(1.5 + 0.002 t) K
where t is the test temperature
Blackbody Radiators
Specific tests or properties
measured
Best measurement capability
Calibration over the following
ranges :­
0 °C to 200 °C
(0.31 + 0.0025 t) K
where t is the test temperature
Infrared Ear Thermometers
Specific Tests or
Properties Measured
Best Measurement
Capability
Calibration over the following ranges :­
34 °C to 42 °C
0.2 °C
Infrared Ear Thermometer
Calibrators
Specific Tests or
Properties Measured
Best Measurement
Capability
Calibration over the following ranges :­
34 °C to 42 °C
0.15 °C