A pyrometer is a non-contacting
device that intercepts and
measures thermal radiation, a process
known as pyrometry. This device can
be used to determine
the temperature of an object's surface.
The word pyrometer comes from
the Greek word for fire, "πυρ" (pyro),
and meter, meaning to measure.
Pyrometer was originally coined to
denote a device capable of measuring
temperatures of objects
above incandescence (i.e. objects
bright to the human eye).
HISTORY OF PYROMETER
Modern pyrometers became available when the first disappearing filament pyrometer was built by L. Holborn
and F. Kurlbaum in 1901. This device superimposed a thin, heated filament over the object to be measured
and relied on the operator’s eye to detect when the filament vanished. The object temperature was then
read from a scale on the pyrometer.
The temperature returned by the vanishing filament pyrometer and others of its kind, called Brightness
Pyrometers, is dependent on the emissivity of the object. With greater use of brightness pyrometers, it
became obvious that problems existed with relying on knowledge of the value of emissivity. Emissivity was
found to change, often drastically, with surface roughness, bulk and surface composition, and even the
To get around these difficulties, the ratio or two-color pyrometer was developed. They rely on the fact
that Planck's Law, which relates temperature to the intensity of radiation emitted at individual wavelengths,
can be solved for temperature if Planck’s statement of the intensities at two different wavelengths is divided.
This solution assumes that the emissivity is the same at both wavelengths and cancels out in the division.
This is known as the gray body assumption. Ratio pyrometers are essentially two brightness pyrometers in a
single instrument. The operational principles of the ratio pyrometers were developed in the 1920s and
1930s, and they were commercially available in 1939.
As the ratio pyrometer came into popular use, it was determined that many materials, of which metals
are an example, do not have the same emissivity at two wavelengths . For these materials, the emissivity
does not cancel out and the temperature measurement is in error. The amount of error depends on the
emissiveties and the wavelengths where the measurements are taken. Two-color ratio pyrometers cannot
measure whether a material’s emissivity is wavelength dependent.
PRINCIPLE OF OPERATION
A pyrometer has an optical system and detector. The optical system focuses the thermal radiation onto the
detector. The output signal of the detector(Temperature T) is related to the thermal radiation or irradiance j* of
the target object through the Stefan–Boltzmann law, the constant of proportionalityσ, called the Stefan-
Boltzmann constant and the emissivity ε of the object.
This output is used to infer the object's temperature. Thus, there is no need for direct contact between the pyrometer and the
object, as there is withthermocouple and Resistance temperature detector
WORKING OF A PYROMETER
There are two basic kinds of
pyrometers: optical pyrometers, where
you look at a heat source through a
mini-telescope and make a manual
measurement, and electronic, digital
pyrometers that measure completely
automatically. Some devices described
as pyrometers actually have to be
touching the hot object they're
measuring. Strictly speaking,
instruments like this are really just high-
temperature thermometers based on
thermocouples. Since they don't
measure temperature at a distance,
they're not really pyrometers at all.
Pyrometer, or radiation thermometer, is a non-
contact instrument that detects an object's surface
temperature by measuring the temperature of
the electromagnetic radiation (infrared or visible)
emitted from the object.
The wavelength of thermal radiation ranges from
0.1 to 100 µm (4 ~ 4,000 µin), i.e., from the deep
ultraviolet (UV) across the visible spectrum to the
middle of the infrared region (IR).
Typical Broadband Pyrometer
The Electromagnetic Radiation Spectrum
Pyrometers are essentially photodetectors which are capable of absorbing energy, or measuring
the EM wave intensity, at a particular wavelength or within a certain range of wavelengths.
TYPES OF PYROMETERS
The following are some of the most commonly and widely used
• OPTICAL PYROMETER
• RADIATION PYROMETER
• DIGITAL PYROMETER
• INFRARED PYROMETER
The Optical Pyrometer is a highly-developed and well accepted
noncontact temperature measurement device with a long and
varied past from its origins more than 100 years ago. In spite
of the fact that more modern, automatic devices have nearly
displaced it, several makers still produce and sell profitable
quantities each year.
In general, opticals, as they are often called, can be described
as fitting into two seperate types, according to the two USA
companies that produce them. However, there are actually
several different types that vary in compexity and cost. A quick
review of the descriptions below will provide some of the
differences and a check of the web sites of the two companies
will yield even more information. We suspect that there are
other makers overseas and we are looking to find more details
about them and their web presence.
•They range between 700 to 3000 c
•Lower limit determined by human eye
•Human eye compares the radiation
WORKING OF A OPTICAL
Optical Pyrometers work on the basic principle of using the human eye to match the
brightness of the hot object to the brightness of a calibrated lamp filament inside the
instrument. The optical system contains filters that restrict the wavelength-sensitivity of the
devices to a narrow wavelength band around 0.65 to 0.66 micons (the red region of the
Other filters reduce the intensity so that one instrument can have a relatively wide
temperature range capability. Needless to say, by restricting the wavelength response of
the device to the red region of the visible, it can only be used to measure objects that are
hot enough to be incandescent, or glowing. This limits the lower end of the temperature
measurement range of these devices to about 700 °C.Some experimental devices have
been built using light amplifiers to extend the range downwards, but the devices become
quite cumbersome, fragile and expensive.
Modern radiation thermometers provide the capability to measure within and below the
range of the optical pyrometer with equal or better measurement precision plus faster time
response, precise emissivity correction capability, better calibration stability, enhanced
ruggedness and relatively modest cost.
• CONVIENTTO USE
• CAN MONITOR MOVEABLE OBJECTS
• NON CONTACTTYPE
• HUMAN ERRORS
• NOT USEFUL FOR MEASURING TEMP OF CLEAR GAS
• AT HIGH TEMP. FILAMENT ERODES FREQUENTLY
• FAST IN RESPONSE
• GOOD SENSITIVITY
NOT USED DUETO :
• POOR PRECISION
• DIFFICULTTO PROVIDE
• LOW RESPONSETIME
• LARGE SENSITIVITY
• LIMITED SPECTRAL SENSITIVITY
•ABILITYTO MEASURE HIGHTEMP
• NO NEED FOR CONTAC
• FAST RESPONSE SPEED
• HIGH O/P
• NON LINEAR SCALE
• EMMISIVITY OF TARGET AFFECT
• ERRORS DUE TO INTERLEAVING MEDIA
Pyrometers are suited especially to the measurement of moving objects or any surfaces that can not
be reached or can not be touched.
Temperature is a fundamental parameter in metallurgical furnace operations. Reliable and
continuous measurement of the melt temperature is essential for effective control of the operation.
Smelting rates can be maximized, slag can be produced at the optimum temperature, fuel
consumption is minimized and refractory life may also be lengthened. Thermocouples were the
traditional devices used for this purpose, but they are unsuitable for continuous measurement
because they rapidly dissolve.
Salt bath furnaces operate at temperatures up to 1300 °C and are used for heat treatment. At
very high working temperatures with intense heat transfer between the molten salt and the steel
being treated, precision is maintained by measuring the temperature of the molten salt. Most errors
are caused by slag on the surface which is cooler than the salt bath.
The Tuyère Pyrometer is an optical instrument for temperature measurement through
the tuyeres which are normally used for feeding air or reactants into the bath of the furnace.
A steam boiler may be fitted with a pyrometer to measure the steam temperature
in the superheater.
Hot Air Balloons
A hot air balloon is equipped with a pyrometer for measuring the temperature at
the top of the envelope in order to prevent overheating of the fabric.
• ^ a b c L. Michalski et al, “Temperature Measurement, Second Edition.(Wiley, 2001), pp.
• ^ a b c d C. Mercer, Optical metrology for fluids, combustion, and solids.(Kluwer Academic,
2003), pp. 297-305.
• ^ a b D. Ng and G. Fralick, ‘’Use of a multiwavelength pyrometer in several elevated
temperature aerospace applications’’, Rev. Sci. Instrum. 72, 1522 (2001)
• ^ a b D. Olinger, J. Gray, R. Felice, ‘’Successful Pyrometry in Investment Casting’’,
Proceedings of the 55th Annual Technical Conference on Investment Casting (Investment
Casting Institute, 2007)
• ^ L. Michalski et al, “Temperature Measurement, Second Edition.(Wiley, 2001), pp. 403-404.