The uses of noncontact temperature sensors are many;
the understanding of their use is, in general, relatively poor. Part of that complication
is often the need to deal with emissivity, or more precisely with spectral emissivity.
In many industrial plants noncontact sensors are not yet standardized to the extent
that thermocouples and RTDs are. In spite of this, there are numerous showcase uses of them
and they more than pay their way in process plants such as steel, glass, ceramics,
forging, heat treating, plastics, baby diapers and semiconductor operations, to
name just a few.
More recently the medical world has adopted the IR ear thermometer
(it has its own set of standards) that is basically a single waveband radiation
thermometer.
However, we believe that limited standardization is hampering
wider use in process and related areas. Standards have
been developed that aid the user in specifying, buying and maintaining such devices,
but they are not widely used. More training and education of the user community
is an obvious need that until now has been provided mostly by equipment vendors.
The advent of the Focal Plane Array, a significant improvement in Thermal Imaging,
is drawing the formerly seperate areas of Thermal Imaging and noncontact spot
temperature measurement closer together. It is likely that the active
training community developed to support Thermal Imagers will begin to provide
more organized, in depth training for all infrared temperature sensors, in addition
to imagers for Thermographers.
Just
to be sure we are addressing the subject you are seeking, please be aware that
these devices are called by a bewildering variety of names. They all work, or
are based on the same law of physics, Planck's Law of the thermal emission
of radiation.
Here's just a few of the names used in current technical and
popular literature (never mind the unprintable names these devices are often called
when the temperatures they report defy all logic-that happens a lot-see our E-missivity
Trail section for a partial understanding of this latter phenomenon): ir thermometer,
radiation thermometer, ir pyrometer, infrared thermometer, spot thermometer, spot
radiometer (our favorite technical misnomer), line scanner, radiation pyrometer,
single waveband pyrometer, dual waveband pyrometer, ratio pyrometer, 2 color thermometer,
2 colour thermometer, two color thermometer, two colour pyrometer, radiometer,
spectral radiometer, IR thermocouple, total radiation pyrometer, fiber optic pyrometer,
disappearing filament pyrometer, quantitative thermal imager, dfp, optical pyro,
multiwavelength pyrometer, and on and on.
It seems that whenever a new
technical or marketing person comes into the "business" a new product
name is coined either out of ignorance of the device history or as an effort to
be technically "pure" (whatever that is) or as a way to differentiate
their product from others. The names used here, as far as we know, do not include
the trademarked names or commonly used product line names. There isn't enough
room on this page for all of them! We shall try to follow the most often used
terminology, that fostered in the excellent work of DeWitt & Nutter in their
1988 book "Theory & Practice of Radiation Thermometery".
The complete citation can be found on the references page.
Good luck and best wishes. If you have some interesting success, let
us know and we'll help you share that with others who visit these pages.
Includes Pyrometers, Infrared
Thermal Imaging Cameras (with temperature measurement capability), line-measuring
thermometers (most of the time they're called line scanners-but all don't scan)
and infrared radiation thermometers, or, perhaps the most-misused term, spot radiometers
(Note: radiometers are calibrated in units of power, such as microwatts, watts,
kilowatts, temperature measurement devices are calibrated in units of temperature).
The
noncontact temperature sensors with many names and many shapes, sizes, prices
and capabilities are well and flourishing. Based on Planck's Law of the thermal
emission of electromagnetic radiation; many industries could not produce goods
as efficiently or quickly were it not for them.
More recently the medical world
has adopted the IR ear thermometer (it has its own set of standards) that is at
heart a single waveband radiation thermometer
The
majority of devices in use are single waveband thermometers (they measure a portion
of the received thermal radiation in a single waveband, or portion of the infrared
part of the electromagnetic spectrum). However, the number of ratio thermometers
(two color pyrometers) on the market has grown considerably in the past ten years, or
so.
Single waveband radiation thermometers are usually designed to measure the
true temperature when they receive all the radiation from an object that has an
emissivity effectively of 1.0, or under blackbody conditions. This occurs most
often when the devices are being calibrated, since they are calibrated under simulated
blackbody conditions. The accuracy of the simulation bears much on the uncertainty
of the calibration of the device.
When
these devices are used under effectively blackbody conditions, and their emissivity
correction is set at 1.0, they can measure very accurately, indeed. Few people
seem to appreciate that blackbody conditions occur regularly in many process applications,
such as in portions of furnaces that are close to thermal equilibrum, such as
glass melters & forehearths, steel mill soaking furnace zones or when a radiation
thermometer is correctly sighted into a closed isothermal cavity, such as a miniature
cavity on the end of a sapphire light pipe or quartz fiber optic.
Quantitative thermal imagers are a special
sub-class of these thermal imaging devices, they measure radiation temperature
distributions as well as shown a false color thermal image. They are basically
single waveband radiation thermometers that measure a two dimensional space instead
of just radiation from a single spot. These are used so widely that they are described
in more detail in a seperate section of this site that is all about
thermography or thermal imaging.
The topic of emissivity is also a broad and complex
one. One cannot mention radiation thermometry without mentioning emissivity.
Some
fundamental understanding of it is essential to successful use and application
of any temperature measuring radiation thermometer. It might be limited to just
the details of one specific application; that's enough in many cases. It is not
magic, it is not unknowable, otherwise all advanced thermal processes in the world
would be running at lower efficiencies than they are.
There are many people who
underated the subject and can explain it. This is our part in that educational
direction. We started a section on this site devoted to
helping people better understand some of the basics of the subject from an applications
perspective. Pardon our cynicism, but the section was initiated after this site
author attended a "Seminar" on Infrared Thermometry a few years ago. The topic
of emissivity came up many times and it was clear that the company representative
giving the presentation had little to no understanding of the subject, unless
the purpose of the talk was to confuse matters. Most people came away, we believe,
with a poorer understanding of the subject at the end than at the beginning. It's
sad when those apparently helping do not do their job competently.
The ratio pyrometer, ratio thermometer
or two color pyrometers (or two colour thermometers, if you prefer) are unique
devices, touted imprecisely by all too many vendor marketing people as being emissivity
independent when they are nothing of the sort. They measure in two separate
wavebands and internally create the ratio of signals (usually that of the shorter
waveband in the numerator to avoid the complication of dividing by zero-because
usually the shorter waveband signal drops out as a function of received radiation,
before the longer waveband signal).
The ratio of radiances
in two wavebands has been shown to be a function of temperature and a function
of the ratio of the spectral emissivity in the two wavebands as well (So much
for the emissivity independence, guys!) When measuring objects that have an emissivity
ratio of 1.0, they can have their emissivity ratio correction set to 1.0, just
like a single waveband thermometer does when measuring under blackbody conditions;
in this latter case one is said to be measuring under graybody (greybody) conditions.
The
old and trusty Optical Pyrometer not only refuses to go away, there's even a new
version on the market. Check out our page and learn about the two USA companies
that still make these devices.(Just between us: These things are really just another
variation of the Planck's Law-based Radiation Thermometers described above, albeit
one of the tried and accepted versions..But these darn things garnered so much
fame and fans over the years that some people just won't settle for anything else.
No matter that the technology can and does produce better devices, but snake-oil
salesmen who can't produce better results with their new devices foster this sort
of conservatism on the part of an undereducated user community.)
There's enough uses and varieties of fiber optic-related
temperature sensors these days to require a separate hyper-link category for them,
To complicate matters a little more, there really are two groups of them contact
and noncontact fiber optic thermometers. They're all covered on this one page.
One of the fabulous uses for these thermometers is to actually provide a temperature
limit signal for operating jet engines in flying aircraft. It's not all that new,
either. Rolls-Royce engines in some European military planes have been flying
for about 20 years using this technology.
Temperature
measurement occasions often seem to stretch the capabilities of existing sensors
and inventive minds continue to create new and/or better ways to measure those
temperatures. There's quite a list of them, the "Other" devices, beginning
with line scanners, two wavelength radiation thermometers, hybrid systems and
multiwavelength pyrometers, already and it's sure to grow.
Other
Resources Some of the key, related pages are at the following links:
Some vendors are
more capable and/or honest than others, just like in every business. The watch
words are, as always: "Buyer beware", until you build up confidence in
their business ethics and technical abilities (and that of the organization they
represent).
There are no standards
for the names of the spot-measuring infrared devices. Most metrologists that work
in this field tend to call them "Radiation Thermometers". Could that
imply, from the enormous variation in names of these devices, that standardization
is non existent or not mature in the noncontact temperature sensor field? Yes,
YEs, YES-there is room for a great deal of work in this area!!
Don't
get caught in the spec trap, thinking that you know what the specification on
a data sheet means. Fact is there are not even any standards for the nomenclature
used to describe technical features of these devices, starting with calibration
uncertainty! Manufacturers know (we hope) but don't always tell all. Few agree
in any aspect of their specifications beyond the temperature range of a given
unit. The ASTM in the USA has brought some order to this with Standard E 1256,
but it is voluntary and not required by many buyers, as of 2003.
The
lack of standards and precise nomenclature in terms of quantitative temperature
measurement (called "radiometrically calibrated devices" by most of
the suppliers) is an obstacle thermal imaging technology needs to overcome to master
many of the more demanding uses such as human body temperature screening and process
monitoring and control. The "Buyer Beware" caution applies doubly or triply with
these devices simply because they are more complex than radiation thermometers,
there is much technical confusion and not a widespread understanding about temperature
calibration and measurement with many of the traditional imager manufacturers.
Most of their attention over the past decades has been in imaging characterization
in terms of contrast.
The entry into
the thermal imager market in the past few years by the top-tier radiation thermometer
companies, who live and thrive on precision and accuracy of IR temperature measurement,
may create some changes and improve this aspect of the business. In addition,
one organization that has attempted to bring some order to the temperature measurement
issue is the Infraspection Institute,
one of the oldest and largest thermography training companies. They have had information
on their website and that of their Symposium Group, IR/Info,
on this issue for the last year, at least. More recently, The Standards organization
of Singapore, known by the acronym SPRING,
has been developing a Technical Standard for basic evaluation of thermal imaging
cameras used in screening humans for possible elevated body temperature as a result
of the SARS scare in early 2003.
Finally,if you have a requirement spelled out and are
sure it is covered by current standards or by a set of detailed specifications
you have developed, are you going to be using the sensor in a ISO900x production
or a process or test that is critical? Then you will most certainly need a traceable
calibration for the devices you seek. Without it any measurements will have errors
that can only be guessed, not verified, nor ever verified or repeated except by
happenstance. Often the last item on a purchase checklist is the most important,
as is this one: Traceable Calibration. In their 2001 book, "Traceable Temperatures"
(see our References page) Nicholas and White observe
that measurements without traceability, are not measurements at all but effectively
some vague effort that, in a critical analysis, is seen not worth the time and
money.
That all implies, among other things that there is
a test or demonstration of capability that a unit must pass to be accepted. Even
devices that will be used to only compare two temperatures (measure a temperature
difference) need to have some reference to which they can be re-tested to verify
that they are meeting the requirement. Often simulating the need under true application
conditions of measurement can be challenging and difficult to do and maintain a
traceable calibration to within the desired norms.
