What is an Infrared Thermometer?

An infrared thermometer, sometimes called an infrared pyrometer or radiation thermometer, is a device used for non-contact temperature measurement. Infrared thermometers are most often applied to measure the surface temperature of an opaque object. They are also used to measure a semi-transparent target such as high temperature combustion gas or molten glass that acquires the measurement into a depth of the target. Non-contact temperature measurement can be made over a wide range of distances, provided the sight path is free and clear of potential sources of error such as steam, dust, mechanical obstructions, dirty windows etc. 


Why should I use an infrared thermometer to measure temperature in my application?

An infrared pyrometer measures temperature in applications where it is not possible or practical to make contact with the target using a thermocouple or other contact sensor.
Non-contact temperature measurement is preferred or required under the following conditions:

  1. The target is moving or is not accessible to a contact probe.
  2. The target or the environment which encompasses the target is too aggressive, hazardous or would otherwise adversely influence the output signal of a contact probe.
  3. Contact with the target would damage the surface or adversely influence the temperature measurement when in contact with the surface.
  4. The temperatures to be measured are too high for the respective thermocouple or other type of contact probe. Infrared pyrometers can reduce maintenance costs dramatically due to their long life in contrast to the costly replacement of high temperature thermocouples which can fail, or are consumed frequently due to process conditions.


How does an infrared thermometer work?

Infrared thermometers detect radiation in the infrared portion of the electromagnetic spectrum which is adjacent to the portion where visible light exists.


Infrared Portion of Electromagnetic Spectrum


The basic components of an infrared thermometer include a lens, filter(s) for the required infrared wavelength response, a detector, and electronics to process the received infrared energy into an output expressed as temperature. Mathematical principals developed in the 1800’s were completed by Max Planck in 1900 who gave us the formula that is still used today to convert infrared radiation into a temperature value.  


Is there a difference between a Pyrometer and a Thermometer?

The terms "infrared thermometer" and "infrared pyrometer" are often interchangeable terms used by end users and vendors alike.


Pyrometer comes from the Greek words meaning fire and measurement, which originally was used to describe an instrument that measured high temperatures, for both non-contact and contact instruments. Thermometers historically read lower temperatures. 


Today, infrared thermometers can measure both high and low temperatures.


The first non-contact pyrometer responded to visible light instead of infrared radiation. This pyrometer is known as an optical pyrometer or disappearing filament pyrometer, and is limited to measuring temperatures high enough for the object to emit visible light. The point at which an object begins to emit visible light is approximately 550 °C when no ambient light is present. A subset of available thermocouples which measure high temperature by contact are also referred to as pyrometers.


What other names are used by our customers to describe infrared pyrometers & infrared thermometers?

Radiation thermometer, infrared radiation thermometer, infrared temperature sensor, non-contact temperature sensor, non-contact thermometer, IR sensor, infrared radiometer, heat gun, IR gun and, although technically incorrect, an optical pyrometer.

What does the laser do when built into an infrared pyrometer?

Infrared pyrometers collect radiated energy from a target area and focus it onto a detector inside the instrument. A laser aiming feature option installed within infrared pyrometers simply identifies where the pyrometer is aimed. Lasers that produce a small single dot (called Pilot Laser by HEITRONICS) identify the center point of the measured spot diameter, not the entire measured spot area.

HEITRONICS offers a unique Focus Laser option that produces a crosshair and circular laser light pattern to identify the location and size of the measured spot diameter.


Focus Laser Option


What are the different categories of infrared pyrometers & thermometers?

There are four categories of infrared pyrometers:

  1. Portable or Fixed Mounted
  2. Single Color, 2-Color and Multiwavelength
  3. Fixed Mounted pyrometers with Fiber Optic Cable between the lens and the detector are available for short wavelength spectral ranges
  4. Linescanning, which uses a mirror to move the single measurement spot along a line

Portable infrared thermometers are mostly available in 1 micron and 8 … 14 micron spectral ranges. There are a small number of 2-color portables available. There are no multiwavelength portables. 

Portable infrared thermometers are mostly available in 1 micron and 8 … 14 micron spectral ranges. There are a small number of 2-color portables available. There are no multiwavelength portables. 

A portable thermometer makes it very easy to measure multiple target areas at various locations. However, the benefit of taking it from place to place, target to target also presents a risk for obtaining unreproducible measurements when aiming at a target at varying distances and angles from one day to the next.  Measurement spot size, target emissivity, sight path obstructions and target geometry are among the factors that need to be carefully considered for optimizing measurement accuracy and reproducibility.

Fixed mounted thermometers provide a continuous output signal for use in automated process control, recording of data for quality control, and for providing operators a temperature display to manage a process manually.


Fixed Mounted Thermometers


Single color thermometers are the most popular, and are also referred to as 1-color, single-waveband, or brightess thermometers & pyrometers.

Infrared Radiation Thermometers (IRT’s) in single color category are available from HEITRONICS in over 20 different spectral ranges. 

2-color pyrometers are only available in short wavelengths and therefore may not measure below approximately 200 °C.   These pyrometers are typically applied to measure greybody emitters of infrared radiation, targets that do not fill the field of view, or are used for certain types of obstructions within the sight path. 

Multi-wavelength pyrometers are only available in short wavelengths and therefore may not measure below approximately 200 °C.  They may use two wavebands plus an algorithm or coefficients, or they may utilize a great many narrowband wavelengths to calculate temperature. Multi-wavelength pyrometers are applied to specific applications involving materials that do not behave as grey body emitters of infrared radiation, or for situations where the sight path causes non-grey transmission of the emitted radiation from the target to the pyrometer.

Fiber Optic pyrometers are only available in short wavelengths and therefore may not measure below approximately 200 °C. The pyrometer’s fiber cable is made of glass or quartz that is protected by a stainless steel coil which can withstand up to approximately 200 °C ambient temperature.

Linescanners are available in two basic configurations.  One uses a rotating mirror with fast scan rates in the range of up to 200Hz. The other uses a reciprocating mirror movement with slow scan rates up to 10Hz per line. The 10Hz HEITRONICS linescanner is available in over 20 different spectral ranges compared to the far fewer number of spectral ranges offered in the faster linescanners made by others. 

Linescanner Pyrometers



What Does Spectral Range Mean?

(Spectral range is also called "spectral response")


The spectral range defines the lower and upper limits of wavelengths that the Infrared Radiation Thermometer (IRT) responds to. Producers of IRT’s are not necessarily using the same basis for defining the spectral range.  A customary definition of spectral range is to use the wavelengths which correspond to 50% of the maximum response to energy that reaches the detector.  FWHM (full width at half maximum) is an acronym used for such a customary definition.


The most popular spectral range used within all the industrial, scientific, medical and commercial market segments is 8 … 14 microns (micron = μm = micrometer) for lower temperatures while the 1 & 1.6 μm spectral ranges are very popular in industrial markets for higher temperatures.  The 8 … 14 μm range is also referred to as a longwavelength wideband spectral range. There is no generally accepted definition of the width of a wideband spectral range.  


The 1 & 1.6 μm ranges are referred to as shortwavelength narrowband spectral ranges.  The term ‘narrowband’ also does not have a generally accepted definition for how narrow it must be.  However, in general practice, a spectral range with approximately < 0.5 μm bandwidth appears to be an unofficial agreed upon definition for what constitutes a narrowband IRT.


The following shows the spectral response for the HEITRONICS 3.9 micron narrowband spectral range.


Spectral Response SP41

Sometimes such narrowband spectral ranges are expressed by the maker using the limits, and at other times, the ranges are expressed using the center-point or an approximate center point of the spectral range.  For example, the 1 μm spectral range is commonly within the limits 0.8 … 1.1 μm.  Comparing one maker to the next, it may be defined with limits that lie between 0.7 … 1.2 μm.

There are many other spectral ranges which are neither wideband nor narrowband.   They will typically reside anywhere between 0.55 μm to 20 μm.  There is no common ‘labeling’ for such spectral ranges other than to simply describe the bandwidth. HEITRONICS offers more than 20 different spectral ranges and is willing to consider providing new spectral ranges on request. HEITRONICS offers the only commercially available ‘total radiation’ spectral range of 0.6 … 39 μm.
There are other wideband spectral ranges in addition to 8 … 14 μm, all of which include long wavelengths. Most of these are found on very low cost portable thermometers.  
There are other narrowband spectral ranges in addition to 1 μm which are available within short wavelengths , mid-infrared wavelengths or longwavelengths. HEITRONICS may likely have the greatest number of standard spectral ranges available in the market.


Why is spectral range important?

For many applications, the spectral range is carefully selected to coincide with the infrared emissions of semi-transparent materials such as glass, thin film polymers or hot combustion gases.  For example, when measuring combustion gas temperature within a combustion chamber, the measurement is acquired via a ‘column’ of gas molecules rather than a particular surface or one particular depth into the gas stream.

Careful selection of the spectral range is also required regarding the issues associated with emissivity variations of metallic surfaces. And, in contrast to measuring combustion gas temperature, there are many applications for which viewing through combustion gas is an important factor to consider which requires a different spectral range.

HEITRONICS has decades of application experience at your disposal to offer recommendations on which spectral range to use and how to apply your HEITRONICS pyrometer. We are always happy to assist you.

For assistance with your application, click here for a Customer Application Requirements Form, complete it as best you can, and email it to win@wintron.com for review by one our experts.

What is the Field of View (FOV) and the measurement spot size?

The optics of an infrared thermometer typically produces a circular measurement spot upon an opaque or solid surface. If a target surface is measured at an angle, the measurement spot will change from a circular shape to an elliptical shape, which then increases the size of the measured area.  Infrared emissions received within the FOV are processed into a single temperature value. The FOV must be smaller than the target size in order to make an accurate measurement when using a single color pyrometer.  If the FOV is larger than the target size, then the thermometer is going to measure some combination of the target temperature and background temperature, which may be a very different temperature.

In addition, mechanical obstructions, dust or other media that are within the sight path will influence the temperature reading. Contact us for recommendations to overcome sight path obstructions found within your application.

The field of view (FOV) of an infrared thermometer is specified in a variety of ways by the various manufacturers because there is no standardized way of describing field of view. There are also differing ways to present the FOV in diagram format, and in addition, the FOV will sometimes be defined as a Distance-to-Spot-Size Ratio.  The following shows an example of a HEITRONICS FOV Diagram.


FOV Diagram KT19/KT15

This FOV Diagram is for the 8 … 14 micron spectral range when a KT15 or KT19 IRT is specified with Detector Type G. The ‘valley points’ in the curves show the ‘focus point’ of each of the five selected FOV’s in this example.  The Y-axis describes the measurement spot diameter in millimeters.  The X-axis describes the distance between the lens and target in millimeters.  


Percent Energy Capture of an Infrared Pyrometer

All HEITRONICS FOV diagrams define the measurement spot size for over 95% of the total energy that the IRT uses to make a temperature measurement.  This is one of HEITRONICS' strengths in the industry! The competition typically specifies only 90% energy for mid-wavelength or long wavelength spectral ranges, or does not even specify the percent energy.  By not specifying the percent energy capture, such competitors are not providing a client with complete information regarding optical resolution performance.  


What percent energy capture means to a user of a pyrometer with a low percent energy capture specification is that the pyrometer will make a measurement from an area larger than what a user may actually require.  This larger area being measured will then include surfaces or objects to the side of or behind the object to be measured.   If the surfaces to the side or behind the object are lower in temperature compared to the object, the output signal level from the pyrometer will be lowered, thereby producing inaccuracies in the measurement.

The percent energy capture specification is particularly important for 1-color pyrometers under the following situations:  (a) measuring objects that are small, or (b) viewing through a sight port or sight tube, or (c) the FOV needs to view between narrow gaps of mechanical obstructions, or (d) applying a Transfer Radiation Thermometer as a Standard in a calibration lab.  (For details on the FOV for 2-color pyrometer please check back for a new forthcoming section.)

If the object to be measured is very large, however, and there is a wide open space between the pyrometer and the object, then the percent energy capture will typically be of no concern to the user.  Other typical qualifying considerations for which percent energy capture is of no concern include these situations:  (a) if the area to be measured is dramatically larger than the specified measurement spot size, or (b) if the entire area can be assumed to be homogenous in temperature,  or (c) if there are no background temperature sources which are significantly different compared to the object’s temperature to enter the pyrometer’s FOV via reflection off the object’s surface. 

In addition to the above points related to applications where large areas are being measured, it is sometimes actually desired by the user to measure a large object with a large spot size to produce an ‘averaged’ temperature reading of a large area,  rather than have temperature anomalies or non-homogenous variations be reported by the pyrometer’s output signal.  The term ‘averaging’ in this example refers to the fact that infrared pyrometers make a single temperature measurement from the energy received within its FOV, regardless of whether the object being measured is homogenous in temperature, or not. 

Getting back to when percent energy is important, if the object is curved or cylindrical in shape, such objects are often best measured with the smallest available measurement spot size.  Smaller measurement spot size will also be important when aimed at objects which are relatively small and that use a significant angle of incidence because the measurement spot size increases as a result of angle of incidence.  (It is also likely that reflectivity of the object will increase at significant angles,  which is why 45 degrees is a typical maximum recommended angle.  Note:  higher reflectivity means lower emissivity.)

To compare a HEITRONICS 95% energy specified FOV with a pyrometer specified at 90% energy, with both described as measuring Ø10mm spot size @ 500mm distance away from an object:  for a 200 °C object with 20 °C background temperature, when viewing an actual Ø10mm flat object perpendicularly, the 90% energy specified pyrometer will produce a measurement which is almost 7 °C lower than the HEITRONICS pyrometer.  The reason for this is that a higher percentage energy capture defined FOV will measure a smaller actual area.  (Note: it is typically recommended that the 1-color pyrometer’s measurement spot size be smaller than the actual object size for reasons that will be discussed in the application section.)

For our clients that desire or require proof of the energy captured by the FOV of a particular pyrometer, HEITRONICS offers a service called SSE Evaluation (Size of Source Effect Evaluation) which follows the method prescribed within IEC TS 62 942.   An example of an SSE Diagram is shown below.  (Note: the example shown below is for the highest performing commercially available SSE in the world for a longwave broadband transfer standard, the HEITRONICS TRT.  The SSE percent energy axis starts at 97% in the below example.)

SSE Size of Source Effect