Why should I use an infrared thermometer to measure temperature in my application?
Non-contact pyrometers allow users to measure temperature
in applications where conventional sensors cannot be employed. Specifically,
in cases dealing with moving objects ( i.e., rollers, moving machinery, or a conveyor belt), or where non-contact measurements are
required because of contamination or hazardous reasons (such as high voltage), where distances are too great, or where the temperatures
to be measured are too high for thermocouples or other contact sensors.
How to Choose an Infrared Thermometer
- Determine the field of view (target size and distance)
- Consider the type of surface being measured and its emissivity
- Analyze the spectral response for atmospheric effects or transmission through surfaces
- Specify the temperature range and the mounting needs
- Don't forget: response time, environment, mounting limitations, viewing port or window applications, and desired signal processing
What should I consider about my application when selecting an IR thermometer?
The critical considerations for any infrared pyrometer include field of view (target size and distance), type of surface being measured
(emissivity considerations), spectral response (for atmospheric effects or transmission through surfaces), temperature range and mounting
(handheld portable or fixed mount). Other considerations include response time, environment, mounting limitations, viewing port or window
applications, and desired signal processing.
What is meant by Field of View, and why is it important?
The field of view is the angle of vision at which the instrument operates, and is determined by the optics of the sensor. To obtain an accurate
temperature reading, the target being measured should completely fill the field of view of the instrument. Since the infrared device determines
the average temperature of all surfaces within the field of view, if the background temperature is different from the object temperature,
a measurement error can occur. OMEGA offers a unique solution to this problem. Many OMEGA infrared pyrometers feature patented laser switchable
from circle to dot. In the circle mode a built-in laser thermometer creates a 12-point circle which clearly indicates the target area being measured.
In the dot mode a single laser dot marks the center of the measurement area.
What is emissivity, and how is it related to infrared temperature measurements?
Emissivity is defined as the ratio of the energy radiated by an object at a given temperature
to the energy emitted by a perfect radiator,
or blackbody, at the same temperature. The emissivity of a blackbody is 1.0. All values of emissivity fall between 0.0 and 1.0. Most infrared
thermometers have the ability to compensate for different emissivity values, for different materials. In general, the higher the emissivity of
an object, the easier it is to obtain an accurate temperature measurement using infrared. Objects with very low emissivities (below 0.2) can be
difficult applications. Some polished, shiny metallic surfaces, such as aluminum, are so reflective in the infrared that accurate temperature
measurements are not always possible.
How can I mount the infrared pyrometer?
The pyrometer can be of two types, either fixed-mount or portable. Fixed mount units are generally installed in one location to
continuously monitor a given process. They usually operate on line power, and are aimed at a single point. The output from this type
of instrument can be a local or remote display, along with an analog output that can be used for another display or control loop.
Battery powered, portable infrared ''guns'' are also available; these units have all the features of the fixed mount devices, usually
without the analog output for control purposes. Generally these units are utilized in maintenance, diagnostics, quality control,
and spot measurements of critical processes.
What else should I take into account when selecting and installing my infrared measurement system?
First, the instrument must respond quickly enough to process changes for accurate temperature recording or control. Typical response times for infrared thermometers are in the 0.1 to 1 second range. Next, the unit must be able to function within the environment, at the ambient temperature. Other considerations include physical mounting limitations, viewing port/window applications (measuring through glass), and the desired signal processing to produce the desired output for further analysis, display or control.
I want to measure the temperature through a glass or quartz window; what special considerations are there?
Transmission of the infrared energy through glass or quartz is an important factor to be considered. The pyrometer must have a wavelength where the glass is somewhat transparent, which means they can only be used for high temperature. Otherwise, the instrument will have measurement errors due to averaging of the glass temperature with the desired product temperature.
What is spectral response, and how will it affect my readings?
The spectral response of the unit is the width of the infrared spectrum covered. Most general purpose units (for temperatures below 1000°F) use a wideband filter in the 8 to 14 micron range. This range is preferred for most measurements, as it will allow measurements to be taken without the atmospheric interference (where the atmospheric temperature affects the readings of the instrument).
Some units use wider filters such as 8 to 20 microns, which can be used for close measurements, but are ‘‘distance-sensitive’’ against longer distances. For special purposes, very narrow bands may be chosen. These can be used for higher temperatures, and for penetrations of atmosphere, flames, and gases. Typical low band filters are at 2.2 or 3.8 microns. High temperatures above 1500°F are usually measured with 2.1 to 2.3 micron filters. Other bandwidths that can be used are 0.78 to 1.06 for high temperatures, 7.9 or 3.43 for limited transmissions through thin film plastics, and 3.8 microns to penetrate through clean flames with minimum interference.
If a part is moving, can I still
Yes. Use infrared devices or direct
contacting sensors plus a slip ring
Can a two-color infrared system
be used to measure low
Only if at high temperature, say,
above 700°C (1300°F).
What error will result if the spot
size of the infrared pyrometer is
larger than the target size?
It would be indeterminate. The
value would be a weighted average
that wouldn’t necessarily be
How is a infrared pyrometer calibrated?
There are basically two types of infrared calibration sources, hot plate blackbody source and cavity type blackbody source. The hot plate style consists of a metal plate (usually aluminium) with or without concentric grooves where the temperature of the-plate is set and controlled using either an inexpensive potentiometer dial or a high-end temperature controller. The temperature of the plate is sensed using either a thermocouple or an RTD probe. The hot plate is usually painted dull black to improve the surface emissivity. The surface emissivity of a hot plate calibration source is typically 0.95.
OMEGA's BB703 Model
is a high-end hot plate blackbody source with a built-in temperature controller. The calibration source with built-in temperature controller has much better accuracy and stability compared to a potentiometer dial type unit.
The cavity type blackbody source consists of a blind hole in a cylinder or a sphere where the temperature of the cavity is controlled by a temperature controller, using a thermocouple probe. The cavity type blackbody source has a higher surface emissivity than a hot plate blackbody unit. The emissivity of a cavity type blackbody source is typically 0.98 or higher.
The OMEGA's BB705 Model
cavity type blackbody typically goes to higher temperatures (over 530°C [1000°F]) than hot plate blackbodies. In addition, having a higher emissivity value makes them ideal for precise calibration tasks.
In order to calibrate an infrared thermometer, a blackbody calibration source is required. There are 3 factors to consider when selecting a blackbody calibration source:
- Type of blackbody (hot plate or cavity type) tells us about the construction and overall performance of the unit.
- Target area tells us how large of an area we can check our infrared thermometers with. The target area should be larger than the field of view of the thermometer; otherwise the infrared thermometer will be measuring the target area plus some of the surrounding cooler areas. Normally, an infrared thermometer is checked against a blackbody source at a relatively close distance (about 0.15 to 1m), depending on the target area).
- The higher the target emissivity, the more ideal is the calibration. At lower emissivity targets, wavelength bandwidth of the infrared thermometer comes into play. Ideally at E=1.00, wavelength bandwidth of the DUT (Device Under Test) is not a factor.