Infrared Thermometers, Part 2
September 1, 2006
When you point and shoot most infrared thermometers, you need to be sure that you are on target and that your sensor is capturing the radiated energy from all of the field of view.
The optical design determines the field of view (FOV). It is the ratio of distance to target/target diameter. Typical values are 12:1 to 200:1.
Your infrared thermometer may or may not include a targeting system to help your aim. A typical targeting system would employ a laser beam, where you direct a light spot to the center of your chosen target before you shoot. Some instruments also can project a ring of laser spots to define the field of view. Your measurement represents the average temperature within that circle.
Bandwidth Selected by Your Optical System. Until recent years, most instruments would use a sensor with a broad infrared band, somewhere within a range of 8 to 14 micron. It receives an ample dose of radiant energy and the sensor delivers a correspondingly large signal. That's good, and economical, in that it does not demand a complex optical and amplifier system. This basic design is still made in large numbers at good prices and does a good job in many non-critical applications.
However, broad-band thermometers have severe limitations in cases of:
- Partially obscured targets.
- Situations where atmospheric moisture is in the line of view of the target.
- When used to sense thin-film plastics or glass surfaces.
- In the presence of CO2 and moisture in flames and gases.
- Situations where the target material has unpredictable or low emissivity.
Narrow-Band (Single-Color) Thermometers. Narrow-band infrared thermometers can go a long way to meet some of the above challenges. With this design, the optical filter and sensor are chosen to respond to a slice (1 micron or less) of the short end of the spectrum. This is a relatively small dose for the sensor to handle but sensors, optics and electronics have advanced.
It is favorable to measurement of temperatures above ~1,100°F (600°C), where the radiant energy curve peaks. Short wavelengths, around say 0.7 micron, show the highest effective emissivity and best tolerance to changes in emissivity.
Applications that benefit from selective spectral response include long-path measurements through atmospheric moisture and measurements on plastic film.
Two-Color (Ratio) Thermometers. This system measures the ratio of radiant energies from two different slices of the spectrum. Their absolute values are not required at this stage. The derived ratio signal is scaled and linearized to accurately represent target temperature in the face of variations of those parameters that plague broad-band thermometers.
If the target's emissivities at the two wavelengths accurately track each other as they change, for various reasons, the ratio measurement is still valid. This kind of target is known as a gray body.
It is an advantage to use a single sensor and a rotating filter wheel so that the detector sees pulses at the two different wavelengths as the two filters pass over it.
The single sensor arrangement cancels any divergent aging and consequent upset of the ratio that employing two sensors would cause.
The design also allows selection of optimum filters for the specific application. For example, you can choose filters that look through water and plasma, or those that suit measurements as low as ~300°F (150°C).
Multi-Wavelength Thermometers. Multi-wavelength thermometers use three or more wavelengths to derive a temperature. For example, a design by Williamson Corp., Concord, Mass., determines the surface emissivity as well as temperature. This data is used to generate application-specific correction algorithms for different materials and conditions. If the emissivity changes, a lookup table corrects the energy signal for that specific emissivity. This design can accurately measure the temperature of just about any material -- gray and non-gray bodies. The company partnered with Pittsburgh-based Alcoa and Purdue University in Indiana in the development of multi-wavelength technology for aluminum. This multi-wavelength development is used on many other applications where single- and dual-wavelength thermometers do not work such as stainless steel, zinc coatings, glass dies and copper.
Emissivity has long plagued infrared thermometry, and this factor can now be addressed with advanced opto-electronic designs.
Additional Features and IssuesAfter all the tough problems at the optical end of the measurement, it is a relief to see the electronic add-ons that designers now take in their stride. Features include:
- A built-in camera to photograph the location.
- USB ports.
- Fast response -- as fast as 5 to 500 ms after signal conversion. It eliminates the sizable time-lag that measurement by thermocouple or RTD would normally introduce. This factor alone may dictate use of an infrared thermometer in some fast control applications.
- Peak picker. When you are monitoring intermittent targets such as parts on a conveyor, the speed of the infrared thermometer allows fast capture-and-hold of the peak temperature until the next sighting. Decay rates can be set up to match the process speed.
- Averaging. A smoothing circuit (averager) can calm dancing digits on the display, render them readable and present less confusion to a controller.
- User adjustments of time-constant. Settings range typically from 5 ms to 5 sec.
- Display hold. The value captured remains on the display when the thermometer is moved away from the target.
- Datalogging, graphing and analysis.
- Maximum and minimum temperature capture.
- Limit alarms.
- Through-lens target sighting, focusing optics plus data readouts.
- Infrared laser that directs a modulated beam at the target to determine the emissivity and distance to target.
- Analog and digital retransmission of measured variables.
- Switchable temperature unit (°F/°C).
- Extended battery life. Some models claim 9 hr some 500 hr, dependent on usage. Watch this on long duration tests.
Final WordI preach this all the time, but this time I have to say:
There are so many things in this field that you cannot do yourself. If you have other jobs than infrared thermometry, you just have to use the experience of those willing people who have made it their career to design, build and apply infrared thermometers. Let them guide you through this application minefield.
I thank all those suppliers whose experience I have called upon: Fluke Corp., Ircon Inc., Land Instruments, Mikron Infrared, Omega Engineering Inc., Raytek Corp., Williamson Corp., and many others.