Infrared Temperature Sensors

Introduction
Interesting links:

1. Infrared Thermometers also known as Pyrometers
2. Infrared Thermometer Basic Presentation Slides
3. How to Compensate for Interferences with an Infrared Thermometer
4. Infrared Energy, Emissivity, Reflection & Transmission
5. Single-, Dual- and Multi-Wavelength IR Thermometer
6. Advantages of Short Wavelength Infrared Sensors

Infrared through the ages

Infrared radiation was discovered by Isaac Newton when he separated the electromagnetic energy from sunlight by passing white light through a glass prism that broke up the beam into colors of the rainbow.

In the 1800's, William Herschel discovered energy beyond the visible.
In the 1900's, Planck, Stefan, Boltzmann and Wien further defined the activity of the electromagnetic spectrum and developed equations to identify IR energy.

This research makes is possible to define IR energy using the basic blackbody (a perfect emitter) which shows that an object with a temperature greater than -273 deg C emits radiant energy in an amount proportional to the fourth power of their temperature. The concept of blackbody emittance is the foundation for IR thermometry.

Basic description of measuring radiation

Infrared radiation thermometers measure temperature without contact. The object being measured essentially broadcasts information about its temperature all the time. An infrared thermometer collects some of the broadcasted radiation, and, if done with reasonable care, can measure the temperature of the object's surface, and for semi-transparent objects, measure the temperature within and/or beyond the object.

Since an infrared thermometer does not contact the object it is measuring, it does not need to be at the same temperature, thus, it can, in theory, measure very rapidly, measure distant objects, measure moving objects, measure very high temperatures, not interfere with the object's temperature distribution and many more very unique things beyond the limits of, and often competitive with (for accuracy), contact temperature sensors. (It's not easy to measure surface temperature of objects accurately, even with thermocouples or other contact sensors! It's very difficult to do it when the object is moving or is above the melting temperature of the sensor materials or both.)
In many industrial applications, there are often application issues that interfere with the amount of infrared energy collected by the sensor; such as: emissivity variations, misalignment, stray reflected infrared energy and optical obstructions.
A general rule of thumb for measuring temperature with infrared sensors suggests that 20% of the problem is picking the right sensor and 80% is installing it correctly.

Application

1. where the object to be measured is moving (example: aluminum extrusion presses, wire manufacturing)
2. The object is surrounded by an electromagnetic field (example: induction heating)
3. The object is contained in a vacuum
4. The application requires a fast response time (example: induction heating)
5. The distance between sensor and heat source is too long (example: flare stack monitoring)
6. The objects temperature exceeds the temperature limits for contact sensors

A high end infrared sensor is capable of "seeing" through intervening media such as smoke, dust, steam, dirty window and so forth.



How to determine emissivity

Before you retreat to the options listed below, please contact us. Many problems can be solved with a quick visit or over the phone.

1. Standardized emissivity values for many materials are readily available
2. Measure a known temperature on a material with a calibrated contact sensor and then compare the value to the measurement with the IR sensor. Then adjust emissivity on IR sensor accordingly.
3. For temperature less than 500�F, stick a masking tape with an emissivity of .095 to the heated surface. Then adjust emissivity on IR sensor accordingly.
4. For temperature above 500�F, drill a hole with a depth 6 times the diameter into the object. This hole may acts as a "blackbody" with an emissivity close to 1.0. Measure the temperature in the hole with the IR sensor and then adjust emissivity on IR sensor accordingly.
5. Paint a portion of the material with black paint (which will have a emissivity close to 1.0). Measure the temperature of the paint with the IR sensor and then adjust emissivity on IR sensor accordingly.