Solving the Emissivity Problem in Real World
Infrared Thermometer Temperature Measurements
 

But the real world is more complicated. For any particular temperature and wavelength, the energy radiated by a surface is directly proportional to the spectral emissivity of the object. Emissivity is the value associated with a surface’s ability to get rid of heat by radiating thermal energy, and different substances have different emissivities. The value of a substance’s spectral emissivity is a number in the range of from 0 to 1.0, which is the ratio of the energy radiated by object’s surface to the energy radiated by a perfect blackbody at the same temperature. The higher the value, the better the surface is at emitting energy.

In physics, a black body is a substance that absorbs all electromagnetic radiation falling upon it. As it is a perfect absorber it is given an emissivity of 1.0. The laws of thermodynamics developed by Max Planck, and others, specify that it must also be a perfect emitter of radiation, and the energy distribution as a function of wavelength is dependent on the absolute temperature.

In practice, the amount of thermal energy a given object emits is directly related to its temperature, wavelength, wavelength band and a number of other factors such as the surface quality, transparency, reflectivity, absorptivity, angle of observation etc. These factors need to be considered when designing and using pyrometers.

Since a pyrometer is not an absolute instrument, it is necessary to calibrate it against a blackbody to convert the electromagnetic radiance received by the pyrometer to its corresponding temperature. If a pyrometer is used to measure the temperature of an object with unknown emissivity, the reading will not be valid, because the object will emit the electromagnetic energy proportional to its emitting ability and temperature shown by the pyrometer will be lower than the actual temperature.

Thus, to deduce the accurate temperature of a given surface from the radiation it is receiving, an operator has to know the emissivity of the material he is measuring. Usually an emissivity control on the pyrometer allows this value to be set on the instrument being used.

Unfortunately, the main difficulty in the practical use of infrared thermometry is the necessity of measuring the temperature of objects whose emissivity is unknown. The lives of scientists, engineers and technicians who must make precise measurements of temperatures in industrial processes or scientific inquiries would be a lot simpler if every substance, regardless of its composition, surface texture or geometry, would emit the thermal radiation at given temperature independent from wavelength . It would also make life a lot simpler for the designer of radiation pyrometers.

There are many publications that list the emissivity values of various materials. It would be relatively simple if these emissivity figures could simply be used as published. However, sometimes this published data lists both total emissivity and spectral emissivity and it is important to pick the right number. Most metals with clean surface or with thin oxide layer have the emissivity that varies with wavelength and using the total emissivity value will cause a significant error in temperature indicated by the pyrometer (if the wavelength band is not wide enough). In order to reduce the temperature error, the effective emissivity and effective wavelength must be used for pyrometer calibration. In each case, the pyrometer operating wavelength and band data has to be matched with the published spectral emissivity table.

There are also other complications.  The emissivity of an object is not a fixed number. It is continuously changing because of changing surface conditions such as oxidation and recrystalization, and these must be taken into consideration if an accurate temperature measurement is to be made. In most cases, temperature has to be measured under a wide variety of conditions presented by objects such as semiconductor wafers, ceramics, clean metal surfaces, partially oxidized metal surfaces, mixtures of molten metal and slag, and semitransparent objects such as glass with surface qualities varying from mirror smooth to perfectly diffuse.

Unfortunately, a universal method suitable for all possible applications doesn’t exist. However, a number of approaches have been developed to overcome some of these difficulties that will produce reliable and consistent temperature measurements. 

With standard single wavelength pyrometers this will often require a certain amount of guesstimation on the part of the operator using the instrument. Rarely is it possible to achieve accurate and repeatable measurements in this manner. The best way to solve typical emissivity problems is to just measure the emissivity. But the emissivity of an object is not easy to measure accurately because it depends significantly on many physical and chemical properties, such as temperature, wavelength, angle, oxidation, and roughness.

Emissivity data can be obtained in number of ways. If the temperature of the object can be measured with a contact thermometer, the pyrometer’s emissivity setting can be varied until it indicates the same temperature, and measurements can then be made of that particular surface using that setting. Any change in the area or surface being measured would require repeating the measurement.

Another technique is to blacken part of the object with soot or special high temperature black paint that will approximate a coefficient of 1.0. The pyrometer measures the temperature of the blackened area, with the instrument set at its highest 1.0 emissivity setting and the temperature is noted. Then the bare surface is measured and the emissivity control setting on the pyrometer is changed until the instrument shows that same temperature. A close approximation of a black body can also be achieved by drilling a deep narrow hole in the object to create what is known as a black body cavity. In this method, the pyrometer must be able to focus into the narrow hole. A reference gold cup pyrometer can also be used to figure the actual object temperature and the pyrometer emissivity adjusted to get the same temperature reading. Finally, a spectrometer and reference source can be used to analyze the emissions of the surface and the pyrometer calibrated accordingly. This is a costly process that usually must be done in a laboratory.

All of these techniques have drawbacks. In real world processes, the emissivity may have changed by the time it has been determined by these methods and the time of the measurement. Other difficulties with these methods include the fact that they can be time consuming and expensive to carry out, and must be repeated each time there is any change in the object being measured or in the measurement setup. In some cases these methods cannot be used at all if the subject of interest is physically inaccessible.

Another factor that can lead to erroneous temperature readings is the geometry of the surface being scanned. A concave surface will tend to concentrate more energy into the scanned area, just as a magnifying shaving mirror focuses sunlight, and presents a higher emissivity. Similarly, a convex surface will disperse the energy for an opposite effect, showing a lower emissivity.

Flat surfaces, especially polished ones, do not emit radiation equally in all directions so the angle at which a flat surface is viewed will have an effect. The more the angle deviates from straight on, or 90 degrees to the surface, the lower the apparent emissivity becomes, and the greater the possible temperature error if this is not taken into consideration. And for highly reflective objects polarization effect has to be taken into account.

Other errors can occur when extraneous energy, such as from the higher temperature inner wall of a furnace or kiln is reflected by the target object and is integrated into the object’s radiation signature. And, finally, but probably not last, in many high temperature industrial processes there can be intervening gases, smoke or vapor that add an obscuring or filtering effect.

Dual and multi wavelength pyrometers use a mathematical technique to sidestep this problem, but are not valid for many applications. Those methods are applicable only for so-called "gray" objects where the emissivity stays constant, independent of wavelength.

Precise measurement and control of temperature is vital in many industrial processes in order to assure product quality and yield, as well as safety. The ePyroCal Emissivity Calculator ( www.pyrometer.com ) can help estimate the possible errors for different types of infrared thermometers, and show that for some processes this error in temperature measurement may not be acceptable.

With all these complicating factors is it ever possible, in the real world, to get accurate and repeatable temperature readings with an infrared radiation pyrometer when an object’s emissivity is not exactly known? It certainly is, when high quality instruments are used that employ special techniques to compensate for the above effects.

The Pyrometer Instrument Company manufactures a complete line of pyrometers that use a patented pulsed laser technique to determine a target's emissivity at the same time, location, wavelength that the infrared temperature measurement is being made. This low-powered, pulsed laser is fired at the target along a dedicated optical path, and both the reflected laser return signal and the infrared signal are detected.  With the known output energy of the laser, the pyrometer will measure how much of the laser energy is returned. Assuming that the target is opaque, the laser energy must either be reflected or absorbed. By measuring the reflectivity of the target at the same location, temperature, wavelength and instant as the radiance measurement, the instrument is able to determine the emissivity and thus the True Emissivity Correct Temperature.

In situations where the temperature reading of the target may be influenced by stray radiation from a hotter surface, such as the walls of a furnace, the laser pyrometer can eliminate this error by taking a radiant measurement of the hotter surface, and knowing the reflectivity of the target.

The Pyrometer Instrument Company’s portable Pyrolaser®  pyrometer incorporates this latest technology and provides laboratory precision temperature measurements in a rugged instrument that is well suited for use in industrial or scientific environments. It has so improved the accuracy of temperature measurement that it received the Photonics Circle of Excellence Award.

Several other instruments using this method of emissivity determination have been developed for special applications. The company’s On-Line Pyrofiber® fiber-optics-based instrument permits temperature measurements to be made where there is a space limitation, electromagnetic interference, a hazardous environment or where a direct line of sight reading cannot be obtained with a standard instrument. Fiber optics based instruments can also be used for industrial or lab environments and can be used for a wide variety of objects from diffuse, using our Pyrofiber® to mirror type objects, using our Optitherm® III providing high accuracy and resolution. They are especially suited for temperature critical semiconductor processing applications.

While determining real-time emissivity in infrared thermometry has long been a serious problem, today Pyrometer Instrument Company's laser equipped pyrometers offer near laboratory accuracy in even the most difficult industrial situations.