emissivity.qxp 5/12/2005 8:13 PM Page 2 LASER PYROMETRY OFFERS PRACTICAL TEMPERATURE MEASUREMENT The physics of measuring temperature pyrometrically can be problematic if the emissivity of a body is not known. Pulsed laser pyrometers help solve this problem.
Dr. Alexander Dmitriyev Pyrometer Instrument Company
very object radiates thermal energy at temperatures above absolute zero. Measuring an object’s temperature using an optical pyrometer is based on E the principle that an object’s thermal radiation is a function of its temper- ature. Measuring that thermal radiation sounds like a very straightforward engineering problem, but the real world is more complicated.
Spectral Emissivity For any particular temperature and wavelength, the energy radiated by a surface is related to the spectral emissivity of the object. Emissivity is the measure of a surface’s ability to disperse heat by radiating thermal
7 energy. Different substances have different emissivities, 10 whose value is expressed as a number from 0 to 1.0. This
6 value expresses the ratio of the energy radiated by object’s 10 3000K surface to the energy radiated by a perfect black body at
105 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 104 800K electromagnetic radiation falling upon it. As it is a perfect
3 absorber it is given an emissivity of 1.0. The laws of ther- 10 modynamics, developed by Max Planck and others, specify that it must also be a perfect emitter of radiation, and the 102 Relative energy energy distribution as a function of wavelength is de- 300K pendent on the absolute temperature. 10 In practice, the amount of thermal energy a given object emits is directly related to its temperature, wavelength, 1 wavelength band, and other factors such as surface quality,
-1 transparency, reflectivity, absorptivity, angle of observa- 10 tion, etc. These factors need to be considered in the de- sign and use of optical pyrometers. 10-2 0.1 1.0 10 100 1000 Wavelength, µm Pyrometer Design Planck’s Law: Radiation intensity as a function of wavelength and Since a pyrometer is not an absolute instrument, it is nec- temperature. essary to calibrate it against a blackbody to convert the elec- 28 HEAT TREATING PROGRESS • MAY/JUNE 2005 emissivity.qxp 5/4/2005 3:20 PM Page 3
tromagnetic radiance received by the pyrometer to its corresponding tem- Pyrometer Instrument Co. Has 77 Year History perature. If a pyrometer is used to Pyrometer Instrument Company is a leader in the development of temper- measure the temperature of an object ature measurement instruments for space, defense, science and industry. of unknown emissivity, the reading During its 77-year history, it has solved many hot-temperature measurement will not be valid because the object challenges. will emit electromagnetic energy pro- It all began in 1928, when Mr. Bollerman opened Pyrometer Instrument portional to its emitting ability, and Company on Lafayette Street, New York, NY. The first products were contact temperature shown by the pyrometer and immersion thermocouple pyrometers for molten iron and steel foundries. will be lower than the actual temper- Five years later, the company introduced a traditional non-contact optical py- ature. rometer that incorporated a photoscreenic wedge, which acts as a neutral Thus, to accurately deduce the tem- density filter that enables the measuring instrument to provide a clear, en- perature of a given surface from the larged view of the target. radiation it is receiving, an operator Steady market growth continued throughout the years of WWII. Soon after has to know the emissivity of the ma- the war, the company moved its headquarters to Bergenfield, NJ. In subse- terial being measured. Usually an emissivity control on the pyrometer quent years a number of new products and innovations were introduced. In allows this value to be set on the in- 1969 the company moved its headquarters to Northvale, NJ. strument being used. In the 1980s, Dr. E. K. Matthews purchased the company from the founding Unfortunately, the main difficulty family. Ownership change was followed by partnerships with other compa- in the practical use of infrared (IR) nies and product line acquisitions, including thermal vision and infrared ther- thermometry is the necessity of meas- mometry equipment. uring the temperature of objects of In 1987, Pyrometer purchased exclusive patent rights from Exxon Corpo- unknown emissivity. The lives of sci- ration to develop emissivity measurement systems. Emissivity is crucial to entists, engineers, and technicians accurate temperature measurement. With the Exxon rights, Pyrometer devel- who must make precise measure- opers went to work on creating an automatic emissivity correcting IR ther- ments of temperatures in industrial mometer that would solve these problems. The result was the Pyrolaser Portable processes or scientific inquiries would automatic IR thermometer.This device so improved the accuracy of temper- be much simpler if every substance, ature measurement that it received the 1988 Photonics Circle of Excellence regardless of its composition, surface Award. texture, or geometry, would emit the Today, Pyrometer Instrument Company claims a leadership role in the de- thermal radiation at given tempera- velopment of temperature measurement instruments for science and industry. ture independent from wavelength. The company is a wholly-owned subsidiary of MAKKE LLC, which owns sev- It would also make life a lot simpler eral companies specializing in the instrumentation field. for the designer of radiation pyrom- eters. matched with the published spectral overcome some of these difficulties Total and Spectral Emissivity emissivity table. that will produce reliable and consis- There are many publications that tent temperature measurements. list emissivity values for various ma- Emissivity Is Variable The use of standard pyrometers terials. It would be relatively simple There are also other complications. will often require a certain amount of if these emissivity figures could The emissivity of an object is not a ‘guesstimation’ on the part of the in- simply be used as published. How- fixed number. It continuously varies strument operator. Rarely is it pos- ever, sometimes this published data because of changing surface condi- sible to achieve accurate and repeat- lists both total emissivity and spec- tions such as oxidation and recrystal- able measurements in this manner. tral emissivity and it is important to ization, and these variations must be pick the correct number. considered if an accurate temperature Measuring Emissivity Most metals with a clean surface or measurement is to be made. The best way to solve typical emis- with a thin oxide layer have emis- In most cases, temperature has to sivity problems is to just measure the sivity that varies with wavelength. be measured under a variety of con- emissivity. But the emissivity of an Consequently, using the total emis- ditions presented by objects such as object is not easy to measure accu- sivity value will cause a significant clean metal surfaces, partially oxi- rately because it heavily depends on error in the temperature indicated by dized metal surfaces, mixtures of many physical and chemical proper- the pyrometer (if the wavelength molten metal and slag, semicon- ties, such as temperature, wave- band is not wide enough). In order to ductor wafers, ceramics, and semi- length, angle, oxidation, surface tex- reduce this error, the effective emis- transparent objects. ture, etc. sivity and effective wavelength must Unfortunately, a universal method Emissivity data can be obtained in be used for pyrometer calibration. In suitable for all applications doesn’t number of ways. If the temperature each case, the pyrometer operating exist. However, a number of ap- of the object can be measured with a wavelength and band data must be proaches have been developed to contact thermometer, the pyrometer’s HEAT TREATING PROGRESS • MAY/JUNE 2005 29 emissivity.qxp 5/4/2005 3:20 PM Page 4
emissivity setting can be varied until Drawbacks Additional errors can occur when it indicates the same temperature, and All these techniques have draw- extraneous energy, such as that from measurements can then be made of backs. In real world processes, a the higher temperature inner wall of that particular surface using that set- body’s emissivity may change by the a furnace, is reflected by the target ob- ting. Any change in the area or sur- time it has been measured by these ject and integrated into the object’s face being measured would require methods. Other difficulties are that radiation signature. Finally, but prob- repeating the measurement. they can be time consuming and ex- ably not least, in high temperature in- Another technique is to blacken part pensive to carry out, and must be re- dustrial processes there can be inter- of the object with soot or special high peated each time there is any change vening gases, smoke, or vapor that temperature black paint that approx- in the object being measured or in the add an obscuring or filtering effect. imates a coefficient of 1.0. The pyrom- measurement setup. In some cases, Dual and multi-wavelength py- eter measures the temperature of the these methods cannot be used at all rometers use a mathematical tech- blackened area, with the instrument if the subject of interest is physically nique to sidestep this emissivity set at its highest (1.0) emissivity set- inaccessible. problem, but are not valid for many ting. That temperature is noted. Then Another factor that can lead to er- applications. Those methods are ap- the bare surface is measured and the roneous temperature readings is the plicable only for so-called ‘gray’ ob- emissivity control setting on the py- body’s surface geometry of the sur- jects, in which the emissivity stays rometer is changed until the instru- face being scanned. A concave surface constant and is independent of wave- ment shows that same temperature. will tend to concentrate more energy length. An approximation of a black into the scanned area, just as a mag- body can also be achieved by nifying shaving mirror focuses sun- Pulsed Laser Technique drilling a deep narrow hole in the light, and presents a higher emis- Precise measurement and control object to create what is known as a sivity. Similarly, a convex surface will of temperature is vital in many indus- black body cavity. Using this disperse the energy for an opposite trial processes to assure product method, the pyrometer must be effect, showing a lower emissivity. quality and yield, as well as safety. able to focus into the narrow hole. Flat surfaces, especially polished The Pyrometer Instrument Company A reference gold cup pyrometer can ones, do not emit radiation equally in (PIC) offers its ePyroCal Emissivity also be used to figure the actual ob- all directions so the angle at which a Calculator on its website at www.py- ject temperature and the pyrometer flat surface is viewed will have an ef- rometer.com. This can help estimate emissivity adjusted to get the same fect. The more the angle deviates from the possible error for different types temperature reading. Finally, a straight on, or 90 degrees to the sur- of pyrometers. spectrometer and reference source face, the lower the apparent emis- With all these complicating factors can be used to analyze the emis- sivity becomes, and the greater the is it ever possible, in the real world, sions of the surface. The pyrometer possible temperature error if this is to get accurate and repeatable tem- may then be calibrated accordingly. not taken into consideration. And for perature readings with an IR radia- This is a costly process that usually highly reflective objects polarization tion pyrometer from an object of un- must be done in a laboratory. effect has to be taken into account. known emissivity? It is when Solution-infrared temperature measurement with automatic emissivity 1875 100
90 1850 80
1825 70
F 60 o 1800
50 1775 40 Emissivity Temperature, Temperature,
1750 30
20 1725 10
1700 13:17:11 13:28:11 13:39:11 13:50:11 14:01:11 13:10:35 13:12:47 13:14:69 13:19:23 13:21:35 13:23:47 13:25:59 13:30:23 13:32:35 13:34:47 13:36:59 13:41:23 13:43:35 13:45:47 13:47:59 13:52:23 13:54:35 13:56:47 13:58:59 14:03:23 Time
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instruments employing special tech- fluenced by stray radiation niques to compensate for the above from a hotter surface (such effects are used for the measurements. as the walls of a furnace), By using a patented pulsed laser tech- use of a laser pyrometer can nique to determine a target’s emis- eliminate this error. It does sivity at the same time, location, and this by taking a radiant wavelength that the infrared temper- measurement of the hotter ature measurement is being made, the surface, and by knowing the precise temperature can be measured. reflectivity of the target. This low-powered, pulsed laser is PIC’s portable Pyrolaser fired at the target along a dedicated pyrometer incorporates optical path, and both the reflected pulsed laser technology and laser return signal and the infrared provides precise tempera- signal are detected. With the known ture measurements in in- output energy of the laser, the pyrom- dustrial or scientific envi- eter will measure how much of the ronments. Several other laser energy is returned. Assuming instruments using this method of sivity in infrared thermometry has the target is opaque, the laser energy emissivity determination have been long been a serious problem, cur- must either be reflected or absorbed. developed for special applications. rent laser-equipped pyrometers By measuring the reflectivity of the The company’s On-Line Pyrofiber offer near laboratory accuracy in target at the same location, temper- fiber optics-based instrument permits even the most difficult indus- ature, wavelength and instant as the temperature measurements to be trial situations. radiance measurement, the instru- made where there is a space limita- ment is able to determine the emis- tion, electromagnetic interference, a Dr. Alexander Dmitriyev is vice president sivity and thus the True Emissivity hazardous environment, or where a of research and development at Pyrometer In- Correct Temperature. direct line of sight reading cannot be strument Company, Northvale, NJ www.py- For situations in which the temper- obtained with a standard instrument. rometer.com Ph: (201) 768-2000; Fax (201) ature reading of the target may be in- While determining real-time emis- 768-2570.
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