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Home , Infrared, Pyrometer, Thermal equilibrium, Thermometer

Thermometry Understanding and using the Infrared

Advances in electronic and detector the curve increases, increasing the area () technology have resulted in a variety of Infrared is part of the electromagnetic beneath it, and (2) the associated spectrum which includes , with the peak energy (highest point of the curve) non-contact infrared (IR) for , visible , , gamma- shifts to the shorter wavelength end of the scale. industrial and scientific use. It is important and X-rays (Figure 2). These various forms of to understand their major differences in energy are categorised by or This relationship is described by Wien’s order to select the proper unit for a given wavelength.* Note that visible light extends from Displacement Law: 3 application. .4 to .7 micron, with ultraviolet (UV) shorter than λmax = 2.89 x 10 /T .4 micron, and infrared longer than .7 micron, where λmax = wavelength of peak Infrared Theory extending to several hundred microns. In energy in microns Energy is emitted by all objects having a practice, the .5 to 20 micron band is used for IR T = in degrees temperature greater than . This . energy increases as the object gets hotter, For example, the wavelength for peak energy permitting measurement of temperature by Planck’s Law emitted from an object at 2617 degrees measurement of the emitted energy, particularly The amplitude (intensity) of radiated energy can (2890 degrees Kelvin) is: the radiation in the infrared portion of the be plotted as a function of wavelength, based on 3 spectrum of emitted radiation. Figure 1 shows Planck’s law. Figure 3 shows the radiation λmax = 2.89 x 10 /2890K = 1.0 µm a typical infrared radiation thermometer. emission curves for objects at two different or . By convention, longer Another illustration involves heating a steel billet.

Dare shownAmp to the right on IR graphs, reverse of At about 600°C (1100°F), a dull, glow is Lens or mirror electromagnetic spectrum charts, such as Figure emitted from the steel. As the temperature D Amp 2. The area under each curve represents the total increases, the colour changes from red to Output to Signal controller, energy radiated atConditioner the associated temperature. and as the peak passes into the Output to Signalrecorder, etc. controller, Conditioner visible light spectrum. Finally, the energy emitted recorder, etc. Note that two changes occur simultaneously as throughout the entire is at such Figure 1: Block diagram, infrared temperature is increased: (1) the amplitude of a high level that white light is given off by the steel at about 1650°C. Because the peak of the curve shifts as temperature increases, selection ed owaves

oadcast oadcast; ed α owaves elevision Infrar Visible Light Ultraviolet X-Rays -Rays

Longwave Radio AM Radio Br FM Radio Br T Micr of the optimum portion of the spectrum is

oadcast oadcast; elevision α 10 KHz 1 MHz Radio 100 MHz AM Radio Br 100 µm Shortwave Radio FM Radio 1 Br µm T Micr Radar Infrar Visible Light Ultraviolet X-Rays -Rays important to achieving satisfactory infrared

The IR to UV spectrum 10 KHz 1 MHz 100 MHz 100 µm 1 µm thermometer performance. is explained as shown.

The IR to UV spectrum is explained as shown. een ellow

Infrared Red Y Gr UV Emissivity is defined as the ratio of the energy 20 µm .7 µm .4 µm radiated by an object at a given temperature to

een the energy emitted by a perfect radiator, or ellow

Infrared Red Y Gr Blue Violet UV blackbody, at the same temperature. The 20 µm .7 µm .4 µm emissivity of a blackbody is 1.0. All values of Figure 2: Electromagnetic spectrum and light spectrum *Wavelength (λ) is inversely related to frequency (f): λ = c/f, where c = velocity of the ; λ is measured in microns (µm); 1 µm = 10-6m. emissivity fall between 0.0 and 1.0.

Emissivity (E), a major but not uncontrollable factor in IR temperature measurement, cannot be ignored. Related to emissivity are reflectivity (R), a measure of an object’s ability to reflect 700C infrared energy, and transmissivity (T), a measure of an object’s ability to pass or transmit IR 25C

Relative Radiation Intensity energy. Since all radiation must be either transmitted, reflected or absorbed:

l 1 µm 10 µm 20 µm A + R + T = 1.0 Figure 3: Radiation intensities derived from Planck’s Law (Blackbody radiation curves)

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R

E Hot Source T

Pyrometer Consider the example in Figure 4a. Object X is a hot block of material, Y is colder; therefore, 1.0 will be radiated from X to Y. Some heat will be absorbed by Y, some reflected, and some transmitted through Y. The three dispositions 5

must equal 100%, represented as 1.0 for ransmission T coefficients of absorption, reflection, and transmission. If A = 1.0, all the heat is Wavelength 0 in microns absorbed; if R = 1.0, then A = T = 0. Usually 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 (µm) some combination exists: Figure 5: IR transmission through

A = .7 (70% absorbed) window to pass a high percentage of the IR 2. Increase the size of object B until it fills R = .2 (20% reflected) energy at that wavelength. the thermometer’s FOV. T = .1 (10% transmitted) 3. Decrease the emissivity compensator Atmospheric Absorption (described later) to compensate for the Sum = 1.0 (100% energy radiated from One of the first considerations in selecting the loss of energy. X to Y) spectral response (wavelength range at which 4. Get a thermometer with a smaller FOV. an instrument is sensitive to IR) of a device is atmospheric absorption. Certain components Field of view is described either by its angle or

of the atmosphere, such as vapour, CO2 by a distance-to-size ratio (D:S). If D:S = 20:1, A and other materials, absorb IR at certain and if the distance to the object divided by the wavelengths, increasing the amount of energy diameter of the object is exactly 20, then the R X Y absorbed with the distance between the object object exactly fills the instrument’s field of view. T and the instrument. Therefore, if these A D:S ratio of 60:1 equals a field of view of 1°. absorbents are ignored, an instrument may read correctly when near the object, but several Since most IR thermometers have fixed focus Figure 4a: Energy budget for all radiated IR energy degrees lower a few feet away because the , the minimum measurement spot occurs displayed temperature represents an average of at the specified focal distance. Typically, if an If an object is in a state of , it the object temperature and the atmosphere instrument has fixed-focus optics with a 120: 1 is getting neither hotter nor colder; the amount 700C temperature. The reading may be affected by D:S ratio and a focal length of 1.5 m the of energy it is radiating must equal the amount of changes in or the presence of steam or minimum spot (resolution) the instrument can energy it is absorbing, so A = E25C (emissivity). By Relative Radiation Intensity certain gases. Fortunately, there are “windows” achieve is 1.5 m divided by 120, or 12.5 mm at substitution: in the IR spectrum which allow these absorption a distance of 1.5 m from the instrument. This is l bands to be avoided. Figure 5 illustrates these significant when the size of the object is close 1 µm 10 µmE + R + T = 1.0 20 µm windows. to the minimum spot the instrument can If any two of these values are known, the third is measure. easy to find. Figure 4b illustrates this relationship. Optics Target size and distance are critical to accuracy Most general-purpose IR thermometers use a for most IR thermometers. Every IR instrument focal distance between 50 cm and 150 cm; R has a field of view (FOV), an angle of vision in special close-focus instruments use a 12.5 mm which it will average all the temperatures it sees to 300 mm focal distance, and may be E Hot Source (Figure 6). equipped with a light-spot aiming device to T ensure that the instrument is measuring the Pyrometer exact spot desired. Some long-range instruments for checking insulators and Figure 4b: Total IR radiation reaching pyrometer transformers on pylons use a 15 m focal Object B Wall distance. Sighting scopes are often used at longer distances or for small spot sizes. Some Transmission Object A IR thermometers use variable-focus optics, In some applications, particularly and thin- especially high performance fixed-mount types film plastics, transmission becomes an important Figure 6: Field of view of instrument with through-the-lens sighting. factor. If it is desired to measure the temperature of these substances using IR, a wavelength Object A fills the field of view of the ; the Fibre optics are alternatively used in special must be chosen where the material appears only temperature seen is that of object A, so the applications where there is not enough space opaque or semi-opaque. Often it is desired to temperature of object A will be accurately to mount a sensing head, or where radio measure temperatures under the surface of an indicated. But if object A is removed, object B frequency interference (RFI) of high intensity object. This is possible when the material is and the wall share the field of view. The could cause erratic readings. somewhat transparent at the measured indicated temperature, somewhere between wavelength. Otherwise, selecting a wavelength that of object B and the wall, will depend on the Emissivity where the material is opaque minimises relative areas of each filling the circular field of The ideal surface for IR temperature measurement errors due to transmitted energy view. If it is desired to measure the temperature measurement would have an emissivity of 1.0. reaching the IR thermometer. If it is desired to of object B, one of four things must be done: Such an object is known as a blackbody, or make measurements of objects through a glass perfect radiator/absorber. For these objects, R or quartz window, a short wavelength must be 1. Move the thermometer closer to object = T = 0. The term “blackbody” is somewhat used to take advantage of the ability of the B, or vice versa. misleading, in that colour is irrelevant in the IR

Calex Electronics Limited Tel: +44 (0)1525 373178/853800 Fax: +44 (0)1525 851319 Online: http://www.calex.co.uk spectrum because coloured light has much emissivity for measurements of this same instruments use a wideband (e.g. 8 to 14 µm in shorter wavelengths. In practice, however, material in the future. Figure 7); because adequate energy is available, most objects are either graybodies (which have 2. For relatively low temperature (up to about only low-gain amplifiers are required. Some an emissivity of less than 1.0 but the same 250°C or 500°F), a piece of masking tape inexpensive units cover most of the .7 to 20 µm emissivity at all wavelengths), or non-graybodies can be placed on the object and the IR spectrum, at the expense of being “distance- (which have which vary with temperature of the masking tape measured sensitive” because they include some wavelength and/or temperature). This last type with the IR thermometer using an emissivity atmospheric absorption bands. A thermometer of object can result in serious measurement setting of 0.95. Next, measure the object which excludes these absorption bands (e.g., 8 accuracy problems because most IR temperature, and adjust the emissivity to 14 µm) avoids these problems. thermometers mathematically translate compensator until the display shows the measured IR energy into temperature. As an correct temperature. Use this emissivity For special purposes, very narrow bands (2.2 µm object with an emissivity of .7 emits only 70% of value for future measurements of this in Figure 7) may be chosen. These instruments the available energy, this would cause the object. are costlier because more stable, high-gain indicated temperature to read lower than actual. 3. For very high temperatures, a hole, the amplifiers are needed to amplify the smaller IR thermometer manufacturers usually address depth of which is at least 6 times the signals which result from reduced energy levels this problem by installing an emissivity diameter can be drilled into the object. This in these narrow bands. However, they can also compensator, a calibrated gain adjustment hole acts as a blackbody with an emissivity be used for general-purpose , as well as which increases the amplification of the detector of approximately 1.0, and the temperature special applications. The ability of narrow band signal to compensate for the energy lost due to measured looking into the hole will be the instruments to measure low temperatures may an emissivity less than 1. This same adjustment correct object temperature. As in example be limited somewhat by the low energy levels can be used to correct for transmission losses 2, use the emissivity compensator to encountered. through viewing ports, smoke, dust, or vapours. determine the correct setting for this For example, setting the compensator to .5 for object’s future measurements. A third type of thermometer is the ratio, or two- an object with that emissivity results in a gain 4. When a portion of the surface of the object colour thermometer. This instrument measures increase by a factor of 2. If a viewing port is can be coated, a dull paint will have the ratio of at two selected narrow used to sight the object in a chamber, an emissivity of about 1.0. Other non- bands. If the change in emissivity at the two and the transmission through the port is 40% metallic coatings such as mold release, selected wavelengths is the same, the effect of (T= .4), the errors are in series, so the net spray baking powder, deodorant, and other emissivity is eliminated, with attendant compensator setting is .5 x .4 = .2. The coatings may also be used. Measure the advantages. resulting amplification factor of 5 will compensate known temperature as before, and use the for all energy losses. emissivity adjustment to determine the Further, the target need not fill the field of view, correct emissivity value. as is the case with single-colour instruments. If a target which just fills the field of view is cut in Emissivity Versus Wavelength 5. Standardised values of emissivity are available For many materials, particularly organics, for most materials. For a detailed listing of half, half the energy will be lost to the detector, emissivity does not vary appreciably with emissivities, refer to “Thermal Radiative and the single-colour instrument will read low. wavelength. Other materials, such as glass and Properties”, (volumes 7, 8 and 9) by Y.S With the two-colour instrument, if the energy at thin-film plastics, present severe transmission Touloukian and D.P. DeWitt, published by IFI/ both wavelengths is cut and the ratio stays the losses at some wavelengths, particularly the Plenum Data Corporation, Subsidiary of same, the temperature reading will not change shorter wavelengths. These will be discussed Plenum Publishing Company, 227 West 17th (Figure 8). The benefit resulting from this feature later. St, New York, New York 10011. is that if a cloud of dust or smoke obscures the target, the radiation reaching the thermometer may be reduced, but the reading will not change Metals, in almost all cases, tend to be more Spectral Response - Wideband, as long as the ratio of energies does not vary. reflective at long wavelengths, hence their Narrowband, and Ratio IR Thermometers emissivity improves inversely with wavelength. One means of categorising IR thermometers is In practice, the emissivities at the two A problem arises with low-temperature metals, by spectral response: the width of the IR wavelengths may not vary in a similar manner. where the shortest usable wavelength depends spectrum covered. The most common design Two-colour thermometer manufacturers address on the point at which insufficient energy exists approach is to select a segment of the IR this problem with a ratio calibrator adjustment, to produce a detector output. In these cases a spectrum, optically filter the units to look only at similar to the emissivity compensator adjustment compromise is necessary. Further discussion is that segment of the spectrum (Figure 7), and of single colour instruments. This adjustment is found in the section on metals applications. integrate the energy falling on the detector for used to calibrate the unit in much the same that segment. Many general-purpose Determination of Emissivity The emissivity of most organic substances Relative (wood, cloth, plastics, etc.) is approximately 0.95. Metals with smooth, polished surfaces can have emissivities much lower than 1.0. The Total energy Total signal "seen" emissivity of a material can be determined in radiated by by 8 to 14 µm detector one of the following ways: hot object

1. Heat a sample of the material to a known temperature as determined by a precise sensor in an oven, and measure the temperature of the object with the IR instrument. Use the emissivity compensator Wavelength in microns (µm) adjustment to force the indicator to display 1 2.2 8 14 20 Figure 7: Distribution of energy as received by filtered IR detectors the correct temperature. Use this value of

Calex Electronics Limited Tel: +44 (0)1525 373178/853800 Fax: +44 (0)1525 851319 Online: http://www.calex.co.uk Energy Curve 2 present.

λ λ 1 2 Two factors limit how short the wavelength can be: (1) the lowest temperature which must be measured; as can be seen from blackbody radiation curves, the shorter the wavelength, the less energy is available at that wavelength,

Curve 1 Wavelength and (2) the width of the temperature range 1 2 10 20 in microns (µm) desired. As wavelength decreases, the energy Figure 8: Two temperatures as interpreted by ratio (two-colour) pyrometer level difference between two given temperatures increases, and an amplifier with wider dynamic manner described earlier for the emissivity metals, and metals near ambient (if reflections range capabilities is required. At some point, compensator. However, this works only for that don’t interfere). This is the only type of IR the gain required to do this becomes particular material, and often only around a given thermometer suitable for measurements below unattainable. For these reasons, a compromise temperature. Therefore, unless the target is a ambient temperature. must be made; the shortest wavelength which true graybody, the ratio thermometer has allows the required temperature range should questionable advantage over a single-colour unit. Spectrum for Mid-Range Temperatures be used. (100-800°C /200-1500°F) In the event of reduced target area (by a target One of the preferred shorter wavelength bands Other considerations in making this choice may not filling the field of view, or being obscured by for penetration of atmosphere, flames and gases be: instrument price and availability, presence of dust or smoke), a single colour unit can read is 3.8 µm. This is the best compromise for low- gases or flames in the line of sight, ability to see properly by adjusting the emissivity compensator temperature metals because shorter wavelength through vacuum chamber windows, etc. The to make up for the loss. This adjustment can instruments are limited to high temperatures. optimum wavelength for high-temperature make up for any kind of loss in the system, metals is the near infrared, around .8 µm. Other provided the loss is constant. The ratio Spectrum for High Temperatures choices are 1.6 µm (where some metals have thermometer has an advantage only when the (Above 300°C /600°F) the same emissivity at different temperatures), loss varies during the process, or in a situation Another window in the atmosphere and flame- 2.2 µm and 3.8 µm (both of which are where changing the emissivity adjustment is not absorption bands ideal for temperature recommended for reading through clean feasible. If the adjustment needs to be made measurement is 2.2 µm. This narrow band is flames). If the metals are coated, well oxidised, only once, the user need not spend the extra especially well-suited for high temperature or can be temporarily improved by adding a cost of a two-colour instrument. measurements. high-emissivity coating, 8 to 14 µm instruments can be used. Other compromises for low To summarise, a two-colour thermometer is Special Purpose Instruments temperature metals are 3.43 µm and 5.1 µm. beneficial in measuring (1) graybodies of varying METALS: Metals present some unique IR or unknown emissivity and (2) targets with a temperature-measurement problems. Spectrum for Plastics varying field of view due to changing size or In general, plastics thicker than 2.5 mm can be distance, varying concentrations of dust or Foremost is the fact that most metals tend to be measured using 8 to 14 µm instruments. In the smoke or sight-window coating. Use of a two- very reflective (unless well oxidised) and thus case of thin films, however, plastic is partially colour instrument is justified economically only have low emissivities. Some of these emissivities transparent in the 8 to 14 µm band. Heat when special circumstances require it. Further, are so low that a large portion of the sensed sources on the other side of the film and in some applications, performance can be less energy is reflected radiation (usually 1.0 from variations in the thickness will result in variations accurate than single-colour instruments if there heaters, flames, refractory walls, etc.). This can in the IR temperature reading. are inconsistent emissivity ratios. result in varying and unreliable readings. For most metals, the problem increases at .5 the Emissivity Spectrum For Low Temperatures (Below longer wavelengths. Fortunately, there are certain resonant points in the IR spectrum at which thin films appear 500°C/1000°F) 0 The most popular band for general purpose The shortest possible measurement wavelength opaque1 µm to an IR thermometer10 µm due to 20 µm measurements up to 500°C is 8 to 14 µm. This should be used. As shown in Figure 9, the characteristics of molecular bonding, eliminating is a wide band, yielding sufficient energy even at emissivities of most metals improve as the transmitted energy completely at certain sub-freezing temperature, and free from wavelength decreases. wavelengths. Some plastics (polyethylene, atmospheric absorption. Uses include Also, as illustrated by Figure 10, a smaller maintenance diagnostics, all organic processes 2000 change in indicated temperature results from the = .8 µm (paper, wood, rubber, textiles, agricultural), thick λ same change in emissivity at shorter plastics, glass surfaces (if reflection from strong 1800

wavelengths, producing more accurate e, ° F heat sources is not a problem), well-oxidised measurements when emissivity variations are λ = 2.2 µm 1600

1.0 1400 Indicated temperatur

.5 1200 Emissivity .2 .4 .6 .8 1.0 Target material emissivity 0 1 µm 10 µm 20 µm Figure 10. Readings obtained on IR with centre wavelengths of .8 µm and 2.2 µm as actual emissivity of tar- get material varies; target temperature is 1100°C, emissivity Figure 9: Typical metal: variation of emissivity with wavelength is 1.0.

Calex Electronics Limited Tel: +44 (0)1525 373178/853800 Fax: +44 (0)1525 851319 Online: http://www.calex.co.uk 2000 λ = .8 µm

1800 e, ° F

λ = 2.2 µm 1600

1400 Indicated temperatur

1200 .2 .4 .6 .8 1.0 Target material emissivity Critical Specifications 100 In addition to optics, spectral response, emissivity, temperature range, and mounting (fixed-mount vs portable), the following list of items should be considered in selecting an

ransmission 50

%T : 1. Response time: The instrument must respond quickly enough to process changes for 2 3 4 5 6 7 8 10 12 14 16 proper recording or control of temperature. Figure 11: line represents transmission characteristics of 25 µm polyethylene film (typical of plastics wtih strong 3.43 µm IR thermometers are usually faster than most absorption) while dashed line illustrates 7.9 µm absorption of polyester film other temperature measurement devices, polypropylene, nylon, polystyrene) are opaque steam and water vapour can be guaranteed with typical response times in the 100 ms to at 3.43 µm; other plastics (polyester, (due to the 5.5 to 7.5 µm ); 7.9 1 s range. polyurethane, Teflon, FEP, cellulose, polyamide) µm is ideal for surface measurement, with no 2. Environment: The instrument must function are opaque at 7.9 µm (Figure 11). Some films reflectance. within the range of ambient temperatures to are opaque at both. In the latter case, a choice which it will be exposed. Special provisions may be based on spectral reflectivity, instrument Spectrum for Flame Measurement/ must be made to protect the instrument price and availability, or whether quartz heaters Combustion Optimising from dirt, dust, flames, and vapours. are used in the process (because these heaters While most IR instruments can be used to Intrinsically safe or explosion-proof may cause severe interference at wavelengths measure “dirty” flame temperatures, a clean instruments may be required. shorter than 5 µm). For plastics opaque only at flame (one with no particulate or smoke) can be 3. Physical mounting limitations: The sensing head must fit in the space available to sight 3.43 µm it may be possible to use the weaker, measured at 4.5 µm where CO2 and NOx are secondary 6.86 µm wavelength to avoid quartz opaque, provided these by-products are the object. If this is a hazardous location, heater interference. present and the IR pathlength through the risk can be minimised by using a head which flame exceeds 25 cm. The same instrument contains the fewest parts (i.e., detector and Spectrum for Glass can also assist in combustion optimising, even ambient sensor only) so that a catastrophic The glass industry is one in which the different for smaller flames, because relative readings loss does not require replacement of the factors involved in IR measurement, particularly can be used (absolute readings are not entire instrument. This type of instrument reflection and transmission, must be well required). typically uses a remotely located electronics understood for optimum results. Figure 12 box containing most of the circuitry, which shows the relationship of transmission to Fixed Mount and Portable IR Thermometers can be mounted a safe distance away from wavelength. In general, pane glass is opaque Fixed mount instruments are generally installed a hazardous location. Alternatives include beyond 5 µm, and becomes progressively in one location to continuously monitor or control use of fibre optics, sight tubes, or front- transparent at shorter wavelengths (as a given process. They operate from the local surface to direct IR energy to the evidenced by the human ). The .8 µm power source (110/220 V AC or 24 V DC), are detector. instrument measures several centimetres into aimed at a single point or scan an area by a 4. Viewing port or window applications: If a molten glass, 2.2 µm about 75 to 100 mm. mechanical aiming device. Often they are vacuum chamber, special atmosphere, or Instruments using 3.8 µm will measure no supplied with a portable case and can be moved other process requires measuring further than 25 or 50 mm, depending on the from one location to another. In manufacturing, temperatures through windows into vessels, type of glass, so this wavelength is excellent for a process can be studied by monitoring several care must be taken to ensure that the averaging “gob” temperatures. (These figures points at different intervals. The sensing head window will pass energy at wavelengths are for non-pigmented glass, and it must be can be mounted on a , and the signal measured by the instrument. Glass will pass remembered that glass nearest the surface will output fed to a chart recorder or data logger for wavelengths shorter than 3 µm, quartz .5 to contribute the most to the temperature reading; later analysis. 4.5 µm, selenide from 2 to 15 µm, pigmented glass will be more opaque, even at 4 to 14 µm. Irtran, a series of short wavelengths.) For panes, , and If a truly portable unit is needed, battery- materials manufactured by Kodak, is other thin-wall glass, the longer wavelengths operated IR thermometers are available to available in several different band pass must be used. Reflection becomes critical at 8 match the features of nearly all fixed-mount wavelengths from .5 to 20 µm. to 14 µm; reflectivity averages 15%. This band instruments except control functions. One of the If visible sighting is required as well as can be used with emissivity settings of .85 with limitations of these units is the need for periodic infrared, a window material which transmits good results. Reflectivity is negligible between battery replacement. Generally, their uses have visible energy as well as infrared must be 5 to 8 µm but 5.1 µm is preferred as most of the been maintenance diagnostics, quality control used. The temperature range of temperature sensed is from a few mils beneath functions, periodic spot measurements of measurement dictates the longest the surface, reducing the cooling effect of temperature critical processes, and energy wavelength to be passed, since peak energy surface convection currents. The 5 to 7 µm surveys. wavelengths increase as temperature band is discouraged unless the absence of decreases. 5. Signal processing: Various signal processing devices are integrated to produce outputs to

1.0 interface with displays, recorders, controllers, data loggers, and computers. Displays,

.5 alarm set points, and PC Interfces are ransmission

T commonly an integral part of the IR 0 thermometer. 1 µm 3 µm 5 µm 10 µm 20 µm

Figure 12: Transmission spectrum of glass (typical 3 mm pane glass)

Calex Electronics Limited Tel: +44 (0)1525 373178/853800 Fax: +44 (0)1525 851319 Online: http://www.calex.co.uk Signal processing features include: Integration of reflected energy Various accessories are available to make Maximum reading: a stored value for the compensation: allows calculation based on IR thermometers convenient to use and highest temperature measured. discrete input for unwanted energy received reduce installation costs. For portable Minimum reading: a stored value for the by instrument. instruments, accessories include: carrying lowest temperature measured. case, wrist strap and source. Difference: maximum minus minimum. Output formats: Fixed instrument accessories include: sight Average temperature: the mean of all mV linear or nonlinear tube, air purge collar, water-cooled housing, temperatures measured in a given time mA linear mounting bracket, swivel bracket and period. equivalents alignment light spots. Variable time constant: enables smoothing RS-485 displayed temperature or output in rapidly USB changing temperature measurements. Contact closures for preset alarm points Self-test or diagnostic outputs.

Calex Electronics Limited Tel: +44 (0)1525 373178/853800 Fax: +44 (0)1525 851319 Online: http://www.calex.co.uk