Infra Red Thermometer FAQ

All objects emit infrared energy. The hotter an object is, the more active its molecules are, and the more infrared energy it emits. An infrared thermometer houses optics that collects the radiant infrared energy from the object and focus it onto a detector. The detector converts the energy into an electrical signal, which is amplified and displayed. 

To take a temperature measurement, just point the unit at the object you wish to measure. Pull the trigger and read the temperature on the units LCD. Be sure to consider distance-to-spot size ratio and field of view. 

Infra Red Thermometers are used to measure temperature of moving objects and objects that cannot be approached. The instrument measures the temperature of the object. We however, have more options like the following:

• You can set the upper and lower limits of temperatures and see randomly if the temperature being measured exceeds the limits set.
• The temperature can be recorded with respect to time.
• You can capture the picture of the product with the temperature measured, written over them along with the time and date mentioned on it, to collect evidence.
• A video can be recorded while monitoring temperature of the object over time.
• You can transfer the temperature being measured to the PC and document it for future use.
• The meters also have option to be used as a contact type temperature-measuring meter using a K type thermocouple.
• You can record the maximum and minimum temperature that has been measured.
• The emissivity can be adjusted to measure temperature of almost all types of materials.

The response time of IR thermometers is faster than most thermometers; approximately 0.5 second. 

This is a function of the optics in your thermometer. Use the distance-to-size ratio and the diameter of your target to determine the maximum distance you can be from the target. Most IR thermometers have a maximum measuring distance of approximately 100 feet (30 meters), depending on atmospheric conditions. 

The infrared spectral range is 0.7 to 1000 μm, the range for wavelength in which infrared radiation is transmitted. For cost reasons, IR thermometers generally operate under 20 μm. Most of the IR thermometers that we carry have a spectral response of 8-20 μm. This range is used because it is minimally affected by CO2 and H2O in the atmosphere. With longer, lower-energy wavelengths, the accuracy decreases with increased distances due to the affects of the atmosphere (humidity). 

Emissivity is the ability of an object to emit or absorb energy. Perfect emitters have an emissivity of 1, emitting 100% of incident energy. An object with an emissivity of 0.8 will absorb 80% and reflect 20% of the incident energy. Emissivity may vary with temperature and spectral response (wavelength). Infrared thermometers will have difficulty taking accurate temperature measurements of shiny metal surfaces unless they can adjust for emissivity. 

1. First, measure the surface temperature of the object to be measured with a surface-type thermocouple probe. Measure the same surface with an IR thermometer, adjusting emissivity on the thermometer until the temperature readings on both the thermocouple and IR meters agree.
2. For temperatures up to approximately 500°F (260°C), place a piece of regular masking tape on the object to be measured. Allow the tape to reach thermal equilibrium with the object. Using an IR thermometer with the emissivity set at 0.95, measure and note the temperature of the masking tape. Then, measure the surface temperature of the object. Adjust the emissivity until the temperature of the object is the same as that of the tape. 

It measures the average temperature of the surface within the measuring diameter.

The emissivity information below was determined under ideal conditions. Note: Surface contamination such as dust, oil films, or other agents will affect the actual emissivity of your material.

Download As PDF

Every object radiates thermal energy at temperatures above absolute zero. Measuring the temperature of an object using an optical pyrometer is based on the principle that the thermal radiation from the object being measured is a function of its temperature, and measuring thermal radiation sounds like a very straightforward engineering problem.

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.

Dew point is a measure of atmospheric moisture. It is the temperature to which air must be cooled to reach saturation (assuming air pressure and moisture content are constant). A higher dew point indicates more moisture present in the air. It is sometimes referred to as dew point temperature, and sometimes written as one word (dewpoint). Frost point is the dew point when temperatures are below freezing.

In simpler terms: the dew point, or frost point, is the temperature at which dew or frost will form should the air temperature fall sufficiently. Other things being equal, as the temperature falls, the relative humidity rises, reaching 100% at the dew point, at least at ground level. Dew point temperature is never greater than the air temperature, since the relative humidity cannot exceed 100%.

Wet Bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin. The adiabatic evaporation of water from the thermometer bulb and the cooling effect is indicated by a "wet bulb temperature" lower than the "dry bulb temperature" in the air.

The rate of evaporation from the wet bandage on the bulb, and the temperature difference between the dry bulb and wet bulb, depends on the humidity of the air. The evaporation is reduced when the air contains more water vapor.

The Wet Bulb temperature is always between the Dry Bulb temperature and the Dew Point. For the wet bulb, there is a dynamic equilibrium between heat gained because the wet bulb is cooler than the surrounding air and heat lost because of evaporation. The wet bulb temperature is the temperature of an object that can be achieved through evaporative cooling, assuming good air flow and that the ambient air temperature remains the same.

Noncontact infrared (IR) thermometers use infrared technology to quickly and conveniently measure the surface temperature of objects. They provide fast temperature readings without physically touching the object. You simply aim, pull the trigger and read the temperature on the LCD display. 

Lightweight, compact, and easy-to-use, IR thermometers can safely measure hot, hazardous, or hard-to-reach surfaces without contaminating or damaging the object. Also, infrared thermometers can provide several readings per second, as compared to contact methods where each measurement can take several minutes.

A solid understanding of infrared technology and its principles lies behind accurate temperature measurement. When the temperature is measured by a noncontact device, the IR energy emitted from the measured object passes through the optical system of the thermometer and is converted to an electrical signal at the detector. This signal is then displayed as a temperature reading. There are several important factors that determine accurate measurement. The most important factors are emissivity, distance to spot ratio, and field- of-view.

Emissivity. All objects reflect, transmit and emit energy. Only the emitted energy indicates the temperature of the object. When IR thermometers measure the surface temperature they sense all three kinds of energy, therefore all thermometers have to be adjusted to read emitted energy only. Measuring errors are often caused by IR energy being reflected by light sources. 

Some IR thermometers allow you to change the emissivity in the unit. The value of emissivity for various materials can be looked up in published emissivity tables. 

Other units have a fixed, pre-set emissivity of 0.95, which is the emissivity value for most organic materials and painted or oxidized surfaces. If you are using a thermometer with a fixed emissivity to measure the surface temperature of a shiny object you can compensate by covering the surface to be measured with masking tape or flat black paint. Allow time for the tape or paint to reach the same temperature as the material underneath. Measure the temperature of the taped or painted surface. That is the true temperature. 

Distance to spot ratio. The optical system of an infrared thermometer collects the infrared energy from a circular measurement spot and focuses it on the detector. Optical resolution is defined by the ratio of the distance from instrument to the object compared to the size of the spot being measured (D:S ratio). The larger the ratio number the better the instruments resolution, and the smaller the spot size that can be measured. The laser sighting included in some instruments only helps to aim at the measured spot. 

A recent innovation in infrared optics is the addition of a Close Focus feature, which provides accurate measurement of small target areas without including unwanted background temperatures. 

Field-of-view. Make sure that the target is larger than the spot size the unit is measuring. The smaller the target, the closer you should be to it. When accuracy is critical make sure that the target is at least twice as large as the spot size.

There are important things to keep in mind while using infrared thermometers:

Measure surface temperature only. The IR thermometer cannot measure internal temperatures.

Do not take temperature measurement through glass. Glass has very distinctive reflection and transmission properties that do not allow accurate infrared temperature reading. Infrared thermometers are not recommended for use in measuring shiny or polished metal surfaces (stainless steel, aluminum, etc.). (See Emissivity.) 

Locate a hot spot. To find a hot spot aim the thermometer outside the area of interest, then scan across with an up and down motion until you locate the hot spot.

Watch for environmental conditions. Steam, dust, smoke, etc., can prevent accurate measurement by obstructing the units optics.

Ambient temperatures. If the thermometer is exposed to abrupt ambient temperature differences of 20 degrees or more, allow it to adjust to the new ambient temperature for at least twenty minutes.

Usually infrared thermometers cannot see through water so the instrument will measure the temperature of the water rather than the temperature of the target. You need to consider if the water would be at a higher or lower temperature then the actual target itself.

Usually the problem is emissivity. The portable is not the same wavelength as the on line instrument. You need to set the correct emissivity for both instruments...and it may not be the same value.

If the scale is tightly bonded to the steel it will usually have no effect because it is the same temperature as the hot target. However, if the scale breaks loose it will cool and cause a cold spot on the target. If the instrument is a single wavelength instrument it will indicate a low temperature. If the instrument is a two color and only a portion of the spot is filled with the cold scale it usually will not affect the temperature indication.

Yes, but you have to consider the problem of reflections. Usually the oven is hotter than the target. The target has some reflectivity so the thermometer measures the emitted energy as well as the reflected energy and indicates too high. This can be eliminated by using a sight tube, or possibly measuring the target at the exit of the oven. For glass, plastic films, and paper applications selection of the right thermometer can eliminate the reflection problem without a sight tube or looking at the exit of the oven.

Yes, the window has to be transparent for the wavelength of instrument you are using. For 1 to 2.6 microns we suggest quartz, for 3.4 to 5.4 microns we suggest Calcium Fluoride

No, the infrared thermometers cannot see thru these interferences. However, in most industrial applications the dust and smoke are rising from the hot object and, if your eye was as fast as the IR thermometer you can see openings where the instrument has a clear line of sight. With the aid of a function called a peak picker the instrument can indicate the target temperature and ignore the cold readings caused by the dust and smoke.

All infrared thermometers have a specific target size they need to see in order to measure the temperature. The target should be 2 times the spot size in order to indicate the correct temperature. If the target is smaller than the spot the instrument will measure any thing that is filling the remainder of the target. This is not true of a two-color instrument.

Targets as small as 0.017 inches is possible. The limitation is the temperature and emissivity. As the temperature goes lower or the emissivity is lower then the instrument cannot go as small in spot size.

As the instrument is placed further from the target the spot size resolved by the instrument becomes bigger therefore the target has to be large enough for the instrument to view it.

For instruments that operate from the visible to 2.6 microns the color will usually change the emissivity. For wavelengths longer than 3 microns the color will not affect the emissivity. However, color does affect heating. Dark objects will get hotter than light colored objects.

You need to check for: A. Incorrect emissivity B. Is the instrument seeing reflections from a hot surrounding source C. Is the instrument being affected by electrical noise entering into the cables or other wiring.

You need to check for:

• Incorrect emissivity
• Dirty lens or window
• The instrument is not focused correctly
• The line of sight is blocked with an obstruction.