8/11/2019 Calculating Emissivity in the Infrared Spectrum

1/2

Home Products Software Services Videos Papers Support Contact Us

Technical Support

Emissivity in the Infrared

Methods of Calculating Emissivity

Optotherm Thermalyze software has the ability to compensate for emissivity so that accurate temperature

measurements can be made of materials with emissivity below 1.00. The accuracy of the measurement, however, is

determined by the precision to which the emissivity value and ambient temperature are known. Additionally, the

temperature of objects in the environment must be uniform. Radiance from objects that are hotter or colder than the

surroundings can reflect off of the target and affect the accuracy of emissivity compensation.

Small changes in an objects emissivity can result in noticeable affects on measured temperature. A 0.02 reduction in

emissivity, for example, can decrease the measured temperature of an object at 100C by approximately 2C. Likewise,

variations in the ambient temperature can affect measured temperature. An increase in ambient temperature of 5C,

for example, can increase measure temperature of an object at 40C with emissivity of 0.80 by approximately 1C.

In order to compensate for the emissivity of an object, its emissivity must first be determined. There are two basic

approaches to determining surface emissivity; surface treatment or material heating. Surface treatment involves

applying a treatment that is of a known high emissivity (usually tape or paint) to the surface of the object and then

heating the surface. Material heating involves uniformly heating the object to a known steady-state temperature that

is above ambient temperature. During both procedures, best results are achieved when the object is heated to a

temperature close to the temperature at which measurements are to be taken during testing. If performed properly,

correct emissivity values can be obtained using either approach. The chosen method will depend on the

characteristics of the surface and size or shape of the object.

Surface Treatment

This method should be employed when the objects size and shape facilitates applying a small section of masking

tape. Masking tape is the preferred surface treatment for object temperatures below 100C due to its uniform

emissivity (0.95) and thickness. Alternatively, a thin dab of paint or white-out can be used on objects with small or

uneven surfaces where tape cannot be applied. The disadvantages of using paint or white-out are the possibility of

deviations in coating emissivity and thermal diffusion due to variations in application thickness. If care is taken during

the application of the coating, however, uniform results can be obtained.

To determine an objects emissivity using the surface treatment method, follow these steps.

Apply a small section of masking tape to the area of interest making sure to leave a section of the original

surface exposed.

1.

Heat the surface to a temperature that is below 100C. Heating can be accomplished by different methods

including powering the device or heating the surface using a heating plate or hot air gun.

2.

Capture a thermal image of the heated surface. Note: Make sure that the heating source is not reflecting off of

the exposed surface when the image is captured.

3.

Draw a small region enclosing the tape and a second small region enclosing the exposed surface.4.

Set the emissivity of the region enclosing the tape to 0.95.5.

Adjust the emissivity of the region enclosing the exposed surface until the temperatures within both regions are

equal. Record the emissivity of the object.

6.

Material Heating

This method should be employed when tape or paint cannot be applied to the surface due to an objects small size or

surface characteristics. Material heating can also be used to determine the emissivity of different materials comprising

a complex object with many different surfaces.

Emissivity

Technology

Links

Planck's Law

What is Emissivity

Physics of Emissivity

Effects of Emissivity

Emissivity Examples

Increasing Emissivity

Calculating Emissivity

Emissivity Tables

Microbolometers

Tutorial Videos

lating Emissivity in the Infrared Spectrum file:///C:/Users/JOSE/AppData/Local/Temp/maftemp-55562986/

2 25/08/2014 0

8/11/2019 Calculating Emissivity in the Infrared Spectrum

2/2

To determine an objects emissivity using the material heating method, follow these steps.

Heat the object to a known uniform steady-state temperature. One of the most common methods of heating

small and thin objects, such as semiconductors chips, is using a heating plate. A thermal chamber can also be

used provided there is an opening or infrared window on the chamber through which to image the object.

1.

Measure the steady-state temperature of the object by measuring the temperature of a high emissivity area in

the thermal image or by using a contact temperature probe.

2.

Draw a small region enclosing each different surface to be measured.3.

Adjust the emissivity of each region until the temperatures within the regions are equal to the temperature of

the object measured in step 2. Record the emissivity of each different surface.

4.

A Note about Contact Temperature Probes

If used in appropriate situations and applied correctly, contact temperature probes such as thermocouples,

thermistors, and RTDs can be used to accurately measure surface temperature. Small objects and thin surfaces,

however, may not contain enough thermal mass to accurately measure using these devices. In these cases, contact

probes can act as heat sinks and lower the temperature of the material, creating erroneous readings. Also, a good

thermal bond must exist between the material and contact probe in order to transfer sufficient thermal energy to heat

the probe to the same temperature as the material. In many cases, poor thermal bonding results in erroneous

temperature measurements that are much lower than true temperature. Measurement errors due to low thermal

mass and poor thermal bonding can result in errors as great 10, 20, or even 30C when measuring an object at 60C.

watch our product and application videos on

Copywrite 2001-2014 Optotherm, Inc., United States About Us Recent News Privacy Policy

lating Emissivity in the Infrared Spectrum file:///C:/Users/JOSE/AppData/Local/Temp/maftemp-55562986/

2 25/08/2014 0