THERMAL SCIENCE AND THE

PRACTICAL USES OF THERMAL IMAGING

There are two “windows” where infrared radiation transmits well. The mid wave IR range is 3-5 Micrometers and the long wave IR range is 8-12 Micrometers.

IR RADIATION CLASSIFICATION WAVELENGTH RANGE

VISIBLE .4-.75 Micrometer

Near IR*Military .7-2.0 Micrometer

Mid Wave* High End Inspection & Military 3 – 5 Micrometer

Long Wave*** Inspection Range 8 – 12 Micrometer

STEPHAN-BOLTZMANN LAW

The amount of electromagnetic radiation emitted by a body is directly related to its temperature.

According to the Stephan-Boltzmann equation, small increase in temperature of a radiating body results in a large amount of additional radiation being emitted.

This is the fundamental concept of how thermal cameras works

Thermal Radiation Properties to Know

EMISSIVITY

ABSORPTION

REFLECTION

TRANSMISSION

All surface materials have these properties

EMISSIVITYEmissivity is the ratio of the actual amount of electromagnetic radiation emitted by an object. This is a calibration that you will need to know to measure more accurate temperatures. Different material surfaces emit radiation differently. The more reflective the surface the lower its emissivity rating. The duller and blacker the material, the closer to emissivity of 1.0, which provides the best readings Most cameras have emissivity adjustments, but how to compute emissivity of the material. More training is needed for this knowledge.

ABSORPTION

Objects will absorb thermal radiation as well as visible radiation. Absorption is the reverse of emissivity in this

matter.

The duller and blacker the surface, the greater amount of radiation absorbed. A high reflective surface will absorb

lower amounts of thermal radiation.

EMISSIVITY = ABSORPTION

REFLECTION

Objects will reflect thermal radiation energy at the surface. What thermal radiation is not absorbed, will be reflected.

How much reflection depends on the surface finish.

Now a change to the previous terms, the duller and blacker the surface, the lower amount of thermal radiation

reflection. A high reflective surface will reflect a large amount of thermal radiation energy.

Now when you measure for temperature, are you calibrating

for reflective radiation? You better!

TRANSMISSION

Some objects will allow radiation to pass through the material like light through glass. This transmission of radiation energy will effect absorption and reflective

calculations. Its when you add absorption + reflective + transmission that you account for 100% of the thermal

radiation energy. Important note- MOST material is not transmissive, meaning IR will not pass through. Paper, paint, and window glass are

some of the common materials in a home.

The properties of thermal radiation energy will determine how well you read your thermal camera for images, and what you see on the screen may not be what is actually happening.

An object may appear cold that is very hot and vis- a – versa. Take a look at the surface material and using the four properties , you can get a better idea of what is truly going on.

WHAT IS HEAT TRANSFER?

Heat transfer describes the flow of heat energy from a body of higher temperature to one of lower temperature.

Heat flow is characterized by a thermal energy per time unit – BTU or WATT/hr

THREE TYPES OF COMMON HEAT TRANSFERSRADIATION

CONVECTIONCONDUCTION

Conduction is the transfer of heat energy through a material (solid, liquid, or gas) by the motion of adjacent atoms and molecules without gross displacement of the particles.

Heat conduction is the only method of heat transfer that occurs in a solid material. Length or thickness of the material will effect conductivity.

With thermal cameras, thermal gradients can be observed with conductivity heat transfer.

CONVECTION – IS THE HEAT TRANSFER DUE TO BULK MOVEMENT OF A FLUID (GAS OR LIQUID)

Convection is a very common form of heat transfer. There must be moving gas or liquid to transfer heat, and there are two types; Natural and Forced.

Convection heat will provide thermal patterns of air flow or liquid flow. Common for building thermal scans.