What is Infrared

A short and sweet course on Infrared and Infrared thermography

The technology of Infrared Thermography is utilized typically to very rapidly infer or confirm the presence or absence of structural or functional defects in buildings, electrical or mechanical systems or sometimes in living critters. The thermal imaging instruments that we use in this enterprise are sensitive to only one thing: Thermal or heat energy radiation or patterns of thermal radiation that occur on the surface being examined (that many times are indicative of what lies underneath).

FLIR T300 high resolution Thermal Imager utilized by Southern Infrared Services

In order to have an idea what we are looking at with one of these devices, it is necessary to understand some basic stuff about heat and how it moves around. So, consider the following observations from the physical sciences:

Thermal or heat energy is conveyed from a point of higher to relatively lower energy level in three different but partially related ways:

1. Conduction (direct contact with another object),
2. Convection (direct contact with a gas or fluid which is heated and then moves to a cooler area or space drawing cooler air in to take its place) and
3. Radiation (the movement of waves of thermal energy through space at light-speed away from a relatively hotter source)

Conduction (Direct Contact)

Convection (in Gas or Fluid)

Radiation (Via Space)

The frequency of occurrence or distance between waves (wavelength) of thermal energy radiation occurs below that of visible light and is therefore invisible to the human eye. The band of frequencies or wavelengths in which thermal energy radiation occurs is called infrared as can be seen in the vivid illustration below that contains the entire electromagnetic spectrum (infrared = below visible red). The Thermal Imager is designed to “see” thermal energy radiation in the infrared region of the electromagnetic spectrum.

Courtesy NASA/JPL-Caltech

The mechanics of creating an image we can see from infrared region radiation is shown in the diagram below:

We might say that the thermographer’s job would be pretty simple (just point the thing and look at the image), and it is, compared to understanding the design principles and physics of the sensor within a thermal imaging camera. However, consider that the apparent heat (or lack of heat) shown by the imager varies with:

• Temperature (actual contact measured temperature)***
• Surface finish of object (dull or painted any color will appear much hotter than a shiny metal!! – A shiny surface actually radiates less!*!)
• Viewing Angle (Shiny/smooth surfaces can reflect heat/cold radiation from other places that mask what is actually being radiated from the surface – as in cold reflections from the sky off a smooth rubber roof)
• Surface shape (Round stuff will have maximum radiation and appear brighter at places that are perpendicular or square with camera lens)
• Wavelength (longer wavelength or lower frequency radiation appears cooler and shorter wavelength or higher frequency radiation appears warmer)
• Thermal Imager frequency response (Short-wave vs long-wave imagers: One or the other is better at certain applications)

And we begin to get the idea that it just isn’t that simple –

 We can see the radiation peculiarities described above very clearly in the examples below of three gallon paint cans filled with ice water, tap water and water heated to approximately 150 degrees left to right respectively.

The three cans have black and yellow electrical tape stripes on either side of a bare metal center as shown in the digital photo. The differences in radiation due to differing temperatures as well as varying surface type are very clear. You can see better than 23 degrees difference between the shiny metal and tape on the hot can at right and both will measure the same temperature! Less obvious are the cold/warm reflections on the sides of the center can from the adjacent cans – look closer and you will see this on the center can of the image at the right –

Note that the color palette can be changed to suit the best presentation of the situation (left side image below). The temperature scale at the image right side can be changed as well (compare scales on the two images).  

Note that color of surface (tape) has little to do with radiation level (less than 0.1degree in the last two photos).

Given what you know now, you can see that your thermographer needs to be acceptably versed in basic heat transfer principles and the peculiarities of thermal imagers as well as having a good understanding of the underlying structure of the object being viewed before making any credible pronouncements about what the imager reveals – Superior imager resolving capacity is very desirable to provide clear and distinct images, however, the capacity of the image interpreter is really the key -

***Note that temperature is a measure of the amount of heat energy that an object has (higher = more). Some more sophisticated thermal imagers are capable of temperature measurement – the EX320 is one of those. Accurate temperature measurement is sometimes necessary in industrial predictive maintenance inspections for data trending but rarely required for energy audit or moisture location purposes.

Pop Quiz

(I told you so): Would the shiny metal flue pipe from your garage natural gas fired water heater heat the garage more or less than if you had painted it red, black or white? What would happen to the garage temperature if you covered your now painted flue pipe with aluminum foil? Shiny side in or out?

Do steam radiators work better as room heaters if they are painted silver or white? Or brown? or black?

If you want to keep the roast or turkey hot longer after it comes out of the oven, would you wrap it in aluminum foil shiny side in or out?

For the Trekkies out there we thought you’d also like to see this amazing infrared image of a section of our night sky that is Courtesy NASA/JPL-Caltech: 

 For more information about how Southern Infrared Services can help you call 404-788-2581 today.