Alfa Exx - Whitepaper on IR-images

1. What is it you see in an Infrared Image?

You see temperature.

Different temperatures are represented by different colors or by different shades of gray. Only if there is a difference in temperature between background and objects you will be able to distinguish them.

If background and objects in a scene have exactly the same temperature you will not be able distinguishthe objects from the background. This never happens in reality, but since the contrast in your IR-image so deeply depends on temperature differences there are other things that influence the quality of your IR-image than ordinary visual images.

2. What influences the contrast in an Infrared image?

The radiation of the sun during the day and the (heat) emission during the night are together the engine that creates the primary contrasts in an IR-image.

3. The sun -The great contrast maker

Most things in nature like trees, rocks, grass, ground, etc do not generate heat themselves. They depend on the sun to heat them up. This is done at different speed due to differences in material parameters as heat capacity and emissivity. Heat capacity determines how fast, or slow, a material heats up or cools down and emissivity tells how well a surface is absorbing or radiating thermal energy.

Other things than the sun can also radiate heat, a fire, a hot engine, an animal or even a heap of decaying biomass and of course do these things heat up closely located objects, but not very much in comparison with the sun.

So if you look at a landscape in infrared you will mainly see the temperature differences that the sun has produced.

4. Contrast dips, twice a day...

When the night comes the sun of course disappears and if there are no clouds everything outdoors is exposed to the naked sky which has a temperature of approximately -273 °C. All objects cool off at different speeds depending on the same material parameters that made them heat up at different speeds. This will create a special swap effect in temperature differences, so that things that actually were hotter than other objects before will become colder. This also implicates that at some point during the night things will more or less have the same temperature and the contrast in an IR image will be poor for a while. The same thing will happen during the day.

5. Clouds, fog and rain, all three factors influence contrast

A cloudy sky will shelter from either the cool space or the hot sun. The heating up or chilling down will go much slower and differences in temperature contrasts will be less. Long periods of cloudy skies and even temperatures will kill most contrasts in an infrared image.

…and rain affects contrast even more

Due to the rinsing effect and the fact that water has very big heat capacity, rain tends to give its own temperature to everything else that comes in its way. This of course removes or diminishes contrast in an IR-image or at least lowers contrast considerably.

Fog does the same thing to object temperature as rain but at a slower rate. It also greatly affects transmission through the air. Depending on drop size the fog is more or less transparent to infrared radiation. In almost every case fog means very poor IR-images, but on very rare occasions, fog can actually be like milk for the naked eye and almost totally transparent for the IR-camera.

6. Low Emissivity Objects

Some objects have very low absorbance, or emissivity, and can almost be considered to be a mirror in the infrared waveband. This kind of objects does not really show their own temperature in an infrared image, they mainly show a reflection of the heat of nearby objects or even the sun. So it is difficult in infrared to measure the temperature of reflective objects merely by looking at their IR image.

One should also remember that things that are not reflective in the visual region might be highly reflective within the Infrared region, and vice versa. Ordinary glass as an example, is not transparent in the infrared area, but white plastic polyethylene bags are more or less totally transparent.

7. Wave bands and Transmissions

Infrared radiation which propagates through the atmosphere experiences strong absorption in some common gases that are present in air. Among the strongest absorbers are water and carbon dioxide. These gases exhibit extremely strong absorption throughout most of the infrared wavelength range. There are some gaps where infrared radiation is not totally absorbed. Usually one divides infrared radiation into three bands, short wave (SWIR 1-2.5 um), medium wave (MWIR 3-5 um) and long wave (LWIR 8-12um) infrared radiation. Fig. 6 shows a typical spectrum where these atmospheric gaps are present.

Every object above the absolute zero (-273°C) acts as a source of radiation. Planck’s law of radiation shows that the radiation peak of an object is located at a wavelength that depends on the temperature. Really hot objects like a jet motor etc will best be seen in the SWIR since it radiates most in that atmospheric transmission gap. Objects in nature like animals as well as ordinary manmade structures usually emit in wavelength regions where LWIR-cameras is the best choice.

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7. Wavelength Bands and Transmission, continued

8. Unexpected Effects

In an IR image one can sometimes get unexpected effects and it may be of help to know about some of them in advance. Roofs like a carports or similar, will shelter from the sky and the heating or cooling of the area beneath it will more or less depend on air temperature. Some examples are areas saturated with water which may look very much different from the surroundings, heat from engines under cars usually reflects in asphalt, ice under a bridge sometimes show an image of the hotter bottom of the bridge, bulged areas can work as a mirror lens and focus heat radiation from any source into a spot. Humans can best be detected by heat from uncovered skin areas like the head and the hands. The hull of metallic boats doesn’t show very well in IR, while their engines do as well as their hot exhaust pipes and sometimes the ventilation shacks of the vessel. Glass windows will act like a mirror while acrylic windows will transmit almost all IR radiation.

9. Examples of Images at different Wavelengths, NIR

NIR

Most ordinary cameras used for visible wavelength are also sensitive to IR but provided with a IR filter. If you remove this filter then you will have a camera that is also sensitive to NIR. Continue.

9. Examples of Images at different Wavelengths, LWIR

LWIR all roomtemperature or temperatures in nature will show best at LWIR. Continue.

9. Examples of Images at different Wavelengths, VLWIR

VLWIR

Really cold environments like the (Ant)Arctic regions are best seen in VLWIR. Satellites sometimes use these wavelengths.