Thermal Camera: What is Black Body?

A black body is one of those concepts in physics that is extremely useful in thermography. In fact, it is a cornerstone of infrared imaging. Keeping a tab on the black body radiation of an object has expanded infrared imaging to greater heights far more than we can imagine. 

For instance, by keeping black body radiation as a reference, astronomers can determine the birth of stars and the formation of a black hole in space. Indeed, it’s not an everyday concept. But taking time to wrap your head around the nitty-gritty of a black body can help push the envelope in your thermal camera detection journey. 

A more widely-used term when it comes to thermal imaging is emissivity. Simply put, emissivity is just the measure of an object’s ability to emit infrared radiation. Every object on the planet from the hottest insides of a raging volcano to the coldest chunk of ice in the Arctic Circle has an emissivity ranging from 0 to 1, with 1 as perfect emissivity. What many don’t know is that emissivity is a function of black body radiation. 

 

Black Body, Max Planck, and Albert Einstein

Simply put, an object is deemed as a black body if it absorbs all instances of incoming radiant energy (i.e., light) hitting it without reflecting any of said energy. The terminology arises as incident light is totally absorbed rather than reflected in any small way. Such a surface therefore will appear black. Technically, however, a perfect black body is theoretical in nature. 

The concept of a black body has been instrumental in the study of radiation of electromagnetic energy. Foremost of this is Planck’s Quantum Theory which explains how energy over a wide array of wavelengths (i.e., electromagnetic spectrum) is reemitted after being absorbed. 

 

Electromagnetic Spectrum

FIG. 1: The Electromagnetic Spectrum 

Thus, by way of demonstration, a practical blackbody is a small hole in a box with an interior that has been blackened. As no light that enters that hole ever escapes, it shows how light is absorbed by a perfect black body. The blackened interior helps hold the light in. In reality, a solid surface covered with lampblack or soot is bound to absorb approximately 97% of all the light making it a near-perfect black body. 

On the other end of the spectrum, a polished metal reflects about 94% of the light that hits it. As it absorbs but 6% of the light, such a metal is a poor black body. Again, it must be noted that a perfect black body is largely hypothetical. As such it would be able to not just perfectly absorb light energy but also perfectly emit such energy. Most objects on the planet are considered grey bodies, unable to absorb all of the radiant energy that befalls them. 

The idea of a black body and blackbody radiation has been around for centuries but scientists were baffled about its nature. Specifically, it was Gustav Kirchoff (1824 - 1887) in 1860 who conceptualized and coined the word black body. But for one, the German physicist himself was confused about how the intensity of the electromagnetic radiation as emitted by a black body depended on the temperature of a black body and the frequency of the radiation. 

Many scientists tried but failed to establish a clear relationship between a black body and blackbody radiation. Although many experiments were explored, no one singular theory was able to explain the experimental values achieved. Wein’s theory by Wilhelm Wein (1864 - 1928), another German physicist, correctly predicted radiation behavior at high frequencies but failed to explain behavior in lower frequencies. 

At least not until Max Planck (1858 - 1947), a Berlin Physics professor, dipped his finger into the fray. Studying his predecessors, the German theoretical physicist, perhaps the first true blue theoretical physicist in history, introduced quantum energy to the world at the turn of the century in 1899. The concept of an indivisible quantum unit of energy earned him the Noble Prize in Physics in 1918. And then some. 

Not only did the quantum theory of energy explained the nature of black body radiation but it also revolutionized the science of physics. So much, quantum physics became a cornerstone of Albert Einstein’s Special Theory of Relativity in 1905 which in turn shook the foundations of science to the core. 

Einstein and Planck were good friends. Oftentimes, Albert would visit Max’s home and play music. Both played the piano. To note, all of the German scientists mentioned here have built their contributions on the basis of scientists before them. All were awarded at least one Nobel Prize for science

 

Blackbody Radiation and Emissivity

Blackbody radiation is the thermal radiation given off by an object at a particular temperature. As this radiation comes off from a mass, it allows the detection of the temperature of the object under study. The radiation, of course, becomes the basis of thermography to produce an image and specify the heat signature. 

But there’s a factor that must be considered for thermography to be considered accurate. To boot, emissivity must be taken into account. As defined, emissivity is the relative ability of a material’s surface to emit heat by radiation. In this regard, a perfect black body has perfect emissivity. Its ability to emit absorbed heat in the form of radiation is total. Every object must therefore be compared to a black body’s emissivity to know how effective is it in emitting thermal radiation. In equation form: 

 

Emissivity () =  Radiant energy of an object / Radiant energy of a black body 

It must be noted that both the black body and the object must be at the same temperature to arrive at the accurate emissivity of that particular object. And as the radiant energy of a black body is total, the radiant energy of an object should be much lesser than that of a black body. Therefore, the emissivity of an object takes on a range from 0 to 1. 

A black body has an emissivity of 1. On the other hand, a perfect reflector of energy has an emissivity of 0. Most objects on the planet are “graybodies’. Meaning, they can but emit a fraction of their total blackbody radiation. 

The emissivity of a Perfect Black Body

Figure 2. The emissivity of a Perfect Black Body. 

As such, water has a near-perfect emissivity or close to 1. The same holds true for most vegetation. However, metals and minerals have a poor emissivity of less than 1. To give you an idea, below are some of the emissivities of the most common everyday things: 

Material 

Emissivity

Ice

0.97

Water

0.95

Soil (Saturated)

0.95

Soil (Dry)

0.92

Concrete

0.92

Glass 

0.92

Sand

0.9

Snow

0.8

Aluminum (anodized) 

0.77

Aluminum (polished) 

0.05

 

You might be surprised why aluminum has two emissivities. Take note that the emissivity of any material is dependent also on the nature of its surface. Polished aluminum metal has a poorer emissivity than that of an oxidized yet rough metal surface. 

It’s important therefore that we take into account emissivity. It’s definitely possible for two objects to have the same temperature but different temperature readings in the thermal camera’s output because of their disparate emissivity. 

The lesser the emissivity of an object the less accurate your thermal reading would be of the object’s temperature. Lower emissivity, therefore, means less ability for the object to tell its true temperature. You will have to find ways to adjust the emissivity of said object to do a better job of finding its temperature. 

The good news is most maintenance inspection work is comparative and qualitative in nature. This means there is no need to know the exact temperature measurement of an object in question. A good rule of thumb is to observe that objects that have an emissivity of 0.6 or lower won’t be able to give you highly reliable results. Usually, shiny metals exhibit such poor range. 

 

To the Stars, Through Black Body

Indeed, blackbody radiation has not been essential only within the planet’s hemispheres, but it is also left, right and center in the study of celestial bodies or astronomy. Take note that planets and stars are not perfect black bodies. Moreover, they are not in thermal equilibrium with their surroundings. But blackbody radiation is instrumental in measuring the energy celestial bodies emit. 

Black holes are the closest celestial phenomena to black bodies. Think of a black hole as a super magnet of a galaxy. It’s a place where gravity is so strong that nothing escapes it. If there was a list of the mysteries of the universe, the ins and outs of a black hole will have to appear on top of the list. 

And yes, Einstein’s general theory of relativity has been successful in predicting the behavior of black holes. That’s telling you how pivotal a black body is. 


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