Infrared is energy waves emitted from any object which is invisible to the naked eye but can be felt physically felt as heat. This energy wave has a length (wavelength) from 700 nanometers (frequency 430 THz), to 1 millimeter (300 GHz) in the visible light spectrum.
To first understand what is infrared we must begin with its origins.
Infrared radiation has given humanity the ability to see beyond what the human eyes can see and explore beyond where any man has set foot before. From a military perspective to a civilian’s, the application of infrared technology is too useful to ignore. It’s no coincidence that someone who wanted to have a clearer view of the heavenly bodies thousands of miles above stumbled upon infrared light. It’s this inquisitive mind that has propelled man to shine a light on some of the darkest scenarios of human ignorance.
Looking into the nature of infrared light is largely advantageous on our part. It gives us a better vantage point in infrared tech’s application. Such knowledge should allow us to extend the uses of thermal technology to the most challenging of man’s endeavors even to the farthest corners of the Earth. Or beyond Earth. As man is expanding his reach to the distant corners of the universe. The world-renowned astronomer Sir William Herschel, the scientist who stumbled upon infrared light in the 1800s, may not have seen it. But his discovery paved the way for the conquest of our known universe, in more ways than one.
To boot, know that the nature of light goes beyond what our human eyes can see. The rainbow may be the most splendid demonstration of how beautiful light is, but it’s not a full picture. Not quite. Ultraviolet rays and infrared radiation cannot be seen in such a vibrant array of colors.
Interestingly, Sir Frederick William Herschel (1738-1822) started his career as a musician in the footsteps of his father, an accomplished musician. Born in Hanover, Germany, William joined the Hanoverian Guard band playing the oboe. A teaching position in music allowed him to move to England in 1755. He settled in Bath.
But it was his hobby that made William a worldwide sensation. Like the greats before him (Sir Isaac Newton, Galileo Galilei), William was immensely attracted to the study of the heavens, or astronomy. At the age of 34, the German immigrant bought a book on astronomy along with a small telescope. That small act would prove to be the turning point of the music teacher’s life.
Herschel studied Newton’s telescope designs and improved on them, casting and polishing mirrors on his own. In the process, he produced much more powerful telescopes allowing him to go “farther” and investigate heavenly bodies in our known universe.
Then came a breakthrough that would make Frederick William Herschel the private astronomer of King George III, the king of England himself. William discovered the planet Uranus in 1781, working by himself.
Eventually, he was knighted and admitted as a Fellow of the Royal Society. Honor and financial backing followed. Eventually, he became the first president of the prestigious British Royal Astronomical Society founded in 1820. Today he along with his sister Caroline is considered one of the pillars of modern astronomy, employing a thorough mathematical approach to the study of the stars.
But Sir Frederick William Herschel’s contribution to humanity did not end in astronomy. The study of the stars fueled an even more practical invention: thermal technology.
In 1800, William began to experiment with the sun’s rays. He used different filters and saw how various colors would be generated with each filter.
Finally, he used a prism much akin to Newton’s experiments centuries earlier. This time he used thermometers to record the temperature of each color of the rainbow produced, a phenomenon later named the electromagnetic spectrum.
Herschel observed that there is an increase in temperature when moving the thermometer from the violet portion to the red part of that rainbow.
Surprisingly, he found out that the region beyond the red portion of the rainbow produced from the sun’s light recorded a much higher temperature. As this light is beneath the red light of the electromagnetic spectrum it was called “infrared light”. The region mentioned is invisible, producing no visible colors. But as he could feel the heat rise in that portion, he called it “radiant heat”.
To note, many contemporary British scientists opposed Herschel’s joining of light and heat as one entity. At this point, the idea of the electromagnetic spectrum has not been hatched. Even William was not too sure about the relationship between the two entities. Eventually, he returned to astronomy, leaving his work on the nature of light somewhat inconclusive.
Today, the European Space Agency’s infrared observatory is named after Herschel.
The Nature of Infrared Radiation
Infrared (IR), oftentimes called infrared light and infrared radiation, is electromagnetic radiation (EMR) that has wavelengths longer than visible light.
Understand that infrared light is invisible to the human eye. A rainbow is the full spectrum of electromagnetic energy produced when light strikes water droplets, much like light passing through a prism. Each color represents different wavelengths.
However, like Heschel’s experiments, a rainbow won’t show infrared light. Infrared is present but is invisible. You only see the colors of the rainbow from the order of their wavelength, longest to shortest:
- Red
- Orange
- Yellow
- Green
- Blue
- Indigo and
- Violet
That list goes to show that ultraviolet rays from the sun which are also invisible to the human eye are shorter in wavelength than the long-wavelength infrared light.
The cones in our eyes can receive radiation within the spectrum or 380 to 700 nanometers. That’s why we see red to purple in a rainbow and all those colors in between.
The wavelengths of infrared radiation (IR), however, are longer than 700 nanometers. It can go as high as one millimeter. But there’s a slim chance that you can see infrared radiation.
Think of the EM or electromagnetic spectrum. A portion of it is visible; the rainbow shows us this (so does the visible light in the prism experiment). Violet is the part of the visible spectrum with the shortest wavelength. Beyond that is ultraviolet. Ultra is Latin for “beyond” or “above” so ultraviolet is above violet in the EM spectrum. And as ultraviolet’s wavelength is shorter than 380 nanometers, we can’t see it.
On the other end of the EM spectrum is the color red. It has the longest wavelength in the visible spectrum. Next to the color red is the infrared spectrum. Infra is Latin for “below” so infrared is literally below red. And as its wavelength is longer than the visible spectrum, we can’t see it too.
However, there are certain conditions when infrared radiation has been visible. IR at wavelengths of up to 1050 nm has been observed from specially-pulsed lasers. The specific scientific explanation for this has not been totally explored.
So technically, IR wavelengths are from 700 nanometers (frequency of 430 THz) to wavelengths of 1 millimeter (frequency of 300 GHz). To note, in near-room temperatures, most if not all of the emitted thermal energy is infrared radiation. Like all the waves in the EM spectrum, IR is radiant energy making Herchel’s theory of radiant energy spot on. Additionally, it also behaves like its quantum particle does, the photon.
This might explain the circumstances of visible infrared light. Experts believe infrared radiation can be seen by the human eye if more than one infrared photon hits the eye all at once.
The presence of infrared radiation therefore connotes the presence of heat. The hotter an object is, the more infrared radiation is. The sun with its intense heat has far longer infrared waves. Another source of intense heat is fire.
Take note that all objects on Earth emit infrared light in the form of heat. Conversely, man has learned to harness infrared light. When you use the remote control for your smart TV, you are using infrared light, or what Herschel described as “radiant heat.”
In Comparison to Other Radiant Energy
Infrared radiation is a part of the bigger electromagnetic (EM) spectrum which includes all types of EM radiation. To note, any radiation has the characteristic of spreading out. So the light from your lamp is EM radiation (visible light) while the music that comes out of your old-school radio is also another EM radiation.
Infrared radiation itself is subdivided into three main portions depending on the particular wavelength. Their bandwidths are:
- Near-infrared (NIR): wavelengths of 0.75 – 3 μm
- Mid-infrared (MWIR): wavelengths of 3 - 50 μm
- Far-infrared (FIR): wavelengths of 50 - 1000 μm
These figures are as specified by ISO 20473. However, astronomy and other international bodies observe a different division scheme.
Right next to infrared radiation is microwave radiation, made popular by microwave ovens used to cook popcorn at homes. Microwave radiation has wavelengths ranging from approximately one meter to one millimeter, carrying frequencies ranging from 300 MHz (1 m) to 300 GHz (1 mm).
The micro in microwaves denotes it has shorter wavelengths in comparison to radio waves which follow microwave radiation in the EM spectrum. Radio waves carry frequencies as low as 30 hertz to as high as 300 Gigahertz.
Note that boundaries between EM regions are fairly arbitrary and may differ in various forms of study. As it belongs to one electromagnetic spectrum, various forms of radiation fade into one another and are interconnected like the colors of the rainbow.
X-ray radiation is found next to ultraviolet radiation in the EM spectrum. As such, its wavelength which ranges from 10 picometers to 10 nanometers is even shorter than that of UV rays. X-ray however is such a powerful penetrating energy.
Lastly, gamma rays, a most powerful energy form, has wavelength even shorter than that of an X-ray. So powerful is its photon that gamma rays have been extensively used as a destructive weapon on nuclear bombs.
NASA believes the biggest generator of gamma rays is the universe itself.
On the other hand, infrared light coming from the sun is responsible for 49% of the heating of the Earth.
But how about the rest of the heat reaching the planet?
To note, infrared is not the only source of “radiant heat”; all electromagnetic bandwidths heat all surfaces that absorb them. Thus, the rest of the heat affecting Earth is caused by visible light that is duly absorbed then radiated once again. This time at longer wavelengths.
In Comparison to Night Vision
Night vision is the stuff of wonder. Featured in many films, night vision goggles and its ability to see in the dark has captured the imagination of many movie buffs.
But how does night vision compare to infrared thermal imaging technology? You may be curious to know as you’d like to see which device can help you drive better at night.
Night vision has been exclusively used in the military for some time there. Today, however, there are smartphones that are capable of night vision technology.
Know that night vision uses the same technology as your standard camera with one exception. Night vision cameras work at greater magnification. So these are capable of absorbing the slightest presence of light in a dark setting. Once captured, this light is then magnified then portrayed as images with greenish hues.
Thus, night vision can be better described as “image intensifiers”. And this is where night vision and thermal infrared technology differs.
Night vision would be rendered useless in pitch-black darkness without the smallest ray of light. Its effectiveness drops precipitously with the decrease of the presence of light. So if a military operation using night vision is done with the moon blocked by dark clouds, visibility can be compromised.
But not if a thermal camera can’t help it.
Thermal cameras use infrared scanning technology. As such, it is capable of measuring the heat signature of objects in front of them. In turn, these heat signatures are translated into images indicating which part is the hottest and which are the coolest. The hotter the object, the brighter it is represented in the thermal scan.
Now, this all means that a thermal camera doesn’t need light to know the presence of an object. Since it’s capable of detecting heat or infrared radiation, it won’t need visible light to know the presence of objects. So, even a most violent storm or the most strenuous weather won’t affect its ability to determine a temperature from a distance.
So even on a pitch-black night, a thermal camera would be able to determine the presence of objects. Or for that matter, the presence of a living individual in a picture.
Even better, thermal scanning technology today is capable of scanning a thousand feet ahead and even more. This makes it a more ideal tool when driving at night compared to night vision.
Most Popular Uses of Infrared Radiation Today
Many technologies use the science behind infrared radiation. The most prominent today is thermal imaging technology.
Infrared Thermography (IR T), also dubbed as thermal imaging or infrared imaging, has been used to determine the temperature of objects from a distance. Initially, this technology was maximized upon by the military. Then it trickled down to firefighting and to industrial purposes. After some time, infrared tech eventually reached the masses.
Today, infrared imaging technology, thanks to much-lower production costs, has become a standard tool in building and home inspections. This is in the form of handy portable cameras dubbed as thermal cameras.
The advantages of thermal imaging technology was exploited to the utmost during the COVID-19 pandemic that rocked the world starting early 2020. Thermal imaging technology became an essential tool in limiting the rapid proliferation of the virus.
By detecting heat energy from a safe distance, infrared cameras have become a standard tool in just about every establishment operating during the pandemic - from government establishments to businesses to homes.
Thermal cameras operate in near-infrared (NIR) and mid-infra red (MWIR) ranges of the electromagnetic spectrum, roughly from 900 to 14,000 nanometers or 0.9 - 1.4 μm.
In the military, heat-seeking missiles used infrared tracking, also dubbed as infrared homing. These missiles are called heat-seekers as they target heat emission from another hot body.
Other popular uses of infrared is as a deliberate heat source such as in infrared saunas. It has also been used to remove ice from wings of aircrafts.
Not to be outdone, the industry has also made use of infrared heating by using it in various manufacturing processes such as:
- Print drying
- Plastic welding
- Annealing
- Forming of plastics
- Curing of coating
Today, many night vision equipment carry infrared-detecting capabilities. This is to offset traditional night vision equipment’s inability to detect objects in situations with insufficient visible light.
In art, infrared reflectography can determine the authenticity of a painting. As infrared reveals underlying layers without destroying the painting, the artist’s underdrawing - his sketches and outlines - will show. In the process, an expert will know if a particular painting is the work of a master or a fake.
On the same token, ancient written documents and objects can be authenticated using infrared. An example of this is the Dead Sea Scrolls.
And Sir Frederick William Herschel’s findings are now a standard methodology in astronomy. NASA is using infrared technology to determine the presence or lack of heat in celestial bodies such as planets, stars and nebulae.
Space infrared telescopes have made the most of infrared astronomy. For instance, infrared is instrumental in detecting protostars or young stars even before they are able to emit visible light.
Herschel may not have seen it in his lifetime but his works bore fruit for his passion in astronomy. He was right after all. His light prism experiment centuries ago has benefited humanity more than ever. Further it has allowed scientists today to continue man’s dream of exploring the universe. Quite simply, that’s how amazing thermal radiation is!