The electromagnetic (EM) spectrum covers the continuum of all electromagnetic energy. I divide the electromagnetic spectrum into three generalized bands. The first is the radio-frequency or RF band. The second is the optical band and the third is the high energy band. The reason I divide the spectrum up in this way is due to how EM waves are discussed in the literature; specifically, the units used.

Electromagnetic signals consist of photons. A photon is a single, quantized packet of electromagnetic energy. The amount of energy is directly related to its frequency; the higher the frequency (meaning the shorter the wavelength), the more energy the photon possesses.

There are two important differences between these bands.

• Sensor Coverage: This is perhaps the biggest difference. The difference is between what type of sensor is used to receive and/or recover the electromagnetic signal. In the RF range, the sensor will be an antenna, typically some type of metal construction. In the optical range, it will be some type of lense, composed of either glass, a glass composite, or some specialized semiconductor material. In the high energy band, it will be either some type of film emulsion or specialized semiconductor.
• Usage: The differences between the bands are also the uses for each. The RF band is used mostly for two-way communication. The optical band is used for sight, thermography, and fluorescence. The high energy band is used for non-destructive imaging and astronomical analysis.

Units for the Spectrum

Electromagnetic waves are based upon the oscillation (back and forth) motion of electromagnetic fields. The spectrum is merely a way to divide up these oscillations based upon the rate of oscillation. The periodic nature of this motion can be measured one of three ways. The first is the frequency, which is merely the inverse of the period of the motion. The common unit for frequency is the hertz, which is abbreviated Hz. One Hz is equivalent to one cycle per second of the EM motion.

The second unit is wavelength, or the distance that an EM wave will travel in one cycle. An EM wave moves through space at the speed of light, which is equivalent to 299,792,458 meters / second, typically just shortened to 3 x 10⁸ meters / second. The faster the EM wave is oscillating, the shorter the distance it will travel before it completes one cycle. Wavelength can be expressed in any unit of length. The most common is the metric system unit of length, the meter. For example, a signal at a frequency of 1 Hz would have a wavelength of 3 x 10⁸ meters. A signal with a frequency of 100 MHz (100 million hertz) would have a wavelength of only 3 meters (3 x 10⁸ / 100 x 10⁶ = 3).

The third unit that can be used is energy, or the amount of energy contained in the photons of which light is made up. The amount of energy is directly proportional to the frequency; the higher the frequency, the greater the energy and vice-versa. The relationship between frequency and energy is proportional to the Planck constant. This relationship is:

E = h f = h c / λ

where:

• E = energy (J)
• h = Planck constant = 6.626 069 57(29) x 10^-34 (J s)
• f = frequency (Hz, or 1/s)
• c = speed of light = 299,792,458 m/s
• λ = wavelength (m)

Another way to think of Planck's constant is by considering that an electromagnetic wave is made up of discrete particles of energy. The energy of each particle is dependent upon its frequency. The higher the frequency, the more energy each "particle" has. For example, consider an FM broadcast transmitter transmitting at a frequency of 90.9 MHz with an ERP (effective radiated power) of 100,000 watts. At this frequency, the energy per photon is E = h f = (6.626e-34 J s)(90.9e6 Hz) = 6.023e-26 J. The transmitter has an output power of 100,000 watts in the mainlobe. This means that, in the main lobe of the antenna, this transmitter will emit 1.660e+30 photons each second.

The three bands, RF, optical, and high energy, typically use frequency, wavelength and energy, respectively, as units of measurement.

The Radio-Frequency (RF) Band

The RF band is subdivided into smaller bands, from the extremely low frequency (ELF) band up to the extremely high frequency (EHF) band, going from lower frequency to higher frequency.

The Optical Band

The optical band is subdivided into three bands, the infrared (IR), the visible, and the ultraviolet (UV), going from longer wavelength (lower frequency) to shorter wavelength (higher frequency), respectively. As already noted, the optical band is divided by units of wavelength rather than frequency. Is it possible to divide it up by frequency? you might ask. Yes, it is. But, historically, scientists and engineers have used wavelength instead. The low end of this band starts where the EHF band stops, which is at 300 GHz. This corresponds to a wavelength of 1 mm, or 1000 um (micrometers, or a millionth of a meter), and is the beginning of the infrared ("infra" meaning "below", which means infrared stands for "below red").

The infrared band goes up in frequency (lower in wavelength) up to the beginning of the visible band, which starts at roughly 0.78 um, or 780 nm (nanometers, or billionths of a meter). The visible spectrum continues up to .38 um (380 nm), which is where the ultraviolet (UV) spectrum starts. The UV spectrum continues to about 7.0 nm, which is where the high energy band starts.

Each section of the optical band is further subdivided into smaller subbands. The infrared band is divided into three subbands, the shortwave, midwave and longwave. The visible band, since it is what we can actually see, is divided into subbands which we simply call colors. The colors typically used are red, orange, yellow, green, blue, indigo, and violet. The common acronym used to remember these colors is ROY G BIV. Finally, the ultraviolet is divided into longwave, midrange, shortwave and extreme subbands.

What's the Difference Between Infrared and Thermal?

This is a common question, or confusion, depending. The difference is that infrared simply refers to the region of the spectrum between the visible spectrum and the RF (radio-frequency) portion of the spectrum. The "zero lux" lights used on some cameras, such as video cameras, emit within the infrared region, but they're not thermal emissions. Thermal, on the other hand, refers to electromagnetic waves emitted by every body with a temperature greater than absolute zero. In other words, everything emits thermal. Now, depending on the temperature, the object may be emitting thermal energy in the infrared band. Most objects of which we humans are familiar, such as the earth itself, our homes, everything around us, is at roughly 300 K. This corresponds to emitting thermal energy based on a Planck curve with a maximum around 10 um (microns). This is right within the infrared band. To summarize, infrared refers to electromagnetic emanations between RF and visible light; thermal refers to EM radiation due to the heat of an object itself.

The High Energy Band

Okay, this is where we run into some problems. Once we get above the ultraviolet range, we're into photons that pack some high energy. Hence, I call this the high energy band. However, it's difficult to find any consensus on how this band is divided up, and where, precisely, it starts.

Where is the UV / high-energy dividing line?

Depends on who you ask. The International Organization of Standards (ISO) lists UV, x-rays and gamma rays as three, distinct bands defined by their wavelengths. The cut-off between UV and x-rays is said to be at 10 nm, which corresponds to a photon energy of 124 eV. Got that? The ISO, an international standards body, says the dividing line is at 124 eV. But the Canadian Centre for Occupational Health & Safety defines the dividing line between UV and x-rays as being at 100 nm (12.4 eV). So, we have a difference between two authoritative sources, the Canadian government and the ISO. Whom do we choose?

What is the difference between x-rays and gamma rays?

Again, just as with the previous question, it depends on your source. The good news is that all of the sources I've looked at agree that both x-rays and gamma rays are photons. Unfortunately, that appears to be as far as the agreement goes.

Outside of that, we have two schools of thought. The US government's EPA (Environmental Protection Agency) talks about the difference between x-rays and gamma rays as being due to their source. According to them, x-rays come from interactions with the electrons around the nucleus, while gamma rays emanate from within the nucleus. This is also the definition provided by the Department of Energy in their "Handbook for Radiation Safety Training". Specifically, they state:

X-rays and gamma rays are a form of electromagnetic radiation. X-rays differ from gamma rays in their point of origin. Gamma rays originate from within the atomic nucleus, whereas X-rays originate from the electrons outside the nucleus and from free electrons decelerating in the vicinity of atoms (i.e., bremsstrahlung).

This is also the definition used by the Nondestructive Testing (NDT) Resource Center, a consortium of companies involved in the NDT field. The Australian government also defines the difference between x-rays and gamma rays in this way.

Then there's the ISO which says that the dividing line between x-rays and gamma rays is not based on their origin, but their specific wavelength. In this case 0.001 nm (1.24 MeV). You might ask, "Well, is it possible that x-rays, even though they originate from the electron shell around the nucleus, only have energies less than 1.24 MeV? And gamma rays only have energies above 1.24 MeV, since they originate from the nucleus itself, and therefore would probably have more energy?" Unfortunately, the answer to that question is, "Nope. It doesn't work that way." The problem is that some atoms, when they decay, emit gamma rays (photons originating from the nucleus) at energies much less than 1 MeV. For example, iodine-125 emits gamma rays at 35.5 keV. And according to the ISO standard, the dividing line between x-rays and gamma rays occurs at 1.24 MeV. Again, which definition to use?

To summarize, the dividing line is either:

1. the source of the photons, which is the electron shell for x-rays and the nucleus itself for gamma rays, -OR-
2. the wavelength / energy of the photon, which is somewhere between roughly 10 eV and a couple hundred thousand eV.

As noted, the high energy band uses the units of energy. Your first question might be, "The energy in what?" Since we're talking about electromagnetic energy, the energy is transmitted by photons. Thus, we're talking about photon energy here. The relationship between frequency, wavelength, and photon energy is given by the following equations:

1. E = h f = h c / λ
2. h = 6.626176 x 10^-34J/Hz (Planck's constant)
3. c = 299792458 m/s (speed of light in a vacuum)

The typical unit for photon energy is electron-volts or eV. The simple equation between wavelength and energy is as follows:

• E (in eV) = 1240 / λ (in nm)
• E (in keV) = 1.24 / λ (in nm)
• λ (in nm) = 1240 / E (in eV)

Thus, since the high energy band starts where the optical band stops, which is about 250 nm, this corresponds to a photon energy of roughly 5 eV. It's important to understand why this is called the high energy band; the photons in this area are quickly getting to the point where they have enough energy to ionize atoms if they should come into contact with them. This is the reason that, starting at roughly 5 eV, electromagnetic radiation is called ionizing radiation. The effects of radiation below this level are much debated. The effects above this level are not. Radiation levels at this extreme energy range require strigent safety measures.

One aspect of this band needs to be understood, if only for clarity. In some cases, people talk about cosmic rays being above the gamma ray band. This is misleading as gamma rays are photons, while cosmic rays are particles. Mind you, they have lots of energy, but they're still particles, not electromagnetic radiation. Therefore, they should be considered separately from our discussions on the EM spectrum.