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Broadcast Transmission Systems 11-7

McPetrie, J. S., and L. H. Ford: “An Experimental Investigation on the Propagation of Radio

Waves over Bare Ridges in the Wavelength Range 10 cm to 10 m,” J. Inst. Elec. Engrs., pt.
3, vol. 93, pg. 527, 1946.

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Broadcast Transmission Systems

11-8 Section Eleven

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Broadcast Transmission Systems

11-9

Chapter

11.1

The Electromagnetic Spectrum

John Norgard

11.1.1 Introduction

The electromagnetic (EM) spectrum consists of all forms of EM radiation—EM waves (radiant
energy) propagating through space, from DC to light to gamma rays. The EM spectrum can be
arranged in order of frequency and/or wavelength into a number of regions, usually wide in
extent, within which the EM waves have some specified common characteristics, such as charac-
teristics relating to the production or detection of the radiation. A common example is the spec-
trum of the radiant energy in white light, as dispersed by a prism, to produce a “rainbow” of its
constituent colors. Specific frequency ranges are often called bands; several contiguous fre-
quency bands are usually called spectrums; and sub-frequency ranges within a band are some-
times called segments.

The EM spectrum can be displayed as a function of frequency (or wavelength). In air, fre-

quency and wavelength are inversely proportional, f = c/

λ (where c ≈  3 ×  10

8

 m/s, the speed of

light in a vacuum). The MKS unit of frequency is the Hertz and the MKS unit of wavelength is
the meter. Frequency is also measured in the following sub-units:

•

Kilohertz, 1 kHz = 10

3

 Hz

•

Megahertz, 1 MHz = 10

6

 Hz

•

Gigahertz, 1 GHz = 10

9

 Hz

•

Terahertz, 1 THz = 10

12

 Hz

•

Petahertz, 1 PHz = 10

15

 Hz

•

Exahertz, 1 EHz = 10

18

 Hz

Or for very high frequencies, electron volts, 1 ev ~ 2.41

  ×   10

14

 Hz

Wavelength is also measured in the following sub-units:

•

Centimeters, 1 cm = 10

–2

 m

•

Millimeters, 1 mm = 10

–3

 m

•

Micrometers, 1 

µm = 10

–6

 m (microns)

•

Nanometers, 1 nm = 10

–9

 m

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Source: Standard Handbook of Audio and Radio Engineering

11-10 Broadcast Transmission Systems

•

Ångstroms, 1 Å = 10

–10

 m

•

Picometers, 1 pm = 10

–12

 m

•

Femtometers, 1 fm = 10

–15

 m

•

Attometers, 1 am = 10

–18

 m

11.1.2 Spectral Sub-Regions

For convenience, the overall EM spectrum can be divided into three main sub-regions:

•

Optical spectrum

•

DC to light spectrum

•

Light to gamma ray spectrum

These main sub-regions of the EM spectrum are next discussed. Note that the boundaries
between some of the spectral regions are somewhat arbitrary. Certain spectral bands have no
sharp edges and merge into each other, while other spectral segments overlap each other slightly.

11.1.2a

Optical Spectrum

The optical spectrum is the “middle” frequency/wavelength region of the EM spectrum. It is
defined here as the visible and near-visible regions of the EM spectrum and includes:

•

The infrared (IR) band, circa 300 

µm–0.7 µm (circa 1 THz–429 THz) 

•

The visible light band, 0.7 

µm–0.4 µm (429 THz–750 THz) 

•

The ultraviolet (UV) band, 0.4 

µm–circa 10 nm (750 THz–circa 30 PHz), approximately 100

ev

These regions of the EM spectrum are usually described in terms of their wavelengths.

Atomic and molecular radiation produce radiant light energy. Molecular radiation and radia-

tion from hot bodies produce EM waves in the IR band. Atomic radiation (outer shell electrons)
and radiation from arcs and sparks produce EM waves in the UV band. 

Visible Light Band

In the “middle” of the optical spectrum is the visible light band, extending approximately from
0.4 

µm (violet) up to 0.7 µm (red), i.e. from 750 THz (violet) down to 429 THz (red). EM radia-

tion in this region of the EM spectrum, when entering the eye, gives rise to visual sensations
(colors), according to the spectral response of the eye, which responds only to radiant energy in
the visible light band extending from the extreme long wavelength edge of red to the extreme
short wavelength edge of violet. (The spectral response of the eye is sometimes quoted as extend-
ing from 0.38 

µm (violet) up to 0.75 or 0.78 µm (red); i.e., from 789 THz down to 400 or 385

THz.)This visible light band is further subdivided into the various colors of the rainbow, in
decreasing wavelength/increasing frequency:

•

Red, a primary color, peak intensity at 700.0 nm (429 THz)

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The Electromagnetic Spectrum

The Electromagnetic Spctrum 11-11

•

Orange

•

Yellow

•

Green, a primary color, peak intensity at 546.1 nm (549 THz)

•

Cyan

•

Blue, a primary color, peak intensity at 435.8 nm (688 THz)

•

Indigo

•

Violet

IR Band

The IR band is the region of the EM spectrum lying immediately below the visible light band.
The IR band consists of EM radiation with wavelengths extending between the longest visible
red (circa 0.7 

µm) and the shortest microwaves (300 µm–1 mm), i.e., from circa 429 THz down

to 1 THz–300 GHz.

The IR band is further subdivided into the “near” (shortwave), “intermediate” (midwave), and

“far” (longwave) IR segments as follows 

1

:

•

Near IR segment, 0.7 

µm up to 3 µm (429 THz down to 100 THz)

•

Intermediate IR segment, 3 

µm up to 7 µm (100 THz down to 42.9 THz)

•

Far IR segment, 7 

µm up to 300 µm (42.9 THz down to 1 THz)

•

Sub-millimeter band, 100 

µm up to 1 mm (3 THz down to 300 GHz). Note that the sub-milli-

meter region of wavelengths is sometimes included in the very far region of the IR band.

EM radiation is produced by oscillating and rotating molecules and atoms. Therefore, all

objects at temperatures above absolute zero emit EM radiation by virtue of their thermal motion
(warmth) alone. Objects near room temperature emit most of their radiation in the IR band. How-
ever, even relatively cool objects emit some IR radiation; hot objects, such as incandescent fila-
ments, emit strong IR radiation.

IR radiation is sometimes incorrectly called “radiant heat” because warm bodies emit IR radi-

ation and bodies that absorb IR radiation are warmed. However, IR radiation is not itself “heat”.
This radiant energy is called “black body” radiation. Such waves are emitted by all material
objects. For example, the background cosmic radiation (2.7K) emits microwaves; room tempera-
ture objects (293K) emit IR rays; the Sun (6000K) emits yellow light; the Solar Corona (1 mil-
lion K) emits X rays.

IR astronomy uses the 1 

µm to 1 mm part of the IR band to study celestial objects by their IR

emissions. IR detectors are used in night vision systems, intruder alarm systems, weather fore-
casting, and missile guidance systems. IR photography uses multilayered color film, with an IR
sensitive emulsion in the wavelengths between 700–900 nm, for medical and forensic applica-
tions, and for aerial surveying. 

1.  Some reference texts use 2.5 mm (120 THz) as the breakpoint between the near and the inter-

mediate IR bands and 10 mm (30 THz) as the breakpoint between the intermediate and the far 
IR bands. Also, 15 mm (20 Thz) is sometimes considered as the long wavelength end of the 
far IR band.

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The Electromagnetic Spectrum