The Visual Spectrum


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1. Radio Spectrum of the Sun, N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF
1. Radio Spectrum of the Sun, N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF


Kirchoff’s three laws of spectroscopy state:

a hot solid object produces light with a continuous spectrum

a hot gas produces light at discrete frequencies

a cool gas absorbs light at discrete frequencies

When an opaque object is heated it produces light with a continuous range of colours (frequencies). Light from an incandescent light bulb is like this and the emitted light energy is spread smoothly over a range of frequencies. The cone cells in our retinas which distinguish red, green and blue coloured light are stimulated at all visible frequencies and we see white.

These continuous spectra can be split into their constituent colours (frequencies), through a prism, a diffraction grating or a raindrop. In the physics lab the spectrascope, invented by Frauenhoffer in 1814, produces the same kind of rainbow image called a spectragraph.

The incident light is spread out by frequency and the intensity of each colour shows the energy at each frequency in the spectrum. In continuous spectra these show a smooth energy distribution, but there are other types of spectra. Hot gasses emit energy in discrete bands depending on their chemical composition, like the single orange of sodium street lighting. These emission spectra look black apart from thin lines of colour that show energy at single frequencies. Cold gasses on the other hand, absorb energy in discrete bands. If white light is shone through a cold gas, absorption spectra are produced which look like a rainbow with black lines across it at frequencies where the light is absorbed by the gas.

The image above is a high resolution spectrum of visible sunlight. It shows a rainbow-like continuous spectrum, with dark lines that are the absorption spectra of all the atoms that make up the sun’s atmosphere.

2. United States Frequency Allocation table
2. United States Frequency Allocation table


A frequency allocation table (FAT) is a representation used by government agencies called spectrum managers to show the uses of different radio frequencies. The tables extrapolate the visual output of the spectragraph’s treatment of visible light to invisible radio frequencies. These are often graphically laid out so that they look very similar to a spectragraph, with rainbow colours arranged in bands, and annotations of frequency information added in small print. Their visual form reminds us of a continuous light spectra with emission and absorption lines overlaid.

But the tables do not show the intensity of energy at each frequency. They show the social use of frequencies rather than their physical properties. Each entry in the FAT represents a government sanctioned use of radio, an application that is frequently tied to a particular radio technology. More importantly, each band represents a user group, and names are attached to the bands in the table; radio-location, detection-of-avalanche-victims, defence-systems; the list is huge. Each name describes a social use and represents a social group who have lobbied successfully for use of radio.

The presentation of social use of radio as a spectragraph, using the visual language of the laboratory, is like an appeal to the higher authority of Physics which occupies a similar cultural role to God or Nature in secular societies. Frequency allocation and spectrum regulation is represented as something beyond mere human construction, as an innate property of the physical world, a natural order which we have been lucky enough to discover with science.

Aesthetically the FAT looks a bit like a satellite image of the mid west. The boundaries look like fields in a huge territory stretching as far as the eye can see and beyond. This image encourages us to think in terms of land metaphors, to think of a place called ‘The Spectrum’, to be divided and ruled, apportioned, allocated, perhaps bought and sold.

We are encouraged to think of ‘The Spectrum’ in terms of other finite resources, like forests or fisheries and to think of spectrum politics as being similar to resource politics. Classic resource politics works out ways to allocate and share finite resources while bearing in mind the pressures of market economics ,the public good and the environment.

Let’s look at what the Frequency Allocation Table actually says.

Each national frequency allocation table represents regulation of radio use by a particular nation. Nations enforce this using laws; not the laws of nature or the laws of physics, but the laws of people.

These laws have effect within a geographical region which is the physical jurisdiction of a state, so in this way an allocation table represents a physical space defined by the administrative boundary of the border.

It also represents a large time period, that is assumed to be ‘the foreseeable future’, or at least until the legislation changes.

Each frequency band within the table represents a colour though it may not be visible to the naked eye. From a physical point of view radio is exactly the same as visible light except that we do not have built in sensors to perceive radio frequencies as we do with our eyes. Frequency is what determines colour so we can say that radio is just light of a different colour. When thinking about spectrum allocation it is worth remembering that:

Radio = Light

Frequency = Colour

Imagine a world where the visible colours were allocated to different social groups by national regulators.

Blue for the police, green for the army, pale yellow for roads-administration, red for detection-of-avalanche-victims and a single very specific hue of violet for the general public. This seems crazy but this is like what a frequency allocation table says.

"Red torches may only be used by, detection-of-avalanche-victim teams, for the foreseeable future, within the boundary of the state – light bulbs under 1 Watt only may be used"

This kind of blanket permission was justified in the past as the only way to maintain the communication right of each radio user. There were technical reasons for this; early radios could only tune for frequency and the propagation characteristics of old high power low frequency radios meant that signal would travel for great distances through all kinds of barriers. Without such blanket legislative protections, different users would interfere with each other’s broadcasts, degrading radio as a medium for everyone.

The technical rationale for frequency allocation also went hand in hand with political control. As the possibilities of radio communication were explored, frequency allocation became a way for societies to control communication between citizens, and to exert control over other sensitive technologies such as radar on and off the battlefield. The complex system of licenses subordinated radio use and communications technology to a political and economic process. Any user group that wishes to communicate via radio still has to go through these processes themselves or by proxy through the device manufacturer.

The many boundaries presented in the FAT are therefore the results of political and economic processes and represent power relationships between users of radio. The groups represented in the FAT are those who have made use of radio during the last century of spectrum regulation. They are the winners of the political processes and the boundaries represent negotiated or enforced settlements. As such the FAT represents an impartial archaeological record of political processes, laid down in the permitted usage of the radio spectrum.

We can analyse the FAT to see who the winners of these processes have been, who has been there longest and who has managed to negotiate the most boundaries. The winners represent all aspects of society, from defence, to commercial interests, emergency services, community media, or public usage of license exempt bands. I explore this in detail in ‘The Political Spectrum’3 but we can see the names of groups, such as ‘defence-systems’ represented many times at the top of the scale with most boundaries set, and smaller groups such as detection of avalanche victims represented few times at the bottom of the scale. The textual frequency of appearance in the allocation table, of a particular group is a metric of that group’s political power.

Frequency = Power

This frequency of use by particular groups varies from country to country so by analysing FATs we can see the relative values placed by different societies on different radio users. In the Political Spectrum I use the European Radiocommunications Office Frequency Information System, which provides searchable FATs for all European countries with harmonised terms easing comparison between countries.4

Dividing radio use up, into colours over whole countries is not however a very efficient or equitable way to divide things. We can easily think of other non-blanket ways of managing co-existence between radio users and these options become greater, cheaper and more feasible as our mastery of radio technology increases. We could;

These techniques can migrate the politics of boundary setting out of traditional political processes and into technical implementations, devices, code and standards. Most current approaches to liberalising radio regulation focus on adding market mechanisms to existing frequency based allocation processes, but technical solutions provide a different possible road map, and it is this that is pursued by the Openspectrum movement. In the EU these new models of regulation are moving towards increased adoption at least in some bands. The EU Radio Spectrum Policy group recently recommended additional space be made for the strategies which are collectively termed Common Use of Spectrum. 5

A change to a technically implemented regulation scheme, can open up possibilities for new participation, and innovation perhaps in free software defined radio. It can also move control from local politically accountable processes to closed private offshore technical teams. As these relationships change, so do the power relationships embedded in them and so it is important to understand and shape the new structures as they emerge.

‘The Spectrum’ represented in the FAT looks less like a physical space to be apportioned, but rather a set of social relationships, some embedded in technology, some in political processes, that we use to negotiate our radio use and electromagnetic communications. ‘The Spectrum’ that we use is created by the interaction of regulators, public, technologies, standards and devices with each other.

The physical description of ‘The Spectrum’ outlined here, describes properties of time, space and frequency. The use of Frequency Allocation is but one possible configuration, of the myriad different possible ways of physically organising transmission and reception. In this sense the FAT frames our degrees of freedom.

There are more recent physical descriptions of electromagnetism from quantum theory accepted among physicists. In the description of Quantum Electro Dynamics, electromagnetic interactions are seen as probabilities or possibilities, even at the level of an individual photon. QED describes light as the probability that a given path will be taken, and that a given interaction will occur. In this description, ‘The Spectrum’ is literally the possibility space of any given electromagnetic interaction.

In ‘The Political Spectrum’, I show a vision of ‘The Spectrum’ as the space of any possible interaction, with a white board, an infinitely re-writable space, Tabula Rasa, to be inscribed temporarily with our communications, structured with our social structures, to be re-used, shared.

The blank white board reflects white light in a continuous spectrum across the frequencies our eyes are sensitive too. The marks of writing are like absorption or emission spectra, structuring the blank space of infinite possibility.

On a calm windless day sit on the ocean beach and watch the huge waves breaking. They approach the shore in a linear harmonic state, large waves with smaller waves superimposed on top of them, lifting the water surface vertically as it reflects the blue of the sky.

As the water shallows the energy of the wave is forced into a smaller physical space. The wave increases in steepness and at some point collapses on itself breaking out of the confines of gravity and surface tension that carries it. The surface changes into a foam of intense and microscopic structure. Bubbles and crests come into being and dissipate the energy. The surface of the water becomes opaque, light scattered in all directions reflecting the continuous energy spectrum of incident light. The crashing wave dissipates its energy in other media, creating heat, the physical movement of water particles and white noise. The beach is filled with the sound of a continuous energy spectrum of intense and chaotic structure beneath our ability to resolve it.

Looking down from above, on the wave at the point of breaking one can see a point where the wave begins it’s break. Travelling out from this point along the break we see the white foam spread away in opposite directions as the wave advances leaving a triangular wake of subsiding trails as the foam subsides into the surface of the water; the energy dissipated into different phase states. The outline that remains shows a patterned structure of the history of the breaking wave.6

Back in the 19th century the social wave of experimentation with electromagnetism began with Orested, Farrady and Maxwell, breaking into a completely new phase space of possibilities. The crest spread out into all areas of society creating a dense and highly organised social structure of political boundaries and control structures.

A century later the energy is dissipated – the activity subsides back into the harmonic state leaving the trails of that structure inscribed in the rules of the information infrastructure, tools and norms that we see etched into the Frequency Allocation Table awaiting new waves to open a new phase space of possibilities for us to inhabit and structure.

1. High Resolution Solar Spectrum, National Optical Astronomy Observatory/Association of Universities for Research in Astronomy/National Science Foundation full-size image 8192 x 5464 2.8 Mb color JPEG

2. U.S. Frequency Allocation Chart as of October 2003, National Telecommunications and Information Administration full-size image scalable 94 Kb color PDF

3. Julian Priest, The Political Spectrum, Waves 2006, Riga

4. ERO Frequency Information System,

5. Draft Radio Spectrum Policy Group opinion on aspects of a European approach to ‘collective use of spectrum’
, June 2008,

6. Shand, Tom; Shand, Roger; Bailey, Donald and Andrews, Connon. Wave Deformation in the Vicinity of a Long Ocean Outfall at Wanganui, New Zealand, pp3