Visible light, the electromagnetic spectrum and satellites

This week we are going to be introducing you to multispectral imagery. We will explore how satellites can see things using light invisible to the human eye, how we can visualise this data in colours we can see, and why this is useful for archaeologists. You will be working with a number of different sources of satellite imagery and visualising them in QGIS.

_{A multispectral satellite image of Dubai’s urban expansion. Sentinel-2 (12/8A/4) image, courtesy of ESA.}

First though, let’s go through an introduction to visible light and how humans see colour.

Visible light

Humans can see a great range of colour (think of a rainbow!), all the way from red, through orange, yellow, green and blue to violet. This order is not arbitrary – it relates to the physical properties of the light itself. Light is transmitted as a wave, and the distance between the peaks and troughs of the wave determines how we perceive its colour. The distances we are talking about here are very small indeed! Red light has a wavelength of around 660 nanometres (660 billionths of a metre!); green is around 550 nanometres, and violet is 400 nanometres.

_{White light is made up of light of all colours – the colour of light is determined by its wavelength. Public domain image.}

The way we see light is similar to how a digital camera works. We have special cone cells in the back of our eyes that are sensitive to light. We have three different types of cones, each sensitive to different wavelengths – shorter wavelengths (purple and blue), medium wavelengths (green and yellow), and longer wavelengths (orange and red). Our brain processes the signal from these three different cells and converts it into the colour image that we see.

_{We also have rod cells, which don’t detect colour but are more sensitive in low light conditions – so when it’s dark but not quite too dark to see, you effectively see in black and white!}

Other animals perceive light differently; dogs only have two types of cone cells, so they can distinguish fewer colours, while some insects and birds are sensitive to ultraviolet light which means they can see things invisible to us.

_{A dog’s eye view! Canines (left) can distinguish fewer colours than humans (right). Public domain image.}

Most imaging satellites detect light across a greater range of wavelengths than the human eye is capable of registering. To understand this better though, we need a bit of background on the rest of the electromagnetic spectrum.

The electromagnetic spectrum

Light that is visible to humans occupies only a tiny part of the electromagnetic spectrum. Other light comes in shorter (beyond violet) and longer (past red) wavelengths that we cannot see. “Electromagnetic radiation” is the more accurate term used to describe these non-visible wavelengths. Visible light lies near the middle of the electromagnetic spectrum. The Sun and other stars emit radiation from across the whole spectrum, not just the visible light we can detect.

_{The electromagnetic spectrum. Courtesy of NASA.}

Beyond violet (shorter wavelengths), there is ultraviolet. This is the light that tans (and can burn!) our skin. Even shorter wavelengths exist such as x-rays, which are used for medical imaging and other similar applications. Lastly there are gamma rays which can be used to sterilise medical equipment or keep food fresh by killing bacteria. Gamma rays, x-rays and shorter wavelengths of ultraviolet can damage living cells and cause cancers because with such a short wavelength, a lot of energy is packed into a really small space!

_{X-rays are very short wavelength radiation that can be used for medical imaging – dense tissue such as bone can absorb them better than other less dense tissue such as muscle. As it is such high-energy, x-ray radiation can damage tissue, or cause cancer, in high doses. Public domain image.}

At the other end, beyond red (longer wavelengths) there is infrared radiation, which is often used in special cameras or digital thermometers to measure heat. Beyond infrared there are microwaves, used (amongst other things!) in microwave ovens and radar. Then finally there are radio waves used for all sorts of telecommunications – when you tune a radio to move between stations, you are actually changing which wavelength it is detecting!

_{Radios, wi-fi and Bluetooth all use radio waves to transmit and/or receive data. Public domain image.}

Satellites and non-visible light

Instruments can be fitted on satellites to detect all parts of the electromagnetic spectrum, allowing us to collect data spanning a wide-range of wavelengths. However, for imaging satellites used to view the Earth (as opposed to astronomical satellites, which look outwards into space), it is not terribly useful to look across the whole spectrum.

Gases in the Earth’s atmosphere absorb almost all of the gamma rays, x-rays and shorter ultraviolet wavelengths radiated towards the Earth by the sun. This is good for life on earth because it stops us being fried by this high-energy radiation! Visible light can pass through the atmosphere although with some distortion. A lot of infrared radiation is also absorbed by the atmosphere. Short and medium length radio waves can pass through the atmosphere without any distortion, but longer wavelengths are blocked.

_{The atmosphere observes different wavelengths to different extents – visible light, short and medium infrared, and most radio waves can pass through most easily. Based on an image courtesy of NASA.}

Because many parts of spectrum are prevented from passing through the atmosphere, there is no point in trying to observe these. You would not see anything! We need to focus our efforts on those parts of the spectrum that can pass through the earth’s atmosphere – visible light, parts of infrared, and short and medium length radio waves.

In the next step, we’ll look at multi-spectral imagery, which uses visible light and infrared, and in Week 4 we’ll explore how we can use radar, which uses radio waves, to study the earth’s surface.

_{Different types of satellite imagery and which wavelengths they use. Based on an image courtesy of NASA.}