Digital images

This week we are going to give you an overview of how satellite imagery is created, and show you how you can find, download, and begin to work with satellite imagery for yourself.

_{A beautiful Blue Marble satellite image of the Middle East. Courtesy of NASA.}

Before we start though, let’s explore the basic principles of how digital colour images are created and displayed.

Digital photographs and other images

To create a high-quality colour image, you need to be able to display every possible colour. This sounds like a tough task as there are an infinite number of them, but every colour can be created by mixing just three colours. With physical pigments, such as paint or ink, these primary colours are red, yellow and blue.

_{Goethe’s colour wheel using pigments from 1810. Public domain image.}

Digital images use light rather than physical pigments to create images and to display them. For light, the three primary colours are red, green and blue. By combining red, green and blue light of varying brightnesses you can create every other colour. You can try this yourself here.

_{By combining red, green and blue light you can make all other colours. Public domain image.}

This forms the basis of all digital imagery. When taking a digital photograph, light reflected from the subject being photographed passes through the lens of a camera and hits a device made up of a grid of millions of sensors. These sensors then individually measure the brightness of red, green and blue (or RGB) light in the portion that hits it. The value outputted from each sensor constitutes one single pixel, with each pixel recording three values – the red, green and blue values – to give the RGB output for that pixel. These pixels are then combined to form a whole image. Usually, useful extra information, such as the date and time it was taken or the settings of the camera, is also added to this image file as metadata (data that provides information about other data).

_{A simplified representation of how digital cameras produce natural-looking photos. Courtesy of William Deadman.}

How detailed a photograph will be depends on the number of sensors in the camera. More sensors means more pixels in the final image, which means that more detail will be visible. This is known as ‘resolution’. A high-resolution image contains more pixels and lots of detail, while a low-resolution image contains fewer pixels and less detail.

_{A high-resolution (top) and low-resolution (bottom) version of the same image. The individual pixels are very clear in the low version and the detail is missing. Public domain image.}

Electronic visual displays – including smartphone screens, computer screens, and televisions – apply the same principles as digital cameras, but in reverse. Colour screens are made up of a grid of thousands of pixels, each individually made up of a red, a green and a blue light-omitting component (these are sometimes even visible to the naked eye in older devices). The colour of any individual pixel is controlled by the brightness of each of its three components. Any colour can then be displayed on the screen. Even greyscale pixels can be displayed using red, green and blue light. For pure black all three will be fully off, for pure white all three will be fully on, and for a particular shade of grey all three will be set to the same level of brightness. The level of detail that can be displayed on a screen depends on its own resolution – the number of pixels it has.

_{Colour screens are made up of thousands of RGB pixels. Courtesy of William Deadman.}