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Chances are, you have a camera near you as you read this—in the smart phone in your pocket or on the tablet or computer you’re using to view this page. Some of you might have a 35 mm film or digital camera nearby. And at some point this week, you probably looked through photos posted by friends or even strangers on the Internet.

In our photo-saturated world, it’s natural to think of the images on the Earth Observatory as snapshots from space. But most aren’t. Though they may look similar, photographs and satellite images are fundamentally different. A photograph is made when light is focused and captured on a light-sensitive surface (such as film or a CCD). A satellite image is created by combining measurements of the intensity of certain wavelengths of light, both visible and invisible to human eyes. Why does the difference matter? When we see a photo where the colors are brightened or altered, we think of it as artful (at best) or manipulated (at worst).

We also have that bias when we look at satellite images that don’t represent the Earth’s surface as we see it. “That forest is red,” we think, “so the image can’t possibly be real.” In reality, a red forest is just as real as a dark green one. Satellites collect information beyond what human eyes can see, so images made from other wavelengths of light look unnatural to us. We call these images “false-color,” and to understand what they mean, it’s necessary to understand exactly what a satellite image is. Satellite instruments gather an array of information about the Earth.

Some of it is visual; some of it is chemical (such as gases in the atmosphere); some of it is physical (sensing topography). In fact, remote sensing scientists and engineers are endlessly creative about what they can measure from space, developing satellites with a wide to tease information out of our planet. Some methods are active, bouncing light or radio waves off the Earth and measuring the energy returned; lidar and radar are good examples. The majority of instruments are that is, they record light reflected or emitted by Earth’s surface. These observations can be turned into that measure everything from plant growth or cloudiness. But data can also become photo-like natural-color images or false color images.

This article describes the process used to transform satellite measurements into images. Most of the electromagnetic radiation that matters for Earth-observing satellites comes from the Sun. When sunlight reaches Earth, the energy is absorbed, transmitted, or reflected. (Absorbed energy is later as lower-energy radiation.) Every surface or object absorbs, emits, and reflects light uniquely depending on its chemical makeup. Chlorophyll in plants, for example, absorbs red and blue light, but reflects green and infrared; this is why leaves appear green.

This unique absorption and reflection pattern is called a spectral signature. To make a satellite image, we choose three bands and represent each in tones of red, green, or blue. Because most visible colors can be created by combining red, green, and blue light, we then combine the red, green, and blue-scale images to get a full-color representation of the world. A natural or “true-color” image combines actual measurements of red, green, and blue light.

The result looks like the world as humans see it. (For tips on understanding true-color images, read on the Earth Observatory.' ) A false-color image uses at least one non-visible wavelength, though that band is still represented in red, green, or blue. As a result, the colors in the final image may not be what you expect them to be. (For instance, grass isn’t always green.) Such false-color band combinations reveal unique aspects of the land or sky that might not be visible otherwise. This series of Landsat images of southeastern Florida and the Northern Everglades illustrates why you might want to see the world in false color.

(A related shows how the images were made.) The visible light image shows dark green forest, light green agriculture, brown wetlands, silver urban areas (the city of Miami), and turquoise offshore reefs and shallows. These colors are similar to what you would see from an airplane. The second image shows the same scene in green, near infrared, and shortwave infrared light. In this false-color band combination, plant-covered land is bright green, water is black, and bare earth ranges from tan to pink. Urban areas are purple.