NASA - National Aeronautics and Space Administration

Light comes to us in waves

Light comes to Earth in waves. Some of the waves are long. Some of the waves are short. The waves are all different in their size. We can see some of the waves in the form of color. But other waves we cannot see with our eyes. But we can see them with special cameras.

Infrared light is one of the lights we can see with special cameras. Infrared light shows us how hot things are. It can also show us how cold things are. But it all has to do with heat.

Our bodies give off heat in waves. Animals like dogs and cats give off heat too. With special infrared cameras, you can see the heat. It makes things that are hot look like they are glowing. In infrared light, hot things look bright yellow and orange. Items that are colder, such as an ice cube, are purple or blue.

We use infrared cameras to help us see things. NASA scientists use infrared to help predict weather. Rescue workers use them to find people who need help.

How can we "see" using the Infrared?
Since the primary source of infrared radiation is heat or thermal radiation, any object which has a temperature radiates in the infrared. Even objects that we think of as being very cold, such as an ice cube, emit infrared. When an object is not quite hot enough to radiate visible light, it will emit most of its energy in the infrared. For example, hot charcoal may not give off light but it does emit infrared radiation which we feel as heat. The warmer the object, the more infrared radiation it emits.

The Electromagnetic Spectrum

To make infrared pictures, we can use special cameras and film that detect differences in temperature, and then assign different brightness or false colors to them. This provides a picture that our eyes can interpret.

Humans may not be able to see infrared light, but did you know that snakes in the pit viper family, like rattlesnakes, have sensory "pits", which are used to image infrared light? This allows the snake to detect warm blooded animals, even in dark burrows! Snakes with 2 sensory pits are even thought to have some depth perception in the infrared!

Many things besides people and animals emit infrared light - the Earth, the Sun, and far away things like stars and galaxies do also! For a view from Earth orbit, whether we are looking out into space or down at Earth, we can use instruments on board satellites.


What does the Infrared show us?

Building and tree - visible light, and same building and tree on infrared film

We know that many things emit infrared light. But many things also reflect infrared light, particularly near infrared light. Near infrared radiation is not related to the temperature of the object being photographed - unless the object is very, very hot.



Infrared film 'sees' the object because the Sun (or some other light source) shines infrared light on it and it is reflected or absorbed by the object. You could say that this reflecting or absorbing of infrared helps to determine the object's 'color' - its color being a combination of red, green, blue, and infrared!



This image of a building with a tree and grass shows how Chlorophyll in plants reflect near infrared waves along with visible light waves. Even though we can't see the infrared waves, they are always there. The visible light waves drawn on this picture are green, and the infrared ones are pale red.
 

To read this entire article, please visit the Imagers Web site.