Seeing What You Can't See

By Kendra Rand

What do night vision goggles, land mine detectors, and studies of the universe have in common?

These may seem rather unrelated, but they have one thing in common. In some way, all of them are connected to a small range of light sandwiched between visible light and microwaves on the electromagnetic spectrum -- infrared light.

Infrared’s place in the world

Human eyes are sensitive to visible light: red, orange, yellow, green, blue, and violet light and all of the shades in between. However, visible light accounts for only a small portion of the total light in the universe.

Radio waves, x-rays, and microwaves are all types of light. The electromagnetic spectrum, shown below, is commonly used to show the different types of light.

By Philip Ronan, http://en.wikipedia.org/wiki/File:EM_spectrum.svg, under GNU Free Documentation License, Version 1.2.

Infrared light, called “IR” in the spectrum above, is just out of range of what the human eye can detect. But that doesn’t mean we can’t use infrared light to see things!

Seeing in the Dark: Night Vision Goggles

Giraffes and turtles from the Santa Barbara Zoo seen through an infrared camera. These images show that warm blooded giraffes glow with more infrared light compared to cold blooded turtles.

Courtesy NASA/JPL-Caltech, Linda Hermans-Killam

U.S. Army photo by Spc. Lee Davis.

Seen through a night-vision device, paratroopers conduct a raid on a suspected terrorist's home in Fallujah, Iraq. The Soldiers are assigned to the 82nd Airborne Division's Company B, 1st Battalion, 505th Parachute Infantry Regiment.

If you need to find someone hiding in the shadows, grab a pair of night vision goggles. The greenish images they produce enable law enforcement, military personnel, hunters, and even science hobbyists to see what is happening even in complete darkness. There are three different types of night vision devices, all which depend on infrared light in different ways.

Thermal Imaging

If you are in complete darkness, you’ll want to use thermal imaging night vision goggles. All people, places, and things give off far* infrared light in an amount proportional to their temperature. Thermal imaging goggles create an electronic image based on the temperature differences in the scene--hotter objects appear brighter than cooler objects. The contrast could be displayed in shades of pink, blue, or any other color, but green is most useful because the human eye can distinguish more shades of green than any other color.

*Far infrared light refers to infrared light near the microwave part of the spectrum.

Low Light Imaging

Low light night vision goggles require that there is some light in the area. They work by intensifying the light signals, sort of like a speaker that amplifies sound when you turn up the volume. The goggles take the ultraviolet, visible, and near infrared light in the scene and use electronics to amplify the signals and create a visible picture of the scene. Low light goggles create the most detailed image of the three types--detailed enough that it is possible to identify specific people in the scene.

Near infrared Imaging

This technique requires two parts--an infrared emitter and an infrared detector. The emitter sends out infrared light, and then the detector records the picture. You can think of the process kind of like using the flash on a camera. When you take a picture the flash lights up the object in focus, and makes the image brighter in the photograph. Only in this case, the flash is in infrared light instead of visible light. This technology is useful in cases where you want to see what is going on without shining visible light on the scene, such as in security systems.

*Near infrared light refers to infrared light near the visible part of the spectrum.