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Displaying Invisible Light: False Color

What would the world look like if you could see light that is currently invisible to us – say infrared, ultraviolet or microwaves? What color would we interpret them as? Remember all electromagnetic energy is colorless—we see color because of functions going on in the eyes and brain.

Some animals can sense energies of light (electromagnetic energies) that are invisible to humans. Bees sense ultraviolet reflected from flowers, crucial to discriminating types of flowers for pollination. Butterflies see from the UV to red, with the added ability to see polarized light. Snakes sense thermal infrared, which they rely upon to locate their prey. These powers of seeing energies invisible to the human eye are critical to their survival. Then there is Superman with his X-ray vision, but that’s another story...

Man-made satellites have sensors that detect energies that are invisible to our eyes, and we use clever ways to display such energies on computers. With the earliest satellites, only one band of electromagnetic information could be visualized at one time by using gray shades. With the development of color display monitors (originally television screens), three sets of measurements could be displayed at once by placing each set of information in one of the primary colors of the display. This technique requires the viewer to understand how colors are created on a computer display monitor. Colors visible on the computer display are based on the overlap of the three satellite detector intensities, which depend on the physical properties of the system object being observed. With satellite sensor data, it’s possible to display images Earth’s surface, gas components of the atmosphere, the Earth’s magnetic field, etc.

 

Overview of the Electromagnetic Spectrum. From smallest to largest wavelength: Gamma, X-ray, UV, visible light, Infrared, Microwaves, and finally Radio Waves

The basics and examples of near infrared digital photography.

The basics and examples of ultraviolet and near infrared digital photography.

Materials

Download DigitalImageBasics, which is available as part of the DEW software bundle.

Download MtStHelens_1973.jpg (image on lower right).

What To Do...

Open the MtStHelens_1973 image in the 'FalseColor' tab panel of Digital Image Basics.

This image was taken of Mt. St. Helens in southwestern Washington by sensors on board Landsat, the first satellite designed to measure and monitor land resources. The sensors could measure a number of narrow bands of electromagnetic radiation, but only three are available for this image: infrared, and visible red and green light. The initial display shows no representation of invisible energy, only the intensities of red and green light detected by the satellite sensors.

Use the drop down menus to make 'False Color' reflect these settings:

 
Image of Mt St Helens in 1973 MtStHelens_1973.jpg from USGS Earthshots website. Download this image by right-clicking and saving the image to your computer.
Satellite Measurement
Computer Color
Red
Red
Green
Green
Blue

Question: In this “blue-less” world, what colors would you expect to see? Do you see these all of these colors on this image? What colors are seen and what colors are missing?

Just for a lark, try making the red satellite sensor energy show as 
display color green and vice versa: green satellite sensor energy 
show as red display color. What do you make of that? False colors!

Making the invisible visible.

Infrared energy is invisible to human eyes but is measured with 
one of the satellite sensors. We can assign this invisible energy 
data to a computer display color as a “false color” that we can see. 
We can assign it to any color: red, blue, green, yellow, cyan, or 
magenta. Try out different display colors for the infrared measurements.

Question: How does the intensity of the infrared data compare to 
that of the visible red and green light?

Go to Characteristic Surface Reflectance

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