Aerial Photography and Remote Sensing – Study Outline

Class 9, Color Infrared

You should be able to:

1. describe the place of near infrared (IR) light in the electromagnetic spectrum

2. describe how near IR light is made visible in a false color IR image

3. tell how CIR can aid in interpreting images, especially of plants

A. the electromagnetic spectrum and color.

1. color vision review

a. our eyes can only detect three color of light: blue, green and red

b. we detect other colors as a mix of these three primary colors, so we see yellow when our red and green detectors are stimulated equally, which can be caused by either yellow light, or an equal mix of red and green. We can “trick” our eyes into see all colors with just these three, and we can’t tell if something is yellow because it is yellow, or because it is equally red and green.

2. infrared light is light with wavelengths just longer than red light .7 micrometers, up to the beginning of microwaves at 1 cm wavelength

a. The wavelengths that can be used for photography with a regular camera are called near IR, just below the threshold of human vision

b. just as objects are different colors because of the visible light they absorb and reflect, they can be different “colors” of IR depending on how much they reflect

B. visualizing near IR

1. The standard film technique involves using special film that is sensitive to IR and filtering out blue light. In the processing (memorize these!)

a. green light from the photographed scene is displayed as blue

b. red light from the scene is displayed as green

c. IR light from the scene is displayed as red.

2. Satellite detectors are normally set to detect the intensity of a small range of wavelengths (called a band). Each band can be assigned a color and the intensity of light from any three bands can be assigned to blue green and red display to create a false color image.

C. IR signatures

1. With the standard false color reassignments above we have to learn new color signatures for objects in an image, some important ones include –

a. water is black

b. urban areas are light blue to grey

c. dry soil and rock are bright, light blue

d. objects painted green are blue

e. residential areas are blue to dark red depending on the amount of grass and trees

2. detecting vegetation

a. because of the size and shape of healthy plant cells they reflect strongly in the near IR and show up as bright red

b. other colors in the leaves will moderate this to give distinctive colors:

i. well watered grass is bright red

ii. a young agricultural crop will show soil in addition to the plants and appear pink

iii. deciduous forest in the middle of summer are dark red

iv. conifers are darker red to purple.

3. vegetation and stress

a. because plant stress changes the water content of cells, a sick plant reflects less IR than a healthy one, even if it is still green.

b. changes in color within a group of plants of the same type on a CIR image can indicate changes in the health of the plants