I. Electromagnetic Radiation
1. Review the wavelengths of radiation and complete the following table:
Division Ultraviolet
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Green Visible
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Red Visible
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Near Infrared
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Far Infrared
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Microwave
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2. Wavelength and frequency of radiation are inversely related -- the higher the wavelength, the lower the frequency. Equation 1.1 (p. 4, Avery & Berlin) relates wavelength (l) to frequency (n). To find the frequency from wavelength, use the second version: n = c / l , where c = the speed of light, 3 x 108 m/sec. Use this equation to find the frequency of:
(a) Blue light (wavelength = 0.4 micrometers)
(b) Microwaves (l = 10 cm)
(c) Near infrared (l = 1.1 mm)
3. Check out the Radiation Laws, Chapter 6, Avery & Berlin, pp. 120-122. Wien's Displacement Law (page 121) relates temperature (T, in degrees Kelvin) to the wavelength (l, in mm) of maximum radiant output of any object: l = 2,897/ T. Any object gives off electromagnetic radiation (EMR) at a great range of wavelengths, but the peak wavelength decreases with increasing temperature. Use this law to calculate the peak wavelength of the Sun and Earth. What division of the spectrum does each wavelength fall into?
(a) Sun (T = 6000 K)
(b) Earth (T = 300 K)
(c) Weather satellites can generate images of Earth at night, when the only energy available is that generated by Earth itself. Based on your answer in (b), what kind(s) of radiation can be used for these "dark-side" pictures of Earth?
II. Atmospheric Interactions
A. Scattering and Absorption
4. Two major types of scattering can reduce the amount of radiation that penetrates the atmosphere. Name each and describe their effects:
5. Look at Photo A in lab. You will notice some areas where the ground image seems obscured. What is the cause of this? Discuss the effect in terms of reflection, scattering and absorption.
6. Refer to Photo B in lab. This is a "high-oblique" aerial photo, tilted enough to include the horizon and part of the sky. What is responsible for the diminishing clarity toward the horizon?
7. Refer to Photo C in lab. This is a color infrared (CIR) aerial photograph. . This is a common means of using color for remote sensing, and we'll discuss it more later. Notice that the colors are not consistent from place to place on the photo. What might be responsible for this "washing out" of colors?
8. Look at Photo D in lab. This is a color infrared (CIR) aerial photograph. From our discussion of films, you recall that instead of recording ordinary visible light (blue-green-red), color infrared film records green, red, and infrared radiation. The blue wavelength reflected from the ground is blocked; so blue you may see on the photo is actually green on the ground. From your knowledge of scattering, why is it useful to omit the blue and take in the infrared? In this photo, what does red represent?
III. Photographic Systems
A. Lenses & Focal Length
9. The focal length of a lens is the distance behind the lens at which light rays, if the rays are coming in parallel to each other, are focused to a point. If the rays are not parallel, which is true with close objects, then the focal plane holding the film must be adjusted to a different distance than the focal length. You can focus most 35mm cameras -- the focus knob varies the distance between lens and focal plane. Unlike 35mm cameras, most aerial cameras cannot be focused -- they are "fixed-focus," meaning the focal plane is at a fixed distance behind the lens. It's certainly not because aerial cameras are cheap! Use the ideas of light rays approaching the lens to explain why aerial cameras can be fixed-focus and still work properly.
B. Shutter Speed & Aperture
10. The diaphragm, or aperture stop, helps control the amount of light reaching the film. We could use the actual size of the diaphragm, but as Avery & Berlin points out, the exposure is also affected by the focal length of each lens (in addition to the shutter speed). To standardize exposure settings from camera to camera, most cameras use a relative aperture measure called the f number, or f-stop. The larger the f-stop, the less light is reaching the film. You can see this from the definition of f-stop:
F = F-stop = lens focal length (f)/lens opening diameter (d) = f/d As the lens opening (d) increases, the F-stop decreases. A standard series of F-stops is used: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, etc. Each higher stop number decreases the exposure by half. If you multiply any F-stop by the square root of 2 (1.414...), you'd get the next higher F-stop; this is because the area of the lens is related to the square of the diameter.
To make lenses with different focal lengths use the same F-stops, manufacturers must vary the lens opening that matches the F-stop. You can figure out the lens opening if you know the F-stop and focal length -- by using the definition of f-stop, d = f / F. Calculate the lens opening used for the following combinations. Please show your work:
(a) F-stop = 11, focal length = 50 mm
(b) F-stop = 4, focal length = 200 mm
(c) F-stop = 2.8, focal length = 400 mm
(d) The longer the focal length of a lens, the more magnification power it has. Based on your calculations above, particularly of (c), why is it difficult to have a small F-stop on lenses with high magnification?
C. Exposure
11. The amount of light that reaches the film, or the exposure, is related not only to the aperture and shutter speed, but also to the brightness of the scene being photographed. The relationship between exposure (E), aperture (d), shutter speed (t) and brightness (s) is
Exposure = sd2t/4f2 Where
s = Brightness of scenet = exposure time, sec
d = Aperture - lens opening, mm
f = Focal length, mm
Using F-stops this equation can be simplified to
Exposure = st/4F2 You can use this relationship to determine proper exposure. For example a film is properly exposed when the lens aperture is set at F/8 and the exposure time is 1/125 sec (condition 1). If the F-stop is changed to F/4, and the scene brightness doesn't change, what exposure time should be used to yield a proper film exposure (condition 2)? Since we want the exposure (E) to be identical for both conditions, we can equate the two exposures:
E1 = s1t1/4(F1)2 = s2 t2/4(F2)2 = E2 Since s (scene brightness) is the same for both photos (s1 = s2), we can cancel it and find:
t1/(F1)2 = t2/(F2)2 Solving for shutter speed (t2):
t2 = t1 (F2)2/F1)2 = 1/125 . (4)2/(8)2 = 1/125 . 16/64= 1/500 sec Hence at F/4, we would use a shutter speed of 1/500 for the same exposure.
Calculate equivalent exposures for different settings in these problems. Please show your work:
(a) Changing from 1/500 sec at F/2, to F/4 -- find shutter speed:
(b) Changing from 1/500 sec at F/2, to F/32 -- find shutter speed:
(c) Changing from 1/60 sec at F/8, to F2.8 -- find shutter speed:
IV. Use of Stereoscopes
The first section is to give you some practice using stereoscopes. If you have never used a stereoscope before, it may take some practice to get the hang of it. Stereo is a great tool for teasing details out of aerial photos, so practice it!
A. Mirror & Binocular
12. Locate the Delph mirror stereoscope and examine the photo pair placed beneath it. Describe the scene in terms of vertical relief -- which areas are higher or lower?
13. Examine the photos set up on the binocular stereoscopes. What is the maximum magnification setting of the stereoscope? At this setting, can you notice the quality of the details breaking down?
B. Pocket Stereoscope
Pick up a pocket stereoscope and a stereo vision chart. Follow the instructions for setting up the stereoscope. Place the vision chart (Figure 3-33, p. 68, Avery & Berlin) under the stereoscope and set it up as you would a photo pair.
Within the rings marked 1 through 8 are designs that appear to be at different elevations. Rank the apparent heights of each item within each circle, with 1 being highest. Two or more designs may be at the same elevation; use the same number for these designs.
14.
Ring 1
Ring 6
Square
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lower left circle
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marginal ring
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lower right circle
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triangle
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upper right circle
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point
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upper left circle
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marginal ring
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Ring 7
Ring 3
black flag
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square
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marginal ring
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marginal ring
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black circle
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cross
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arrow
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lower left circle
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tower with cross
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upper center circle
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double cross
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black triangle
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black rectangle
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15. Indicate the relative elevations of the rings 1 through 8:
( )
( )
( )
( )
( )
( )
( )
( )
highest
lowest
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