Visible light 0.4 to 0.7μ
Transmitted white light composed of three
additive colors: red, green,
Other colors are mixes of these
An additive primary color is one that canot be created by adding other colors
colors: cyan, magenta, yellow
Optically subtracting two primary subractive colors creates from white light one of the primary additive colors. (See Plate I)
Can measure color in terms of brightness, hue (color) and saturation (See Figure 14.1)
Sequence of events for color: source → modified by reflectance or transmission → then reaches the optical and physiological components of vision
Why are objects a particular color?
An object reflecting all light is white (a mixture of red, green, and blue)
An object that doesn't reflect light well is dark or grey
An object absorbing green and blue, but reflecting red appears red.
Looking through a filter is different. A red filter absorbs blue and green and transmits red. A filter that transmits red and green and absorbs blue will appear yellow. (a Minus-blue filter) (See Table 14.1)
Filter: a piece of material placed between the film and the reflected light rays coming from the source, which absorbs unwanted light rays, not allowing them to reach the film
In aerial photos, classified by their use:
narrow band-pass filters
Compensates for tendency of wide angle lenses to have darker areas on the edges. Slightly darkened central area that gradually diminishes from the center to the outside edge
Used to penetrate haze and reduce reflections from surfaces such as water.
Effective when the angle of reflection of light is approximately 35d.
Eliminates all rays of light except those vibrating in a particular direction
Rotate one lens against the other.
Haze caused by weather conditions or pollution. Scattering of blue light by
particles in the atmosphere.
Filters remove various amounts of blue light. Range from almost clear to dark yellow. Need to be darker at a higher altitude.
Used in color printing
Often used in color-infrared
Used to produce color-separation filters for special projects
May have to adjust exposure (f-stop and/or shutter speed) since filter blocks light
Black and white films
Panchromatic -- sees in the human range 0.4 to 0.7microns
Black and white infrared -- includes the near infrared 0.4 to 0.9microns
True infrared -- uses a black filter to absorb all visible light (See Figure 14.7)
Normal Color -- sees in the human range 0.4 to 0.7microns
False color -- healthy vegetation appears red, dying or diseased vegetation is white or greenish white (See Plate II)
IR film doesn't show details in shadowed areas
(See Table 14.2)
Photographic film easy to use, can also store a vast amount of information
consists of a film base covered with a light sensitive emulsion, may also have anti-curl coating and anti-halation backing
Silver halides, the light sensitive substance. Suspended in gelatin.
Average size important for
1. amount of light needed to form an image
2. Graininess of the image which influences the resolution.
Latent image -- the pattern made in the light sensitive emulsion that can be made visible by development. Is not permanent until development.
Exposure -- the total amount of energy reaching the film. Depends on the film speed. Speed rated by ASA, ISO, or AFS (Aerial Film Speed) standards. Nine differnt atmospheric components of sunlight
Aerial Exposure Computer -- an exposure tool developed by Kodak
Grain -- the relative size of the silver halide crystals
Resolution -- In photos often measured in line pairs per millimeter
that can be just barely distinguished. (See Figure 14.9)
Many factors affect resolution such as contrast, lens quality, etc.
Resolution proportional to:
1. Contrast ratio between object and background
2. Ration between the length and width of an object.
3. the number of objects in the pattern
Resolution inversely proportional to:
1. Graininess of film
2. Amount of image motion during exposure
3. Amount of atmospheric haze
Resolution can also be described as the size of the smallest object that can be distinguished.
Negatives formed when millions of exposed silver halide crystals are converted to silver metal in the developing process
Areas on the film receiving the most light are darkened by silver. Areas receiving no light remain transparent on the developed negative because they contain no silver. That's why the image in a negative is reversed
In brief, the developing process consists of:
Positives the black and white negative is reversed to expose a positive image. Most aerial photos use 9 inch film. Prints are made by a contact printing procedure with no enlargement.
Aerial color films can be either color reversal films (makes color slides without a negative) or negative color films (makes a color negative which is then used to make a color print)
Negative color films produce color negatives similar to b&w film except that three separate color emulsion layers are used. Color negatives reversed as to scene brightness and geometry and the negative colors are complements of the original scene. Prints made from color negatives are correct as to geometry, brightness, and color of the original scene.
Color transparencies (slides) have to be seen on a light table. The final product is hard to replace if damaged. Transparencies are very fine-grained and reveal a maximum of detail.
Can also make b&w prints from color negatives with special papers.
(See Figure 14.10)
Color infrared films similar to normal color negative films with some crucial differences (See Figure 14.11)
Color IR gives better contrast between hardwoods and softwoods (Plate
Color IR allows the distinguishing of healthy from unhealthy vegetation.
Advantage of color over B&W is that people can distinguish thousands or millions of colors and only 200 shades of gray. Greater levels of discernible difference.
Disadvantages are that:
Read on your own.