Notes on Chapter 14 Films, Filters, and Photographic Process

14.1 Theory of Color

Visible light 0.4 to 0.7μ

Transmitted white light composed of three additive colors: red, green, blue
Other colors are mixes of these

An additive primary color is one that canot be created by adding other colors

Primary subtractive colors: cyan, magenta, yellow
Optically subtracting two primary subractive colors creates from white light one of the primary additive colors. (See Plate I)

14.1.1 The Munsell System

Can measure color in terms of brightness, hue (color) and saturation (See Figure 14.1)

14.1.2 The Phenomenon of Color

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?

 

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)

14.2 Filters for Aerial Photography

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:

14.2.1 Antivignetting 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

14.2.2 Polarizing Filters

Used to penetrate haze and reduce reflections from surfaces such as water. (Figure 14.3)
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.

14.2.3 Haze-Cutting Filters

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.

14.2.4 Color Correction Filters

Used in color printing
Often used in color-infrared

14.2.5 Narrow Band-Pass Filters

Used to produce color-separation filters for special projects

14.2.6 Special Characteristics of filters

May have to adjust exposure (f-stop and/or shutter speed) since filter blocks light

14.3 Photographic Film Types

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)

Color films
  
     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)

14.3.1 Advantages of Black and White and Normal Color

14.3.2 Advantages of Black and White Infrared Film

IR film doesn't show details in shadowed areas
(See Table 14.2)

14.4 Photographic Processes

Photographic film easy to use, can also store a vast amount of information

14.4.1 Black and White Film

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:

  1. immersion of film in a chemical developing solution
  2. immersion in another chemical solution to stop the developing process
  3. fixing to remove unaffected silver salts
  4. washing to remove all processing materials
  5. drying

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.

14.4.2 Color Film

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.

14.4.3 Normal Color Negative Film

(See Figure 14.10)

14.4.4 Color Infrared Film

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 V)
Color IR  allows the distinguishing of healthy from unhealthy vegetation. 

14.5 Advantages and Disadvantages of Color Films Compared to Black and White Films

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:

  1. Both production and storage to keep colors from fading are more expensive
  2. Images on photographic prints are generally not as sharp. But film technology is continually improving.

14.5.1 Choice of Film

Read on your own.