The Photographic Process - Lecture Note - Completely Remote Sensing tutorial, GPS, and GIS - facegis.com
The Photographic Process

Before beginning this page, a review of the answer to the first question, concerning the human eye, in the Quiz at the end of the Introduction may be helpful. With this in mind, use this diagram to compare the components and functions of the eye with that of a photo camera:

Comparison of the operation of the human eye in obtaining an image with the function of a film camera in recording an image.

Black and white (b & w) photographs start with exposing a light-sensitive film to incoming electromagnetic radiation (light), selected from the spectral range between ultraviolet through visible, and into the near infrared. The optical system of the camera focuses the light, reflected from the target, onto the focal plane (plane of focus). The film is held flat at the focal plane, and the light activates positions in the film area in the same spatial relation that the radiation photons had from the surfaces within the scene. The recorded exposure is a function of many variables, of which the three principal ones relate to the scene, the camera, and the film.:

1) The scene usually contains various objects that contribute their spectral character (reflected wavelengths) and intensities of the reflected radiation.

2) In the camera, we can vary the lens diameter, D, and the effective size of the aperture opening, d.

The aperture depends on the diaphragm width for admitting light. An open/shut shutter controls the duration of light admission. The optical characteristics of the lens vary the distance from the lens to the film (focal length, f) at which the focus is sharpest. This light-gathering system is adjustable for film response (ISO, formally ASA values);

3) In the film, its properties vary, e.g., which wavelengths it is most sensitive to, and under which conditions does it develop best as a negative and then printed.

For most cameras, the four variables that we normally adjust are:

1) The focus, by moving the lens back and forth, relative to the focal plane, so that the target image is in focus in the plane of the film;

2) The F-stop, defined as f/d, the focal length divided by the effective diameter of the lens opening. Typical values of the F-number are F/1 (the lens opening is the same size as the focal length), F/1.4, F/2 (the lens opening is half the focal length), and F/2.8 to F/22. The denominator increases by approximately the square root of 2 (1.414...), so that each decrease of F/d (i.e., denominator increases), leads to a decrease in the amount of light admitted by a factor of 2. Thus the F-number increases as the lens diameter decreases, and therefore we photograph dark scenes at low F-stops, e.g., F/2, and bright scenes at high F-stops, e.g. F/22.

3) The shutter speed (typically, in a sequence from 1/2000, 1/1000, 1/500, 1/250, 1/30, 1/15, 1/8, 1/2, 1/1, 2, 4 ...., in seconds), which controls film exposure times;

4) The film speed, i.e., the exposure levels over which the film responds. The ISO (ASA) rates film properties. High ISO numbers refer to "fast" film (high speed), e.g., ISO 1000, which requires less radiation (hence, a shorter exposure time or a smaller aperture) to achieve a given response. "Slow" film, e.g., ASA 64, requires a longer exposure or a larger aperture, but provides higher resolution. For aerial film, the AFS (Aerial Film Speed) is more commonly used.

One general equation for exposure is:

exposure equation

where

  • E = exposure in Joules (J) mm-2
  • s = intrinsic scene brightness, in J mm-2 sec-1
  • d = diameter of lens opening in mm
  • t = time in seconds
  • f = lens focal length, mm

(see Ch. 2 in Lillesand & Kiefer, 2000). Changes in any one or combination of these variables brings about variations in photo response characteristics. These differences can be favorable and we actuate them by adjusting one or more camera settings.

Black and white film consists of a base or backing, coated by an emulsion composed of gelatin, in which are embedded tiny crystals of silver halides (commonly, Silver Chloride, AgCl) together with wavelength sensitive dyes. The dyes respond to radiation from segments of the electromagnetic spectrum, such as, ultraviolet, visible,and visible/near IR. Special films respond to photons from shorter or longer wavelengths; for example, X-ray film. When a number of photons strike a halide crystal, they knock loose electrons from some of the silver (Ag) atoms, ionizing them (remember Einstein's photoelectric effect; Introduction). The number of electrons thus activated depends on the brightness (intensity) of the radiation. We can control the part of the spectral range to be recorded by using color filters over the lens. These filters admit radiation from limited segments of the spectrum. This process is a photochemical reaction which conditions the halide grains for later chemical change, forming an intermediate latent image (invisible but ready to appear when we develop it).

Developing begins by immersing the film in an alkaline solution of specific organic chemicals that neutralize the electrons and reduce Ag+ ions into minute grains of black silver metal. The number of such metallic grains in a given volume determines the film (negative) density. For parts of the emulsion receiving more light, the density (darkness) of the film is greater. In the developing process, we must stop the ion conversion at some point using an acidic stop bath. We remove any silver halides that remain undeveloped by chemical fixing. Volumes in the thin film that saw little exposure (fewer photons) end up with minimal silver grains and thus appear as light and clear in the film negative. We can control and modify the development process, and hence relative densities in the negative, by changing such variables as solution strengths, developer temperatures, and times in each processing step.

Next, we must use the negative to make a separate, positive, black and white print, in which the negative's dark tones correspond to lighter areas in the scene, and light tones (fewer Ag+'s) to darker areas. We do this during the printing process. A print (or a positive transparency) consists of an emulsion, backed (in a print) by paper. We pass white light through the negative onto the print material. Clear areas allow ample light to pass and strike the print, which produces high densities of dark (silver-rich) tones. Thus, the initial low levels of photons coming from the target (relative darkness) ultimately produce a print image consisting of many silver grains that make the areas affected dark. Bright target areas in turn, being represented by dark areas in the negative (more Ag+'s) that prevent light from passing, are expressed as light (whitish to light gray) tones in the print (little silver, so that the whiteness of the paper persists). Once again, we can control the relative levels of gray, or increasing darkness, in the development process by changing the same variables as above, by modifying exposure times, by using print papers with specific radiation responses, and by using filters with different spectral responses (minimizing passage of certain wavelengths) or light transmission. Thus, we can choose different average tonal levels of the print, and, more important, we can adjust the relative levels of gray (tones) to present a pictorial expression, called contrast. Contrast determines whether a scene with variable colors and brightnesses appears flat (low contrast) or presents wide ranges of light-dark areas (high contrast) that aid in discriminating features. Contrast is the ratio of density to the logarithmic value of exposure. We can plot this ratio in the Hurter-Driffield (H-D) curve, which is a straight line with a slope angle for a range of exposures but becomes curved at high and low exposures.

We can expose b & w films under a wavelength-controlling condition that converts them into multispectral images. We do this by using color filters that pass-limited ranges of wavelengths (bandpass filters) during exposure. As we explained in the Introduction, a red filter, for example, passes mainly radiation representing the red region of the visible spectrum. Reddish objects produce high exposures that appear in dark tones on a negative and reappear as light tones in b & w prints or in red on color positive film. We describe why this is so different from the response of b & w film in the following paragraphs. Green appears as dark in a b & w multispectral image representing the red region, and as dark or subdued green in a color multispectral version. We can project multispectral positive transparencies for different color bands using several color filters onto color print paper to produce natural or false color composites, as described in the Introduction.

As an aside, transparencies representing different bands can be combined in a projection system, using filters to determine the colors sought, that result in a color composite. Commercial systems are available, as exemplified by this Color Additive Viewer made by International Imaging Systems, Inc.

A color additive viewer; the transparency film, with different bands, aligns any three multispectral bands, each with a unique color filter, into a single projection system that produces a color composite image on the viewing screen.

The use of filters to produce individual color photographs is one of two principal ways to make this product (the other uses multiple color-sensitive layers in the film itself). Much as does Landsat and other systems utilize filters on the sensors to subdivide the light received into wavelength intervals (the bands), so will a multiple camera array have filters of different bandpass intervals over the different lens involved. Here is a plot that shows the spectral properties of such filters:

Spectral response curves for several filters.

How we use color film to produce color images involves some different concepts, although many of the same factors and mechanisms are still valid. Starting with the three additive primary colors, red, green, and blue, or the subtractive primary colors, yellow, cyan and magenta, we can make other colors by using the principles of either the color addition or the color subtraction process. Look at these diagrams:

Color Models
Color additive and subtractive models, using overlapping color circles.
Additive Color Model Subtractive Color Model

Color addition works when we superimpose the primary colors on one another. For example, if we shine a green light and a red light on the same spot on a white wall, we will see some shade of orange or yellow, depending on the relative intensity of the red and green illumination. If we add a third blue light to the spot, we will see white or a shade of gray. Computer displays work this way. Of course, the absence of any amount of all three primary colors leads to black. To create a color shade (i.e., some deviation from an arbitrary primary color value, such as pink or purple or yellow) in the photographic printing process a mix of the three primaries in some non-equal proportion is the procedure. (In a computer-generated color image, we can typically choose a number between 0 and 255 to indicate how much of each of the three primary colors we want. If our display board has sufficient memory, we will have 2553 (16,581,375) colors to choose from.)

In subtractive color, we use filters to remove colors. For example, a yellow filter removes colors other than yellow, as do cyan (bluish) and magenta (reddish) filters. If one superimposes all three filters, little or no visible light gets through, so either black or dark gray results. By combining pairs of the subtractive primary colors, we can create each of the additive primary colors. Magenta and yellow produce red. What corresponds to mixing cyan and magenta; yellow and cyan?

The principles of color subtraction apply to color-sensitized film. This film consists of emulsion layers containing silver chloride treated with light sensitive dyes, each responding to a limited wavelength range. These layers act as subtractive filters during development. Thus each layer of the film responds to different sections of the scene's spectrum. These layers are stacked, respectively, as follows: a blue-sensitive layer on the top, then a yellow filter layer (to screen out ultraviolet and blue from passing into the next layers; omitted from the diagrams below), and finally, green- and red-sensitive layers.

Effects of color filters that permit transmittance or absorptance of light of different wavelengths.

From F.F. Sabins, Jr., Remote Sensing: Principles and Interpretation. 2nd Ed., 1987. Reproduced by permission of W.H. Freeman & Co., New York City.

Referring to the above diagram, when a shade of red passes through a color layer sensitized to cyan (a blue-green, the complementary color to red; the sum of any primary color and its opposing complement always equals white), its absorption activates the dye/silver grains in that layer to produce, in a negative, cyan tones in areas associated spatially with reddish objects in the scene. In color film, the three subtractive color layers stack together (a fourth serves a special purpose, described below) on top of a clear base. To guide you in reasoning through production of other colors, check this schematic diagram:

 Schematic diagrams representing both color negative and positive film, showing how different color emulsion layers respond to light of different wavelengths; the process is two-step when prints are the final product (a different response pattern determines color transparencies).

From F.F. Sabins, Jr., Remote Sensing: Principles and Interpretation. 2nd Ed., 1987. Reproduced by permission of W.H. Freeman & Co., New York City.

Thus, in a similar manner, light from a blue subject reacts with the yellow layer to produce a yellow shade (red and green make this complementary color) for its area on the negative.

Additive and Subtractive Color Triangle diagram.

From F.F. Sabins, Jr., Remote Sensing: Principles and Interpretation. 2nd Ed., 1987. Reproduced by permission of W.H. Freeman & Co., New York City.

As evident in the diagram, each primary color activates the layer containing the subtractive color opposite it. Several other rules or observations apply:

1) A given primary color does not directly activate the other two film layers.

2) Note that yellow + magenta = red. The red is common to each of these subtractive colors, with blue and green being filtered out. The same rationale applies to the other two combinations of subtractive colors.

3) White light exposes all three subtractive layers in the negative. The sum of these three layers (the center of the color diagram on the right) on a positive is black. Conversely, black (absence of light) objects produce a clear (not colored) area in the three layers of film

4) We must insert a fourth, special yellow filter layer below the yellow layer, because the dyes in the red and green sensitive layers below are also sensitive to blue, which this filter layer screens out and then dissolves away during developing.

To comprehend how to make a color print, follow this exposition, which is an exact quote from F. F. Sabins, op. cit., 2nd Ed., pp, 44-46; we reproduce one of the above figures to minimize your need to scroll up the screen:

In the negative film, "the silver halide salts of the green-sensitive and red-sensitive layers are also sensitive to blue light. A yellow filter layer beneath the upper emulsion layer prevents blue light from exposing the green-sensitive and red-sensitive layers. The yellow filter layer is dissolve are removed during film processing. On negative film the red-sensitive, bottom layer produces a complementary cyan image of a red subject. Green and blue subjects produce magenta and yellow images respectively. The white subject exposes all three layers, resulting in an image that transmits no light; the black subject results in a clear image because none of the layers is exposed. The negative film records the color of a subject as its complementary color. The image on a negative color film is projected onto photographic paper coated with sensitive emulsions that is developed to produce a color print. Positive color film records a subject in its true color, not its complementary color. Looking at the positive film part in the above diagram, a red subject forms a clear image on the red sensitive cyan-colored layer. The red subject also forms a magenta and a yellow image respectively on the green-sensitive and blue-sensitive layers. When viewed with transmitted white light, the yellow and magenta images absorb blue and green respectively and allow a red image to be projected. A white subject forms clear images on all three layers." (Similar reasoning applies to other color combinations - NMS)

Other systems of color production have been devised. One mentioned briefly here is the IHS system, in which:

  • I = the color intensity or brightness,
  • H = the hue, comprised of a dominant wavelength, averaged from a limited range of adjacent wavelengths
  • S = saturation, the purity of the color relative to gray.

This system is sensitive to controllable transformations (computer-aided) that optimize and enhance color representations.

Source: http://rst.gsfc.nasa.gov