Color Aerial Photography: Use of Additive Color (Hue, Saturation and Brightness) (Especially for GATE-Geospatial 2022)

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Color aerial photography may be carried out by taking pictures with multilayer aerial film simultaneously in the blue, green, and red regions of the visible portion of the electromagnetic spectrum. It may also be performed by using three separate aerial films and later combining the monochromatic images obtained during processing to produce a full-color image. The second method makes it possible to obtain the most accurate differentiated color reproduction of features but, overall, is more complicated and expensive than the first method at the present time. Aerial photography in transformed colors, which is called multiband aerial photography, is sometimes regarded as a type of color aerial photography.

History of Color Photography

  • Color photography from the air was first performed in 1936 in both the USSR and Canada. In both countries, motion-picture cameras were used rather than aerial cameras.
  • In the USSR, the photography was carried out by the Leningrad Division of the Central Scientific Research Institute of Geodesy, Aerial Photography, and Cartography.
  • Color aerial photography proper was first used for scientific and economic purposes immediately after World War II. By the late 1950՚s, it had been used extensively.

Applications of Color Photography

  • Color aerial photography is effective in the general geographic study of the earth (especially studies of seasonal changes in the earth՚s appearance) . It is used in:
    • Geological mapping of outcrop areas
    • Management of coniferous and deciduous forests
    • Inventories of tree stands damaged by industrial pollution or harmful insects
    • Compilation of soil maps of croplands
    • Surveys of planted fields
    • Study of continental shelves, especially with respect to topography, shallow-water bottom types and vegetation, water pollution, and ice conditions
  • Color aerial photography is also employed in the planning of urban reconstruction, in socioeconomic and archaeological research, and in topographic surveys of densely populated regions.
  • In surveys from spacecraft, color photography is used as a new means of studying the earth՚s surface and phenomena that occur on the surface.

Principle of Color Reversal

This colour reversal concept may sound confusing, and it is, but with a little practice, force memorization, one can learn it. Just we need to remember that the colors (wavelengths) of light mix differently than the watercolour paints we used in school.

Additive Colour

First described by James Clark Maxwell in the mid-1800s, the colour additive theory describes how he perceive colour and how colors are created. Essentially white light is a combination of many different colors, a continuum of wavelengths organized into “bands” which we label with names (blue, green, red etc.) . When equal parts of each of the three major bands are combined, we get white light. White light is the sum of red, green and blue.

Additive Components

Red, green, and blue are the “primary” colors of white light. All three colors will result in white; the absence of all three will produce black. When two primary colors of light are added together, we get a colour that is brighter than either of its components.

These are the “additive” combinations:

  • Red + Green = Yellow
  • Red + Blue = Magenta
  • Blue + Green = Cyan

By using unequal amounts of red, green, and blue light we can create new colors. Using red, green and blue, the entire spectrum of visible light can be created.

Spectrum of Visible Light Created

Two primary colors of light are added together, we get a colour that is brighter than either of its components

A TV monitor uses additive colour. Three beams of electrons corresponding to red, blue and green are projected onto a fluorescent screen. The pixels of the screen are made of triads which are sensitive to the three colors, based on the proportion of red, blue, or green light striking the triad the pixel can appear in any single colour.

Spectral Response

So far light has been referred to with terms like “color” , “wavelength” and “photon” . These terms will be reviewed in greater detail in another module but a basic explanation is required to understand the usage of these terms here. Basically, the physics explanation of what light is involves two ideas; the first is that light has wavelike properties (refraction of colors in a prism) and particle-like properties (the heating of a surface in the sun) . The wave explanation allows for the splitting-up of the electromagnetic spectrum into colors (UV, Blue, Green, Red, Infrared, etc.) and the particle (photon) explanation allows for the explanation of the energy imparted to a surface by light of different wavelengths (UV photons have the most energy, shortest wavelength, and infrared have the least, longest wavelengths, etc.)

Molecular scattering takes place in the atmosphere between the camera and the ground and has the tendency to reduce clarity. UV and blue wavelength photons that have been scattered by molecules in the atmosphere, called “diffuse skylight” , enter the camera lens along with the photons reflected from scene and blur the image. Filters are used to reduce the film՚s exposure to these shorter blue and UV wavelengths. Infrared red films (longer wavelengths) are less sensitive to atmospheric scattering and are often used in conjunction with filters to increase clarity.

It is necessary to know how to read spectral response graphs. Along the x-axis is wavelength in micrometres (µm) or nanometers (nm) usually for visible wavelengths, and along the axis is the percent reflectance (or absorption) . Having such graphs allows for the separation of characteristic materials within an image based on the “brightness” or percent reflectance; if we know very little about an area, but have near-infrared wavelength imagery, it is possible for us to “classify” certain features based only on their reflectance, and it is possible to discriminate healthy vegetation, tree types, soil/rock types etc. based solely on how bright they appeared in the image using the spectral response curves for these materials as an interpretation aid.

Hue and saturation are independent characteristics of colour. Hue refers to the wavelength of light, which we commonly call “colour” , while saturation indicates; how pure the colour is, or how much white I mixed in with it. For instance, “pink” can be considered a less saturated version of “red” .

Hue and Saturation Lightness

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