Remote Sensing - a tool for environmental observation

The colours seen by the human eye are in fact

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The colours seen by the human eye are in fact described by three variables intensity, hue and saturation (IHS): - Intensity (also referred to as value or brightness) relates to the total brightness of a colour (amplitude of the wave); - hue (colour tone) refers to the dominant or average wavelength of light contributing to a colour (frequency); - saturation (or chroma) specifies the purity of the colour. Hence, a three dimensional space (international colour cone) is required to describe these three variables (figure 1.14). Image processing normally uses additive colours likewise televisions. Additive primary colours The visible part (0.4-0.7 μm) of the spectrum can be divided into six gradational colours: violet, blue, green, yellow, orange and red. Almost all colours can be synthesized by adding different portions of blue, green and red. These are called the three additive primary colours because synthetic white light forms when equal amounts of blue, green and red are superimposed. The range of each additive primary colour spans one-third of the visible spectrum. A colour image can be produced by adding these three primary colours on a television screen or computer monitor. Subtractive primary colours Colours on e.g. film can also be produced by subtracting specific parts of the spectrum. In order to mix colours for films, the three subtractive primary colours must be used: yellow, magenta and cyan. Colour films Colour prints are colour photographs with an opaque base. Colour film is a transparent medium that may be either positive or negative. On conventional negative film, the colour present is complementary (!) to the colour of the object photographed and the density on the film is the inverse of the brightness of the object. Colour negative films consist of a transparent base coated with three emulsion layers (figure
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20 1.14). Each layer is sensitive to one of the additive primary colours: blue, green or red. During developing of the film, each emulsion layer forms a colour dye that is complementary to the primary colour that exposed the layer: the blue-sensitive emulsion layer forms a yellow negative image, the green-sensitive layer forms a magenta negative layer and the red-sensitive layer forms a cyan negative layer. A more comprehensive description of colour film technology is given by Lillesand and Kiefer (1994), pp. 76-94. Colour infrared film (CIR) The spectral sensitivity of the three layers of colour emulsion on a film may be changed to register the electromagnetic energy of other wavelengths than the visible ones. A shift of the wavelengths towards the near infrared is most widely used. Such a film is called IR colour film or false colour film. It was designed for military purposes and hence it was earlier called camouflage detection film. The sensitivity of the film is shifted as follows: the blue-imaging layer is exposed by green light, the green-imaging layer is exposed by red light and the red- imaging layer is exposed by reflected near infrared. Hence, a false colour film is not sensitive for thermal infrared!
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  • Winter '12
  • JOHN
  • Remote Sensing, Electromagnetic spectrum, µm, Infrared

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