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Understanding Electromagnetic Spectrum and Imaging Techniques

Explore the world of electromagnetic waves, from visible light to infrared and microwave bands. Learn about measuring light, thermal infrared, and atmospheric effects, as well as remote sensing techniques, image corrections, and thematic data analysis in the context of multispectral imaging.

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Understanding Electromagnetic Spectrum and Imaging Techniques

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  1. Lecture 4 Content Electromagnetic wave

  2. Electromagnetic wave • Visible light, radio waves, heat, ultraviolet, and X-rays are all forms of electromagnetic energy • All forms of electromagnetic energy travels at the speed of light which is 3 x 108 m/sec • The type of energy being detected is characterized by its position in the electromagnetic spectrum • The spectrum is commonly divided into units of wavelengths measured in micrometers (μm) which is one millionth of a meter

  3. Wavelength Visible ˜ 0.7 ˜ 0.4 micrometers Measuring Light • Light can be classified according to the length of the wave The Electromagnetic Spectrum (EMS) Microwave Gamma Rays X-Ray Ultraviolet Infrared TV/Radio

  4. Measuring Light: Bands • Human eyes only ‘measure’ visible light • Sensors can measure other portions of EMS Bands

  5. Electromagnetic Spectrum

  6. Electromagnetic spectrum characteristics • Visible portion: • Region to which the human eye is sensitive • Extremely narrow range of wavelengths • Wavelength: 0.4 μm to 0.7 μm ( the blue and red regions) • The ultra-violet region has the shorter wavelengths next to the visible blue region

  7. Infra-red portion • Cannot be seen directly with the human eye • Wavelength: 0.7 μm to 0.8 μm • Longer than the red rays in the visible portion • Useful for penetrating hazy weather conditions

  8. Thermal infra-red portion • Cannot be detected by the human eye nor by photographic film • Wavelength: 0.8 μm to 14 μm • Electronic detectors cooled by liquid nitrogen are used to measure the energy emitted at these wavelengths

  9. Microwave portion • Very low intensity • Wavelength: 1mm to 1m • Useful for penetrating hazy weather conditions

  10. Emission of energy from a blackbody • Area under the curve represents the total amount of energy emitted • Sun’s highest energy levels are emitted at the wavelength 0.5 μm • Atmospheric effects • Atmospheric windows are the regions of the spectrum that are used for remote sensing • The systems operating in wavelengths longer that the visible range makes use of such windows for which the atmosphere has relatively high transparency

  11. Swath • Sensors collect 2D images of the surface in a swath below the sensor • Example: Landsat has a 185 km swath

  12. Two types of images • REMOTELY SENSED images Continuous data • Measured Values (light) • Quantitative • THEMATIC Images Discrete data • Values only indicate class • Qualitative

  13. Multispectral ( 2 or more Bands) • Panchromatic ( 1 Band/layer) Continuous data • Two types:

  14. Thematic Mapper Band 1 Band 4 Band 5 Viewing continuous images • Each band or layer is viewable as a separate image

  15. Monitor color guns Part of spectrum Band 4 Band 1 Band 4 Blue Green Band 3 Band 5 Band 2 Red NIR Band 2 Band 3 Band 3 SWIR Viewing images • Three bands are viewable simultaneously

  16. Urban Vegetation Band Combinations • Features can become more obvious 4,5,3 (RGB) 2,3,1 (RGB) 4,3,2 (RGB)

  17. Image Geometric Correction • Raw imagery has no reference to the ground Location in image only Defined by raster grid only

  18. Image Geometric Correction • Correction is the process of giving an image a Real World coordinate system 116 41 52.20 W, 33 50 03.23 N (Latitude, Longitude)

  19. 116 28 55.20 W, 33 00 22.43 N 116 47 27.20 W, 33 50 50.09 N Resampling Method 116 28 07.20 W, 33 55 43.98 N 116 45 14.54 W, 33 43 05.65 N Ground Control • Raw images need ground control • Measured control points are used to correct entire image

  20. Discrete data (Thematic images) • Deal with categorical information about an area • Used and created by analytical operations • Used to extract information from multispectral images

  21. Viewing thematic images • Thematic images • Each pixel contains a class value • Each class value is associated with a specific color

  22. Viewing thematic images • Each class can have associated attributes: • Name, Color, Area etc. Exe…

  23. Vector Raster Polygons Lines Points Vector Coverages • Different means of representing features

  24. Vectors colored according to attributes Vector Coverages • Vectors can also have attributes

  25. … The End …

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