Wednesday, September 24, 2008

Image Quality

Image quality criteria can be divided into two categories: basic criteria that apply to all imaging system, image defects-some of this are unique to television images.

Basic criteria:

  • Image definition. Definition is the degree of 'in-focus' appearance of the image. It is primarily determined by the shape of the transitions between dark and light areas of the picture. (EIA standard resolution test chart)
  • Limiting resolution. (?)
  • Gray scale. It can be specified by three parameters: 1. highlight brightness, the brightness of the brightest areas of the image; 2. contrast ratio, the ration of the brightness of the brightest areas of the image to the darkest; 3. gamma, the slope of the transfer characteristic of image brightness as a function of scene brightness.
  • Signal-to-noise-ratio

Image defects:

  • Flicker: when the frame rate is not high enough to cause the eye to see continuous image.
  • Aliasing: sampling problem.
  • Geometric distortion.
  • Hum: ex. a horizontal hum bar moves vertically through the picture, interference caused by power source voltage.
  • Ghost: a duplicate of the main mage, slightly displace from it.

Image quality factors:

  1. sharpness: how to measure-use a bar pattern, use 'rise distance', the distance for the pixel level to go from 10% to 90% of its final value.
  2. noise
  3. color accuracy
  4. tonal response & contrast
  5. exposure accuracy
  6. light
  7. lens distortion: r_u = r_d + kr_d^3

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