Imaging, signalling and polarisation

• Real image


  One that can be projected onto a screen
• Wave model of light

  • The wave fronts lie perpendicular to the direction of the wave's motion
• Plane wave fronts


  Wave fronts that are parallel to each other and don't appear to have any curvature
• Condition for plane wave fronts

  1. The wave should come from a very distant source

  2. Curvature = 1/radius

  3. As the radius tends to infinity, the curvature approaches zero
• Converging lens


  Adds a curvature of 1/f to the wavefronts of light
• Electromagnetic spectrum


  Transverse waves
• Lensmaker's Equation


  1/v = 1/u + 1/f, where v = image distance, u = object distance, f = focal distance
• Electromagnetic waves in a vacuum all travel at the same speed of 3 x 10^8 ms⁻¹
• Structure of electromagnetic waves

  • An alternating magnetic field that oscillates perpendicular to an alternating electric field. Both of which oscillate perpendicular to the direction of wave travel.
• Polarisation


  Oscillations of a wave are limited to a single plane
• Power of a lens

  Power = 1/f
• Rotating two polarising filters relative to each other

  1. When the two filters are aligned, light is clearly seen through them

  2. As one of the filters is rotated through 90 degrees, the light intensity decreases to zero as no light can pass through
• Unit of lens power

  Dioptres, D
• Object distance values


  Negative, as they are measured in the opposite direction to the image and focal distances
• Linear magnification equations


  Linear Magnification = Image Height / Object Height

  Linear Magnification = Image Distance / Object Distance
• Modern camera image storage

  Charged Coupled Devices (CCDs)
• CCD


  A screen covered with pixels that store charge when light is incident on them
• Binary system


  A system that can only take two possible values, such as on or off
• Calculating number of alternatives from bits

  N = 2^b, where N is the number of alternatives, b is the number of bits
• Calculating bits required for a given number of values

  b = log2N, where N is the number of alternatives, b is the number of bits
• Image resolution


  The smallest distance between which two points can be distinguished
• Equation for image resolution

  Resolution = Width of Image / Number of Pixels
• Main ways of processing an image

  • Removal of Noise
  • Edge Detection
  • Increasing Contrast
  • Changing Brightness
• Noise removal


  The value of each pixel is replaced by the mean of the 8 pixels immediately surrounding it, removing random disturbances and producing a smoother image
• Edge detection


  The average value of the 8 pixels around a given pixel is subtracted from that pixel, removing areas of uniform colour and producing clear outlines
• Increasing image contrast


  Multiply each pixel value by a fixed number, so the image spreads across all available values, making the image more vivid
• Edge detection


  1. The average value of the 8 pixels around a given pixel is subtracted from that pixel

  2. This removes areas of uniform colour and produces clear outlines
• Increasing image contrast


  1. Multiply each pixel value by a fixed number, so the image spreads across all available values

  2. This will make the image more vivid
• Increasing image brightness


  1. Add a fixed number to each pixel's value

  2. This will brighten the image
• Analogue signals


  Can continuously vary between values
• Digital signals


  Can only take discrete values
• Converting analogue signals to digital signals

  Sampling: Taking frequent measurements of analogue signals, and then rounding them to the nearest value/quantisation level
• Sampling can produce quantisation errors, which is where there is a difference between the actual level and the quantisation level
• Positives and negatives of analogue signals

  • Positive: More detailed than digital signals
  • Negative: They cannot be easily amplified due to noise
• Advantages of digital signals

  • They are noise resistant
  • They are easy to send, store and receive
  • Their transmission is faster
  • They can be easily compressed
• Disadvantages of digital signals

  • They can have a lack of detail
  • They are easily scrambled
• The minimum sampling rate must be at least twice the highest frequency in the signal
• If the sampling rate is too low, aliasing can occur and a low frequency signal may be produced from a high frequency signal due to it being sampled too infrequently
• Bit rate


  The rate of information transmission
• Relationship between bits and bytes


  A byte is a string of 8 bits