Imaging and Signaling

Imaging and Signaling

1. The nature of waves

   • Uses of waves - medical imaging, Communications, seeing and heat cameras
   • A wave transfers energy away from its source and since it is carrying energy away from its source, its source is losing energy.
   • Reflection of a wave - The wave is bounced back when it hits a boundary
     • Refraction of a wave - The wave changes direction as it enters a different medium. The change in direction is the result of the wave slowing down or speeding up.
   • Wave equation

2. Transverse waves and polarisation

   • A transverse wave is a wave where the angle of oscillation is at 90 degrees to the direction of propagation. EM waves are transverse, another example of a transverse wave are ripples on water.
   • A polarized wave only oscillates in one direction.
     • Electromagnetic radiation oscillates in two planes. One is magnetic and the other electric. A polarising filter blocks out one plane and leaves the EM wave polarised.
       • Light is part of the EM spectrum and is partially polarised when reflected back off an object.
     • Uses of polarised light
       • Communications satellites use different polarisations of the same frequency band to reduce interference.
       • CD players use polarising crystals to transmit light polarised in only one direction
       • Polaroid sunglasses block glare from reflected light.

3. Forming images with lenses

   • When light hits a boundary between one medium and another some light is reflected back into the first medium whilst some light is transmitted through the second medium and gets refracted
     • The absolute refractive index of material is (n) is the ratio between the speed of light in a vacuum (c) and the speed of light in that material (v) so n=c/v
   • Converging lenses add curvature to the wavefronts of waves
     • The power of a lens is 1/f. f being the focal length. The power of the lens is measured in diopters
       • Curvature = 1/radius of curvature
   • The lens equation can be used to find where an Image will be found
   • magnification = size of image/size of object

4. Information in images

   • A single binary digit is called a bit. a bit can either be a 1 or a 0. A group of 8 bits is called a byte.
     • Number of alternatives = 2^number of bits
     • Number of bits = Log2(number of alternatives)
   • Multiplying all the pixels by a fixed value improves the contrast
   • Adding false colour highlights features
   • Replacing pixels with the median of their neighbour reduces noise
   • The Laplace rule is used to find edges. You multiply a pixels value by 4 and then subtract from it the value of the pixels directly above, next to and below it.

5. Sampling

   • Analogue signals vary continuously and so are more likely to pick up noise
     • Analogue signals can be digitised by taking the value of the signal at regular time intervals and then finding the nearest digital value. How well a signal is digitised depends on two factors:
       • resolution
         • This is the number of digital value intervals there are. The more digital alternatives there are the more closely the digitised signal will match with the original analogue signal. The greater the number of bits the better the resolution.
       • Sample rate
         • This is the time from one sample to the next. The maximum sample rate should be twice the maximum frequency of the signal.
   • Digital signals are resistant to the effects of noise because the number of values the digital signal can take is limited
     • Digital signals have 4 advantages of analogue signals: they can be sent and received more easily than analogue signals, they are more resistant to noise, they can be used to represent different forms of information and they are easier to process with computer.

6. Signal spectra and bandwidth

   • Signals are not just made up of one frequency they are made of of a spectra.
     • The bandwidth is the rang of frequencies within a signal eg. 2Hz - 1Hz = 1Hz bandwidth
       • Bandwidth limits the number of signals that can be transmitted because there has to be a gap between carrier frequencies to accomodate them
   • Carrier waves carry the signal spectra of frequencies with transports the information.
   • Rate of transmission (bits per second) = samples per second x bits per sample

Images and files in the map