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AS Physics (Unit 1: G491) > Imaging > Flashcards

Flashcards in Imaging Deck (17)
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What are some uses of imaging?

1. Medical imaging: Ultrasound can be used to build up images of unborn foetus as well as broken limbs...
2. Astronomical imaging: Light and other wavelengths of the EM spectrum can be used to create images of distant stars and galaxies.
3. Seeing: Our eyes detect light reflected off other objects. We rely on this to see. Different objects reflect different wavelengths of light, forming different colours.
4. Infrared radiation can be picked up by special cameras. This radiation is detected by hot objects and can be used to measure temperature.


How does ultrasound work?

- Ultrasound is able to penetrate through amniotic fluid in the wound but doesn't penetrate flesh or bone.
- A piezoelectric crystal is vibrated by an electric current to generate the ultrasound signal which passes through the womb and reflects off the foetus.
- The signal then travels back to the piezoelectric crystal and another electric signal is generated.
- The pulse takes different times to travel different distances.
- A computer uses these time delays to assemble a 3D image of the foetus on a computer screen.


How are images stored and displayed in a computer?

Images are stored as pixels in a computer. A screen is made out of thousands of pixels. An image is stored as a series of binary digits that refer to the colour of each pixel. These pixels are then assembled on the screen and the image is formed.


What is the wave equation?

v=fλ, (speed = frequency*wavelength).


What is resolution?

Resolution is the smallest distance two objects can be apart and still be distinguishable as 2 separate objects. Resolution can be limited by 2 factors:
- The resolution of the detector that created the image.
- The number of pixels the image is stored as.
For example, if the distance between the 2 objects is smaller than the wavelength of the radiation the detector uses, then the two objects would be indistinguishable. However, if the distance between the 2 objects is smaller than a pixel length, then the 2 objects will also be indistinguishable.


What is the importance of using other wavelengths in the EM spectrum as an imaging medium?

Using other wavelengths means that we are able to detect things we would otherwise not be able to. For example, black holes do not emit light; however, they do emit radio waves that can be detected with radio telescopes.


What is a CCD and how does it work?

- A CCD, or Charge-Coupled Device, is the modern way of recording optical from things like digital cameras and optical telescopes rather than conventional photographic film.
- The device consists of thousands of silicon picture elements that store a charge proportional to the amount of light that falls onto it.
- The charges are shunned to the edges where they are read as voltages. The image is then stored as a series of potential differences that are converted to binary and displayed as pixels on a screen.


How does a Scanning Tunnelling Microscope (STM) work?

- An ultra sharp needle is scanned across the surface of the material. Electrons are able to tunnel from the tip of the needle to the material and a current is produced.
- The current is proportional to the distance between the surface of the material and the needle.
- A control device is used to move the arm up and down to keep the tunnelling current constant.
- The movement of the arm is recorded and an image can be produced electronically.
- This technique is so precise that it is able to pick up the contours of individual atoms.


What is the binary digit system?

The binary digit system is a number system that uses a series of 0s and 1s to represent numbers which can be translated into other types of information.


What is a bit and what is a byte?

A bit is a number with one digit and has two possibilities; i.e. 0 and 1. Two bits can be put together to form 4 different possibilities; i.e. 00, 01, 10, 11. n bits can be put together to form 2^n possibilities. A byte is 8 bits.


What is the significance of bits and bytes?

Digital information is stored as bits and bytes. However, the more bits that are assigned to each piece of information, the more detailed the information can be. For example, a digital camera contains millions of pixels that each have a colour. This information of colours is then stored as bits in the memory card. However, if more bits were assigned to each pixel, then a greater variety of colours can be stored, making the image more detailed.


What are the bit equations?

1. Number of possibilities (I) = 2^n.
2. Number of bits = log (n)/log (2).


What are the 3 image processing techniques?

1. Smoothing edges: Replace the value of each pixel with the mean of its value and its eight neighbours.
2. Removing noise: Replace the value of each pixel with the median of its value and its eight neighbours.
3. Laplace rule (finding edges): Multiply each pixel by 4 and subtract the values of its N, E, S, and W neighbours.


What does a converging lens do?

A converging lens adds curvature to wave fronts. This happens because the lens is thicker in the centre compared to the sides, so light takes longer to travel through the centre than the sides. However, light from a lens always converges at a point called a focal point. The distance between the focal point and the lens is the focal distance.
Thicker lenses add more curvature to waves so the focal length is shorter, making the lens more powerful.


What is the power of the lens?

Power (dioptres) = 1/focal length.


What is the lens equation?

1/image distance = 1/object distance + 1/focal distance.


What is the nature of wavefronts in relation to the object?

- Close objects have negatively curved wavefronts.
- Far objects have almost parallel wavefronts.