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Chapter 1 and 2 - Imaging & Signalling
Terms in this set (22)
How does the radius (distance from the source) effect curvature of wave fronts and why?
Curvature = 1/r, so as r tends to infinity, the curvature tends to 0. This means that the further a lens is away from a source, the less curvature created on the wave fronts passing through the lens.
How many shades of colour are available in a stored image, and which numbers are used to measure these levels?
There are 256 shades of colour, and these levels are measured using 0-255.
How is brightness changed in an image?
The value of each pixel is increased by the same amount, until the brightest pixel has reached level 255.
How is noise removed in an image?
Noise can be removed my smoothing the image, which is where the value of each pixel is replaced by the median or mean of the pixels that surround it.
How is edge detection carried out in an image?
Edges are enhanced by removing the average value of a pixel's neighbours from said pixel. This removes uniform areas of brightness and picks out where the brightness changes abruptly (the edges).
How is the contrast changed within an image?
Images with little contrast may only use the values 75-150, so to increase contrast these values are spread out across the whole 0-255 range. Here, the value 75 becomes 0 and 150 becomes 255.
What are the advantages of digitising signals?
Noise can be removed easier from digital signals,
Digital signals can transmit data much faster than analogue signals,
Digital signals can be regenerated accurately after having noise removed, unlike analogue where the signal loses detail.
What is the difference between a polarised wave and an unpolarised wave?
Polarised waves oscillate in one plane only, and unpolarised waves oscillate in all directions.
How do polarisation filters work?
Filters only allow waves through that are oscillating in one direction, so all of the other directions are blocked out.
What is the relationship between frequency, f, and the time period, T?
f = 1/T
What is the equation for information in an image?
Information in an image = no. of pixels x bits per pixel.
What is the equation for linear magnification?
Linear magnification, m = image height/object height = v/u.
What is the relationship between Power of a lens and focal length?
Power, P (in Dioptres) = 1/focal length.
What do the terms in 1/v = 1/u + 1/f mean?
v = distance from the lens to the image,
u = distance from the lens to the source,
f = focal length of the lens.
Because 1/r = curvature, 1/v shows the curvature of waves leaving the lens and 1/u shows the curvature of waves entering the lens.
1/f shows the curvature added by the lens.
What do the terms in v=f(lamda) mean?
v = wave speed,
f = frequency of wave,
lamda = wavelength.
How do you calculate the number of levels/alternatives in an image?
N = 2^b, where b = number of bits.
How do you calculate the number of bits used in an image?
b = Log2(N), where N = number of alternatives.
How do you calculate the minimum sampling rate?
Minimum sampling > 2 x maximum frequency of signal.
How do you calculate the rate of transmission of a digital signal?
Rate of transmission = samples per second x bits per sample.
How can the number of bits be calculated using Vtotal and Vnoise?
b = Log2(Vtotal/Vnoise).
What shape is a converging lens and what does this do to the wave fronts?
It adds curvature to the wave fronts, focuses then into one point and has positive power.
What shape is a diverging lens and what does this do to the wave fronts?
It removes curvature from the wave fronts, makes them spread out and has negative power.
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