IMAGING
- Created by: NadiaRahman123
- Created on: 23-04-15 20:13
FREQUENCY, WAVELENGTH & WAVESPEED
PERIOD: Time taken for one complete oscillation
FREQUENCY Htz: The number of complete cycles of oscillation each second
AMPLITUDE: The magnitude of the maximum departure of the oscillating quantity from its mean value
WAVELENGTH λ: The distance from one point on a wave to the next exact point
T = 1/f and v = fλ
ELECTROMAGNETIC SPECTRUM
GAMMA: wavelength smaller than 1nm, sourced from radio active decay and detected by a geiger counter. Used in sterilising
XRAY: wavelength smaller than 1nm, sourced from inner shell of atomic electron transitors and detected by a geiger counter. Used in X rays
ULTRA VIOLET: wavelength between 1-400nm, sourced by atomic electron transition and dectected by a photo cell. used in sunbeds
VISIBLE: wavelength 400-700nm, source by outer shell of atomic electron molecules and detected by a thermophile. used for seeing
INFARED: wavelength 700nm-1mm, sourced from vibrations of atoms and molecules and dectected by a thermopile. used for thermography
MICROWAVES: wavelength 1mm-0.1m, sourced from electrons in resonant cavities and dectected by a mircowave diode. used in cooking
RADIO: wavelength bigger than 0.1m, source and detected by electrons moving in aerial. used in communication
COVERGING LENS
Lenses used to form images. The effect of lens is to change the curvature of the wavefronts passing through it
CONVERGING LENS adds to the curvature of wavefronts of light falling on it, making light from a point object converge
FOCAL LENGTH, POWER + MAGNIFICATION
FOCAL POINT F: Point where light from distant object is brought to a focus by the lens
FOCAL LENGTH f: The distance from the centre of the lends to F (+ for converging lens, - for divergent lens)
POWER = 1/ focal length (measured in dioptres. shorter f = more power
1/v = 1/u + 1/f (u = 0 then v=f and v=0 then u=-f)
LINEAR MAGNIFICATION: height of image/ height of object or v/u as heights proportional to v and u
PIXELS
Digital images consists of a rectangular array of picture elements (pixels)
each pixel is stored as a number
1 bit = 0 or 1. 8 bits = 1 byte (2^8 = 256) alternatives)
more bits = better resolution
3 bytes used in colour images, 1 byte for each primary colour
IMAGE PROCESSING
NOISE REDUCTION: Noise in an image can be reduced by replacing he byte representing a pixel with the median of the values of that pixel and its neighbours
EDGE DETECTION: Edges can be located + enhanced; it is a place where the gradient of light intensity chnages sharply. A difference between the value of pixel & average of neighbours = edge
SMOOTHING: Smooting of sharp edged can be achieved by replacing a pixel with the mean of its value and neighbours
FALSE COLOUR: Useful to enhance some of image by assigning different colours to different ranges of brightness
AVERAGES AND LOGS
1. arthmetic mean
2. median (used for noise reduction)
3. mean ( used in smoothing)
n = b p, then p = log b n
The base 2 logarithm of a number n is p = log2 n, where n = 2p.
1. z = x y, then log z = log x + log y
2.z = x / y, then log z = log x - log y.
3. For p = log10 n, then 10 p = n. Also log10 10 p = p
log scale = where a quantity is multipled by a constant at each step
BITS, BYTES AND INFO
smallest amount of info expressed in digital form
bit represented as 0 or 1
byte = 8 bits (2^8= 256 alternatives)
A sequence of n bits has 2^n alternatives
For a certain number of alternatives N = 2I , then the amount of information I = log2 N.
digital camera =1m pixels. each pixel generates 3 bytes (1 of each primary colour) would need a storage capacity of about 3 megabytes
Digital cameras use image compression methods to reduce this to less than 1 Mbyte per image.
Information I= log2 N where N is the number of alternatives.
Number of alternatives N = 2I
ULTRA SOUND + CCD
ULTRA SOUND: used to form images of a very high frequency. at the boundary of 2 substances a proportion of the wave is reflected, depending on the density difference between the substances, which is detected by a transducer
a USS emits US pulses & detects reflections at the boundaries. the dectection reflection is used to reconstruct an image of inside the object
higher US frequency = less difraction = better resolution
CCD: each dector stores a large proportional to the light that has fallen on it & is read as a sequence of voltage pules to recreate a visual images
signals are used to generate a bright spot at the pixel position corresponding to the position of the orginal element
STM
Makes an image of an electrically conducting surface, by allowing electrons to tunnel across a gap between the surface and a fine conducting tip above the surface
Due to QB there is small probability for an electron to cross gap
With a small constant PD between the tip and surface = the probability of transfer from one surface to tip is much greater than for transfer from tip to surface, giving a net tunnelling current between them
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