# 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 = log_{2} 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 = 2^{I} , then the amount of information I = log_{2} 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= log_{2} N where N is the number of alternatives.

Number of alternatives N = 2^{I}

## 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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