Imaging Flashcards
What is curvature?
1/r
As r tends to infinity, curvature tends to 0. This means waves from a distant source have no curvature. They are called plane wave-fronts.
Ways of modelling light
Ray point of view; arrows parallel to direction of travel.
Wave point of view; lines perpendicular to direction of travel.
Lens power
Lens power (D) = 1/focal length (m)
The Lens Equation
1/v = 1/u + 1/f (u is negative)
Curvature of waves leaving the lens equals the curvature of the waves entering the lens plus the curvature added by the lens.
Position of image from varying sources
Very distant source = image at focus
Nearby source beyond focus = image passed the focus
Source at the focus = very distant image
Source inside the focus = virtual image
Describing an image
Upright or inverted
Real or virtual
Magnified or diminished
Image when the object is:
Greater than 2f away from lens
Between f and 2f
Within f of the lens
(with examples)
e.g eye or camera
inverted
diminished
real
e.g. projectors
inverted
magnified
real
e.g. magnifying glass
upright
magnified
virtual
Magnification
m = image height/object height
= image distance/object distance
CCDs
Grid of picture elements that store charge proportional to the light falling on them.
Wires read off the charges to form a digital image.
Bits and Bytes
A bit is either a 1 or a 0
A byte is 8 bits
Decimal values can be represented by the number of various powers of 2 that make it up using 1s and 0s
Relationship between number of bits and number of possible arrangements
N=2^b
b=log2(N)
Resolution
Smallest distinguishable object distance on an image.
i.e. the distance represented by the width of each pixel
Information in an image
Information in an image = (# of pixels) x (bits per pixel)
Types of image processing
Changing brightness
Removing noise
Edge detection
Changing contrast
Changing brightness
Increasing the value of each pixel until the brightest has the max value (usually 255)
Removing noise
Replacing the value of each pixel with the mean or median of its value and the value of surrounding pixels
Edge detection
Where adjacent pixels are subtracted to remove areas of uniform brightness and only pick out areas where the gradient of the brightness changes abruptly; e.g. at the edges
Changing contrast
Spreading a narrow range of pixel values so the min is 0 and the max is 255 but in the same proportion
Wave speed
speed = frequency x wavelength
Frequency
Frequency = 1/Time period
EM wavelength values (powers of 10 in m)
>6 radio -1 microwave -3 infrared (700nm) visible light (400nm) ultraviolet -8 x-rays -10 to -13 gamma rays
EM waves
EM waves are waves of oscillating magnetic and electrical fields perpendicular to each other and the direction of travel
All EM waves can be polarised
Unpolarised waves
Transverse oscillations in all planes
Polarised waves
Once put through a polarising filter, waves oscillate in one plane
Demonstrating polarisation
Putting light through a vertical and then a horizontal filter will result in no transmitted light
As polarised light is transmitted through another filter a detector behind it will see minimums and maximums every 90 degrees
Light can be polarised by shining a narrow beam through a tank of water containing a few drops of milk
