omi term 2 Interferometry Principles

Question 1

interference

Answer 1

means completely in phase and constructive interference leading to bright light

path lenght longer for blue
in phase
constructive interference
slightly less bright light

path length longer for blue
completely out of phase
destructive interferance
no light

Question 2

michelson interferometer measurement pricniples

Answer 2

measurement principles

light split into two beams using beam splitter
both beams have same amp freq and wavelength
only diff is distance beam travels from source to detector so this changes interferance pattern
measurement based upon appearance of interferance patter and the number of lines the interference pattern changes from constructive to destructive when adjusting mirror

Question 3

physical principles of michelson interferometer

Answer 3

coherent light beam splits in two then brought back together
resulant light intensity depends on the path diff(phase shift) between two beams and builds on the basis of length/depth measurements
as mirror moves back and forth the combine beam moves in and out of phase shift varies continuously

Question 4

interference in opthalmic imaging

Answer 4

fixed mirror replaced by part of eye (retina)

interference produced using low coherance light sources(length around 10 microns or less)

Question 5

coherance light cal

Answer 5

L= (central wavelength of source)^2/n (change in spectral width of source)

Question 6

adv of using low coherent light source

Answer 6

creation of fringe pattern useful to asses structures
less powerful light source with respect to intensity-reduces possiblity of harm to ocular tissue
low coherence means limited small range wavelength not single wavelength

wehn reference mirror is move back and forth 3 interference fringe patterns are created, spacing of fringe patterns give position of tissue layers in depth

Question 7

optical coherance tomography

Answer 7

light source used in OCT near infrared as tissues partially transparent to near infra red light
some light will still be scattered back to detector but some is transmitted to deeper layers before its reflected
transmission allows us to get a depth profile of tissue
longer wavelength of light means greater tissue penetrration and deeper it trabvels through tissue

Question 8

using interferometry-ocular imaging

Answer 8

cornea-curvature thickness and shape
anterior chamber depth
lens-thickness and shape
retina-thickness and layer
measurement of axial distance

Question 9

types of OCT one subset

Answer 9

fibre based time domain oct
fibre base fourier domain
swept-source oct

the key diff are between set up and adv/disadvantages

Question 10

fibre based time domain oct

Answer 10

oct system with moveable mirror are called time domain system
low coherance near infrared light acts as light source
this light source produced using superluminescent diode
the measurement beam is reflected or backscattered from object with diff delay times adn these are depnedant on the optical properties of ocular tissue
axial profile of reflectivity vs depth is obtained by movement of reference mirror
this changes the apth length in reference aim
for each point on retina the magnitude of intensity of the resulting interference is recorded for each position of the reference mirror and computer processing can determine depth of tissue
scanning light beam on tissue enables cross sectional imaging of tissue

Question 11

fibre based fourier domain oct

Answer 11

intensity is recorded as a function of wavelengths or frequencies of the light
intensity modulations when measure as a function of frequency called specttral interference
all axial info in a scan of tissue can be acquired simultaneosuly
scan is 20-40x faster and enables more detailed info

Question 12

swept source oct

Answer 12

type of fd-oct
wavelength of light source is turned and a single photo detector is tuned and a single photodetector is used and this records each wavelenght sequentially
axial resolution 3-5 microns, low signal to noise ratio, fastest imaging speed
loses sensitivity with increaseds canning depths

Question 13

equipment example

Answer 13

zeiss IOL master measures

axial length
anterior chamber depth
corneal radii or curvature
central corneal thickness
pupil size
provides an image of the lenticular structure

Question 14

why OCT

Answer 14

we asess ocular structures for IOL power calculation, basis for refractive surgery, research

adv compared to ultrasound

non invasive, no pressure applied on globe
more accurate