1. Properties of light and visual function Flashcards
optical radiation
lies between x-rays and mircowaves
how many subdivisions
seven
UVC, UVB, UVA, visible , IRA, IRB, IRC
wavelength for uvc
200-280nm
wavelength for uvb
280-315nm
wavelength for UVA
315-400nm
wavelength for visible
400-780nm
wavelength for IRA
780 - 1400
wavelength for IRB
1400-3000NM
wavelength for IRC
3000-10,000nm
shorter wavelength
greater energy
cornea and sclera absorb
very short UVB and UVC
very long IRB IRC
crystalline lens absorbs
UVA
what passes through
visible light and IRA
400 - 1,400nm
white loght
mixture of all visible light
colour and cone
Short = blue
Middle = green
Long = red
Blue, green and red wavelength length
blue 440
green 535
red 560
Green colour blind
deuteranopia
red colour blind
protanopia
blue colour blind
tritanopia
Clinical testing of colour vision
Farnsworth-Munsell FM hue 100
D-15
Ishihara - congenital red-green
Lanthony new colour test - paeds
Retinal photoreceptors are ALSO sensitive to
UVA (near UV)
350-400
UVA is normally absorbed by
the crystalline lens
retinal damage
350-441nm
UV to visible light
define fluorescence
property of a molecule to spontaneously emit light of a longer wavelenght when stimulated by light of a shorter wavelength
–> longer = less energy
Fluorescein
Excited by blue light (465-490)
Emits yellow (520-530)
fluorescein angiography assess
retinal and choroidal circulation
Describe FA
white light flash
goes thorugh blue excitation filter
blue light on fundus
most absorbed, some reflected unchange some is changed by fluorescence
seperate the reflected (unchanged + yellow)
camera only dtets the yellow (which is flureoscein leak from vessels)
indocyanine green
absorbs 805nm
emits 835nm (IR)
How does indocyanine green work
RPE doesn’t absorb ICG thf you see chorodial circulation
path of light
is always straight
wave motion
is a disturbance of energy passing through a medium
medium does not move but it vibrates at right angles to the direction of the wave
wavelength
distance between two symmetrical points
cycle
one complete oscillation (up and down)
white light
waves inphase = coherent
light waves out of phase
incoherent
two waves are in phase
CONSTRUCTIVE interference
resultant wave = summation of the two
two waves (of equal amplitude) byt out of phase by half cycle
DESTRUCTIVE intereference
cancel each out out
two waves out of step (less than half a cycle)
intermediate summation
when does destructive interence occur
in the stroma
collagen bundle eliminate light with destructive interference
define defraction
when a wave fronts encouters a narrow opening or the edge of an obstruction - the wave motion spread out on the far side of the obstruction
fixation in babies
central, steady and maintained
between 2-5months
visually directed reaching develops
catford drum
white cylinder marked with blacks dots of increasing size
can measure from 6/6 to 2/60
when viewed at 60cm
what are visually evoked potentials
electrical responses generated in the occiptal cortex by stimulation of the eye
Optotype testing
for children between 18 and 24 months
match symbols
Logmar
more prescise than snellen
near vision
distance of 25-33cm
potential visual acuity testing
used e.g. when cataract in the way
pin hile testing
contrast sensitivity
ability to see low contrast objects, e.g. faces
lost in cataract, glaucoma, optic neuritis
principle of contrast sensitvity measurement
sine wave grating, gradual transition between light and dark bands
narrower bands - higher spatial frequency
how is contrast sensitivity measured clinically
Pelli robson chart
glare testing
test VA in the prescence of a source of glare
how is polarized light made
ordinary light encountering a polarizing substance ar agenet
polarising agent
calcite crystals - only transmit light going in one plan
how does a polarising agent affect light
reduces the radiant intensity but does not affect the spectral composition
how can light also be polarised
reflected from a plane surface e.g. water if the angle of incidence if equal to the polarising angle for the substance
birefringence
molecular structure which transmits light waves lying parallel to its structure but which selectively slows and therefore redirects light waves vibrating in a plane prependicular to its structure
why do things that use biregringence have two refractice indcies
as they split inciden unpolairsed light into two polarised beams travelling in different directions
define dichroism
causes light to be split up into different wavelengths
or
waves of different polorisations are absorbed by different amounts
what effect does a dichroic substance have on light
completely blocks trasnmission of light waves not aligned with its structre by absoprtion
only one beam of polarised light emerges (much weaker)
steroscopic vision
ability to fuse slightly dissimplar images procuding depth perception
how should the retina be stimulated for stereoscopic vision
different parts of the retina that are WITHIN Panum’s fusional areas
how is steroscopic vision graded
measured in seconds of arc
least horizontal disparity of the retinal image that evokes depth preception
what is normal steroracuityu
60 seconds of arc or better
what grading system measures steroacuity
Frisby stereotest range 600-15 seconds (minimum thf is 15 seconds)
what is good steroacuity the product of
central single binocular vision
what does steroacuity worse than 250 seconds suggest
amblyopia
vectograph
two superimposed views presented in such a way that the light entering each eye is plan polaised
meaning that the light from one view being at right angles to that from the other
the compsoite pictreu must be view with polrising glasses
Titmus / Writ fly test
graded sets of anumals and circles one of which is disparate and appears to stand foward
must be viewed be 40cm
range 3,000 to 40 seconds of arc
Frisby test
three clear plastic plates of different thciknes
on each plate if four squares filled with random shapes
one scare has a ‘hidden’ circle
viewined at 40cm
what seconds of arc can frisby give
340, 170 and 55 –> plates
by adjusting the viewing distance can give a range of 600 to 15 seconds
TNO test
Random dot test using anaglyphs
480 to 15 seconds
anaglyphs
sterogram in which two disparate views ae printed in red and green on a white backgroun
red green glasses are worn
through red fliteter only see green as balck and vice versa
two views may be fused to give a steroscopic effect
Lang sterotest
fine vertical lines which are seen alternately by each eye when focused through bulit in cylindical lens elements
must be hled parallelt to the patient’s face to avoid uniocular clues
what arcs does lang measure
1200 to 550 seconds of arc
uses preferntial looking for paeds
define radiometry
quantifies radiant energey in all parts of the EMS
define photometry
quanitfies only the raidant energy the evokes a visual reponse
quantity of light emitted
radiant flux watt = joule / sec
luminous flux = lumen (lm)
light emitted per unit solid angle
raidant intensity = watt / steradian
luminonous intensity candela = lm/sterraidna
light refelcted or emitted by surgae in a ginve direction per solid angle per unit area
radiance = watt / steraidan/m2
luminance = candela / m2
light per unit area incidence at surface
irradaince= watt /m2
illuminance = lux = lm/m2
Emitted form light source
Intensity
Amount falling on a surface
Reflected
Radiant flux / luminous flux
radiant intenisty / luminois intensity
irradiance / illliuminance
radiance / luminance
what is the peak photopic sensitivyt of the eye
555nm yellow / green
1 wat of monochromati light
has the photometric equivalent of 685 lumens
555nm has the
maxiumum luminous effencivty
luminous efficienty is the
conversion factor specific for each wavelegnght determinded by the sensitivity of the eye to it
this is where radimetric and photmetric units are related
what happens towards the end of the EMS in the eye
eye is progressivel y less sesnsitivt to wavelenghts towards each end of the EMS spectrum
when does the conversion factor fall towards zero
outside the range of 400-700nm (visible light)
STERADIAN
unit of solid angle (Resemblinga cone)
define a steradia
anlge at the centre of a spehe which subtends an area onthe surgface of the spehe meauring the square of the radia
defined perimetry
measures the light sensitivity of point on ht ertina by the ability of a patient to detect light stimuli of varuing intensity presented at corresponding points in the visual fields
what is the standard backgroun luminance of most perimeters
31.5 apostilbs (asb)
varaition of light intesnity in perimtery
between 0.8 to 10,000 asb
