Chapter 1: Light properties and vision Flashcards
1
Q
Between which type of electromagnetic waves does optical radiation lie?
A
x-ray and microwaves (includes UV, visible light and infrared light)
2
Q
What is the range of wavelengths of visible light?
A
400nm - 780nm
3
Q
What is the relationship between electromagnetic wave wavelength and energy of photos?
A
short wavelength - greater energy of photons of optical radiation
4
Q
What is the fate of the following types of radiation:
- UVA
- UVB and UVC
- IRA
- IRB and IRC
A
- UVA: absorbed by crytalline lens
- UVB and UVC: absorbed by cornea and sclera
- IRA: passes through ocular media with visile light to fall on retina - causes thermal effects (e.g. eclipse burns)
- IRB and IRC: absorbed by cornea and sclera
IRA and UVA are those wavelengths closest to optical radiation
5
Q
What are the 3 populations of cone photoreceptors in the retina which perceive colour?
A
- Short wavelength light: blue
- Middle wavelength light: green
- Long wavelength light: red
6
Q
What are 3 groups of congenital colour vision defects?
A
- Green cone pigment absent / shift in sensitivity: deuteranopia / deuteranomaly
- Red cone pigment absent / shift in sensitivity: protanopia / protanomaly
- Blue cone pigment absent / shift in sensitivity: tritanopia / tritanomaly
7
Q
On which chromosomes are the genes encoding red, green and blue pigments found?
A
- Red: X chromosome
- Green: X chromosome
- Blue: chromosome 7
8
Q
What is commonest congenital colour vision defect?
A
deuteranomaly
9
Q
What are 2 sites of acquired colour defects?
A
- acquired optic nerve defect
- acquired retinal disease
10
Q
Which colours tend to be affected by acquired optic nerve colour defects and what are the 2 exceptions?
A
red-green defects
Exceptions: glaucoma and autosomal dominant optic neuropathy (predominantly blue-yellow deficit)
11
Q
Which colours tend to be affected by acquired retinal disease colour defects and what are the 2 exceptions?
A
blue-yellow defects
Exceptions: cone dystrophy and Stargardt’s disease (predominantly red-green)
12
Q
What is the most comprehensive test of colour vision?
A
Farnworth-Munsell (FM) hue 100 test (84 coloured discs, groups arranged in rows in order of hue)
13
Q
What is a colour vision test that can be used by children?
A
Lanthony New colour test
14
Q
What colour vision defects do Ishihara pseudoisochromatic test plates test for?
A
specific test for congenital red-green defects
15
Q
What is a less comprehensive test for colour defects than the Farnsworth Munsell hue 100 test?
A
D15 test - doesn’t distinguish mild colour defects
16
Q
What do aphakic (no lens) eye or pseudophakic eyes with IOLs give rise to blue or violet colours?
A
usually UVA is absorbed by the lens of the eye but retinal photoreceptors are also sensitive to wavelengths 350-400nm (UVA)
17
Q
How can some ophthalmic instruments reduce exposure of the retina to damaging wavelengths?
A
yellow filters
18
Q
What is the definition of fluorescence?
A
= the property of a molecule to spontaneously emit light of longer wavelength when stimulated by light of a shorter wavelength
19
Q
How does fluorescein angiography work?
A
allows retinal + choroidal circulation to be studied by photographing passage of fluorescein through the vessels
- white light from flash unit of fluorescein camera passes through a blue excitation filter, illuminates fundus with blue light
- wavelights transmitted by the excitation filter approximate to the absorption spectrum of fluorescein
- blue reflected light and yellow-green fluorescent light leaving the eye are separated by a yellow-green ‘barrier’ filter in the camera
- blue light is blocked and exposes the camera film only to yellow-green light from the fluorescein to delineate vascular structures/dye leakage
20
Q
What is pseudofluorescence?
A
- Occurs if there is overlap in the spectral transmission of excitation and barrier filters
- Allows reflected wavelengths at green end of blue to pass through barrier filter and appear as fluorescence
21
Q
What is another fluorscent substance in addition to fluorescein?
A
indocyanine green dye
22
Q
What is the function of indocyanine green dye?
A
Absorbs 805nm, emits 835 nm infrared radiation - RPE does not absorb these wavelengths, therefore possible to observe fluorescence of choroidal circulation
ICG also used to photosensitise vascular lesions to diode laser photocoagulation
23
Q
How does the fluorescence of ICG compare with fluorescein?
A
4% of 805nm radiation absorbed by ICG is emitted at 835nm, vs total (100%) fluorescence of fluorescein
24
Q
What is the definition of the amplitude of a wave?
A
maximum displacement of an imaginary particle on the wave from the baseline (see A)
25
What is the definition of destructive interference?
if two waves of equal amplitude are out of phase by a half cycle, they will cancel each other out
26
What is the result of waves that are out of phase by \< half a cycle?
wave of intermediate amplitude and phase; final effect is as if waves were superimposed and added algebraically to each other
27
What type of wave interference occurs in the corneal stroma and why? What is another practical example?
destructive interference - collagen bundles are spaced so any deviated light is eliminated by destructive interference
same principle as low reflection coatings on lens surfaces - light reflected from superficial and deep surfaces eliminate each other
28
What is the definition of diffraction?
= when wave front encounters a narrow opening or the edge of an obstruction, the wave motion spreads out on far side of an obstruction
as if edge of the obstruction acts as a new centre from which secondary wave fronts are produced which are out of phase with the primary waves
29
What is an Airy disc?
when light passes through a circular aperture, a circular diffraction pattern is produced, with a bright central disc and surrounding alternate dark and light rings
the bright central zone = the airy disc
30
How does aperture size influence diffraction?
diffraction effects are most marked with small apertures
31
What is the main source of iamge imperfect when the pupil is **small**?
**diffraction** (vs aberrations of refractive elements of the eye in large pupil with reduced diffraction)
32
What is the definition of the limit of resolution and resolving power?
the s**mallest angle of separation (w) between two points** which allows the formation of **2 discernible images** by an optical system
limit of resolution is reached when **2 airy discs** are separated so the **centre of one** falls on the **first dark ring** of the other
33
At what age in infants should visually-directed reaching develop?
2-5 months
34
What visual acuity does the ability of a 15-month child to pick up tiny coloured hundreds and thousands sweet?
6/24 or better (i.e. absence of serious visual defect)
35
What are 5 types of visual acuity tests?
1. Catford drum
2. STYCAR
3. Preferential looking
4. Visual evoked potentials
5. Optotype (e.g. Kay's/Cardiff/STYCAR/Sheridan Gardiner cards, Snellen)
36
What is the Catford drum?
white cylinder with black dots of increasing size corresponding to visual acuities 6/6 to 2/60, viewed at 60cm. spots oscillate horizontally, stimulates corresponding eye movement if seen
37
Does the catford drum over or under-estimate visual acuity and why?
over-estimates it - because target is moving and test conducted at short working distance
38
What is STYCAR testing?
Screening Test for Young Children and Retards
10 white polystyrene rolling balls 3.5mm - 6cm diameter, rolled across contrasting floor 3m from child - look for pursuit eye movements
39
What is preferential looking?
assesses VA of infants based on turning head/eyes towards patterned vs uniform target
black and white square grating + plain grey target presented simultaneously
40
What are visual evoked potentials?
**electrical** responses generated in the **occipital cortex** by visual stimulation of the eye - stimulus is black + white square grating or chequerboard pattern, pattern **reverses** at a set frequency
41
What is an optotype and what are 5 examples of tests?
symbol, identificatino of which corresponds to a certain level of visual acuity
1. Kay's pictures test
2. Cardiff cards
3. STYCAR letter tests
4. Sheridan-Gardiner test
5. Snellen visual acuity
42
What is Kay's picture test?
pictures of objects e.g. cat, train, house
43
What are Cardiff cards?
pictures similar to Kay's pictures test - makes use of preferential looking
44
What are STYCAR letter tests?
using the 5 letters first recognised by children: HOTVX
tests children up to approx 3 years
45
What is the Sheridan-Gardiner test?
7 letters used to test visual acuity, HOTVX UA (2 more added to STYCAR letter tests)
46
How does the Snellen visual acuity test work?
based on theory that smallest object which can be resolved by the eye subtends the same visual angle at the nodal point of the eye as a cone photoreceptor i.e. one minute arc
chart with rows of letters of diminishing size, each row accorded a number indicating the distance in metres at which person with normal visual acuity should correctly identify the letters
47
How is the result of the Snellen visual acuity test interpreted?
should be read from 6m (12ft)
subject who identifies letters on the 12 line from 6m has 6/12 vision - numerator indicates the viewing distance, denominator is distance at which someone with normal visual should be able to read the letter
normal is 6/6 (young adults can achieve 6/4)
48
What are LogMAR visual acuity charts?
regular progression in size and spacing of letters from one line to the next, same number of letters on every line
therefore more precise than snellen test
comparable results at any distance
49
What is Vernier acuity?
measured using square wave grating, measures smallest offset of a line which can be detected (normally 3-5 seconds of arc discerinble)
50
How is Near Visual acuity tested?
distance 25-33cm, chart comprising of unrelated words or passages of text
51
What is the British N system?
type of near vision test; based on typesetters' point system to specify the size of the metal block on which letters were traditionally set
each block is 1/72 of an inch
uses Times New Roman
52
What is potential visual acuity testing? What are 3 examples?
tests visual acuity when not possible to see macula e.g. cataract;
1. pinhole test
2. blue field entoptic phenomenon
3. interferometers
53
What is the **blue field entoptic** phenomenon?
ability to see moving white dots when blue light diffusely illuminates the retina - thought to represent white blood cells in the perifoveal capillaries.
if present - macular function grossly intact
54
What is an interferometer?
projects **laser** light from **2 sources** onto the retina
interference occurs where the 2 sources meet, this is seen as a **sine wave grating** if the macula is functioning
55
What is the overarching phenomenon used to test contrast sensitivity and what are 2 more specific ways to test this?
**sine wave** grating
1. television monitor
2. chart e.g. **_Pelli-Robson contrast_** test chart (letters), VISITECH chart (must indicate orientation of circles)
56
What is glare?
scattered light which reduces visual function, may be predominant symptom of corneal oedema or scarring, cataracts or opacification of the posterior lens capsule
57
What is glare testing?
measurement of visual function (e.g. visual acuity, contrast sensitivity, colour vision) in the presence of a source of glare.
58
What is polarised light?
individual wave motions of light in a beam of light which are **parallel** to each other when seen in cross section
59
How is polarised light produced?
ordinary light has encounter with polarising substance or agent
60
How do polarising substances such as **calcite crystals** work?
only transmit light rays which are vibrating in one particular plane
thus only a proportion of incident light is transmitted onwards and the emerging light is polarised
61
What are 2 things that polarising media do to light?
1. **reduce** radiant **intensity**
2. do not affect spectral composition
62
When is light polarised on reflection from a plane surface and what happens otherwise?
* it is polarised if the **angle of incidence** is **equal to the polarising angle** for the substance
* at other angle of incidence the reflected light is **partly polarised** i.e. a mixture of polarised and non-polarised light
63
What does the polarising angle of a polarising substance depend on?
the **refractive index** of the substance comprising the reflecting surface
64
Where is the **plane of polarisation** of reflected light from a plane surface?
parallel with the surface
65
What is birefringence?
property possessed by some substances whereby they can transmit light waves lying **paralell** to its structure, but selectively slow and therefore redirect light waves vibrating in a plane **perpendicular** to its structure
e.g. crystals of **_quartz_**
66
How do birefringent substances split light and what measurements does this generate?
split incident unpolarised light into **two polarised beams travellining in different directions**
therefore they have **2 refractive indices**
67
What is dichroism?
property possessed by substance wherein its molecular structure completely **blocks** transmission of **light waves not aligned** with its structure, by absorption
therefore only **1 beam of polarised light** emerges of weaker intensity
68
What are 2 examples of dichroic substances?
1. tourmaline
2. polaroid (fine iodine and quinine sulfate crystals embedded in plastic)
69
What are 4 uses of polarised light?
1. polaroid in sunglasses
2. assessment of binocular vision e.g. Titmus test
3. pleoptics to produce Haidinger's brushes
4. manufacture of optical lenses to examine them for stress
70
What is stereopsis?
ability to fuse slightly dissimilar images which stimulate disparate retinal elements within Panum's fusional areas in the two eyes, with the perception of depth
71
How is stereopsis measured and what is normal steroacuity?
* graded according to the least horizontal disparity of retinal image that evokes depth perception, measured in seconds of arc
* normal stereoacuity is approx **60 seconds of arc** or greater (if very good, better than 15 seconds of arc)
* maximum steroacuity is when images fall on the macular area
72
What level of stereoacuity can exclude amblyopia?
**250 seconds of arc** or lower excludes significant amblyopia
73
What are 4 clinical tests of stereoacuity?
1. Titmus test
2. TNO test
3. Frisby test
4. Lang stereotest
74
What is the Titmus test?
vectographs used - biggest is the Wirt fly. viewed at 40cm
## Footnote
A vectrograph consists of two superimposed views presented in such a way that the light entering each eye is plane **polarised**, the light from one view being at right angles to that from the other. The composite picture must be viewed through a polarising visor or spectacles.
75
What is the Frisby test?
clear plastic plates of different thicknesses, each plate has 4 squares filled with small random shapes. One square on each plate contains **hidden circle**
76
What is the TNO test?
computer generated random dot anaglyphs - stereogram in which 2 disparate views are printed in red and green respectively on a white background. red-green spectacles worn
eye looking through red filter sees green picture and vice versa. 2 views may be fused to give stereoscopic effect
77
What is the Lang stereotest?
targets made up of fine vertical lines seen alternately by each eye when focused through built-in cylindrical lens elements.
displacement of random dot images creates disparity ranging from 1200 to 550 seconds of arc
78
What is radiometry vs photometry?
* **Radiometry** - quantifies radiant energy in all parts of the EM spectrum as an *absolute* value
* **Photometry** - quantifies part of the spectrum in terms of the *visual response* it produces i.e. the spectral sensitibity of the eye
79
What are 4 important measurements related to radiometry?
* radiant flux = how much light is emitted from a source
* radiant intensity = intensity of light
* irradiance = amount of light falling on a surface
* radiance = amount of light reflected from a surface
80
What are 4 importance photmetric measures?
1. luminous flux = amount of light emitted from a source
2. luminous intensity = light intensity
3. illuminance = amount of light falling on a surface
4. luminance = amount of light reflected from a surface
81
How are radiometric and photometric units related?
the luminous efficiency of the radiation, a conversion factor specific for each wavelength determined by the sensitivity of the eye to it
82
What is the peak photopic sensitivity of the eye in terms of wavelength, and how much radiant flux and luminous flux this equates to?
* peak photopic sensitivity of eye is **555nm** (yellow-green)
* at this level **1 watt** of monochromatic light has photochromatic equivalent of **685 lumens**
83
What are the units for radiant flux vs luminous flux, when measuring total flow of light?
radiant flux = watts, luminous flux = lumens
84
What is light intensity a measure of?
the flow of light per unit solid angle of space extending away from it
85
What are the units for radiant intensity vs luminous intensity?
* radiant = watts per steradian
* luminous = candelas (lumens per steradian)
86
What is the luminous intensity of one candela equivalent to and why?
one candela = 4π lumens.
A steradian is the unit of solid angle (resembling a cone) and defined as the angle at the centre of a sphere which subtends an area on the surface of the sphere measuring the square of the radius (r) i.e. r2.
Since the surface area of a sphere is 4πr2, it follows that a point source whose luminous intensity is one candela emits a total of 4π lumens.
87
What units are used for irradiance and illuminance?
* irradiance = watst per square metre
* illuminance = lumens per square metre
88
How can illumination of a surface be calculated?
E = Icosi / d2
E=illumination, I = luminous intensity, i = angle of incidence, d= distance between source and surface
so **illumination is inversely related to distance**
89
What is a perfect diffuser?
surface that is uniformly diffusing (**reflects light equally in all directions**) AND reflects **all** the light which is incident
90
What are radiance and luminance measured in?
* radiance = **watts per steradian per square metre**
* luminance = **candelas per square metre**
* OR often measured by comparison to uniform diffuser which emits total flux in all directions of 1 lumen per unit area
* 1 **apolstilb** = 1/π candelas per square metre.
* **troland** = retinal illumination when surface luminance of 1 candela per square metre viewed through entrance pupil measuring one square millimetre
91
What is the definition of luminance?
luminous intensity per unit projected area in a given direction (measures light emitted in only **one direction**, NOT total amount leaving the surface in all directions)
92
What is automated perimetry?
* [https://www.youtube.com/watch?v=g11hC2HuApg&ab\_channel=WillsEyeGlaucomaApp](https://www.youtube.com/watch?v=g11hC2HuApg&ab_channel=WillsEyeGlaucomaApp)
* Perimetry measures the **light sensitivity** of points on the retina by the ability of the patient to **detect light stimuli of varying intensity** presented at corresponding **points in the visual field.**
* Currently, most perimeters have a standard background luminance of **31.5 apostilbs** (asb)
* Eye is positioned at the centre of the hemisphere
* Spots of light are projected on to the inner surface of the hemisphere.
93
What is indocyanine green used for?
provides view of choroidal circulation, useful in detecting occult choroidal neovascularisation e.g. polypoid CNV (not visible in fluorescein)
94
How does indocyanine green fluorescence work to visualise the choroidal circulation?
not absorbed by the RPE (unlike fluorescein); dye absorbs wavelength of light 805nm and emits 835nm wavelength light (both in infrared spectrum)
