Chapter 1: Light properties and vision Flashcards
Between which type of electromagnetic waves does optical radiation lie?
x-ray and microwaves (includes UV, visible light and infrared light)
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What is the range of wavelengths of visible light?
400nm - 780nm
What is the relationship between electromagnetic wave wavelength and energy of photos?
short wavelength - greater energy of photons of optical radiation
What is the fate of the following types of radiation:
- UVA
- UVB and UVC
- IRA
- IRB and IRC
- 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
What are the 3 populations of cone photoreceptors in the retina which perceive colour?
- Short wavelength light: blue
- Middle wavelength light: green
- Long wavelength light: red
What are 3 groups of congenital colour vision defects?
- 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
On which chromosomes are the genes encoding red, green and blue pigments found?
- Red: X chromosome
- Green: X chromosome
- Blue: chromosome 7
What is commonest congenital colour vision defect?
deuteranomaly
What are 2 sites of acquired colour defects?
- acquired optic nerve defect
- acquired retinal disease
Which colours tend to be affected by acquired optic nerve colour defects and what are the 2 exceptions?
red-green defects
Exceptions: glaucoma and autosomal dominant optic neuropathy (predominantly blue-yellow deficit)
Which colours tend to be affected by acquired retinal disease colour defects and what are the 2 exceptions?
blue-yellow defects
Exceptions: cone dystrophy and Stargardt’s disease (predominantly red-green)
What is the most comprehensive test of colour vision?
Farnworth-Munsell (FM) hue 100 test (84 coloured discs, groups arranged in rows in order of hue)
What is a colour vision test that can be used by children?
Lanthony New colour test
What colour vision defects do Ishihara pseudoisochromatic test plates test for?
specific test for congenital red-green defects
What is a less comprehensive test for colour defects than the Farnsworth Munsell hue 100 test?
D15 test - doesn’t distinguish mild colour defects
What do aphakic (no lens) eye or pseudophakic eyes with IOLs give rise to blue or violet colours?
usually UVA is absorbed by the lens of the eye but retinal photoreceptors are also sensitive to wavelengths 350-400nm (UVA)
How can some ophthalmic instruments reduce exposure of the retina to damaging wavelengths?
yellow filters
What is the definition of fluorescence?
= the property of a molecule to spontaneously emit light of longer wavelength when stimulated by light of a shorter wavelength
How does fluorescein angiography work?
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
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What is pseudofluorescence?
- 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
What is another fluorscent substance in addition to fluorescein?
indocyanine green dye
What is the function of indocyanine green dye?
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
How does the fluorescence of ICG compare with fluorescein?
4% of 805nm radiation absorbed by ICG is emitted at 835nm, vs total (100%) fluorescence of fluorescein
What is the definition of the amplitude of a wave?
maximum displacement of an imaginary particle on the wave from the baseline (see A)
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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
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
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
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
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
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How does aperture size influence diffraction?
diffraction effects are most marked with small apertures
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)
What is the definition of the limit of resolution and resolving power?
the smallest 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
At what age in infants should visually-directed reaching develop?
2-5 months
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)
What are 5 types of visual acuity tests?
- Catford drum
- STYCAR
- Preferential looking
- Visual evoked potentials
- Optotype (e.g. Kay’s/Cardiff/STYCAR/Sheridan Gardiner cards, Snellen)
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
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