Properties of light and visual function Flashcards
What is the optic radiation
Optical radiation lies between X-rays and microwaves in the electromagnetic spectrum
How many wavebands are there in optic radiation
7
What are the 7 wavebands of the optic radiation
UV-C 200-280nm
UV-B 280-315nm
UV-A 315-400nm
Visible light 400-700nm
IRA 700-1400nm
IRB 1400-3000nm
IRC 3000-10000nm
Relationship between wavelength and energy
The shorter the wavelength, the greater the energy of the individual quanta, or photons, of optical radiation.
What structures in the eye absorb the short and long wavelengths
Cornea and Sclera (absorb UVB,C, IRB,C)
Where is UVA absorbed
Lens which absorbs 400-1400nm wavelengths
What is the colour of an object determined by
the wavelengths emitted or reflected from the surface
Which cone photoreceptors determine colour
three populations of cone photoreceptors in the retina which are sensitive to light of short (blue), middle (green), or long (red)
What is deuteranopia
Absence of green cone function
What is protanopia
Absence of red cone function
What is tritanopia
Absence of blue cone function
Which chromosome carries genes encoding for red and green pigment
X chromosome
Which chromosome carries genes encoding for blue pigment
7
What colour defect does acquired optic nerve disease cause
red-green defects
What colour defect does autosomal dominant optic neuropathy and glaucoma cause
blue yellow. it has recently been found that visual field loss in glaucoma is detected earlier if perimetry is performed using a blue light stimulus on a yellow background.
Colour defect in acquired retinal disease
blue-yellow defects
Colour defect in cone dystrophy and Stargardt’s disease
Red-green defect
What is the FM hue 100 test
most comprehensive method. It comprises 84 coloured discs, numbered in sequence on the undersurface and divided into four groups of 21. The colours of each group occupy a portion of the colour spectrum. The colours differ only in hue and have equivalent brightness and saturation. Each group must be arranged in a row with the reference colours at each end and the intervening discs in order of closest colour match. The order of placement indicates the nature of the colour defect
What is the D-15 colour vision test
uses colours from all parts of the spectrum which must be arranged in order from a single reference colour. The test does not distinguish mild colour defects, but for most purposes those passing the test are unlikely to have problems with hue discrimination
What is the ishihara test
test plates specifically test for congenital red–green defects, the most common abnormality of colour vision. The test plates consist of random spots of varying isochromatic density. Numbers or wavy lines (for illiterates) are represented by spots of different colours. A patient who is colour blind will see only a random pattern of spots or incorrect numbers. The figures can only be distinguished from their background by their colour and not by a difference in contrast
What is the Lanthony New colour test
tests hue discrimination and can be used by children
Which wavelengths in the visible spectrum are shown to cause retinal damage under normal conditions
350nm to 441nm. Desirable for intraocular lenses to have UV filters on them
What is fluorescence
property of a molecule to spontaneously emit light of a longer wavelength when stimulated by light of a shorter wavelength. For example, the orange dye fluorescein sodium when excited by blue light (465–490 nm) emits yellow–green light
How does fluorescein angiography work
White light from the flash unit of a fluorescein camera passes through a blue ‘excitation’ filter to illuminate the fundus with blue light The wavelengths transmitted by the excitation filter approximate to the absorption spectrum of fluorescein. Most of the light is absorbed, some is reflected unchanged, and some is changed to yellow–green light by fluorescence. The blue reflected light and yellow–green fluorescent light leaving the eye are separated by a yellow–green ‘barrier’ filter in the camera. This blocks blue light and exposes the camera film only to yellow–green light from the fluorescein, thereby delineating vascular structures and leakage of dye
What is pseudofluorescence
occurs if there is an overlap in the spectral transmission of the excitation and barrier filters. This allows reflected wavelengths at the green end of blue to pass through the barrier filter and appear as fluorescence
What wavelength does ICG absorb and emit
absorbs 805 nm and emits 835 nm
Uses of ICG
1) delineate occult choroidal neovascularisation not visible with fluorescein
2) photosensitise vascular lesions to diode laser photocoagulation
Define wavelength
distance between two symmetrical parts of the wave motion
Define a cycle
One complete oscillation is called a cycle
Define amplitude
maximum displacement of an imaginary particle on the wave from the base line
Define phase difference
If two waves of equal wavelength (but not necessarily of equal amplitude) are travelling in the same direction but are ‘out of step’ with each other, the fraction of a cycle or wavelength by which one leads the other is known as the phase difference
What are out of phase light waves termed
incoherent
