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
What are in phase light waves termed
Coherent
Define constructive interference
2 light waves in phase to result in a summation of the 2
Define destructive interference
two waves of equal amplitude are out of phase by half a cycle they will cancel each other out: destructive interference
Where does destructive interference occur in the eye
cornea. The collagen bundles of the stroma are so spaced that any light deviated by them is eliminated by destructive interference
What is diffraction
when a wave front encounters a narrow opening or the edge of an obstruction, the wave motion spreads out on the far side of the obstruction. It is as if the 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. This phenomenon is called diffraction
What is an airy disc
When light passes through a circular aperture, a circular diffraction pattern is produced. This consists of a bright central disc surrounded by alternate dark and light rings. The central bright zone is known as the Airy disc
What is the limit of resolution/resolving power of the eye
the smallest angle of separation (w) between two points which allows the formation of two discernible images by an optical system
When is the limit of resolution reached
when two Airy discs are separated so that the centre of one falls on the first dark ring of the other.
Visual acuity tests for babies
Fixation with either eye should be central, steady and maintained (CSM). The best target is a face (especially that of the mother), a toy, or a television cartoon. A strong preference for one eye, indicated by an aversive response to occlusion of that eye, squint, nystagmus, roving gaze, and eye poking, all suggest poor visual acuity
When does visually directed reaching develop
between 2 and 5 months of age
What is the Catford drum
comprises a white cylinder marked with black dots of increasing size corresponding to visual acuities ranging from 6/6 to 2/60 when viewed from 60 cm. his spot is made to oscillate horizontally and stimulates corresponding eye movement if seen by the child.
What is the STYCAR test
rolling balls are ten white polystyrene spheres ranging in size from 3.5 mm to 6 cm in diameter. They are rolled across a well illuminated contrasting floor 3 m from the child. Pursuit eye movements indicate that they are seen. The Worth’s ivory ball test is similar.
Other methods of measuring VA in preverbal chldren
depend upon preferential looking and the measurement of visually evoked potentials
What is an optotype
a symbol, the identification of which corresponds to a certain level of visual acuity. All tests employ black letters or pictures on an opaque or retroilluminated white background in order to maximise contrast
What is the Kay picture test
uses pictures of objects such as a cat, train or house. The Cardiff cards also use pictures.
What are the first letters recognised by children
HOTVX. The Sheridan–Gardiner test uses seven letters, adding U and A
On the Snellen chart what is the angle subtended by the bars and spaces of each letter
1 minute of degree (1 arc minute).
1 minute of a degree= 1/60th of 1 degree
Why are LogMAR and Bailey Lovie charts more precise than Snellen
because they have a regular progression in the size and spacing of the letters from one line to the next and the same number of letters on every line
What is the nodal point of the eye
at the junction of anterior 2/3 to posterior 1/3 of lens lies the nodal point from which 2 images are discerned by minimum 1 minute of angle
What is vernier acuity
smallest offset of a line which can be detected. An offset of 3–5 seconds of arc is normally discernible.
What is blue field entopic phenomenon
ability to see moving white dots when blue light diffusely illuminates the retina. They are thought to represent light transmitted by white blood cells in the perifoveal capillaries. When this phenomenon is present, macular function is grossly intact
How is contrast sensitivity measured
using a sine wave grating. This is a pattern in which there is a gradual transition between alternating light and dark bands, i.e. the edges of the bands appeared blurred.
What is the Pelli-Robson chart
Pelli–Robson contrast test chart displays letters that have decreasing levels of contrast to their background
What is polarized light
a beam of light in which the individual wave motions are lying parallel to each other. Such a beam is said to be polarized.
What are some examples of polarising substances
Calcite crystals. polarising medium reduces radiant intensity but does not affect spectral composition
How is light polarized
on reflection from a plane surface, such as water, if the angle of incidence is equal to the polarising angle for the substance. The polarising angle is dependent on the refractive index of the substance comprising the reflecting surface. At other angles of incidence the reflected light is partly polarised, i.e. a mixture of polarised and non-polarised light
What is birefringence
Some substances have a molecular structure which transmits light waves lying parallel to its structure but which selectively slows and therefore redirects light waves vibrating in a plane perpendicular to its structure. Crystals of quartz have this property, which is known as birefringence. Because they split incident unpolarised light into two polarised beams travelling in different directions, they have two refractive indices.
what do dichroisim substances do
completely blocks transmission of light waves not aligned with its structure by absorption. Thus, only one beam of polarised light emerges, much weakened in intensity compared with the incident non-polarised light
Examples of dichroic substances
Tourmaline and polaroid (the latter made from fine iodine and quinine sulphate crystals embedded in plastic) are dichroic substances, polaroid being commonly used in sunglasses.
Other examples of the use of polarised light in ophthalmology are the assessment of binocular vision in which polarising glasses may be used to dissociate the eyes, e.g. in the Titmus test
What is stereopsis
Stereopsis is the 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.
It s measured in seconds of arc.
What is normal stereoacuity
approximately 60 seconds of arc or better (slightly different values are quoted by different workers). An individual with very good stereoscopic vision may have a stereoacuity of better than 15 seconds of arc which is the smallest disparity offered in the Frisby stereotest
Range of stereoacuity tested by Frisby test
600–15 seconds of arc
Where on the retina is the max stereoacuity achieved
when the images fall on the macula area
What stereoacuity is said to exclude significant amblyopia
A stereoacuity of better than 250 seconds of arc. A stereoacuity of worse than 250 seconds of arc may be an indicator of amblyopia.
Stereoacuity tests
Titmus test, the TNO test, the Frisby test and the Lang stereotest
What range of stereoacuity does the Wirt fly test
The test must be viewed at 40 cm, and covers a range of stereoacuity from approximately 3000 to 40 seconds of arc
What range of stereoacuity does the TNO test
480 to 15 seconds of arc
What range of stereoacuity does the Lang stereotest
1200 to 550 seconds of arc. The test is viewed at a normal reading distance.
How does radiometry measure light
in terms of how much is emitted from a source (radiant flux), its intensity (radiant intensity) and the amount falling on a surface (irradiance) or reflected from it (radiance)
What are the equivalent photometric measurements of radiometry
luminous flux, luminous intensity, illuminance and luminance
What is the peak photopic sensitivity of the eye
to the wavelength of 555 nm (yellow–green), at which 1 watt of monochromatic light has a photometric equivalent of 685 lumens. This wavelength is therefore said to have maximum luminous efficiency
What is luminous efficiency
Radiometric and photometric units are related by the luminous efficiency of the radiation, a conversion factor specific for each wavelength determined by the sensitivity of the eye to it. The eye is progressively less sensitive to wavelengths towards each end of the visible spectrum. In other words, the luminous efficiency of the radiation becomes lower and the same energy flux (radiometric unit) is equivalent to a lower luminous flux (photometric unit). The conversion factor falls towards zero outside the range 400–700 nm (visible light).
Units for radiant intensity
watts per steradian
units for luminous intensity
candelas (lumens per steradian)
units for irradiance
watt/metre squared
units for illuminance
lux=lumens/metre squared
Units for radiance
watt/steradian/metre squared
Units for Luminance
candela/metre squared
What is an apostilb
A luminous flux of one lumen per square metre corresponds to a luminance of one apostilb. (An alternative definition is 1 apostilb = 1/π candelas per square metre.)
