optics Flashcards
how is light focused onto the fovea of the eye
what is cornea and lens power
- A clear view to the retina.
- The length of the eye.
- The power, measured in diopters (D), of the refractive components of the eye,
mainly the cornea (40D) and lens (20D).
what is axial length and when does it lengthen
eyeball is measured from the corneal surface to the retinal pigment epithelium (RPE)/Bruch membrane.
first 3-6 months of life
Newborn: 16 mm
3 years: 22.5 mm
13 to 18 years (adult length): 24 mm
length from lens to retina in adults is
17mm
what is myopia & how can it be classified
principle focus of light lies before reaching the retina.
low (−6D).
myopia can be caused by
● Axial myopia: Large eyes (axial length >24 mm). Most common cause.
● Index myopia: High refractive power as seen in conditions such as
keratoconus or nuclear sclerotic cataract.
methods that may be helpful in slowing myopic progression
● Use of atropine (lower doses of atropine were more effective with fewer side effects than higher doses [1]) and pirenzepine drops.
● Outdoor activity: It is thought that too much near work may contribute in myopic progression (2).
● Bifocals and progressive lenses.
Mx for myopia
● Spherical concave lenses (glasses or contact lenses).
● Keratorefractive surgery: Uses laser to reshape the cornea and so changing
its refractive power. In myopia, the central corneal tissue is ablated, making the central cornea flatter. Common procedures used are photorefractive keratectomy (PRK), LASIK or LASEK.
Caution: Contact lens wearers should withhold wearing their lenses for at least 14 days for soft lenses and at least 1 month for rigid gas-permeable (RGP) lenses. Similarly, this precaution is used for hypermetropic surgery.
what is hypermetropia and its classification
the principle focus of light lies beyond the retina.
low (+5D)
hypermetropia can be caused by
● Small eyes (axial length <24 mm)
● Low refractive power: As seen in aphakic patients (absence of the lens) and
patients with flat corneas
ass of hypermetropia
esotropia
angle-closure glaucoma, retinoschisis
uveal effusion syndrome (nanophthalmos)
amblyopia in children.
Mx of hypermetropia
● Spherical convex lenses.
● Keratorefractive surgery: Procedures used are similar to those used in
myopia; however, the peripheral corneal tissue is ablated in hypermetropia resulting in a steeper central cornea.
what are spherical convex lenses
These are also called minus lenses, and they resemble two prisms placed base to base
how do spherical convex lenses work
They work by converging light and are used in managing hypermetropia by bringing the image formed behind the retina closer to land on the retina.
what are spherical concave lenses
These are also called plus lenses and they resemble two prisms placed apex to apex
how do spherical concave lenses work
They work by diverging light and are used in managing myopia by bringing the image formed in front of the retina further to land on the retina.
how do you calculate the power of lenses
The power of the lens is measured in diopters. It is equal to the reciprocal of the focal length (f) in metres:
Power = 1/f
what is astigmatism
refractive power of the eye is not the same in all meridians (directions) due to a change in the shape of the lens or cornea’s curvature, frequently described as ‘rugby ball-shaped’
image is formed as a Sturm’s Conoid
what is diurnal variations
in corneal shape, flattest in the morning, as a result of changes in eyelid pressure and muscle tension
what is regular astigmatism
The principle meridians, termed steepest and flattest meridians, are 90° from each other.
Further classified as:
With-the-rule astigmatism: Occurs when the vertical meridian
(90°) is the steepest. It is corrected with a plus cylinder lens between 60° and 120°.
Against-the-rule: Occurs when the horizontal meridian (180°) is the steepest. It is corrected with a plus cylinder lens between 150° and 30°. Oblique: Occurs when the principle meridians are neither at 90° nor 180°. It is corrected with a plus cylinder lens between 31° and 50° and 121°–149°.
what is irregular astigmatism
Principle meridians are not perpendicular to each other. Occurs in conditions such as keratoconus or corneal ulcers.
Mx of astigmatism
● Soft toric lenses: Combination of spherical and cylindrical lenses.
● RGP contact lenses are usually used for irregular astigmatism.
what are cylindrical lenses
contain a cylinder in a single plane surface
focus the length into a line rather than a point
one meridian only
what is transposition
converting a minus cylindrical lens to a plus cylindrical lens and vice versa.
where is transposition used
frequently used in toric lens prescriptions.
steps of transposing
● Step 1: Add the cylinder and sphere power; this becomes your new sphere power.
● Step 2: Change the sign of the cylinder.
● Step 3: Change the axis by 90°: If the axis is ≤90° then add 90°, but if it is
>90° then subtract 90°.
what is presbyopia
age-related loss of accommodative ability of the eye.
pathophysiology of presbyopia
increase in lens size and hardness, or due to ciliary muscle dysfunction
lens cannot thicken or flatten properly, and accommodative power is lost.
accommodative power at age
8
60
14D
<1D
when does presbyopia occur
40 years
what is amplitude of accommodation
maximum increase in diopter power the eye can achieve through accommodation.
● The near point of the eye is the closest point where the image remains clear.
● To achieve comfortable vision, at least 1/3 of the amplitude of accommodation should be kept in reserve.
what is esotropia
eviated nasally and moves temporally on cover testing to fixate.
what is exotropia
deviated temporally and moves nasally on cover testing to fixate.
what is the angle of deviation
an be measured objectively via prism cover testing.
how do you prescribe prism and where are they placed
they should be placed for both eyes with the power of prisms split evenly between the two eyes. The base of the prism should point away from the deviation. For example, the base is pointed temporally in an esotropic eye and the apex nasally.
what is hypermetropia
ye is deviated superiorly and moves inferiorly on cover testing to fixate.
what is hypotropia
where the eye is deviated inferiorly and moves superiorly on cover testing to fixate.
how should the prism places in hyper/hypotropia
split evenly between the two eyes. The base of the prism should be away from the deviation. Prisms should be pointing in opposite directions for both eyes. For example, for a right hypertropia, the right eye has the prism base down and the left eye has the prism base up.
how does a prism work
transparent medium bound by two planes that are at an angle to each other. They do not focus light. They bend light (refraction) towards the base of the prism. To an observer, a virtual image is formed that is erect and displaced towards the apex
what is snell’s law
when light moves from one transparent medium of higher density to another of lower density (e.g. water to air), the light refracts. This concept applies to the eyes and is the basis for lens manufacturing.
what is the angle of incidence
angle the light travels as it hits the boundary of another medium.
what is the angle of refraction
dependent on the angle of incidence and is the angle the light travels as it crosses the boundary.
as angle of incidence increases the angle of refractiom
gets closer to 90 degress
what is the critical angle
When the angle of refraction is equal to 90°, the angle of incidence
what is total internal reflection
angle of incidence is greater than the critical angle; the light will not pass through the medium, that is, it is completely reflected.
what instruments rely in total internal reflection
prisms and gonioscopy
what is one prism power (PD)
produces a deviation of a light ray of 1 cm measured at 1 m from the prism. It can be calculated using one of two equations:
(1) P = Fd (2.2)
where P is the prism power (PD), F is the lens power (D) and d is the distance (cm)
of the pupil from optical centre.
Or
P = 2 × angle of deviation (2.3)
where the angle of deviation is measured in degrees (°).
what is the snellen chart
most commonly used tool to subjectively assess visual acuity.
placed 6 metres away from the patient (this represents the numerator). The denominator represents the distance at which an average person or eye can read.
what is the duochrome test
uses chromatic aberrations of the eye to refine the best vision sphere following optical correction. The test is comprised of black
letters positioned on two backgrounds, red and green.
As red is a longer wavelength than green, it will focus behind the retina, while green will focus in front of the retina.
what is the ishihara chart
This is a test used to screen for red-green colour vision defect. The test comprises of plates containing dots of various colours and sizes and other dots which form certain numbers that should be visible to a patient with normal colour vision.
what is the LogMAR chart
Due to limitations of the Snellen chart, a new chart was developed by Bailey and Lovie called the logarithm of minimum angle of resolution (LogMAR) chart
MORE POPULAR CHOICE OF RECORDING VA IN CLINICAL TRIALS
key features of LogMAR chart
● It has five letters on each row with equal spacing, ensuring the task is equivalent at each level and accounts for the crowding phenomenon (difficulty of recognizing letters when they are presented with other neighbouring letters. This occurs in amblyopia).
● The letter spacing is equal to one letter width.
● The row spacing is equal to the height of a letter from the row below.
● Each correct letter is worth 0.02 log units. Since there are five letters a
line, a correct line is worth 0.1 log units.
● Patients are usually positioned 4 metres away. The scoring is from
1 to 0, so that each letter read correctly will result in subtraction of 0.02 from 1. In comparison with the Snellen chart, a 6/6 Snellen acuity is equal to 0 LogMAR and a 6/60 Snellen acuity is equal to 1 LogMAR.
what is retinoscopy
objective method of measuring VA. It is useful in uncooperative patients.
process of retinoscopy
Light from a retinoscope is shone into the patient’s retina at a certain distance. The aim is to observe the patient’s red reflex while adding plus/minus lenses until a complete red reflex is observed. In myopic patients, the direction of the reflex is against the direction of the light. In hypermetropic patients, the direction of reflex is with the direction of light.
Once the perfect reflex is observed, the examiner should subtract the dioptric equivalent of their working distance from the correcting lens. If the working distance is a 67 cm (2/3 m), then the examiner should subtract 1.5D (Power = 1/f = 1/0.67) from the lens power used to achieve perfect vision correction.
average VA of newborn 3 months 6 months 9 months 1-2 years
● Newborn: 6/200–6/60 ● 3 months: 6/90–6/60 ● 6 months: 6/30 ● 9 months: 6/24 ● 1 to 2 years: 6/18–6/6
who is forced preferential looking charts
(e.g. Keeler or Cardiff): Used for children less than 1 year of age.
what is cardiff cards
● Cardiff cards: Used for children aged 6 months up to and including 2 years.
what is kay pictures
● Kay pictures: Children aged 2–3 years.
what is sheridan-Gardiner
children aged 3-5 years
snellen and Log MAR is used by which age group
over 4 years old and
literate adults. The Keeler LogMAR or the illiterate E test (either Snellen or LogMAR) can be used for preschool children and illiterate adults.
what is slit lamp
two microscopes positioned at 13–14° from each other to provide a binocular view with stereopsis.
what is direct focal illumination
Here the beam focusses on the part of the eye being examined.
what is diffuse illlumination and used for
This is where the illumination light is out of focus and diffusely illuminating the area being examined. Used for general examination of external eye structures
what is reteroillumination and used for
Uses the light reflected from the iris to look for corneal opacities, or from the fundus to examine the red reflex, patency of iridotomies and lens opacities. Iris transillumination uses light reflected from the retina in an undilated pupil to view iris abnormalities, such as in pigment dispersion or pseudoexfoliation.
what is specular reflection and used for
Best way to view corneal endothelium, for example, in Fuchs’ corneal dystrophy.
what is sclerotic scatter and used for
Light is directed at the limbus, which is in turn scattered through the cornea. Used to evaluate general corneal opacities.
what does direct ophthalmoscope produce
● An image with ×15 magnification and an area of about 2 disc diameters.
● An image that is virtual and erect.
● No stereoscopic vision.
what is indirect opthalmoscope
light attached to a headband and uses a handheld biconvex aspheric lens ranging from 15–40D in power. Generally, a 20D is routinely used.
optics of indirect opthalmoscopy produces an image
● Has a 2–5 times magnification. As the power increases, the magnification decreases; a 20D lens has about ×3 magnification, whereas a 30D lens has about ×2 magnification.
● Has a wide view of about 8 disc diameters.
● Is real and inverted vertically and horizontally.
● Note: The field of illumination is largest in cases of high myopia and smallest
in hypermetropia.
visible light wavelenght
390 nm and 760 nm.
how much light does cornea and lens refract
cornea two - thirds of the focusing power of the eye, the crystalline lens for one - third.
why do light rays converge in the eye
cornea is thicker than air
lens medium thicker than the vitreous tumor
therefore higher refractive index means it bends towards the normal more hence converging
what is trichromacy
- all 3 types of cones working
- normal colour vision
what is dichromacy
2 types of cones working
1 type of cone isn’t
red-
if a cone colour is missing what do you call it
red - protanopia
green - deuteranopia
blue - tritanopia
what is monochromacy
none or only 1 type of cone working
total colour blindness
what is anomalous trichromacy
all 3 cones are used to perceive light BUT 1 types of cone is malfunctioning
most common anomalous trichromacy
GREEN- deuteranomaly
where is red/green gene
X chromosome
where is blue colour gene
chromosome 7
What is visual acuity
measure of the ability to discriminate 2 stimuli separated in space
If VA is 6/6 on snellen chart what does that mean
at a viewing distance of 6m the smallest leter that can be resolved subtends 5 mins of arc
what does each stroke of the letter represent
1 min of arc
snell’s law of reflection
angle of incidence = angle of reflection
incident ray, reflected ray and normal all lie in the same plane
if object is outside the centre of curvature what image is formed by concave mirrors
real
inverted
diminished in size
lying between C and F
if object is between centre of curvature C and prinicipal of focus F what image is formed by concave mirrors
real
inverted
enlarged
lying outside the centre of curvature
if object is inside principal focus F what image is formed by concave mirrors
virtual
erect
enlarged
what image is formed by convex mirror
virtual
erect
diminished
in a prism light is deviated towards what
base of the prism following snells law
image formed by prisms
erect
virtual
deviated towards the apex
what is angle of deviation
net change in direction of the ray angle D
for a prism in air, the angle of deviation is determined by 3 factors
(1) The refractive index of the material of which the prism is made.
(2) The refracting angle, α
, of the prism.
(3) Angle of incidence of the ray considered.
what is angle of minimum deviation
angle of incidence = angle of emergence
therefore angle of deviation is half the refracting angle of the prism
what is prentice position
one surface of the prism is normal to the ray of light whereas the other surface does the deviation
deviation of light in the prentice position > angle of minimum deviation
prism dioptre
1/f (m)
which way does light deviate when it enters dense medium from a dense medium
light deviates towards the normal
if object outside F1 where will image form by thin convex lens
image real
inverted
outside F2
if object at F1 where what image will be formed by thin convex lens
virtual image
erect
at infinity
if object inside F1 how will image be formed by thin convex lens
virtual image
erect
maginfied further from lens than object
image formed by thin concave lens
virtual
erect
diminished image
inside F2
what is spherical abberation
The peripheral parts of spherical lenses have a prismatic effect as rays of light passing through the periphery of the lens are deviated more than those passing the paraxial (central) zone
what features does the eye have to reduce spherical abberation and how does it do it
anterior corneal surface is flatter peripherally than centrally - aplanatic
the lens nucleus has a higher refractive index than the lens cortex therefore axial zone has greater refractive power than periphery
the iris stops light entering from the periphery
stiles crawford effect
what is stile crawford effect
Retinal cones are more sensitive to light entering the eye paraxially than obliquely through the peripheral cornea.
what is chromatic abberation
hen white light is refracted at an optical interface, it is dispersed into its component wavelengths or colour -> shorter λ = more deviation
when wavelengths of color are focused at different positions in the focal plane
the emmetropic eye focuses on which colour light
yellow-green as this is the peak wavelength of the photopic relative luminosity curve
in the duochrome test if the pt was myopic or hypermetropic what will they see
myopic - red
hypermetropic - green
Types of lenses
spherical
astigmatic
what are astigmatic lenses
- All meridians do not have the same curvature, and a point image of a point object
cannot be formed Ø2 types:
• CylindricalLenses
• Toric (sphero-cylindrical) Lenses
what are cylindrical lenses
- have one plane surface and this meridian has no power this is called the axis of the cylinder
meridian at right angles to the axis, the cylinder acts as a spherical lens
• The total effect is the formation of a line image of a point object - called the “focal line” parallel to the axis
what are toric lenses
behave like a spherical and cylindrical
lens w different optical powers and focal length in 2 orientations perpendicular ot each other
what is the sturm conoid
between the 2 line foci the rays of light form this fugure
what is the sturm interval
distance between the 2 line foci
what is the circle of least confusion
two pencils of light intersect
whay is speherical equivalent and its role
calculate the power of the spherical lens of closest overall effect to a given toric lens
tells whether eye is hypermetropic, emmetropic or myopic
focal point of the speherical eq coincides with circle of least confusion
how to calculate the spherical equivalent form the toric lens description
sphere + 1/2 cylinder
