OL1 term 1 content Flashcards
(23 cards)
All about light what slows it down and describe white light
light travels left to right, refracts when entering a mediujm w greater density or a greater refractive index. all transparent media slows down light as they have a n of greater than 1. light travels in a straight line through a homogenous medium
higher n= less of the lens you need to bend the light. less thick it needs to be.
white light splits into consituent wavelengths, red is long wavelength and green or yellow is medium and short is blue. blue bends best in dispersion bc short wavelengths are refratced the most bc they are slow= speed of light varies for each wavelength
lenses
nidpic- negative is diverging positive is converging
vergence- path of curvature of light rays convergence adds vergence light rays move towards optical axis and divergence is opposite
plano convex- front positive back no power
biconvex- front positive back positive
convex meniscus- front pos back neg overall more pos
convex= front has the power
concave- opposite
concave- minifies near and distant objects convex magnifies. concave image movenet is with and convex= against.
meniscus lenses what do they have thats better and toroidal lenses
and why are thick lenses more expensive
Meniscus lenes have better image qualities especially in the lens periphery.
Toroidal lenses curved cylinder form
Tyre- GAC in middle. Barrel- AR in middle. Capsatan- VOG in middle.
Tyre= most common, mass produced and cheap
Barrel= good off axis performance not mass produced
Capstan= rarely used, sometimes for solid bifocals
Thick lenses= use more off axis so more expensive
what does the focimeter measure and what does it consist of
measures the front and back vertex power of a lens or lens system= used to derive sphere cyl prescription and to locate optical centre of a lens and any prismatic effect it could have
consists of a focusing system (collimator) and observation system (telescope)
when can targets only be imaged by the telescope and how can this target be moved +how can we work this out
targets can only be imaged in focus by the telescope if the light rays entering it are parallel
in order to meet this the target must be moved using the power wheel. we can work out the amount of movement needed= x.
positive lens and negative lens what happens to targets
positive lenses- target moves towards eyepiece
negative lens- target moves away from eyepiece
describe the cornea, lens and the properties of the eye
cornea has 2 surfaces: radius of curvature is higher anteriorly. lens changes w age grows throughout life and gets axially thicker. lens has 4 surfaces 2 cortex and 2 nucleus, n higher in nucleus.
cornea has highest n
eye is 22.22mm long
power=60D
n=1.33
emmetropia k is 0
myopic and hyperopic eye further spectacle lens is away from the eye
Myopic eye- further spectacle lens away from the eye= the stronger or more powerful the lens must become to correct the eye
Hyperopic eye- further spectacle lens is away from the eye, weaker less powerful the lens must become to correct the eye.
prisms base and describe single binocular vision and how they maintain it
ophthalmic prisms what is the apical angle
base- opposite the apical angle, flatter part= base. light rays deviate towards the base but looks as if towards apex
single binocular vision occurs when image forms on fovea of both eyes prism can cause diplopia when it displaces image in one eye away from fovea, single binocular vision can be restored if eye behind prism rotates towards prism apex. eye can cope w low powered prisms
ophthalmic prisms- apical angle is less than 10
uses of prisms
Assessment of oculomotor balance- how much eye rotates=single vision
Increase prism power to see how flexible eye is at maintaining single vision
Treatment of heterophoria- reduces rotation
Treatment of strabismus- moves image to fovea of deviated eye where it should be
Cosmetic- equalises thickness of upper and lower portion of lenses
Prism compensation- there to cope w differences between the eyes
prism notation
Base up, down, in and out. Vertical specify eye. Horizontal= in and out vert=up and down
Horizontal= hsa. Vss. Horizontal same add opposite subtract. Vertical same subtract
If they cancel out- light is still parallel in the same direc nothing changes. Diff directions the eye can rotate to fix this difference.
positive vs negative prisms
Positive lenses- bases in contact. Thicker in middle and thinner at top (Against)
Negative lenses- bases away from each other. Thicker at top thin in middle. (With)
can px always cope w prismatic effect
Px can’t always cope with prismatic effect and it may lead to eye strain. Optical centre usually but not always coincides w geometrical centre of uncut lens. Decentration= movement of optical centre on a lens to either incorporate prism or to ensure there is no prismatic effects.
where is lens shape measured from, what would decentering a lens do
positive and negative lenses if we decentre them
radius is measured from furthest point away from horizontal centre line to get your uncut, if you want the pupil to be in the middle then decentre it but this would probably increase diameter of lens blank bc larger diameter.
Positive lenses are thicker in the middle than the edge= middle needs to be thicker so the edge isn’t as thin. Negative lenses are thicker at the edges so the edges need to be thick to ensure the middle isn’t too thin so that’s why bigger uncuts need to be also be thicker.
so as decentration increases
when no decentration is desired where are lenses decentered towards
why may it not be possible to incorporate lenses by decentration
So as decentration increases, the msu or lens blank increases too.
When no prismatic effect is desired it is usual for lenses to be decentred inwards.
It may not be possible to incorporate lenses by decentration: lens may be too low powered, too much prism may be required, lens is aspheric. If so you can grind f1 and f2 at an angle to each other.
fresnel prisms and lenses
lenses constructed from pvc using hydrostatic force. These are temporary and these reduce weight and reduce lens thickness= circles of power lots of prisms in a row and it goes to high powers. +-30
what is h’
h’= retinal image size. How big an image is on the retina. When the object is brought closer the visual angle increases so size appears bigger=h’
what is h’
h’= retinal image size. How big an image is on the retina. When the object is brought closer the visual angle increases so size appears bigger=h’
x always 16.67
depth of field vs focus and factors affecting it
how much of the image is actually in focus and depth of focus is how sharp the focus across the entire image so overall focus.
Things can look in focus even if there is a blur circle that just means there isn’t enough of a blur circle to appear blurry, blur circle smaller than critical size of blur circle= don’t realise
Factors affecting dof- Pupil size increases more blur circle so dof decreases
Target size increasing increases dof
Ocular refract= retinal image size differs
Illumination increases= dof decreases bc easier to spot contrast. But then miosis
rsm
comparing how much the image is magnified or minified by a spectacle lens.
In hyperopes RSM is positive so the image is magnified by the lens. Myopes rsm is negative so image minified.
When spectacle lenses are brought closer to the eyes RSM increases for hyperopes and decreases for myopes. Hyperopes rsm increases more than myopic lenses. Hyperopes see objects smaller with their glasses=h’ smallerand myopes see objects bigger.
rsm for contact lenses vs glasses
Rsm for contact lenses is generally less than glasses bc cl are closer to eye so provide a smaller h’ lower retinal image size for hyperopes.
Myopes have a larger retinal image size so if you went from contact lenses to spectacles for a myope the px would go from larger h’ to smaller h’ so warn px
. Cl to spectacles for myopes would cause an increase in rsm. Glasses tend to have a more magnified image for hyperopes than contact lenses.
astigmatism
anisometropia
aneisokinia
Astigmatism= mag varies between meridians.
Anisometropia is differences btwn ocular refraction of the eye which can lead to aniseikonia which is differences in cortical image size. Leads to headaches, eye strain, poor stereopsis and possible suppression. Knapps law- axial anisometropia spectacles gives a more equal image size and in refractive ani cl more equal image size but clinical findings= cl best for all forms of it.
aphakia
extreme change in prescription eg cataracts scatters light- have lens removed and place an aphakic eye. Worth about 20D but lens really gives px about 12D of change.
Correcting this= can use spectacles, cl or IOL. Specs are heavy magnified image significantly and hard to cope, can misjudge distances. CL= preferable to spectacles, much less noticeable, size more similar but cost and infections and other complications. Most cases IOL px is now pseudophakic and there will be very little image mag compared to pre op, little aneisokonia or problem w depth. Can aim for a particular refraction.
