Random Flashcards
Contact lenses VS spectacles
Advantages and disadvantages
cosmetically more appealing no weight less spec mag no fogging wider fov - no Peripheral aberration Good for Sports refractive alsometropes keratoconus no differential prism- as lens moves with eye no abberations/ Surface reflections increased VA CL - bandage good for hyperopes for near vision - less accommodation
Disadvantages can't wear in water less oxygen infection risk restricted RX Allergies / Hygiene FB time AC/ EE costly can become lost or damaged Good level of manual dexterity I & R
Colour defect
most common
Deuteranomaly
Detective green pigment
what is deuteranomaly
Defective green pigment
colour defect
what is Protanopia
struggle to see Dark reds reds blue green looks grey
cover Test
most useful binocular assesment anomalies
only best to differentiate between Strabismus ( hetertropia )from heterophoria
6m distance
33cm near
hirschberg Test
Gross binocular uncooperative kids moderate or large mainfest deviations only fixate on Pen torch @ 33 cm @ eye level reflections on cornea and any asymmetry esotropia = reflection temporal exotropia = reflection nassally
20 base out Prism tests
6 month and above
Positive response demonstrates an adequate convergent fusinal range and absense of global Supression
look @ fixationstick 20 Prism placed in front of each eye In turn Prism causes image to shift should cause eye to adduct to keep focus on Image other eye should also move
if global suppression Present when prism placed infront no movement shift of image undetected.
near point vergence accomodation formula’s
near point vergence = B = K-A Age= 13+( 13 - A ) 4. ( Duanes formula ) near point = b = 1/ B Accomodation = A = K-B Add hsp - ( 2/3 A )
B near point vergence
b =near point
Amp= Amplitude of accomodation
A / ACC = Accomodation
K= k = k ' = K ' = Fe=
K = ocular refraction vergence required at the eye
k = far point - a point conjugate with the macular in the unaccomodated and uncorrected eye
k ‘ = axial length of eye
k’ = Droptric length vergence required after refraction in order to focus the image on the retina
Fe =Power of the reduced surface
k’ = K + Fe ( image Vergence = object vergence +refractive power )
Depth of field
the retinal Image is only Sharp when the object Position is conjugale with the retina.However very Small amounts of blurring are not noticed by the eye so the object can be moved a small distance nearer or further away without appearing to be out of focus. This range of object Positions is the linear depth of field.
As the depth of field depends on blur dimensions which is a function of pupil size , it is increased with miosis (pupil constriction ) and decreased with mydrasis ( pupil dilation)
How emsley modified Gullstrands Simplified schematic eye to produce the reduced eye
the simplified schematic eye has 3 Surfaces
A Single surface Cornea and 2 Surface crystaline lens
Emsley calculated the single surface that would be equivalent to these using refractive indices of4/3 for Aqueous and vitreous and 1.416 for the lens
Define Anisometropia
Anisometropia is the condition where the right and left eyes have a different Ocular ametropias
if one hypermetropia and other Myopic = Antimetropia
Low = 0.25 - 0.75 D
medium = 1.00 - 2.00D
High = 2.25 Or more
can cause vertical differential Prism
can cause eyes to focus unevenly Subjects vision may blur due to different size images through each eye
can result in strabismus and amblyopia
Spec Mag
S M defined as a ratio
Retinal image size in the corrected eye h ‘
__________________________________________
uncorrected retinal Image size h ‘ u
value calculated using
SM =K
—–
Fsp
SM = h ‘ c
—— Thus h ‘ c = h ‘ u x S M. change as a % ( Sm - 1.00 ) x 100
h ‘ u
h ‘ e = 1000 x tan w x k
‐——————- —-
k ‘ Fsp
Definition of Spectacle magnification and the derivation of the formula
SM = K
—–
Fsp
The relationship assumes that the Spectacle lens is thin and that the object is at infinity
SM important - calculates the change in retinal Image size experienced by the eye when Spec lens is placed in front of it
influenced by vertex distance + by the form and thickness of the lens
L A S E R
Light Amplification Stimulated Emission Radiation
laser produces light which is
Monochromatic - targeted at tissue which absorb Particular wavelengths
Coherent -all particals in phase - arrive same place Same time
Directional - can be accurately positioned
can be Switched on and off rapidly - allow precise amounts of energy to be delivered
