Chua 6 Flashcards
Which refractive state has presbyopia earlier?
Hypermetropia
How much accommodation kept in reserve for comfortable near vision
1/3
What is the franklin bifocal design?
Split bifocals = distance lens is mounted on near lens
What are fused bifocals
Heat fusing near potion made of flint glass into a dperession on a crown glass with lower refractive index
What is prismatic jump related to
Power of lenses and distance between interface from optical centre
How can image jump be reduced
Moving optical centres towards junction of the two potions
Base up rpsim incorporated into reading section
Using executive glasses
What do soft designs of varifocal lenses have
Wuder progressive corridor, smaller distance, near sections
Larger intermediate portion for VDU work
What determines power of CL
Anteiror and posterior curvature
Thickness
Refractive index
What is CL prescription
BC/diameter/power
BDP
What determines CL fitting
Posterior surface - base curve - central posterior curve
Radiuscope is used to measure radius of spherical base curve
Determines tightness of lens fitting and movement with blinking
What is CL truncation
Cutting off lower part of sphericla lens to prevent lens rotation
What is piggyback CL
USe of soft CL to provide fitting surface for RGP lens
What do CL do to amount of accommodation and convergance required compared with glasses in myopes vs hypermetropes
Myopic patient require greater convergence and accommodation in CL wear (myopic spec have base in prism which reduces the amount of convergence required for near)
This may cause eye strain in presbyopic myopes
CL decrease the amount of convergence and accommodation needed in hypermetropes
How can you reduce CL movement?
Increase contact lens diameter
Decrease CL base curve
Increase thickness of CL
What does changing from specs to CL do in hypermetropic patients
Image minification
Increased field of view
What does changing from specs to CL do in myopic patients
Image magnification
Reduced field of view
What are disadvantages of using magnifying glasses
Reduced depth of focus
Reduced visual fields
Object need ot be placed near the eye
Unesteady image if not held steady
What is field of view realted to in convex lens
Distance between lens and eye - greater = smaller visual field
Diameter of lens - greater = larger visual field
What lens has more power in Galilean telescope
Eyepiece lens - high power convcave minus
Objective - low power plus convex
How can constricted visual fields be aided?
Minification - concanve lens or inverted Galilean telescope
Image relocation - relocates visual information form teh scotoma closer to area fo fucntioning visual field - prisms with base in direction of scoptoma
