SVEN Flashcards by louisa reavy

COMBINATION FRAMES

front made of both plastic + metal

side materials are irrelevant

How well did you know this?

Not at all

Perfectly

CONCOMITTANT VS INCOMITTANT

concomitant - same in all directions of gaze

incomittant - varies with direction of gaze

How well did you know this?

Not at all

Perfectly

ASTHENOPIA

eye-strain

How well did you know this?

Not at all

Perfectly

LENTICULAR LENS

collection of magnifying lenses

used for very high powers

How well did you know this?

Not at all

Perfectly

PRESBYOPIA

as we age, the lens continues to grow until it becomes stiff and close to the ciliary muscle

this means we can’t change focus enough to see at all distances

> 45

How well did you know this?

Not at all

Perfectly

3 TYPES OF FOCIMETERS

conventional (in labs)
projection
automated

How well did you know this?

Not at all

Perfectly

HOW MANY FOCAL LINES DO SPHERO-CYLS HAVE?

two line foci (as there are two power meridians)

How well did you know this?

Not at all

Perfectly

FRONT-TORIC LENSES

front of lens contains base curve/ cross curve

back of lens has sphere curve

convex front surface

base curve/ cross curve
————————————
sphere curve

How well did you know this?

Not at all

Perfectly

BACK-TORIC LENSES

front surface contains sphere curve

back surface contains base curve/ cross curve

this form has a concave back surface

sphere curve
———————
base curve/ cross curve

How well did you know this?

Not at all

Perfectly

MEAN SPHERICAL EQUIVALENT

sphere + half of cylinder

How well did you know this?

Not at all

Perfectly

LENS MEASURE

used to measure dioptric power of a lens surface directly

measures curvature of surface

assumes lens has a particular RI

finds surface sag between 2 points

(also called lens gauge/ clock)

How well did you know this?

Not at all

Perfectly

ACCOMMODATION

ability of eye to alter the crystalline lens shape to change its focal power

How well did you know this?

Not at all

Perfectly

WHEN WERE VARIFOCALS FIRST CREATED?

been around since 1920s

first successful varifocal came into UK market in 1970 - varilux 2

How well did you know this?

Not at all

Perfectly

MINUS LENSES GET WHAT PRISM?

base in prism

How well did you know this?

Not at all

Perfectly

PLUS LENSES GET WHAT PRISM?

base out prism

need to converge more

How well did you know this?

Not at all

Perfectly

WHAT IS THE USUAL PANTOSCOPIC TILT IN VARIS?

about 10 degrees

WHAT ARE ISO-MEAN POWER LINE DIAGRAMS IN CONTEXT OF VARIS?

imaginary lines on lens surface, taking form of a contour plot, showing where specified steps in equivalent sphere of local lens power are

PANTOSCOPIC TILT

angle that the spectacle front/ lens makes with the vertical when being worn

can only be measured when frame is on px

ASPHERICAL SURFACES

historically meant not spherical i.e. all varis were aspherical

now for ophthalmic lenses - rotationally symmetrical

DIFFERENCE BETWEEN DEGRESSIVE (OCCUPATIONAL) + CONVENTIONAL VARIFOCALS?

conventional have large distance + smaller near area - aberrations concentrated in upper half

occupational have large near + smaller distance area - aberrations concentrated in upper half of lens

INWARD ANGLE DROP

should be parallel to head (angle of end of side towards head)

~5-10 degrees

HOW TO CHECK FOR A 4-POINT TOUCH?

place frame top down on flat surface

ear points (i.e. close to where the drop of the side starts) + top of front above each lens should all sit on the flat surface at same time

NEURAL DENSITY TINT

one that transmits all visible wavelengths equally

WHAT IS LUMINOUS TRANSMISSION IN BOTH UV + INFRA RED?

zero

SOLID TINT

tint all the way through the lens

DIPPED TINT

dye soaks into lens (plastic only) concentration decreases with distance into lens

WHY ARE SOLID TINTS VERY SELDOM USED FOR RX SUNGLASSES?

tint darkness varies with lens thickness lens darkness varies where lens is thicker i.e. in plus lenses it’ll be in the middle of lens large range of tints then have to be in stock to to compensate for both power + uncut size (centre thickness) different material has to be stocked for every tint type

EQUITINT

trade name (of zeiss) for a lens made up of a plano powered solid tinted layer (often photochromic) + a powered untinted layer made up of two lenses bonded together to give lens with an even tint over whole area now means any tint that is even over lens area I.e. independent of lens power so includes both vacuum + dipped tints

HOW THICK IS ANTI-REFLECTION COATING?

1/4 wavelength thick - single layer allows destructive interference to occur in reflected light

MIRROR COATS

1/2 wavelength thick allows constructive interference to occur in reflected light

PLASTIC ANTI-REFLECTIVE COATING CANNOT BE APPLIED WITHOUT WHAT?

a hard coat

UV ABSORBING LENSES

mid + high index plastics absorb almost all UV not available for glass

WHAT CAN AN INCREASE IN UV EXPOSURE CAUSE?

increase in cataract or AMD

POLARISING LENSES

reduces reflections max transmittance is 50% even with AR coating good for fishing, skiing, driving

HOW TO POLARISING LENSES CUT DOWN REFLECTIONS?

light waves oscillate in all directions but after reflection, only waves oscillating parallel to surface are left polarising lenses act to eliminate this acting like a vertically oriented “wire grid”

WHICH RANGE OF WAVELENGTHS ARE PLASTIC PHOTOCHROMIC LENSES BETTER AT PROTECTING THE EYE FROM THAN GLASS ONES?

UVA i.e. longer wavelength UV 315-400nm

PATIENTS SHOULD BE WARNED OF WHAT PROBLEM ASSOCIATED WITH PLASTIC PHOTOCHROMIC LENSES?

they remain tinted when they are older, potentially causing serious problems under low-light level conditions

PULFRICH EFFECT

delayed image processing from one eye e.g. pupil size differences