BIO: larger or smaller FOV than DO

larger

BIO: larger or smaller depth of focus than DO

larger

BIO: larger or smaller mag than DO

less mag

BIO: erect or inverted image

inverted

light from the standard lens of the lensometer must converge at the ___________ focal point of the test lens

primary

formula of lensometer to determine Rx if given power of standard lens and distance target is moved

x = f^2(Fv) x = distance target moved (+ if towards you, - if away) Fv = test lens power f = focal length of standard lens

moving the target towards us (towards the standard lens) to get neutrality the lens is a _________ patient lens

plus

moving the target away from us (away from the standard lens) to get neutrality the lens is a _________ patient lens

minus

start lensometry by blurring the target in the ______ direction plus minus

plus

+25.00 standard lens move target 2 mm forward to achieve focus what is the back vertex power of the lens being tested?

0.002 = (1/25)^2Fv solve Fv Fv = +1.25 D

the target is displaced up from the reticle (OD) what is the base direction

BU

Center of the cross hairs fall to the left of the center of the bulls eye for OD lens, what is the base direction?

BO

the target is displaced right from the reticle (OS) what is the base direction

BO

A prescription calls for 3 BO OD. How should the lens be centered in the lensometer for verification?

Position crosshairs to left of bulls-eye by 3 rings

two prisms base to base

plus lenses

two prisms apex to apex

minus lenses

you are doing hand neutralization the object moves with the direction of the glasses myope or hyperope?

myope (- lens)

you are doing hand neutralization the object moves against the direction of the glasses myope or hyperope?

hyperope (+ lens)

you are doing hand neutralization there is no motion myope or hyperope?

emmetrope

How do you hand neutralize lens of unknown power?

Add another lens of known power to neutralize motion effect. When motion is stopped, unknown lens power is opposite of known lens.

measures BC of GP contact lens

radiuscope

which surface of a CL is the base curve?

back

what is the first clear image in the radiuscope?

surface of lens

what is the second clear image in the radiuscope?

center of curvature

the distance between the two positions of focus (the clear images) corresponds to the ______________ of the GP CL

base curve / radius of curvature

Formula for keratometry radius of curvature?

F = (1.3375 -1) / r F= .3375 / r

The readout on a keratometer is 4.25 DK. What is the radius of curvature of the cornea?

45.25 = .3375 / r r = .00746 m

where is a minus lens thicker? center edges

edges

where is a plus lens thicker? center edges

center

What is a lens clock used for?

To measure the sag of a lens, and determine power

If a lens clock is calibrated for a lower index of refraction, will it over or under estimate the actual power of the lens?

Under

Formula for an incorrectly calibrated lens clock?

FL = [(nL-1)/(nLC-1)] * FLC FL: Lens power nL: index of lens nLC: index used for calibration FLC: Power readout of lens clock

You use a lens clock calibrated with n = 1.5. You want to measure the power of a lens made from material n = 1.7. The power readout of the lens clock is +6.00D. What is the actual power of the lens?

FL = [(1.7-1)/(1.5-1)] * 6 FL = +8.40 D

sags are + if they look like a

C

When using fundus lenses, what kind of image is created? Higher powered fundus lenses affect magnification and FOV how?

Real, inverted image Lower mag, Higher FOV

Placing a fundus lens in front of the eye creates a telescope system. What is the formula for magnification?

M = -(Foc) / (Fobj) M = -(Feye) / (Flens)

Calculate the magnification produced when a +90D fundus lens is placed in front of the eye?

Assuming the power of the eye is +60D M = - (+60) / (+90) = -0.67x, image is inverted and minified

ANSI Z87.1 Safety Standards for Impact High mass impact: High velocity impact:

High Mass (drop ball test): pointed projectile, 500g, dropped from 50 inches High velocity: steel ball, 0.25 inch diameter, fired at 150 ft/sec

What do these markings mean on safety eyewear: H? + W U R L V What 3 markings must safety frames/lenses carry?

Smaller head Impact rating Welding UV filter IR filter Visible filter Variable tints Manufacturer logo, impact mark (+), standard mark (Z87-2)

What kind of front and back surface does a meniscus have?

Convex front surface Concave back surface

Where is base curve on single vision lenses?

Front surface

Plus cylinder lens Which curve is the base curve? Cross curve? Sphere curve?

Base curve = flatter of the front/toric curves Cross curve = other front/toric curve Sphere curve = back curve

Minus cylinder lens Which curve is the base curve? Toric base curve? Cross curve?

Base curve = front sphere curve Toric base curve = back flatter curve Cross curve = other back cruve

Where is the base curve on a contact lens?

Back surface

A contact lens has a base curve measured to be +42.00 DK. What is the radius of curvature of this base curve?

F = 0.3375/r r = 0.3375/42 r = .00804m

formula for lens with two curved surfaces

Tc- Te = s1 - s2 Tc: center thickness Te: edge thickness s1: sag of first surface of lens s2: sag of second surface of lens

formula for CT for plus lens

CT = ET + s1 + s2

formula for CT for minus lens

CT = ET - s1 - s2

formula to find power of lens given n2 = index of lens s = sag h = half chord length/half diameter of lens

F = 2(n2 - n1) *s / (h^2)

A biconvex lens is clocked. The lens has the following properties: diameter = 44mm, sag of the front surface = 4.5mm, sage of the back surface = 3.2mm, n = 1.50. What is the power of each lens surface?

Both surface powers are positive because the lens is biconvex F= (2(n2-n1) * s) / (h^2) F1 = (2(1.50-1)*.0045) / (.022^2) = +9.30D F2 = (2(1-1.50)*(-.0032) / (.022^2) = +6.61D

equation which relates thickness to lens power

CT - ET = (F1 + F2)h^2 / (2(n-1))

sag formula, given h = half chord r = radius of lens

s = h^2/(2r)

power formula, given n2 = lens index r = radius of lens

F = n2 - n1/(r)

describe the continuum of converting from thickness to power

find sags by drawing picture s = h^2/(2r) solve r F = n2 - n1/(r)

a biconvex lens d = 44 mm s1 = 4.5 s2 = -3.2 n = 1.5 power of each surface

front: 4.5 = .022^2/(2 * r) solve r F = n2 - n1/(r) solve F

sag will be 0 for what lens type?

pl

horizontal lens length (eyesize)

A box

vertical lens length

B box

midpoint between A and B

GC

distance from GC of one lens to other

Frame PD or GCD

shortest horizontal distance between lenses

DBL

Formula to calculate frame PD (GCD)

A + DBL = GCD

longest diameter of the lens

ED

Minimum blank size Definition? Formula?

Smallest size lens blank needed to make the lens. M = ED + 2(d) + 2mm M: minimum blank size (mm) ED: effective diameter (mm) d: decentration per lens (mm)

decentration per lens, d =

d = FPD - PPD/(2) d = (frame PD - patient's PD) / 2

point on lens through which line of sight passes would be optic axis if no prism

major reference point (MRP)

for an Near Vision Only pair of glasses what PD is used

near PD

distance from top of bifocal line to optical center of FT 25 (28 or less)

5 mm

distance from top of bifocal line to optical center of executive (Franklin)

0 (OC is at seg line)

Distance from top of bifocal line to OC: Flat top 35? Round segs (Kryptok) Flat top larger than 35? Curve-top, Panoptic, Ribbon-B? Ribbon R-seg?

4.5mm r (radius of the seg) 0mm (OC at seg line) 4.5mm 7mm

Progressives: Corridor length and high/low add power? Hard design: Soft design:

Hard: Short corridor and/or high add power Soft: Long corridor and/or low add power

distance from top of bifocal line to optical center of RD 22

11 mm (r)

Trifocals have an additional intermediate add, what is its power?

1/2 the power of near add

vertical distance between MRP and top of bifocal segment

seg drop

What is inset? Formula?

horizontal distance from GC to MRP frame PD - distance PD / 2

What is seg inset? Formula?

horizontal distance from MRP to center of seg seg inset = (distance PD - near PD) / 2

What is total inset? Formula?

horizontal distance from GC to center of seg (near reference point) total inset = (frame PD - near PD) / 2 OR total inset = seg inset + inset

A frame has A=54mm and DBL = 18mm. The patient's distance PD is 66mm and the near PD is 62mm. What is the seg inset? What is the total seg inset?

Seg inset = Distance PD - near PD / 2 = 66mm - 62mm / 2 = 2mm Total seg inset = frame PD - near PD / 2 = 72 - 62 / 2 = 5mm

What are adjustments that can be made if the seg is too high on a patient (5)

1. Increase panto 2. Decrease vertex distance 3. Spread the pads 4. Move pads up by adjusting pad arms 5. Stretch the brdige

For every ____ degrees of increased panto, the seg heigh must be raised by ____, to keep the segment at the proper fitting height relative to the patient's eyes.

2, 1mm

What is the difference when using a lensometer to measure the power of a multifocal?

Measure the front vertex instead of back vertex

n and abbe of crown glass

n = 1.523 abbe = 58.8

n and abbe of CR-39

n = 1.498 abbe = 58

n and abbe of polycarbonate *Characteristic?

n = 1.586 abbe = 30 high chromatic aberration

n and abbe of trivex *characteristic?

n = 1.53 abbe = 44 light and impact resistant

rays close to the middle

paraxial

rays close to the edges

marginal

are paraxial or marginal rays bent more bent more = come to a focus closer to the lens

marginal

why do longitudinal spherical aberrations occur?

marginal rays come to a point of focus before paraxial rays

in longitudinal spherical aberrations, a point object is no longer forming a _____

point image

t or f - longitudinal spherical aberrations occurs for both on and off axis point sources

t

longitudinal spherical aberrations contribute to what refractive error?

nocturnal myopia

coma only occurs for.. on axis points off axis points

off axis points

aberration with comet shaped blur

coma

due to rays of light hitting a lens surface obliquely

radial/oblique astigmatism

3 curved surfaces formed in radial/oblique astigmatism

tangential sagittal petzval

how do we minimize oblique astigmatism?

pick correct base curve

what does the tscherning ellipse plot? What is its use?

BC vs back vertex power Shows the best value of BC for a back vertex power for eliminating oblique astigmatism and curvature of field. It consists of an elliptical set of points made of two curves: Wollaston and Ostwalt curve (flatter curve)

What happens if an incorrect base curve is used, according to the Tscherning ellipse?

Vision will be reduced in periphery of the lens

what curve of the tscherning ellipse plot is used to minimize oblique astigmatism?

ostwalt curve