D: Vertometry - Week 3

Question 1

List the roles of the vertometer (5-7)

Answer 1

Measure:
- vertex power (incl. near add)
- cylinder axis
- check current Rx (compare with new or order)
- check if ordered lens powers are correct

Determine prismatic properties:
- power and base orientation (prescribed and induced)

Mark optical centre: for lens incorporation into frame at correct orientation
- check PD

Question 2

How does vertometry work? Briefly explain and state the formula to find back vertex power

Answer 2

Place an ophthalmic lens at the front focal point of the collimator, resulting in a disruption of collimation hitting the telescope. Then move the target until collimation is re-obtained (i.e. image is clear again).

Fv = xFc^2

(back vertex power of lens = displacement of target from back focal point in mm times the power of the collimating lens squared)

Question 3

How do you set up a vertometer? (4)

Answer 3

1. Inspect lens (identify if SV, BF, PAL)
2. adjust power drum to zero (so target is in its nominal position)
3. adjust eyepiece so afocal
4. rotate eyepiece until target is just in focus (rotate first away from instrument then slowly toward instrument)

Question 4

Name the components of the vertometer (5-8)

Answer 4

• eyepiece (focus dial)
• lens rest/platform
• lens clamp
• power wheel
• axis wheel
• lens dotting/marking device
• prism compensating device
• internal/external scale

Question 5

Explain the process involved in positioning the lens in the vertometer

Answer 5

• place lens concave side down on lens rest
• measure R lens first
• lower clamp
• position lens so target is central on graticule/cross-hairs
• must lift clamp as you maipulate lens (or else could scratch lens)
• ensure lens is not tilted

Question 6

Why place lens concave side down in vertometer?

Answer 6

Because that’s the typical orientation when worn by px

Question 7

What do line targets look like?

Answer 7

2 sets of lines representing sphere and cyl respectively. - cyl axis represented by 3 widely spaced lines
- sphere axis represented by either 1 line or 3 closely spaced lines (depending on model)

Question 8

How do line targets work? What does it mean if both sets of lines are in focus?

Answer 8

• find the most positive meridian first
• start with high plus to blur the target
• rotate drum in more negative power direction
• rotate power drum and axis until part or all lines are in focus
• if all lines are in focus, you have a spherical lens
• if only 1 set clear or both unclear by different amounts, you have spherocyl

Question 9

How do you find the cyl axis with line targets?

Answer 9

It’s the axis orientation when cyl lines clear

Question 10

Why should you read the axis off the internal scale of the vertometer?

Answer 10

To avoid error

Question 11

How do dot targets work?

Answer 11

Same as line targets
- find most positive power (or just rotate until target mostly clear)

• if circle of clear dots (not dashes) visible, then it’s a spherical lens
• if there is a cyl, this will be shown by the orientation and elongation of the dots

Question 12

How do we perform vertometry on bifocal lenses? How does this help us find the near addition?

Answer 12

• measure distance as normal (optical centre)
• keep target aligned on 1 principle meridian
• release clamp and elevate lens platform to measure near segment (optical centre of near segment/the D-seg)

The difference between distance and near measurements is our near addition

i.e. near add = Fv’near - Fv’distance

Question 13

Should we flip the lens to do both sides when doing the near segment?

Answer 13

No. “We will not be doing that”. It only makes a difference at high power anyway.

Question 14

How do we locate the optical centre with vertometry? How can we use this to avoid unwanted prism?

Answer 14

• the vertometer marks the lens with usually 3 dots. The centre dot is the optical centre
• we then compare the distance between the 2 optical centres and the px’s PD to ensure they match (to avoid prism)

Question 15

How do we use vertometry to determine the amount of vertical prism?

Answer 15

• centre and measure 1st lens
• position contralateral lens on platform without moving platform height
• ensure target clear (adjust power/cyl axis)
• vertical prism is indicated by amount of target displacement

Question 16

How do we calculate amount of displacement when measuring vertical or horizontal prism?

Answer 16

Generally a gradation on graticule = 1 diopter of displacement

(i.e. normally the target (dot or lined) will be in the inner most circle, but if there was 1 prism base down for instance, the image appears lower for vertometry and rests on the edge of the 2nd smallest circle)

Question 17

How is the image deflected by prism in vertometry? Is this different from what you’d normally expect?

Answer 17

BU deflects image up, BD down, BI nasal, BO temporal

Yes

Question 18

How do we use vertometry to determine the amount of horizontal prism?

Answer 18

• gently dot the optical centre of lenses on vert as measure each eye
• if desired PDs do not match OC position, mark desired OC locations with marking pen
• position lens on platform and position marking dot in centre of vertometer rest
• estimate amount of prism by amount of decentration of target

Question 19

How can vertometry identify that prism is present? (2 indications)

Answer 19

• displaced target; cannot get target to centre of lens easily
• target not quite as clear as normal (even if looking through centre) b/c prism subtly reduces image quality

Question 20

What could you do if you cannot get the target into the centre in vertometry?

Answer 20

You might need a ‘prism compensator’: a device in the vertometer which translates your image either up, down, left or right by adjusting magnitude of prism compensator.

Question 21

How do progressive lenses differ in vertometry?

Answer 21

They have a distance an near reference point (DRF/NRF) which we measure from
- be wary of distortion and aberration off to the sides of the lens

Question 22

List advantages of auto-vertometers (4-5)

Answer 22

• reduces fatigue (e.g. from screen display, reduce observer focusing errors, automatic reading)
• fast
• detects lens design
• can easily check with aus standards + provide record to identify failed aspects
• UV transmittance

Question 23

List possible sources of error in vertometry (6-9)

Answer 23

• failure to adjust eyepiece focusing (over 1D)
• misalignment of platform
• lens not clamped
• move platform when measure PD and prism
• not checking both meridia when checking prism
• instrument alignment out of calibration
• different filters
• vertex shift
• problem with aspheric and progressive lenses