D: Vertometry - Week 3 Flashcards
List the roles of the vertometer (5-7)
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
How does vertometry work? Briefly explain and state the formula to find back vertex power
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)
How do you set up a vertometer? (4)
- Inspect lens (identify if SV, BF, PAL)
- adjust power drum to zero (so target is in its nominal position)
- adjust eyepiece so afocal
- rotate eyepiece until target is just in focus (rotate first away from instrument then slowly toward instrument)
Name the components of the vertometer (5-8)
- eyepiece (focus dial)
- lens rest/platform
- lens clamp
- power wheel
- axis wheel
- lens dotting/marking device
- prism compensating device
- internal/external scale
Explain the process involved in positioning the lens in the vertometer
- 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
Why place lens concave side down in vertometer?
Because that’s the typical orientation when worn by px
What do line targets look like?
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)
How do line targets work? What does it mean if both sets of lines are in focus?
- 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
How do you find the cyl axis with line targets?
It’s the axis orientation when cyl lines clear
Why should you read the axis off the internal scale of the vertometer?
To avoid error
How do dot targets work?
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
How do we perform vertometry on bifocal lenses? How does this help us find the near addition?
- 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
Should we flip the lens to do both sides when doing the near segment?
No. “We will not be doing that”. It only makes a difference at high power anyway.
How do we locate the optical centre with vertometry? How can we use this to avoid unwanted prism?
- 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)
How do we use vertometry to determine the amount of vertical prism?
- 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
How do we calculate amount of displacement when measuring vertical or horizontal prism?
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)
How is the image deflected by prism in vertometry? Is this different from what you’d normally expect?
BU deflects image up, BD down, BI nasal, BO temporal
Yes
How do we use vertometry to determine the amount of horizontal prism?
- 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
How can vertometry identify that prism is present? (2 indications)
- 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
What could you do if you cannot get the target into the centre in vertometry?
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.
How do progressive lenses differ in vertometry?
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
List advantages of auto-vertometers (4-5)
- 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
List possible sources of error in vertometry (6-9)
- 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
