Focimetry - Intro Flashcards
(21 cards)
What does the Focimeter do?
- measures the FVP & BVP of lenses (NOT principal powers)
- measures the power for meridians in the cylinder and toric lenses = cross cyl powers
- if the power of meridians is the same, gives spherical power (a circle of dots)
- works out the sphere/cylinder axis in the two meridians
- measures the prismatic effect, at the optical centre
What are the two types of Focimeters?
- Fixed Target
2. Rotating Target (this has a rotating wheel)
Which type of Focimeter is more accurate?
- Rotating Target is more accurate when finding cylinder axis
What is the eyepiece used for?
- used to observe/ look through
What is the power wheel used for?
- moves the target
- can twist to see the power of the lens
What is the lens table used for?
- moves up/ down
- used to rest the glasses/ lenses
What is the lens lamp used for?
- holds the glasses being observed in place
What does the Focimeter consist of?
- Observation system
2. Foccussing system
What are the features of the Foccussing system?
- collimator
- produces an image of the target
- projected at infinity
- light rays will leave the focussing system as parallel to the axis
- power wheel moves the target
What are the features of the Observation system?
- graticule
- eyepiece
Define ‘ZERO position’
- you adjust the lens wheel so that the reading is zero
- the target is in front of the collimator so that the parallel rays of light enter the completely relaxed eye
- both graticule and target are in focus
- no lens in the way
What does the graticule show?
- consists of a scale
- in 1 prism dioptre steps
Define ‘optical centre’
- the target is at the centre of the graticule
How do we know if the target is NOT at the optical centre?-
- lens behaves like a prism = prismatic effect
How can we measure the prismatic effect?
- measured using the scale going up in 1 PRISM DIOPTERS
What are the benefits of the prismatic effect?
- helps people to see comfortably in single vision
How do we determine vertex power?
- rotate the power wheel
- moves the target from its ‘zero position’
- if POSITIVE LENS (target can only be imaged if the light enters is parallel) so MOVE TARGET TOWARDS the eyepiece
- if NEGATIVE LENS (target can only be imaged if the light enters is parallel) so MOVE TARGET AWAY from the eyepiece
*twist power wheel
What does a Toric lens do to a Fixed Target?
- both toric & cyl lens produces 2 line foci
- light is focused on either the power/ axis meridian (maximally defocused) for each foci
- meridians are always 90 degrees apart
- in between these 2 foci, an elliptical image is produced (this is not focused on any meridian)
- we focus the focimeter target to find the tangential & sagittal foci.
What does a Cylinder lens do to a Focimeter target?
- we find the cylinder axes by looking at the orientation of the lines
- each dot of the focimeter target will form two lines in two major meridians
- neutralizing the power in the meridians, will allow these lines to come into focus 90 degrees from each other
What does the Focimeter Power tell us?
- We don’t measure sph or cyl power, but power meridians & axes.
- We convert to sph/cyl form, either + or - (very easy to convert)
- Tells us nothing about lens form
What is important to note when reading the cyl axis off the external wheel?
- The 2 sets of lines on a rotating target from the centre are different lengths.
- When reading the cyl axis from the external wheel, it gives the axis of the longest line.
