Focimeter Flashcards
What is the function of a focimeter
The focimeter is used to measure the back vertex power of a lens
What measurements can a focimeter give?
the sphere, cyl (power and axis), and near add of a pair of bifocal spectacles. It can also be used to measure the amount and direction of prism that may have been incorporated into the lens
How to set up the focimeter at first?
Turn the focimeter on and set the power wheel to zero. Then turn the viewing eye- piece fully anticlockwise, look down the eyepiece, and turn it clockwise until the dots and graticule are in focus (this reduces instrument accommodation which will give a false recording). It’s also important to perform this procedure with both eyes open as using one eye only will also induce instrument accommodation whilst focusing.
How to set up bifocals on the focimeter
check that the upper distance segment is orientated on the focimeter. You may need to move the lens around until the target is centralised on the graticule.
When is a ring of dots target seen?
Once the spectacles are placed on the focimeter, a ring of dots is only seen if the lens only contains a sphere and when the collimating lens is focused.
Therefore, rotate the focusing wheel until a crisp ring of dots is seen, and then note the power value and sign (+or-) on the wheel.This will give the distance spherical prescription.
What does it mean when a ring of lines is seen?
(Ring dot target Focimeter)
As with most cases, the prescription will have an astigmatic element, so rather than a ring of dots, a ring of lines is observed.
How to calculate the near add from the bifocal near segment?
Move the spectacles so that the lower near segment is orientated on the focimeter. Rotate the power wheel until the dots (or lines in the case of astigmatism) are in focus and record the power. Subtract the distance prescription from this near value to give the near add.
If the dots are in sharp focus at -3.00 ds for the distance segment and -1.50 ds for the near segment, what is the nead add?
+1.50 ds.
(-1.50 - -3.00) = +1.50 DS
How to identify prisms in lenses?
When trying to centre the dots on the graticule, it may become apparent that the dots cannot be centralised. This is due to a prism being incorporated into the lens.
Where do the dots deviate to if a prism is incorporated into a lens?
The dots will be deviated towards the base of the prism, since although prisms deviate images towards their apex, the focimeter eyepiece viewing system inverts this view.
How to calculcate power of a prism from the focimetry?
The power of the prism is equal to the number of spaces (denoted by the
graticule) that the dots are deviated.
What does it mean if the dots are deviated by 2 spaces upwards on Focimetry of?
there is a 2 PD BU
prism in that lens. If the dots are deviated by four spaces to the right when the right lens is being assessed, there is incorporated 4 pd BI prism
How is the prismatic correction balanced between the 2 lenses?
Typically the prismatic correction is halved between the two lenses, and the bases will be in the same direction for horizontal deviations and opposite directions for vertical deviations.
How can a 13PD esodeviation be corrected?
6 pd BI correction in front of the right eye and 7 pd correction in front of the left eye.
How can a 4PD right hyperdeviation be corrected?
a 2 pd BD correction in front of the right eye and a 2 pd BU correction in front of the left eye. The apex of the correcting prism is always in the direction of deviation.
