Focimetry- Astigmatic, Multifical, Bifocal and Prisms Flashcards
What is the use of focimetry?
Focimeters allow accurate determination of the spherical and cylindrical power of lenses and also indicate the axis of the cylinder.
Focimeter is used to determine the..
Power of the lens
Focimetery for spherical lenses method
For spherical lenses it is simple put glasses in so legs are facing away then move dial until dots are clear. And the target in the centre.
All the target lines will come into focus at the same time (Figure 3)
Simply read off the power drum to determine the spherical power
Focimetry for astigmatic lens method
Only some lines are clear at a given power, Most +ve is sphere and most -ve is axis
Start with highest plus and reduce until lines are clear
Turn axis wheel until three parallel target lines are straight and unbroken (Other lines a re broken )
Or turn axis wheel so that the central dots lie on the meridian
Then clear other line and find difference
Focimety for astigmatic lens 2
Only some of the lines are clear at a given power
Turn the power wheel to a high plus reading and slowly decrease the power until some of the target lines just become clear.
Turn the axis wheel until the three parallel target lines are straight
The number on the power wheel at this point is the most positive meridian of the lens. This will be the spherical power when you write the astigmatic lens prescription.
Slowly turn the power wheel to decrease the power until the other line is clear. The number on the power wheel will now tell you the power of the least positive meridian of the lens.
Find the cylindrical power of the lens: this is the difference in powers, ie the more positive power subtracted from the more negative power (negative cylinder)
Find the axis of the cylinder. The axis of the cylinder is the direction of the second power reading, ie the least positive power. The direction of this line is measured by aligning the graticule and looking at the axis numbers on the graticule inside the eyepiece.
Example: Reading 1 is +3.00D x 90 and Reading 2 is +2.00D x 180. The lens is: +3.00/-1.00 x 180
Focimetry for vertical prisms
The vertical prism is determined by keeping the lens table at the same height on swapping from right lens to left lens
If the focimeter target (the green cross) is not centred vertically about the reticule, then there is vertical prism present
If the focimeter target is below the horizontal then there is base-down prism
If the focimeter target is above the horizontal then there is base-up prism
The size of the prism can be read from the reticle markings
Note: this method determines the total vertical prism between the two lenses. In order to determine the precise contribution of the left and right lenses to the total prism it is necessary to know the position of the patient’s pupils relative to the optical centres of each lens. In practice, the total vertical prism is usually divided evenly between the two lenses
Focimetry for horizontal prisms
To determine horizontal prism from decentration
Mark the optical centres of each lens
Determine the patient’s inter-papillary distance (IPD)
Mark the left lens at the IPD distance away from the optical centre of the right lens
Clamp the left spectacle lens on the lens rest so that the PD mark that you made on the lens is over the centre of the lens rest.
Measure the horizontal prism and direction using the reticule. This is the total horizontal prism. Remember that:
if the target is to the left (nearer the right lens) the prism is base-in
if the target is to the right (away from the right lens) the prism is base-out
Note: this method determines the total horizontal prism between the two lenses. In order to determine the precise contribution of the right and left lenses to the total prism it is necessary to now the position of the patient’s pupils relative to the optical centres of each lens. In practice, the total horizontal prism is usually divided evenly between the two lenses
To begin
Position the focimeter at a comfortable angle for viewing by using the locking lever or inclination control at the side of the instrument (Figure 1, label 10) to adjust the tilt of the focimeter. This is also a good time to ensure the prism compensator is at 0.
Focusing on the eye piece
It is essential to focus the eyepiece for your eye if someone else has used the focimeter before you. The video in the Multimedia Tab of this article called ‘Setting up a Focimeter’ demonstrates how to focus the eyepiece. The steps are:
Keep the focimeter power off.
Rotate the eyepiece counter-clockwise until the reticle is blurred. A white card or piece of paper held behind the eyepiece may make the reticle lines more visible.
Turn the eyepiece clockwise until the reticle is just clear. The lensometer is now adjusted for your eye. Note: Do not turn past the point at which the reticle is first clear.
Positioning the spectacles
Turn the spectacles so that the front of the spectacles is facing towards you. The temples (the arms of the spectacle frame) should be pointing away from you.
Put the spectacles on the frame table. The bottom rim of the spectacles should rest on the frame table.
Clamp the spectacle lens to keep it pressed against the lens rest. Note: always start with the right lens (for consistency)
Look through the eyepiece and move the spectacles side-to-side and up-and-down until the target is in the centre of the black graticule.
Change the height of the frame table to keep the frame horizontal in this position (that is, with the bottom rims of the spectacle against the table to make sure one side does not drop down)
Marking the optical centre
When the lens is positioned as above - where the centre of the focimeter target is over the centre of the graticule - then you are at the optical centre of the lens. Note: if there is a ground prism in the lens, despite movements of the lens up-and-down or side-to-side, the focimeter target will not be centred over the graticule - so there is no optical centre of such a lens.
Mark the optical centre. Focimeters usually have an ink well and marking pins. When the lens is centred correctly, you can use the marking pins to put a mark (usually small dots) on the lens surface. If the vertometer does not have an ink well or marking pins, you can use a marking pen (felt tipped pen) to mark the optical centre of the lens yourself. You will need to make the mark on the lens directly over the lens rest.
Focimetry steps summary
1- focus the eye piece
2- check power calibration by turning wheel to plus
3- position the spectacles
4- mark the optical centres
5- find the right power and turn dial
to a high plus reading and then decrease until clear
5- determine which lens; sphere, astigmatic, prism, bifocal
6- determine the near add
Determining the best add
Turn the spectacles around. The near addition is a measure of the front vertex power - as opposed to the distance prescription which is a measure of back vertex power. It is therefore necessary to turn the spectacles around so that the arms of the spectacles point towards you
Measure the power of the distance section and compare this to the power of the near section - the difference is the near addition
For astigmatic lenses, simply compare one meridian in the distance to the equivalent meridian in the near - again the difference is the near add
Astigmatic lens will
Only some of the lines are clear at a given power
Sphere only lens will
All the target lines will come into focus at the same time
