Focimetry Flashcards
What are some of the reasons why we might use a focimeter?
1) to check spherical and cylindrical lens power
2) to check patients’ existing spectacles, to check and verify new spectacles (finding the current prescription is useful for knowing if the prescription has changed, it can also help with retinoscopy as a rough guideline)
3) can be used to mark the actual optical centres of the lenses, the required optical centre of the lens if prism is required, and to find the axis of direction that the cylindrical power lies along
PREP - optical centres
Before even touching the focimeter you decide where you want to measure the prescription from, you do this by marking either the box centre of the frame or the optical centre of the lenses.
If its an exam you are usually told whether you should be measuring at the optical centre or box centre.
Optical Centre - centre of the lens
Box Centre - centre of that half of the entire frame (excluding the connecting bridge)
Both the optical centre and box centre can be found using the City Rule and marked on with a marker pen.
PREP - markings
An important thing to do as soon as you walk into your exam space or even just in practice, is take your marker and label on the lenses with ‘L’ and ‘R’ so that later on you don’t get confused which one is left and right.
Remember its LEFT and RIGHT FOR THE PATIENT, imagine PUTTING THE GLASSES ON, which one is for the left eye and which is for the right? Mark this on the lens so you don’t get confused later on.
This is particularly important during focimetry because when we are focimetering the spectacles they are facing away from us, and this inversion means the lens on the right will actually be the left lens and vice versa.
what are the TWO TYPES OF FOCIMETERS?
1) TRADITIONAL - CIRCLE OF DOTS
2) WHEEL - CIRCLE OF DOTS, TWO SETS OF PERIPHERAL LINES, ONE SHORT, ONE LONG
STEP ONE - TURN IT ON (TRADITIONAL)
First start by turning the focimeter on, there is a small button on the front, press this and it will turn green, make sure this button stays green which means the focimeter will stay on throughout.
STEP TWO - CALIBRATION (TRADITIONAL)
Next focus the eyepiece. To do this turn the EYEPIECE LENS ANTICLOCKWISE ALL THE WAY and then CLOCKWISE until the BLACK NUMBERS ON THE GRATICULE BECOME CLEAR AND FOCUSED.
Place your glasses in the focimeter with the ARMS FACING AWAY FROM YOU, and then CLAMP THE SPECTACLES IN PLACE SO THAT THE MARKED CENTRE LINES UP WITH THE FOCIMETER.
Then calibrate the focimeter using the POWER DIAL (this is the black dial at the back of the focimeter). Whilst looking at the TARGET OF SMALL GREEN DOTS, twist the power dial until this comes into sharp focus.
Once the target is in focus, look through the lens at the bottom of the display, the POWER READING (ORANGE BACKGROUND) SHOULD READ ZERO, if this reading is NOT ZERO, then you will have to take this into account in your final reading as your focimeter is not properly calibrated.
STEP THREE - FINDING MERIDIANS (TRADITIONAL)
For astigmatic lenses, there will be a different power in each of the perpendicular meridians. Find these meridians by twisting the power dial until the TARGET (circle of dots) becomes a STRETCHED SET OF LINES, there will be two sets, 90 DEGREES APART.
STEP FOUR - SPHERICAL COMPONENT (TRADITIONAL)
After finding the meridians, decide which one is the MORE POSITIVE IN POWER TO BE OUR SPHERICAL COMPONENT. To do this, turn the power dial back and forth to each of the meridians and read off the POWER READING FROM THE BOTTOM OF THE DISPLAY, whichever meridian has the MORE POSITIVE READING should be written down as your SPHERICAL COMPONENT.
Remember we always want to work in NEGATIVE CYL so this is why we find the MORE POSITIVE POWER to be our SPHERICAL COMPONENT and then subtract this from the MORE NEGATIVE TO FORM OUR CYL.
STEP FIVE - CYLINDRICAL COMPONENT (TRADITIONAL)
After writing down your spherical component, turn the dial back so that you can see your MORE NEGATIVE MERIDIAN AGAIN. Write down the power you see on the reading as we are going to be using this in a CALCULATION in order to work out our CYLINDRICAL COMPONENT:
(more negative power) - (more positive power) = cylindrical power
The calculation above will ALWAYS PRODUCE A NEGATIVE CYL COMPONENT, which is what we desire. Remember, the result should always be negative so if you’re getting a positive cyl you messed up somewhere.
STEP SIX - AXIS (TRADITIONAL)
First make sure you are on the MORE NEGATIVE (SECOND) READING. To measure the axis, rotate the SILVER COLLAR BEHIND THE EYEPIECE, this will ROTATE THE EYEPIECE GRATICULE, rotate this until it is LINED UP PARALLEL WITH THE DIRECTION THAT THE LINES ARE BEING STRETCHED IN ON THE TARGET.
The axis can then be read off from the NUMBERS AT THE TOP WHERE IT INTERSECTS.
WHEEL FOCIMETERS - summary
Focus and calibrate in exactly the same way, find the meridians in the same way. After finding the MORE POSITIVE MERIDIAN, before recording it, TURN THE SILVER DIAL AT THE BOTTOM OF THE FOCIMETER WHILST LOOKING INSIDE, UNTIL THE SHORT SET OF PERIPHERAL LINES BECOMES CLEAR. You can now leave that dial alone.
Record your SPHERICAL COMPONENT off of the POWER READING SCALE, then record the NEGATIVE CYL by finding the difference between the two powers observed and putting MINUS IN FRONT.
Find the axis by lining up the GRATICULE along the direction that the CENTRE CIRCLE WAS STRETCHED IN, there is also a DIAL ON THE MACHINE that MAGNIFIES THE NUMBER OF THE AXIS, you can check your INTERNAL READING USING THIS EXTERNAL READING, if they aren’t the same then you messed up somewhere.
FINDING PRISM
If the spectacles contain PRISM, then the CENTRE CIRCLE OF DOTS OR LINES WON’T BE IN THE CENTRE OF THE DISPLAY, IT WILL BE DISPLACED. We can measure HOW FAR OFF THE TARGET IS DISPLACED and this displacement is the VALUE OF THE PRISM.
If the target has moved:
UP - the prism is BASE DIRECTION UPWARDS (UP)
DOWN - the prism is BASE DIRECTION DOWN (DN)
TOWARDS THE NASAL SIDE - the prism is BASE DIRECTION IN
TOWARDS THE EAR - the prism is BASE DIRECTION OUT
To determine the value of the prism, we use the GRATICULE which has INTERSECTING LINES AT VARIOUS POINTS. Each of these intersections REPRESENTS 1 PRISM DIOPTRE (the first tiny intersection does not count), to find the prism value, simply COUNT HOW MANY OF THESE INTERSECTIONS SEPARATE THE TARGET FROM THE CENTRE. Count the number of intersections between the CENTRE OF THE TARGET and the CENTRE OF THE DISPLAY
If you had 4 INTERSECTIONS BEFORE THE CENTRE OF THE TARGET, we would record the prism value as:
4 PRISM DIOPTRES, BASE UP or 4 UP.
