Figure 6.1 A focimeter
Recording distance prescription
Turn the focimeter on and set the focusing wheel to zero. Then turn the viewing eyepiece fully anticlockwise and look down the eyepiece, turning it clockwise until the dots and graticule are in focus (this reduces instrument accommodation, which will give a false recording).
Place the spectacles on the focimeter with the arms facing backwards, to ensure that the focimeter measures the back vertex power of the lens. Conventionally, the distance then near prescriptions are established for the right lens and then for the left lens.
If the spectacles are bifocals, check that it is the upper distance segment that is orientated on the focimeter. You may need to move the lens around until the ring of dots is centralised on the graticule. If this is not possible, this is due to a prism in the lens (see p. 75).
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 then note the power value and sign (+ or –) on the wheel. This will give the distance spherical prescription.
As with most cases, the prescription will have an astigmatic element, so, rather than a ring of dots being observed, a ring of fine lines is observed. Turning the focusing wheel will bring these lines into focus and turning the wheel further will bring a set of perpendicular lines into focus (the previous lines will become blurred or will disappear). It is necessary to adjust the axis of the graticule so that the lines are made linear. Once the axis has been corrected, turn the focusing wheel to bring the lines into sharp focus. Failure to first match the axis will result in an inability to sharply focus the lines. Record the power and the axis – this is the value of the cylindrical component in one of the two principal meridians. Then turn the focusing wheel until the perpendicular lines appear. Again, fine-tune the axis of the graticule until the lines are linear then alter the focus wheel until in sharp focus. Record the power and axis of this perpendicular principal meridian.
It is quite simple to convert the two cyl recordings into a spectacle prescription. If working in plus cyls:
- the sphere is the most negative recording
- the cyl is plus and is the difference between the two recordings
- the axis is the same as the most plus recording.
Some examples follow.
| Two cyl recordings from focimeter | Prescription |
|---|---|
| +3.00 @ 030, –2.00 @ 120 | –2.00/+5.00 @ 030 |
| –1.75 @ 145, –3.25 @ 055 | –3.25/+1.50 @ 145 |
| +1.50 @ 060, +6.25 @ 150 | +1.50/+4.75 @ 150 |
Note that the focimeter records the cyl axis and not the orientation of the cyl power (perpendicular to the axis). This is important to appreciate if using power crosses to obtain the prescription, rather than the simple three-step process described here.
For example, if the two cyl recordings from the focimeter are +3.00 @ 135 and –1.75 @ 045, this would give the following power cross:
This gives –1.75/+4.75 @ 135 (equivalent to +3.00/–4.75 @ 045).
See ‘Power crosses’, in Chapter 4, to see how to obtain the prescription from the power cross. Although academically it is useful to appreciate power crosses, you may well find it practically simpler to use the three-step method already detailed.
Recording near add value
To measure the near add of the bifocal segment, move the spectacles so that the lower near segment is orientated on the focimeter. Rotate the focus 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.
For example, if the dots are in sharp focus at –3.00 sphere for the distance segment and –1.50 sphere for the near segment, the near add will be +1.50 sphere. When establishing the near add for a sphero-cylindrical lens (used to correct astigmatism) ensure that the lines brought into focus are at the same orientation as those lines used to give the power value for distance that is subtracted from the near recording. For example, if the lines are in focus for the distance segment at +3.00 @ 030 and –2.50 @ 120 and the 030 line is in focus at +5.00 for the near segment, the near add is +2.00 sphere. The 120 lines would then be in focus at –0.50 for the near segment.
In most cases, the near add value will be the same for each eye. However, do not assume this, since they may be the spectacles of a young (pre-presbyopic) patient that has had unilateral cataract surgery. In this case, a near add may not be required on the eye that has not had surgery; however, a near add may be required on the side that has had cataract surgery.
Recording the prismatic correction
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.
The dots will be deviated towards the base of the prism because, although prisms deviate images towards their apex, the focimeter eyepiece viewing system inverts this view. The power of the prism is equal to the number of spaces (denoted by the graticule) that the dots are deviated.
For example, if the dots are deviated by two spaces upwards, 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 a 4 pd BI prism in that lens.
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. For example, a 13 pd exodeviation will be corrected by a 6 pd BI correction in front of the right eye and 7 pd BI correction in front of the left eye. A 4 pd right hyperdeviation will be corrected by 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.
Symptomatic ocular deviations can be corrected by incorporating prisms into the spectacle prescription, which can be measured by the focimeter as described earlier. However, it is important to note that ocular deviations can also be controlled in another way – through ‘lens decentration’. This is where the optical axis of the lens is purposefully decentred relative to the patient’s pupil. The prismatic power (pd) is equal to the power of the lens (dioptres) multiplied by the distance of decentration (cm). If this has been done, it will still be possible to centre the image on the focimeter. Such prismatic correction could, therefore, be overlooked. The only way to detect lens decentration is by checking the lens for a marking that indicates this or by using a lens marker to mark the position of the pupil centre whilst the patient is wearing the spectacles. This mark should then be placed in the centre of the focimeter stop and any decentration will be evident. Fortunately, in the Refraction Certificate Examination, since you are only provided with a pair of bifocal spectacles and not with their owner, you are not expected to mark the pupil centre and assess for lens decentration.
