astigmatism using certain formulae. The most commonly used formula is Javal’s rule, which is:

Spectacle astigmatism 1.25 (corneal cylinder) ( 0.50 D 90).

The 1.25 factor was proposed to account for the change in cylinder power due to the vertex distance of spectacles, and the 0.50 of against-the-rule astigmatism is used to account for the astigmatism believed to be from the lens. Note that the 1.25 factor is theoretically incorrect as, although it works for a 1.00 cylinder with a 8.00 sphere, a factor of 0.75 is required for a 1.00 cylinder with a 8.00 sphere. A modification to Javal’s rule has been proposed (Grosvenor et al. 1988):

Spectacle astigmatism 1.00 (corneal cylinder) ( 0.50 D 90).

These estimates of astigmatism can be very inaccurate (by up to 2.00 DC) for some patients and are much less accurate than either retinoscopy or autorefraction (Elliott et al. 1994, Mote & Fry 1939). They are particularly inaccurate for small degrees of astigmatism and should only be used for patients with astigmatism greater than about 1.00 D (Elliott et al. 1994).

4.3.7 Most common errors

1.Not maintaining mire image focus when attempting superimposition of the mire image.

2.Not getting the patient to keep their head against the headrest.

3.Forgetting to focus the eyepieces.

4.Not centring the mire images.

5.Not getting the patient to fixate properly.

6.Not determining the correct axis.

7.Regularly estimating spectacle astigmatism from keratometry readings. This is usually only valuable for high astigmatism, when subjective responses and other objective assessments such as retinoscopy and autorefraction are unavailable or unreliable.

8.Forgetting to calibrate the instrument on a regular basis.

4.4 FOCIMETRY

Focimeters measure the vertex power, axis direction and optical centres of ophthalmic lenses. These devices are also referred to by trade names in some countries, including lensometer or lensmeter (America) and vertometer (Australia). Automatic focimeters are available that measure the lens characteristics mentioned above once the lens has been appropriately positioned and provide a printout of the results. These are very simple to use and the measurement procedure will not be explained. Their main disadvantage is that they break down more often than non-automated focimeters (Steele et al. 2006).

4.4.1 Spectacle lens identification

It is often important to determine the spectacle details. You may want to check that they have been made to the specifications you ordered, or you may wish to determine the parameters of the spectacles that a patient is wearing. It is important to know the spectacle prescription so that you can compare it with the optimal refractive correction that you will subsequently determine during refraction.

4.4.2 Advantages and disadvantages

Focimeters provide simple and accurate measurements of vertex power, axis direction and optical centres. Focimeters do not provide information about all the important features of spectacle lenses, however, and it is important to consider that changes in other spectacle characteristics could cause patients problems and need to be checked. These include base curve and lens form, segment style, height, size and inset, centre or edge thickness, optical and surface quality and the presence of a lens tint and/or surface coating (antireflection, anti-scratch).

4.4.3 Procedure

1.Explain the test to the patient: ‘I am going to measure the power of your spectacles.’

2.Set the power of the focimeter to zero and focus the eyepiece (turn it as far anticlockwise as possible, then slowly turn it clockwise until the target and graticule first come into sharp focus).

3.Measure the back surface power (BVP) by placing the spectacles on the focimeter with the back (ocular) surface away from you. Position the middle of the right lens against the lens stop.

4.Look into the focimeter and adjust the lens position vertically (using the lens table) and horizontally until the illuminated target is placed in the middle of the reticule. If the lens is high powered, you may need to turn the power wheel to bring the target into focus before it can be centred.

5.Fix the lens into position using the lens retainer.

6.To obtain the power of the sphere, turn the power wheel to bring the target into focus.

a)If the entire target is focused at the same time (Fig. 4.3), the lens is a sphere and there is no cylindrical component. Record the sphere power for the right eye from the power wheel or the internal scale and go to step 8.

b)If parts of the target are in focus at different powers and to record in the standard negative cylinder format, turn the power wheel until the meridian with the most plus power (or least minus power) is brought into focus.

c)With focimeters using line targets, rotate the axis wheel until the sphere line (Fig. 4.3a) is in focus and the line is continuous without breaks. You may need to use the power wheel to gain best focus.

d)Record the sphere power from the power wheel or internal scale.

7.To obtain the power and axis of the cylinder:

a)Focus the image in the meridian at 90° from the first meridian by turning the power wheel towards the most minus (or least plus) power.

b)Read off the power when this meridian is in focus. With focimeters using line targets, the cylinder lines will be in focus.

(b)

Fig. 4.3 The entire focimeter target is in focus at the same time, indicating a spherical lens. The graticule scale allows measurement of prism. (a) A focimeter that uses a cylindrical (3-line) and spherical (1-line) target. The graticule scale is numbered 1–5. (b) A focimeter that uses a circle-of-dots target. The graticule scale is indicated by the intersecting lines and runs from 1–5 horizontally and 1–3 vertically in both directions from the centre. With an astigmatic lens, the dots become lines orientated along the two principal meridians.

c)Record the difference between the sphere power from step 6.d) and the new meridian power as the cylinder power.

d)Record the orientation of the second meridian from the eyepiece protractor or the axis wheel as the cylinder axis. With focimeters using line targets, this will be the orientation of the cylinder lines

(Fig. 4.3a).

8.Make sure the target is centred in the graticule and dot the right lens using the focimeter’s marking device. This could be just one spot (the lens optical centre) or three dots (the middle is the lens optical centre, the other two indicate the horizontal line).

9.Release the lens retainer and repeat steps 4 to 7 for the left eye. Do not change the vertical position of the lenses between

measurements of the right and left lenses as you need to determine if any vertical prism is incorporated in the spectacles.

10.Move the lens horizontally until the target is in the same vertical plane as the centre of the graticule and dot the left lens using the focimeter’s marking device.

11.If the target is above or below the centre of the reticule, vertical prism is present and

should be recorded to the nearest 0.5 using the graticule scale (Fig. 4.3).

12.Remove the spectacles from the focimeter and measure the distance between the right and left optical centres to calculate the distance between centres (DBC). Record the DBC in mm.

13.For front-surface solid multifocal lenses, the reading add must be measured using front vertex power (FVP). Turn the lens around so that the ocular surface faces you and reposition the spectacles in the focimeter. Measure the FVP along one meridian in the distance portion of the spectacles. Measure the FVP along the same meridian in the near portion of the spectacles. The difference between these powers is the reading addition. Repeat the measurement in the left lens. For low-powered lenses, the FVP approximately equals the BVP, and the BVP add can be measured.

14.For progressive addition lenses (PALs), the appropriate position on each lens to measure the distance and near prescription, optical centres and any prism must first be found (Fig. 4.4). A faint mark is etched into both the nasal and temporal sides of each lens, and this must be found and marked with a non-permanent marker. The mark may also indicate the PAL manufacturer and the power of the addition. Use the manufacturer’s marking up card, to find the appropriate distance and near centres and measure the spherocylindrical power as previously described. Use the card to determine where to mark the optical

Fig. 4.4 An example of the important points and areas of a progressive addition lens.

centres and where to check for any prism (Fig. 4.4).

15.Compare the distance DBC and the patient’s distance interpupillary distance (PD). If these distances are different, calculate the induced horizontal prism using Prentice’s rule (induced prism Fc, where F is the power of the lens along the horizontal meridian and c is the difference between the DBC and PD in cm). The direction of the prism also needs to be deduced.

4.4.4 Recording

Record the spherocylinder correction in minus cylinder form for both eyes and the reading addition power if a multifocal. Also record any prism, the type of lens, any tints or coatings etc. Use ‘x’ rather than the word ‘axis’. Record the spherical and cylindrical power to the nearest 0.25 D, and the cylinder axis to the nearest 2.5°. The axis should be between 2.5° and 180°. Use 180 rather than 0 degrees. For example:

D28 segment bifocal, CR39, MAR coat

RE: 2.00/ 1.00 35

LE: 2.25 DS

Add: 2.00 DS

NV spectacles, CR39

OD: 2.25/ 0.75 80

OS: 2.50/ 0.50 105.

4.4.5 Interpretation

One of the most common errors in focimetry is an axis reading incorrect by 90° (Steele et al. 2006). Given that the cylindrical axes in the two eyes are often mirror images of each other (for example, both axes 90° or both axes 180°; right axis 175°, left 5°; right 20°, left 160°; right 45°, left 135°, etc.; Solsona 1975), if axes are 90° different to this (for example, 180° and 90°; 175° and 95°; 20° and 50°; both axes 45°; both axes 135°) then recheck the two cylindrical axes. Reading additions are typically the same in both eyes, so that if they are read as different, they should be rechecked.

4.4.6 Most common errors

1.Not focusing the focimeter eyepiece. This can lead to inaccuracies for high-powered lenses.

2.Not measuring the reading addition using FVP measurements for front-surface solid multifocals.

3.Reading one or both of the cylindrical axes incorrectly by 90°.

4.Ignoring the relative vertical position of the target between the right and left lens, thereby missing vertical prism.

5.Changing the vertical position of the lenses between measurements of the right and left lenses, thereby incorrectly reading vertical prism.

4.5 ANATOMICAL INTER-

PUPILLARY DISTANCE

The interpupillary distance (PD) is the distance between the centres of the pupils of the eyes.

4.5.1 Interpupillary distance

The PD is measured for two reasons:

1.To place the optical centre of the phoropter/trial frame lenses in front of the patient’s visual axes to control prism and avoid aberrations.

2.So that the optical centre of spectacle lenses can be placed in front of the patient’s visual axis to avoid unwanted prism and aberrations or deliberately placed elsewhere to produce desired prism.

4.5.2 Advantages and disadvantages

Measurement of the anatomical PD is quick and convenient to use during an eye examination and it requires no instrumentation other than a simple millimetre ruler. The repeatability of anatomical binocular PD measurements is similar to that for a pupillometer (Holland & Siderov 1999, Osuobeni & Al-Fahdi 1994). A pupillometer could be considered when refracting or dispensing a patient with a large amount of ametropia, where slight discrepancies in PD could lead to induced prism, and for monocular measurements when dispensing progressive addition lenses.

4.5.3 Procedure

1.Keep the room lights on.

2.Explain the test to the patient: ‘I am going to measure the distance between your eyes so that I can put your lenses in the correct position for your eyes.’

3.Face the patient directly at the distance desired for the near PD (usually about 40 cm).

4.Rest the PD ruler on the bridge of the patient’s nose or on the forehead so that the millimetre scale is within the spectacle plane. Steady your hand with your fingers on the patient’s temple to ensure that the ruler is held firmly in place.

Distance PD

5.Close your right eye and ask the patient to look at your left eye. (It is usually easiest to indicate with your finger the eye that you want the patient to fixate). To allow a patient with unilateral strabismus to fixate, you may need to cover the fellow eye.

Fig. 4.5 (a) Measurement of near PD. (b) Measurement of distance PD.

6.Choose a point of reference on the patient’s right eye. The temporal pupil margin is usually most convenient, although the centre of the pupil or the temporal limbus margin may also be used and the latter may be essential with patients with dark irides. Align the zero point on the ruler with this reference point.

7.Close your left eye, open your right and ask the patient to change fixation to your open right eye. Take care not to move the ruler or your head position. By sighting again to the appropriate reference point on the patient’s left eye, you will obtain a reading for the distance PD (Fig. 4.5). This would be the left nasal pupil margin if you used the temporal pupil margin of the right eye.

Near PD

8.Move laterally to place your dominant eye opposite the patient’s nose.

9.Ensure that you are still at a distance from the patient equal to their near working distance. Normally this is done at 40 cm but, if desired, the near PD can be measured for a closer or farther working distance.

10.Using your dominant eye only, choose a point of reference on the patient’s right eye and align the zero point on the ruler with this reference point.

11.Look over to the patient’s left eye and note the reading on the ruler that aligns with the corresponding reference point on the left eye (Fig. 4.5).

4.5.4 Alternative procedures

The near PD can also be measured during the distance PD measurement. During step 6, after aligning the zero mark on the ruler with the reference point, look over to the patient’s left eye and note the reading on the ruler that aligns with the corresponding reference point on the left eye. The only difference to the procedure described above is that the target is not placed at the patient’s midline. Any error is likely to be negligible.

When measuring monocular distance PDs you must use the middle of the patient’s pupil as the reference point. The right monocular PD is measured from the middle of the patient’s right pupil to the centre of the bridge of the nose. To determine the left monocular PD, you need to subtract the right monocular PD from the total distance PD. For example, if the total distance PD is 65 mm and the right monocular PD is 32 mm, the left monocular PD is 33 mm. It is not advisable to move the ruler to measure the left monocular PD (i.e. making the centre of the bridge of the nose the zero reference point) as this will lead to an additional error.

4.5.5 Recording

The values are normally recorded as distance PD/near PD (in mm). For example, PD: 63/60.

4.5.6 Interpretation

For women, the distance PD is most commonly in the range of 55–65 mm, and for men, 60–70 mm. Young children may have PDs as low as 45 mm. The distance PD value is usually 3–4 mm greater than the near PD at 40 cm. Inaccuracies in anatomical PD can occur due to parallax error when there is a large difference between your PD and the patient’s PD. However, the error is slight, with an 8 mm difference in the examiner and patient’s PDs leading to a 0.5 mm error in the measured patient PD (Brown

1991). The repeatability of anatomical PDs taken by an experienced practitioner is approximately1–2 mm (Holland & Siderov 1999, Osuobeni & Al-Fahdi 1994). Repeatability between practitioners is slightly poorer at about 1.5 2 mm (Holland & Siderov 1999).

anatomical PD section (Brown 1991). Inaccuracies can occur if the pupillometer sits higher or (usually) lower on the bridge than the intended spectacle frame and the nose is not straight, so that the monocular PDs can be shifted to one side.

4.5.7 Most common errors

4.6 PHOROPTER OR TRIAL

FRAME?

1.Moving the ruler during the measurement. Make sure it is held firmly and steadily in position. After taking the distance PD reading, it is a good idea to re-open your left eye, have the patient switch fixation back to it and check that the zero mark on the ruler is still aligned with the original reference point on the patient’s right eye.

2.Using an inaccurate near test distance. Most commonly, unwittingly drifting in closer than 40 cm so the near PD turns out to be lower than it should be. The test distance should not affect the distance PD measurement.

3.Using a PD ruler that is not accurately calibrated, such as some give-away rulers provided by optical companies.

4.5.8 Alternative procedure: corneal reflection pupillometer

Pupillometers allow monocular PDs to be measured more accurately than an anatomical measurement (Holland & Siderov 1999). This is beneficial when ordering spectacles for high refractive errors or for progressive addition lenses where precise centration of each lens along the patient’s visual axes is necessary. In addition, the procedure is quick and simple and could be performed by a clinical assistant and the examiner does not need to be binocular. The PD measured with a corneal reflection pupillometer will typically be 0.5–1 mm smaller than the anatomical PD (Holland & Siderov 1999, Osuobeni & Al-Fahdi 1994). This is because pupillometers measure the ‘physiological PD’, the distance between the two principal corneal reflexes, and locate the visual axes, whereas the anatomical PD locates the lines of sight or optical axes. Note that many pupillometers use a correction for the parallax error mentioned in the

A phoropter is a unit that is placed in front of the patient’s head and contains all the equipment necessary to measure a patient’s ametropia, heterophoria and accommodation. It can also be called a refractor, refractor head or refracting unit. A trial frame is an adjustable spectacle frame that includes cells into which all the various lenses required to measure a patient’s ametropia, heterophoria and accommodation can be placed.

4.6.1 Advantages of a phoropter

The use of a phoropter (Fig. 4.6) is the preferred technique for distance vision refraction of the majority of patients. A video clip introduction to the phoropter is provided on the website . The main advantages of phoropters are:

■A quicker refraction: As the lenses are all contained within the phoropter, it is much quicker to change lens powers for both retinoscopy and subjective refraction than with a trial frame. This may also provide less back strain for the examiner.

■Comfort: The trial frame containing several lenses can become uncomfortably heavy, particularly for older patients.

■Jackson cross-cylinder alignment: On all modern phoropters, the Jackson crosscylinder (JCC) is automatically aligned with the cylinder axis in the phoropter.

■No lens smear: Trial case lenses can become covered with fingerprints, and require regular cleaning. The trial frame should also be regularly cleaned.

■Risley prisms: These are standard on phoropters and make measurements of

subjective heterophoria and fusional reserves faster and easier and allow for easy use of the binocular prism dissociated accommodative balance technique.

■Computerisation: Computerised phoropters are available and can include data links to an automated focimeter (lensmeter) and/or autorefractor.

■High-tech: Some patients may prefer hightech phoropters rather than the ancientlooking trial frame.

4.6.2 Advantages of a trial frame

A video clip introduction to the trial frame is provided on the website . In the routine refraction of presbyopic patients, the trial frame (Fig. 4.7) is preferred for the final determination of the near addition, as the test can be performed at the patient’s preferred working distance and position, and the range of clear vision can be easily measured and compared to the near vision requirements of the patient. A trial frame is also useful to illustrate the improvement in distance vision in the ‘real world’ that a pair of spectacles could provide. For example, the new refractive correction can be placed into

the trial frame and the patient shown the improvement of their vision while looking through the window of the practice.

A trial frame is required for refractions during home (domiciliary) visits and is preferred when refracting:

■Patients who provide poor subjective responses: Some patients, despite normal or near normal visual acuity, provide poor subjective responses and cannot seem to discriminate between the view provided with and without a 0.25 DS lens or a 0.25 Jackson cross-cylinder (JCC). Using larger dioptric changes in sphere ( 0.50 or 0.75 DS) and a higher-powered JCC ( 0.50 or0.75) can sometimes elicit better subjective responses, and these changes are more easily made with a trial frame than with a phoropter. Many phoropters have a 0.25 JCC that cannot be changed.

■Patients with high refractive error: The back vertex power (BVP) of a combination of lenses in the trial frame or phoropter is not necessarily the algebraic sum. It depends on the power, thickness, form and position of the lenses used. After refracting a patient with high ametropia in a trial frame, you should