Use of a focimeter for checking spectacle lenses is an important part of the work of optometrists and dispensing opticians, but is not always carried out correctly or well understood. This article provides some reminders of key points to be taken into consideration when using these instruments.
Types of focimeter
There are three main types of focimeter:
? Conventional eyepiece focusing instruments. These were the original design and are still widely used and cost effective
? Projection focimeters. These eliminate the eyepiece focusing of the first type as the image is focused on a screen
? Automatic electronic focimeters. Measurement of the carefully positioned lens is fully automated with these instruments.
Each type has its advantages and disadvantages and all must be used with care to get accurate results.
Conventional eyepiece focusing focimeters
Before using these instruments it is important to focus the eyepiece to ensure accurate measurement:
? Set the power control so that the instrument is reading zero dioptres
? While viewing the target, unscrew the eyepiece until the image goes out of focus
? Screw in the eyepiece until the image is just in focus
? Adjust the power wheel and check that the target and eyepiece graticule are in focus at zero dioptres power.
This procedure is necessary to ensure that parallel light is leaving the telescopic viewing system of the instrument. As the main variable is the reserve of accommodation of the user; this procedure becomes more important the younger the user.
Measuring power
It is first necessary to determine where on the lens the power is to be measured. Ideally, this should be at the centration point of the lens, the point at which the dispensing practitioner has indicated that the prescription is to be correct. In the majority of cases, this will be the optical centre of the lens, but see discussion later on prescribed prism.
When measuring spherical power lenses, the procedure is straightforward as there is a single focus where the target is equally clear in all meridians. The difficulty can arise when it is not clear if there is a 0.25D cylinder. This is where a rotating target focimeter is useful, as this type of target is more sensitive to measuring small power cylinders.
Lenses containing a cylinder will require two measurements to focus on the two mutually perpendicular principal powers. Those new to using a focimeter can get confused by the fact that although we always talk about lens prescriptions in terms of their sphero-cylindrical power, this type of focimeter measures the principal powers, which then have to be converted into sphero-cylindrical form. This can be confusing, particularly if the principal powers are of opposite sign. For example, in Figure 1, a schematic view of the two principal powers of a lens is shown.
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This could be recorded as -0.50x45/+0.75x135, the cross-cylinder form, but more usually as either -0.50/+1.25x135 or +0.75/-1.25x45, the sphero-cylindrical form. The ability to be able to quickly transpose from one form to another is obviously essential when using these instruments. Although the fact that these instruments do not give a direct indication in sphero-cylindrical form may be considered a disadvantage, it is a benefit when applying lens power tolerances, as we shall see later in this article.
When using a rotating target focimeter, it is essential to only measure the power when the target is perfectly aligned with the axis, and has one meridian in focus. Initially, the appearance may be as in Figure 2, where the target is out of focus, but also misaligned. The orientation of the small segments in the centre of the target give a clue as to the orientation of the required meridian, in this case 180 degrees.
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It can also be confusing when using manually focusing instruments that the graticule used for measuring the axis can be aligned either at the correct axis or at 90 degrees to it. For this reason, it is always useful when measuring an axis to first think what the axis is in standard notation before reading the scale.
Projection focimeters
The above discussion on eyepiece focimeters is also relevant to projection designs, with the exception that there is no eyepiece to be focused. Apart from this, these focimeters are often preferred when used continuously over a long period as they can be less tiring to use for the operator.
Automatic electronic focimeters
These instruments were first introduced some 40 years ago and may seem to answer all the problems involved in measuring lens powers. But they must be used with care to get meaningful results:
? Measurement must take place at the correct point on the lens. This is particularly important with prescribed prism, and also progressive power lenses
? Has the Abbe number of the lens material been correctly set where necessary? Measurement of lens power should ideally take place using the same wavelength of light as that used for measuring refractive index. By International Standard,1 this is either the helium ‘d’ line (587.56nm) or the mercury ‘e’ line (546.07nm). Although conventional focimeters can be filtered to give one of these values, automatic electronic focimeters generally use lasers at different wavelengths. Thus a correction factor has to be used in the instrument to convert the power to that which would be achieved at a standard wavelength. But this correction factor depends on the chromatic properties of the lens material, so the Abbe number must be known for the lens material being measured
? Although most instruments of this type can be set to indicate measurement in steps of 0.01D, this does not indicate that they are accurate to this value. A typical example of this is a Nidek LM 500 (Figure 3), which guarantees an accuracy of ±0.06D.
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Centration and prism
To verify a prescription using a focimeter, it is essential to know the centration distance for which it has been produced. If this is not known, then a complete specification cannot be given. For example, if we have a pair of +5.00DS lenses, with optical centres glazed 66mm apart, then there will be no prism at a centration distance of 66mm. If, on the other hand, the patient’s centration distance is required as 60mm with these lens powers, then by Prentice’s rule we can calculate that there will be a prismatic effect of 1.5? base out in each eye due to the 3mm outwards decentration in each eye.
Sometimes in a prismatic prescription the optical centre will be displaced off the lens entirely. For example:
R + 0.50 DS 3? base-in
L +0.75 DS 3? base-in
Here again a knowledge of the correct centration distance is important as checking at the wrong point could induce an incorrect prismatic effect.
Some lens prescriptions can cause confusion when trying to centre the target in a focimeter due to the transposition in which they are written. For example:
R + 4.00/- 4.00 x 90
L + 4.00/- 4.00 x 90
If transposed to:
R Plano/+4.00 x 180
L Plano/+4.00 x 180
It will be apparent that these are a pair of axis horizontal plano cylinders and that no sideways lens movement will induce a prismatic effect.
In the case of large prismatic prescriptions and also when checking near additions in multifocal lenses with a high distance prescription it can be useful to use a focimeter with a built-in prism compensator. But always remember with these instruments to check that the compensator has been zeroed before inserting a lens!
Multifocal and progressive power lens additions
In the current standard for measurement of spectacle lenses2 it states that when measuring near additions the following should be considered:
Unless otherwise stated by the manufacturer, the surface to be placed against the focimeter support shall be the segment or progressive side. This means that for a front surface addition lens, the addition is the difference between near and distance front vertex powers, and if the addition is worked on the rear surface, the addition is the difference between near and distance back vertex powers. Why is this? The reason is basically that when multifocal or progressive power lenses are made the initial stage of manufacture is often to produce a semi-finished lens blank. Here only the addition and the distance curve on which the addition is worked are fully finished. From this semi-finished form, various finished lenses can be produced with different centre thicknesses. If the addition is worked on the front of the lens, then the back vertex power addition will vary with the finished centre thickness.
As an example, consider a 1.5 refractive index semi-finished lens blank with a +6.00D distance curve and a +3.00D addition bifocal segment worked on this surface. From this, a thin, plano distance power lens could be made with a centre thickness of 2.00mm and a rear surface power of -6.00D. The exact back vertex power for this would be +0.05D for distance with a +3.06D addition. If, on the other hand, the same semi-finished blank was used to produce a lens with a plano rear surface and a centre thickness of 9.0mm, then this would produce a distance back vertex power of +6.22D but with an addition of +3.29D. The design addition of +3.00D would only be measured correctly if the difference in front vertex power was measured.
Note that in the case of progressive power lenses the manufacturer may give specific information as to how the addition should be measured, particularly if the lens powers have been compensated.
Progressive power lenses
When checking progressive power lenses it is essential to measure values at the points specified by the manufacturer. When new, these points may be indicated on the lens by removal markings. For other lenses it is necessary to reconstruct the required points using a template which is aligned with the permanent lens engravings. Figure 4 shows typical temporary and permanent markings on a progressive power lens.
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The removable markings are:
? Checking point for distance sphere, cylinder and axis (marked ‘a’ Figure 4)
? Fitting cross – generally indicates pupil centre position ‘b’
? Prism reference point ‘c’
? Near power checking point ‘d’.
Two permanent circles are shown which indicate the horizontal reference. These are conventionally 34mm apart, although other symbols may be used instead. The addition is sometimes engraved beneath the temporal circle, and a trade mark or indication of lens design or material beneath the nasal circle (TM).
Note that when drawing the checking points on a lens using a template it is important not to use a thick pen which will obscure the focimeter image. This is particularly in relation to point ‘d’, where a small dot rather than a circle is often preferable.
Optical centration and prism must only be measured at point ‘c’. If non-prescribed prism which has the same value and vertical base direction is found in each eye, then this is thinning prism designed to improve the weight and appearance of the lens. Sometimes a combination of prescribed and thinning prism might be found.
For example:
R 2? base-down
L 5? base-down.
This would signify a prescribed prism of 3? base-down left combined with 2? base-down right and left thinning prism.
Compensation of lens powers
It is becoming increasingly common for manufacturers of progressive power lenses to compensate the near addition, and/or the distance, to allow for the position of the lens in the ‘as worn’ position. For example, two lenses from different manufacturers were ordered as plano distance with a +2.00D addition. One of the supplied lenses is marked on the packet as having a compensated distance power of + 0.03 / +0.04 x 180, with a +2.00D addition. The second lens was supplied with a packet showing +0.09DS for the distance, with +1.77D addition.
Thus different manufacturers are using different design criteria for compensation, to compensate for the difference between the powers found in the consulting room with a trial frame or a refractor head, and the powers experienced by the wearer looking through a glazed lens.
Figure 5 shows a diagrammatic representation of the factors which are sometimes taken into account at near:
? Near vision effectivity – the change in form between the trial lens and the finished lens affects the effective power.
? Oblique aberrations – off-axis viewing through the lens will induce power differences
? Vertex sphere error – the back vertex distance increases as you look down, changing the effective power.
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There are other factors, such as the pantoscopic tilt of the lens.
All this assumes, of course, that the manufacturer knows exactly how the lens is going to be fitted relative to the eye.
Tolerances
When checking prescription lenses it is important to be aware of the current standard for lens tolerances.2 This document is quite complex and is unfortunately not always well understood. Unlike early standards for spectacle lens powers, tolerances are not applied to the prescription in sphero-cylindrical form. The standard first requires you to note the ‘Power of the principal meridian with higher absolute back vertex power’.
If we take the single-vision prescription discussed earlier of +0.75/-1.25x45 then this will be +0.75, as the principal powers are +0.75 (sphere) and -0.50 (sphere+cylinder). As 0.75 is less than 3.00, we are directed to the first row of tolerances within a table in the standard. This tells us that the ‘Tolerance on the back vertex power of each principal meridian’ is ±0.12D. But in addition, as the cylinder is between 0.75D and 4.00D the tolerance on the cylinder is given as ±0.12D. The reason for this is to prevent the situation where one meridian is 0.12D under power and the second meridian 0.12D over power, giving a cylinder of 1.50D.
References
1 BS EN ISO 7944:1998 ‘Optics and optical instruments — Reference wavelengths’. British Standards Institution, London.
2 BS EN ISO 21987:2009 ‘Ophthalmic optics — Mounted spectacle lenses’. British Standards Institution, London.
? Colin Fowler is a senior lecturer, School of Life and Health Sciences, Aston University, Birmingham
Model answers
(The correct answer is shown in bold text)
1 Which of the following statements concerning set up of a conventional eyepiece focimeter is true?
A Presbyopes are more likely to experience error due to lack of accommodation
B Eyepiece focusing makes the target easier to see but has minimal impact on the result
C Light leaving the focimeter needs to be parallel
D A +0.25/-0.25DS error is allowable on older instruments
2 Which of the following differs from the other three in power?
A -0.50 x 130/+1.75 x 40
B -0.50/+2.25 x 40
C -0.50 x 40/+1.75 x 130
D +1.75/-2.25 x 130
3 A pair of -2.00DS lenses are glazed at a centration distance of 62mm for a patient requiring a centration distance of 66mm. What is the measured induced prism in each lens?
A 2? base out
B 0.2D base out
C 0.4? base out
D 0.4? base in
4 What problem might arise when checking a 4? base in prism on a +0.50DS lens?
A The prism is large enough to distort the focimeter image
B The centration point will be displaced beyond the edge of the lens
C Prism may only be assessed on paired lenses
D This should not present any problems
5 Where on a marked up progressive lens might the addition power be seen?
A Near power checking point
B Temporal marked circle
C Nasal marked circle
D Fitting cross
6 What is the tolerance for the power of a -1.25DC cylinder?
A None
B +/- 0.12D
C +/- 0.25D
D +/- 0.50D