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Heights are only taken after fitting the frame
Lenses

The importance of getting the right fit

10/08/2018
By Bruce Wain
No matter the type of optical appliance, the fitting of lenses is a vital part of the overall dispensing process. BRUCE WAIN explores the most important aspects in this special two-part series.

An important aim of any optical appliance is to provide the highest quality vision possible for the person wearing it.

A central task in this process is the measurement of the eyes – namely the pupillary distance – distance between the visual axes – referred to as PDs. This measurement needs to be accurate as this will be the point where the finished lenses need to be aligned.

Any deviation may result in poor optical performance of the appliance. It’s also important to note the higher the power on the 180-power meridian, the more accurate these measurements need to be.

Another important measurement is for heights and these are taken once the frame has been properly adjusted to fit the face – facial fitting (not standard alignment) – as the customer’s frame is positioned exactly where there is best fit and comfort.

How are these important measurements taken? Are the ‘PD rule’ – ‘sticky tape’ and ‘permanent markers’ still used? One would hope not. Today, more sophisticated equipment is used in a practice, suggesting this measurement should be very accurate – but how many jobs are remade due to inaccurate centration for PDs and/or heights?

The foundations

"Experience in the use of the focimeter is valuable, as it is not only used to set up jobs but also to quality check the fitted appliance"

Like building a house, get the foundations correct and the builder has a good starting point; in optics, get the PDs/heights and frame fit correct and the manufacturer will be able to produce an appliance to the specifications provided.

Heights – optical centres, segment position and fitting cross, require the accuracy of measurement equal to that when measuring PDs.

Most practices utilise the latest technologies to undertake this measurement, as many of the clip-on devices provide very accurate measurements. However, others may still use the “look over my left shoulder”, or worse still a piece of sticky tape and black marking pen. I would strongly advise against this.

Crucially, the wearer needs to be positioned at a height equal to the measurer and be seated in a ‘normal’ position. For those of you wondering, “does it really matter if the heights are a little off?” – it certainly does, just ask wearers of progressive lenses.

Proper focimeter use is very important in any practice, regardless of whether the manual or fully automatic digital varieties are available. Image targets for manual focimeters range from from cross-line, to ring of dots and combinations of both types. By calling on their lens theory knowledge, the accomplished dispenser should be able to move freely between the various manual focimeter designs and use them all accurately.

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Experience in the use of the focimeter is valuable, as it is not only used to set up jobs – that is dotting uncut lenses – but also to quality check the fitted appliance.

Certificate IV students are taught to ‘dot’ uncut (finished lenses) on axis and some with prescribed prism. Here they learn to ensure the ‘dots’ are clear and crisp – not some blob that is already 2 mm wide that leads to centration errors.

While this may not be an issue for today’s dispenser – it does matter if that dispenser works in their workshop. At the manufacturing facility this dotting or layout of lenses would probably be completed automatically where very little errors occur – but then why are there remakes for incorrect PDs and heights?

Blocking and edging

After the uncut lenses are ‘dotted’ they need to be blocked. This involves applying a ‘fitting block’ to the front surface of the lens – on axis and at the correct position for centration and heights. There are set calculations that need to be carefully followed to achieve this task. The manufacturing facility will depend heavily on the accuracy of those initial measurements taken and frame details provided either by measuring, tracing or from a frame library.

Patternless edging requires accurate tracing of frames and/or frame libraries containing accurate frame details – both systems, if properly managed and maintained, provide accurate measurements for these calculations.

At our workshop the students use a manual edger therefore, they are required to make an accurate ‘machine’ and ‘handmade’ template, both of which need to be ‘boxed’ centred – that is, equal distances at nasal and temple distances – as this ensures accurate horizontal centration.

Equally important is equal distances at top and bottom of template, which ensures accurate heights are achieved. Dispensers should not only learn the importance of having accurate templates for centration/heights but also that templates need to be on axis and with an accurate shape of the eyewire. These three variables can result in either a very good fitted appliance or a poorly fitted appliance.

Part two of Bruce Wain’s fitting tutorial will focus on the next steps in the process: blocking, edging and quality control checks.

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