Prismatic corrections – a communication challenge between the industry and opticians

Prismatic corrections – a communication challenge between the industry and opticians

Picture by Michael Dziedzic on Unsplash

In my career of over 30 years as a lecturer in ophthalmic optics, I have been through all the stages from student to teacher to developer to expert authority – trusted with confidential knowledge about the industry. I would like to thank everyone for the trust they have shown in me, because this was the only way I was able to explain complex optical issues to opticians in a clear and understandable way. Here aberrometry with its wavefront measurement is a case in point.

By Fritz Paßmann

Knowledge means recognizing connections and developing links to topics that are already understood. The conversion of prismatic prescriptions into spectacle lenses that a wearer is comfortable with is something I care deeply about. Both from the point of view of the end customer, i.e. the improvement of his vision and possible alleviation of asthenopic complaints, as well as from the point of view of the optician, who needs support in exercising his skills and offering his – sometimes emotional – recommendations. This calls for broad backing by the industry. This article is aimed at bringing about a willingness to offer opticians more information, education and transparency.

This is a huge challenge: converting values ​​determined in the refraction room into spectacle lenses that are comfortable for the end user. It requires close cooperation between all the parties involved. The optician is the link between the lens manufacturer and the customer.

Let’s start at the beginning

When looking through a spectacle lens, a prismatic effect is created outside the optical center. Normally this does not pose a problem for the spectacle wearer, as the prismatic effects of different versions and base settings mutually diminish each other resulting in an “almost zero” overall prism; except where there is anisometropia, in particular in the principal meridian of the spectacle lens in the respective direction of view.

To calculate this, the optician uses the “Prentice law”: P = c * S`. Charles F. Prentice is considered as the father of optometry in the USA and in 1921 he recognized the connection between the distance of the path of the light beam in the spectacle lens from the optical center to the vertex power for calculating a prismatic effect.

If someone wants to achieve a prismatic effect by decentering the lens, they must not use the Prentice law as the formula does not take account of the rotational direction of the eye when looking through the lens. For more accuracy, the ‘Weinhold formula’ or ‘extended Prentice law’ (Fig. 1) which includes the BVD (back vertex distance) must be used.

‘extended Prentice law’

Technically the distance between the eye center of rotation and the lens vertex b` should be taken into account, as the eye rotates around its optical ocular center Z` in the opposite direction to the base setting of the prism. But how can you measure the distance to a virtual point? This raises three fundamental questions for the lens manufacturer:

  1. Is the BVD or b´ always included when ordering prismatic lenses? In which case a specifically developed order form including all the necessary data is recommended.
  2. What distance does the industry use to calculate b’? The ‘standard eye’ according to Gullstrand specifies 13.5
  3. Is there any conversion made between ‘Prentice’ and ‘Weinhold’?

This leads to a further issue: the difference between the measurement position and the position of use of the spectacle lens which needs to be communicated precisely, both for the clarity and understanding of the optician and for the technical details to be printed on the lens bag. Admittedly the differences in calculation will mostly have an effect ‘after the decimal point’, i.e. in the decimal range.

However in problem cases several factors often occur together, or the customer is particularly sensitive, in which case precise communication can minimize misunderstandings.

Prismatic effect through decentration and its limits

The application of the Prentice law is already taught during training and thus opticians are able to autonomously decenter the lenses and determine the reference point in order to achieve the desired prismatic effect. I was already proud to be able to do this in the early days during my apprenticeship. At a time when considerable subsidies were still paid by the health insurance for spectacle lenses (editor´s note: that was the case in Germany until 2004), ordering a spherical or toric single vision lens (multifocal lenses naturally did not apply) and achieving a prismatic effect with the help of a larger uncut lens diameter was a great art and did not involve changing the prescription or even cheating the health insurance.

The desired effect was achieved and afterwards it was not possible to see by the lens how this had been done. However, this approach does have its limits, which should be clearly communicated by the industry:

  1. The vertex power and uncut lens diameter are mutually dependent. That means that, where S` is rather low, the blank will not have the required diameter. My recommendation for maximum dioptric power is the number “4”. This means that above a vertex power of0 D and prismatic effect of 4 cm/m, the prism should be created industrially, i.e. by tilting the blank when blocking. This procedure is not known to all opticians, but it explains the limitations of what is feasible when the blank only has a limited edge thickness.
  2. For toric lenses the effect in the meridian must be taken into account. This means that where the principal meridian and the base setting of the prism to be generated are not parallel to each other but intersect obliquely, this approach tends to lead to approximate results.
  3. If the lens manufacturer is not aware of the prismatic effect to be achieved, the increased aberrations that inevitably occur will not be corrected or even minimized. At this point the occurrence of ‘astigmatism of oblique bundles’ in wedge-shaped lenses should just be mentioned. An optimized prismatic correction for the comfortable vision of the spectacle-lens wearer requires the aberrations to be corrected at the reference point.

Mandatory: BVD and PD

During training to become a refractionist (not a refraction assistant), a distinction is made in the correction of a latent squint (heterophoria & associated heterophoria – explaining the difference would require a chapter of its own) between the pupil-center centration (PMZ) and the formula-case [1] when using a trial frame [2]. When centering on the pupil center using the Viktorin method, the PD setting is retained during the entire measurement process.

On the one hand, this is a straightforward procedure, on the other hand, it contains inherent inaccuracies and may even reach its limits. As already mentioned, after inserting a prism the customer no longer looks through the optical center of the existing sphero-cylindrical correction. Two prismatic effects occur: that of the measuring prism and the unwanted prismatic power of the spherical and toric lenses, leading to a weakening or strengthening of the overall prismatic effect.

 

The refractionist does not know the overall prism value and has to rely on the expertise of the lens manufacturer to produce the exact prismatic effect. However, to do this, as already mentioned, the PD and BVD need to be known. A prismatic prescription cannot be followed precisely without specifying the PD and BVD.

Furthermore, the visual acuity/vision of the customer may be impaired during refraction due to the resulting aberrations, which may lead to the refractionist giving up on further prismatic correction, although the status of binocular vision of the customer could still be improved. In addition, the binocular field of vision becomes smaller.

When using the equation case for lens centering, the far distance centration (not the customer´s PD) is adjusted. Two similar rules of thumb are used which are by no means only marginally different: for every 3cm/m or every 4cm/m, the set distance in the trial frame is corrected in the opposite direction to the base setting of the inserted prism. This minimizes the prismatic effect of the main lens but by no means removes it completely.

Furthermore, new questions arise as to the practical procedure used: if the formula is only used from 3 cm/m or 4 cm/m, what procedure is to be followed below these values? In the case of higher vertex powers, a noticeable prismatic side effect quickly becomes apparent, even if the trial lenses used are of low prism. What is the right setting for 5 cm/m on the trial frame? The required accuracy of 1.67 mm or 1.25 mm cannot be achieved.

In practice, this results in a combination of the two variants: partly further adjustment, partly accepting the prismatic side effects of the main lens. I recommend making the size of the ‘readjustment’ dependent on b`. The larger the VD, the more likely is it necessary to make readjustments. This results in the next demand on the measurement protocol/order form for spectacle lenses with prismatic effect: the indication of the PD at the beginning and at the end of measurement as well as the BVD. As previously stated, progressive lenses must always be ordered with the prismatic effect.

Potential sources of communication errors between the optician and the lens manufacturer

The question sometimes arises for the optician: “If the customer’s PD is readjusted already during refraction, (I am deliberately using this imprecise wording here because it is so often used; which does not make it any more correct. The correct terminology would be to readjust the centration point distance in the trial frame) then do I no longer have to take it into account when readjusting?” The answer is: “It depends.”

The crux of the matter is that it depends on the optician’s workflow. A refraction is usually carried out after taking down the case history and assessing the requirements. The PD setting of the trial frame is rarely recorded. The optician takes the centering data from his centering system using the – hopefully correctly adjusted – trial frame.

However, at this point the prismatic correction is not in front of the eyes, so they take the ortho position. This data is then sent to the lens manufacturer. In this case, the centration correction must be carried out over the entire prismatic effect, not just over the ordered prism, regardless of whether readjustment has already occurred in the refraction room.
If the optician specifies the readjusted PD when ordering the lens, only the small difference to the newly calculated total prism needs to be changed. In my opinion, it is desirable to indicate this correction on the lens bags as the new “distance centration distance”.

Some lens manufacturers have recognized this potential source of error in communication and have for some time now been taking a new approach, at least for progressive lenses: relocating the stamp by moving the engraving. The lens manufacturer calculates the new position of the reference point exactly, i.e. to an accuracy of 1/10 mm, thereby offsetting the centration marking of the stamp which makes sense. The optician has one less thing to worry about and continues to work as usual. However this is not a general solution to the problem.

New questions arise: “Can this approach be used for all types of lenses in the portfolio?” Is the centration correction clearly shown in the price list? At what point in time was the change made?” This question may be of key significance when handling complaints. Since specialist opticians usually work with a number of lens suppliers: “Which lens manufacturers offer this, and which do not?”

Prism distribution

Personally, I prefer asymmetrical prism distribution. I find it satisfying when aesthetically pleasing lenses can be achieved by adjusting the thickness. It is down to the refractionist to decide whether a symmetrical or asymmetrical distribution of the prismatic effect is more appropriate. Only he knows the visual acuity and the tolerance of any prism combinations as well as the influence of the customer’s “dominant eye”.

Only the lens manufacturer can calculate the absolute effective thickness ratios for ground lenses with a sphero-cylindrical and/or progressive surface. From these two points of view, an interaction is recommended through which all possibilities can be envisaged.

Taking account of the refraction conditions in the trial frame

The lens manufacturer should have a further skill and needs to communicate this to the optician: namely modification of the overall prism in the finished lens. In the refraction trial frame, lenses are placed in different holders, whereby the prismatic trial lens with its base setting is usually inserted into the front lens holder for practical reasons. The lens is thicker and would otherwise scratch the subsequent lenses or would not even fit into the lens holder. However, in the end only one lens is manufactured and ground to fit into the customer’s spectacle frame, with the resulting effect corresponding to the sum of all the individual lenses.

This poses additional questions: Are the various distances between the trial lenses taken into account when calculating the spectacle lens? Is the forward prism wedge of the trial lens converted into the same effect on the spectacle lens in which the prism wedge protrudes backwards? How is the prismatic trial lens labeled? According to DIN, the basic deflection of the prism is defined in such a way that the principal beam hits the front surface at a right angle and deflection only takes place at the rear surface. Has the trial lens already been adjusted in this regard and, with the wedge protruding forward, does it have the same effect as is required in the position of use of the corrective lens for the main direction of view, despite the principal beam being refracted twice?

Measurement position versus position of use

In addition, there is the well-known problem of distinguishing between the measuring position and the position of use. The requirement for the measurement protocol/order form for spectacle lenses with prismatic effects should therefore include information on the respective slots used. In this context, the position of the edge of the ground-in spectacle lens is also important. Usually, only the optician has exact knowledge of the spectacle frame in question. Only the details of the lens shape are passed on to the lens manufacturer.

However, if the details of the entire frame – with its material, type of manufacture and the possibility of hiding the thickness of the lens rim – are passed on, new possibilities of producing aesthetically pleasing spectacle lenses by shifting the facet are opened up.

In addition, the position of the spectacles on the face is important. Are there any anatomical features such as strong eyebrows or deep-set eyes or protruding cheekbones to be considered? If you want the standard facet to deviate from parallel to the front surface, i.e. to be incorporated in the middle or even in the rear third of the edge thickness, this option must also be made clear on the order form.

Interplay between lens thickness, transparency and color fringe

I would also like to see more information about different lens materials and their Abbe number. This knowledge should then be used to advantage by the optician during the consultation. Thinner and more expensive does not always mean lighter and better!

Using easily understandable graphic charts, the interplay of lens thickness, transparency and color fringe of spectacle lenses can be used as a sales-support material to find the ideal combination, thus leading to the greatest benefit for the customer.

Taking account of the reference point

In the next point, too, I would like to plead for a commitment to sustained communication by the industry to the optician: “How can I (the optician) check lenses for prismatic effect?” This is only possible at the reference point. For this reason, single vision lenses should always be delivered with the reference point marked on them.

In the case of progressive lenses, the corrective prism effect cannot be measured solely at the prism reference point. At this point, the prescription prism and the calculated thickness reduction prism of the lens manufacturer have to be considered together. Often the optician is not aware of this combined effect. In addition with a height prism, the difference between the prismatic effects must be calculated separately from right to left.

One question after another

With regard to the previous point, the optician needs the expertise of physicists and physiologists from the industry. What does the spectacle wearer need to know about prismatic corrections? What tips and recommendations can the optician pass on to his customer? The comment “You’ll just have to get used to it!” is simply not good enough and certainly not professional.

How do increased distortions and dispersions affect vision? What can help to reduce these aberrations? How can it be that a customer reports seeing colored objects in different layers due to color stereopsis? Can anything be done about this? Can micropsia (objects appearing to be smaller) and macropsia (objects appearing to be larger than they are) be identified and avoided during eye testing for spectacle lenses?

Questions upon questions that can only be satisfactorily answered through a cooperative partnership where both sides bring in their expertise.

In conclusion, I hope that this article will create a better understanding of the vital need to collect all this data. A certain amount of transparency is desirable for calculating ideal lenses in order to demonstrate both the quality of the lenses and to stand out from the competition. This applies equally to lens manufacturers and opticians.

Footnotes:
[1] Formula case means that the pupil distance in the trial frame is corrected by 1 mm against the base for every 4 cm/m. An example: Initial situation; PD 32 mm, 4cm/m base-in, then the trial glasses are readjusted to 33 mm.
[2] A Trial Frame is an adjustable spectacle-like device containing cells used to hold multiple trial lenses during subjective refraction.