Understanding Vision

How modern prescription eyeglass lenses maximise your vision potential

In conversation with ZEISS on holistic eyeglass lens determination and prescription spectacle lenses optimised for binocular vision.

16 October 2017
  • How modern prescription eyeglass lenses maximise your vision potential

All glasses wearers are familiar with vision tests or subjective refraction. The eye care professional fits a trial frame and asks us to read letters, numbers or state the opening direction of rings in different sizes projected onto a surface. After he inserts different measuring lenses in the trial frame, we indicate whether they make us see better or worse. In this step-by-step process, the eye care professional finds exactly the prescription eyeglass lenses which allow us to see clearly and distinctly. But is there a better and more precise way? BETTER VISION spoke to the Product Manager for ophthalmic optical instruments at Carl Zeiss Vision, Carsten Kress.

BETTER VISION: Eyeglass lens determination is the most important part of the process of buying glasses. What characterizes modern subjective refraction today?

Carsten Kress:  That's right. At ZEISS, our goal for eyeglass lens determination is to maximize the vision of every individual patient. That might sound obvious, but we really want glasses wearers to get precisely the prescription eyeglass lenses that allow them to use all of their available vision. We don't try to reach what is commonly termed "100%" visual acuity - instead we aim for the maximum and this can vary greatly depending on the individual.

We don't try to reach what is commonly termed "100%" visual acuity - instead we aim for the maximum and this can vary greatly depending on the individual.

Carsten Kress

Product Manager for ophthalmic optical instruments at Carl Zeiss Vision

BETTER VISION: What are the criteria for this?

Carsten Kress: The test conditions for subjective refraction are crucial. Traditionally, eyeglass lens determination is still frequently done with projectors which project the optotypes onto a wall. However, the vision test room must be darkened to see the optotypes with as much contrast as possible. That can be a disadvantage, as the light conditions generally differ from those experienced when wearing the subsequent correction.

As a result, at ZEISS we have been developing measuring instruments with backlit optotypes since 1958. That means that subjective refraction can be performed in daylight conditions. The device currently used for distance measurement is the electronic i.Polatest®, which shows the optotypes on a specially modified LCD. Another advantage of this vision testing instrument is that new optotypes are generated every time. Accordingly, the tester can offer patients a new selection to read whenever required, effectively preventing fatigue or memorizing. It also allows eye care professionals to use different optotypes, such as letters, numbers, Landolt rings, Snellen E-types or even symbols for children or illiterate persons.

A similar instrument is also available for near vision eyeglass lens determination, for reading glasses or  glasses for the workplace. A vision test is performed with a mobile hand-held device, Polatest®  N Classic, at the patient's usual reading and working distance. For example, picture a musician who has to be able to read their score precisely at a certain distance. Prescription eyeglass lenses can be optimized precisely for this reading distance if it is accurately measured during the subjective refraction test.

BETTER VISION: Is it true that with i.Polatest® eyeglass lens determination can be done on both eyes simultaneously? Why is that so important?

Carsten Kress: The workflow is as follows. The eye care professional first optimizes the prescription lens for one eye with i.Polatest®, and then separately for the other – a standard monocular measurement. However, as the patient generally has two eyes involved in the vision process, the interaction of both eyes must always be tested. That is why we offer polarizing tests for Polatest®  devices - hence the name Polatest®.

We also integrate binocular testing which enables the eye care professional to diagnose any associated heterophoria. At the end of the day, the binocular tests have one goal: good, fatigue-free vision with both eyes at the same time. The lens measurements obtained can be transferred to prescription eyeglass lenses for day-to-day use.

Many people don't even know that they have binocular problems.

Carsten Kress

Product Manager for ophthalmic optical instruments at Carl Zeiss Vision

Carsten Kress:  Many people don’t even know that they have binocular problems. For example, there are patients who suffer from serious strain-related complaints (such as headaches, burning eyes or great sensitivity to light), reading problems, reduced depth perception or permanent muscle tension due to a slight, unnoticed tilt of the head, which have not been diagnosed in spite of in-depth examinations. These people often suffer from an undiagnosed associated heterophoria, which can frequently be corrected with prismatic glasses. Don't worry though; the prescription eyeglass lenses are not cosmetically unattractive for the wearer. A polarizing or binocular eye test, in conjunction with a comprehensive medical history can clarify matters.

BETTER VISION: How do these tests work?

Carsten Kress:  As with monocular measurements, the patient is examined with a vision testing device. A special technology – similar to 3D projection – shows only parts of the test to one eye at a time, while the test environment is viewed normally with both eyes. Corrective steps which can improve measurement of binocular vision are derived from the statements of the patient on how they perceive this test, i.e. whether all test components are seen simultaneously or parts of the test are offset from one another.

A method for correcting binocular vision which is common in German-speaking countries was developed by H.-J. Haase as early as 1953 and named after him: the Haase measurement and correction method, abbreviated to MKH in German. However, depending on the medical history findings, eye care professionals can also use other binocular tests and methods with i.Polatest® to balance binocular vision.

BETTER VISION: Can children also suffer from associated heterophoria?

Carsten Kress:  Of course. In general, it is extremely important to check children's and infants' eyesight regularly. It is incredible how many vision problems our brain simply compensates by itself – especially in children. The eye with better vision generally does the work and the poorer visual impression is simply suppressed. The earlier it is detected and measures are taken, the better the chances of specifically treating vision deficiencies and helping children to develop binocular vision. It doesn't even always have to be a comprehensive eye test. For example, parents can test eye tracking motions themselves with mobility tests: just move an object through the child's field of vision from various directions and observe how both eyes follow the object, evenly, unevenly or with jumps. If one of the eyes reacts differently, we recommend having it tested by an ophthalmologist.

It is now known that conditions such as illiteracy, restlessness and lack of concentration or ADHD in children can be related to vision problems.

Eye tests can be performed with children as soon as they can speak. Before this, observation or other measurement methods can be used, for  example for premature babies.

BETTER VISION: Why do many eye care professionals perform objective sight testing before subjective refraction?

Carsten Kress: Autorefractors are generally used for objective sight testing. For this, the patient looks into the device and the ametropia is automatically measured consecutively for each individual eye. At ZEISS, the autorefractor is called the i.Profiler® and uses wave-front technology to do the measurements. This technology facilitates the measurement of higher-order aberrations, which in turn can be used to optimize prescription eyeglass lens correction for low-light situations (such as dusk and twilight). One of the advantages of performing an objective vision test first is clear - patients are not subjected to excessive strain in the vision testing room with lengthy examinations. The i.Profiler® gives eye care professionals very revealing measurement results very fast, which allows them to start the subjective refraction immediately and to comprehensively optimize prescription eyeglass lenses with binocular measurement as well.

Legend of the eye test results:

During eye tests, the refractive value of the optical correction of the eye is determined, without letting the eye create a clear image via accommodation (individual dynamic adjustment of the refractive power of the crystalline lens).

0.00 dpt (dioptres)  = Normal vision (emmetropia)

Not 0.00 dpt (dioptres)  = Ametropias

e.g. sph +2.00 dpt (dioptres)  = Far-sightedness or longsightedness (hyperopia) – also for presbyopia; this is then called a near addition (Add)

e.g. sph -1.00 dpt (dioptres)  = Myopia

cyl 1.00 dpt (dioptres)  = Cylindrical ametropia (astigmatism) - the eye views points as bars or lines

What is associated heterophoria?

If binocular vision is normal, both eyes see the object in question identically. With associated heterophoria, there is an imbalance of the eye movement muscles, which requires a great deal of energy from a person with the condition to make vision possible. If the eyes were to take up the most strain-free position, the result would be double vision. Sufferers of associated heterophoria are subject to the (involuntary) compulsion to compensate this vision problem, which results in a wide range of complaints. The closest comparison is a deviation from the normal bodily structure, e.g. one leg is slightly shorter than the other. This often results in back pain and tension, which are easy to compensate with an appropriate insole in the shoe. Prismatic eyeglass lenses can compensate for associated heterophoria.

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