What does prism
Prism glasses | Advice before buying
What are prism glasses?
There are two types of prism glasses that use two different modes of operation of the prism optical component:
- Prismatic spectacle lenses based on the principle of the dispersion prism for the correction of latent (heterophy, ametropia) or manifest strabismus
- Reflection prisms for redirecting the image with special glasses, for example for climbing
1. Prismatic glasses for the correction of latent (heterophy, ametropia) or manifest strabismus
Prism lenses for the correction of latent (heterophy, ametropia) or manifest strabismus
Source photo: Wikipedia
The principle of dispersion prisms is used for the prismatic effect of the glass. These prisms refract the light depending on the wavelength at the entry and exit surfaces towards the base of the triangular prism body. So they deflect the light beam, whereby color fringes form when the light emerges. The prismatic effect, i.e. the deflection of the light beam from such a glass, is expressed in the unit centimeter per meter (cm / m).
Refraction in the prism
This deflecting property of a prism is made use of by prismatic spectacle lenses of prism glasses to correct latent (heterophoria, ametropia) or manifest squint, in which the light beam emanating from the fixed object is refracted in such a way that the image takes place where the corresponding retinal points of the deviating eye meet are located. In this way, binocular vision can be restored, in which the eyes can merge the two individual images into a three-dimensional individual image.
Corrective lenses with a prismatic effect have a completely different purpose than spectacle lenses, which are intended to compensate for ametropia such as nearsightedness or farsightedness by reducing the refractive power of the imaging system in the former case or increasing it in the case of hyperopia. Today, however, corrective effects for the above-mentioned ametropia can be combined with a required prismatic effect in one lens.
2. Reflection prisms for deflecting the image with special glasses (e.g. climbing glasses)
Prism glasses can help with climbing
These prism glasses use the property of reflection prisms to deflect light through total reflection, so it is a deflecting prism. With these prismatic glasses you can practically “see around the corner”. Spectacles with such prisms have nothing to do with prismatic spectacle lenses to compensate for squint, heterophoria or ametropia. So their purpose is not to compensate for ametropia or a muscular eye muscle imbalance.
These glasses are used, for example, as safety glasses for sport climbing, as reclining glasses for watching TV or reading while lying down, but also as deflection glasses for various experiments in research. They don't actually have glasses. Erecting prisms are built into them in a kind of glasses frame. Often these prism glasses can also be worn over normal prescription glasses.
Light deflection through total reflection in the prism
These optical aids are only to be used for the precisely defined purposes, such as climbing goggles. It has nothing to do with ametropia, disturbed binocular vision, strabismus or eye diseases.
Prism glasses for work are usually calculated for a very narrowly defined special visual situation and, thanks to the prismatic effect, enable special viewing angles with a comfortable head position. In most cases, corrective effects for the ametropia of the wearer can also be incorporated.
Your optician is also the competent contact for these glasses.
When do I need prism glasses?
In the case of a manifest, permanent squint that can also be recognized by laypeople, an ophthalmologist will probably prescribe prismatic glasses for you or your child.
In the case of a latent, or hidden, squint, you will experience abnormal sensations called "asthenopic complaints" such as headaches or double vision and therefore consult your optician. A thorough eye exam will reveal heterophoria / ametropia that your eyes can no longer compensate for when they are strained. The prism glasses then ensure that you can see individual images again and work again symptom-free, even when the working day is long.
Prism glasses can help with latent or manifest strabismus
There is the possibility that prism glasses may be dispensed with if the squint angle is small. However, this only works if the effect can also be achieved by decentering the glass and the required prism is not more than 2 cm / m thick. How is that possible?
In the case of heterophoria or heterotropy, the main visual point of the eye does not coincide with the optical center of the lens. If the prism is not too strong, a spectacle lens can also be decentered so that the optical center points are placed on the main visual points. The decentration takes place according to the rule that for every 1 cm / m of effect, the glass must be decentered by 0.33 mm in the opposite direction to the base position. This means that for every 3 cm / m of prism effect, a glass must be shifted by 1 mm against the base position.
However, this method also has serious disadvantages. Even if the prismatic effects are still relatively small, the lens diameter has to be larger, which results in increased weight of the glasses and thicker glasses. In addition, lenses outside of the center have poorer imaging properties. This is calculated and taken into account when manufacturing prismatic lenses.
How do prism glasses work?
Prism glasses are prescribed for disorders of the two-eye vision such as latent or manifest strabismus. Prismatic glasses help to restore the muscular imbalance of the outer eye muscles and prevent double vision and sometimes also enable spatial vision.
That is why prismatic spectacle lenses contain sections of a prism in addition to their corrective effect. That makes them thicker on one side that corresponds to the base of the prism. In contrast to prism bodies, however, prism lenses do not have any flat (planar) surfaces, but rather slight curvatures in order to minimize imaging errors.
How prismatic glass works
With these prism parts, prism glasses are able to shift the image of a fixed object so that it falls on areas of the retina that can be fused. This enables both eyes to see individual images despite the misalignment of the eyes.
The greater the ametropia or squint angle, the thicker and therefore heavier the prism lenses and thus the glasses. The image quality of prismatic spectacle lenses also decreases with increasing thickness. Then the "chromatic aberration" creates color fringes when the light is refracted and the image sharpness decreases.
In the glasses pass you will usually find the necessary information in this form:
- P or Pr = prism; next to it there is a number that indicates the strength of the prism of prism glasses.
- B or Ba = base; it indicates the direction of the prismatic displacement and thus designates the position of the base of the prism.
Prism lenses for which types of prescription glasses?
Prism lenses can be used for almost all types of prescription glasses and the most varied of visual requirements. The prismatic effect can be combined with the corrective effect for ametropia, nearsightedness, farsightedness and presbyopia. Prism lenses can be manufactured as single vision lenses as well as multifocal lenses. This means that prismatic lenses can also be manufactured for varifocals or special screen glasses.
There is only one limitation: the technical limit of what is feasible in the production of varifocal lenses is prism thicknesses of around 8 cm / m per lens.
However, with prismatic standard varifocal lenses with the base position inside or outside there is slight deterioration in the distance range, the surface shifts towards the base, which means that the view of closer objects is no longer optimally centered in the progression canals. In the case of varifocal prism lenses with a base at the bottom or at the top, there is also a slight deterioration in the far range, but without surface displacement. This means that when you lower your gaze to a nearby object, it goes through the center of the progression channel. These effects increase with higher glass thicknesses and result in slight reductions in the quality of vision.
It is different with individually optimized progressive lenses. With this modern type of lens, there are now techniques to achieve optimal image quality and thus the highest degree of compatibility, even with prismatic varifocal lenses.
Prism foil - use, function?
It doesn't always have to be a new spectacle lens with a prismatic effect when trying out the compatibility of a prismatic correction of a latent or manifest squint. A form that can be implemented quickly and easily, at least for a short time, is the application of a prismatic film.
The principle of this prism film is based on the Fresnel prism and the Fresnel lenses, as they are used in lighthouse optics. A Fresnel prism has a linearly arranged prism-like structure with a constant prism angle. It redirects parallel light with a constant deflection angle.
The Fresnel prism deflects parallel light with a constant deflection angle
In principle, nothing else happens here than the dissolution of the prism body into many small individual prisms that are arranged in rows.
The prism foil has the advantage that it can be easily applied and removed again. It consists of a highly transparent, elastic plastic material that is less than 1mm thick and can be cut to size. After your optician has cut it exactly to fit your spectacle lens, the prism film is placed on the moistened rear surface of the lens facing the eye. The film can be positioned exactly on the wetted surface. Any air bubbles that form can easily be brushed out. The film sticks there simply by adhesion. This simple process makes it easy to remove the film if it is dirty and put it back on again after it has been cleaned. It is therefore also important that your optician cuts the film precisely. This ensures that you can put the film back on in the exact position after cleaning.
In this way, each lens can also be given a prismatic effect.
The prism film is thus a simple and inexpensive correction option to find the best possible compatible prism correction. But this process also has serious disadvantages. Above all, the quality of the image will not satisfy any wearer of glasses in the long run, visual acuity is significantly reduced with prism foils. Therefore, prism foils are used almost exclusively for interim correction.
Prism glasses - tolerance, getting used to?
Prism glasses provide you with completely new visual impressions, above all the spatial impressions have been completely changed, this applies to both the assessment of distance and the perception of widths. Looking down, in particular, can be deceptive and at first you might think you're stepping into a hole, you are a little unsafe when walking upstairs, or you don't really notice edges. So it is necessary to exercise some caution in the beginning when moving around the room. It is similar with the shorter distances, for example when you are sitting at the table and want to pour a drink into the glass. So at first it is better to feel with a finger to see if the neck of the bottle touches the glass in order to avoid "flooding" the table.
Other entry complaints such as a slight headache or neck tension can occur because you suddenly adopt a different work posture. Relaxing after practicing constrained postures must also be practiced anew.
During the entry phase, you should only use your new glasses in traffic when you are sure that you can cope with the new sense of space. If you have just received your new prismatic glasses, it is important to wear them regularly and to increase the wearing times continuously so that you get used to the new visual impressions. However, please only wear the new glasses for as long as you do not experience any discomfort. If you have any questions, are unsure or do not feel comfortable with the glasses, please contact your optician.
However, all initial symptoms will usually disappear after a short period of application and you will soon find the new vision as completely normal. But even if you have no problems with your prism glasses, you should visit your optician for a check-up after a few weeks.
Prism glasses - advantages and disadvantages?
Your new prismatic glasses take the strain off your vision if you suffer from ametropia / heterophoria, because your eyes are not constantly overstrained trying to fuse the images on non-corresponding retinal areas into a single image. This is especially true for the eye muscles, which constantly try to align the visual axes with additional muscular effort so that no double images arise.
With the prism glasses, your eyes can see again without any additional effort. Your prism glasses cause the images in the eye to hit corresponding retinal locations by deflecting the imaging beam. The prism glasses make it possible to see with both eyes.
Correction with prism lenses guarantees you and your eyes relaxed vision without double images. If your cross-eyed child is given prism glasses, it will primarily help him to maintain binocular vision by not switching off the cross-eyed eye from the visual process. Otherwise amblyopia would develop, i.e. functional one-eyedness.
Disadvantages of prism glasses
- With prism values above 5 cm / m, glasses with prismatic lenses can be less aesthetically pleasing than spectacle lenses without a prismatic effect due to the thicker edges on the prism base.
- Entry problems can occur with prism lenses.
- Especially in the case of heterophoria / ametropia, it cannot be guaranteed that all previous complaints will disappear. The term asthenopic complaints includes a whole collection of even diffuse abnormal sensations that could also have other causes.
- In the case of heterophoria, it may be necessary to correct it again, because here only the correction is used that leads to the merger being possible again without problems.
- Ophthalmologists' associations report that after a few months of full correction of the ametropia with the measurement and correction method according to Hans-Joachim Haase, the prisms may have to be reinforced more and more.
Advantages of prism glasses
- Well-being and joie de vivre usually increase spontaneously, because the exertion complaints from headaches to concentration problems to tension decrease and double images disappear.
- The work performance increases because the fatigue caused by eye strain is eliminated.
- In road traffic, distances can be estimated more accurately, which brings safety and helps when parking and leaving.
- For sports enthusiasts, a completely new feeling of spatial vision arises.
Most prism lenses are in a strength range where they cannot be recognized as such
Prism glasses - costs?
If your optician or ophthalmologist advised you or your child to correct a latent squint (also heterophoria or angular ametropia) or a manifest, overt squint through prism glasses, the question of costs follows. This in turn leads to the question of whether and, if so, to what extent the health insurance company will contribute to the costs of prism glasses for you or your child.
What does the health insurance company cover for prism glasses?
In general, these rules apply in Germany:
- For children up to the age of 14, the statutory health insurance companies cover the cost of glasses if their eyesight deteriorates in old age. This is generally the case with prism lens prescriptions due to strabismus.
- In the case of adolescents between the ages of 14 and 18, the statutory health insurance companies will pay for the glasses if they are prescribed by a doctor for medical reasons. If the glasses are also used and prescribed for sports lessons, the statutory health insurance will also cover the costs for plastic lenses.
- Insured persons who have reached the age of 18 are only entitled to a health insurance benefit if they have poor eyesight or blindness which, according to Section 33 (2) SGB V according to the classification of the World Health Organization, has at least level 1 visual impairment in both eyes. In plain language, this only means that the health insurance fund is obliged to pay if the fully corrected pair of eyes with the better eye does not achieve more than 30% visual performance.
- The health insurance companies cover the costs for lenses of glasses that are used to treat eye diseases and eye injuries.This service category also includes prism glasses with a prism thickness of at least 3 cm / m for horizontal base positions and 1 cm / m for height prisms expressly because of "strabismus". They are generally considered to be therapeutic glasses. These glasses must be prescribed by a doctor. The diagnosis of orthopedic ametropia is not included in the catalog of services, as the method does not belong to the scientifically recognized procedures and the orthopedic ametropia is not considered a disease.
It must be emphasized that the statutory health insurances only have to assume the nationwide uniform fixed amounts for glasses in the amount of around 10 to 110 euros. They pay nothing for glasses frames. The health insurance companies generally cover the costs of the eye examination. If you need prism lenses for your work glasses, your employer is the right contact.
But in addition to the fixed amounts in the health care system, there are also the additional benefits of the health insurance companies, which can also relate to the provision of glasses. According to comparison portals on the Internet, these contributions can amount to up to 200 euros. It is therefore worthwhile to compare these additional services in a family of glasses wearers.
Prism glasses when climbing?
Do you also climb with prism glasses?
Prism glasses for climbing usually use a special reflection prism called the Bauernfeind prism. This prism ensures light deflections of 45 to 60 degrees with prism glasses as with the safety glasses for climbing, whereby the light beam is reflected twice.
The prism of the 60-degree prism shown below is fully mirrored on the area marked in green below. There the exit beam is steeper than the critical angle of total reflection and would emerge from the prism if it were not mirrored towards the base. According to the equation angle of incidence = angle of reflection reflected, it hits surface 2, where total reflection occurs again and the beam hits surface 1 at an angle of 90 degrees, passes through and reaches the viewer's eye. In total, the beam is deflected by 60 degrees.
If you try to put your head back 60 degrees, you will quickly notice how exhausting it can be on a longer climbing tour. In this case, these very special prismatic glasses ensure tension-free vision during sport climbing.
Beam path in a Bauerfeind prism for 60 degrees deflection. Green side requires mirroring
These prismatic glasses are used as safety glasses in sport climbing. With the help of these prism goggles, the climber who is securing the rope partner can keep an eye on the climbing partner without bending his head back extremely and putting extreme strain on the neck and neck muscles. The higher the lead climber climbs, the more extreme the neck flexion becomes on the partner belaying further down.
With the prismatic goggles, the securing climber simply looks ahead and through the prismatic goggles with the reflective prisms, he sees the partner climbing ahead high up on the climbing wall. Many climbing goggles are designed so that they can also be worn over prescription glasses or sunglasses.
Reflection prism glasses while climbing
There are a few things to keep in mind when using these safety glasses while climbing:
- The brain usually needs a little acclimatization phase with these prismatic glasses, so you should put them on a few times at home and get used to the feeling of sight.
- The field of view through the glasses is a little restricted, so all safety-relevant points and the surroundings should be checked before putting on the glasses.
- Don't look at the sun! The glare can be very strong. Also, due to the changed field of vision, the sun is suddenly in a different place than expected.
- Do not put on the glasses until your partner is so high up in the wall that he is completely in the field of vision. The entire length of the rope is not in view, so it should be secured against fraying with a knot at the end.
- Make sure that you can take off the glasses quickly in an emergency. It is recommended to attach the protective case to the strap with a snap hook so that the glasses can be stowed away quickly and safely when not in use.
Exemplary model for climbing goggles
Your optician will be happy to advise you on the subject of climbing goggles, including the interaction of climbing goggles and prescription glasses. He will explain to you which type of climbing goggles are best for people who wear glasses.
How does my optician determine the right prismatic glasses for me?
First, your optician will ask you exactly whether you have problems with your vision, what you are using the glasses for, what your vision requirements are and much more. If you mention double vision when asked about specific complaints, the specialist will immediately know what to look out for. But maybe you just said something about tired eyes because you work long hours? Or did you not mention any complaints because you did not associate your discomfort with your eyes at all or thought that your glasses were no longer strong enough?
Nevertheless, your optician will always also perform a heterophoria test in the course of the careful and complete objective and subjective refraction determination after determining the full correction of your ametropia. A simple masking test also provides information as to whether heterophoria could be present. Then we continue to work carefully with various test configurations until the prismatic correction is determined that you need in order to be able to see again without double images or similar complaints - even if the working day is long.
Some opticians will do an initial test to determine if there is a phlegm and then continue with the measurement and correction method according to Hans-Joachim Haase. At the end of each of these test procedures, the complete lens strengths are determined, which include all components of your farsightedness or nearsightedness, plus, if necessary, the astigmatic values and, if available, also the additive for presbyopia and the prismatic effects. Your optician will then advise you which type of glasses is recommended for you.
Refraction determination by your optician
What should you consider when buying prism glasses?
First and foremost, if you wear glasses, the following should apply to you: If something is not clear to you, please ask twice and have complicated issues explained to you. If you know exactly what is going on, you can simply make a better decision. A good consultation about your new prism glasses should enable you to do just that.
Let your trusted optician advise you in detail and explain to you how you will see with the new prismatic glasses. He can also explain to you how you should behave in the entry phase, what problems you have to reckon with and how common these are.
Your optician will also advise you on the glasses frame and the recommended type of lens. First he will explain to you what your new glasses with the built-in prismatic effect will look like. It tells you where the base is and how thick the glasses will approximate there. The desired frame should also come into play now. Even extremely fashionable glasses can be selected in such a way that they not only look great on you, but also divert your gaze from a possibly thick edge through the prism.
Then you can choose together which glass material is the most advantageous. Please also bear in mind that plastic glasses are not only lighter than glass and that the high-index variants are also thinner. You also have a higher level of safety when doing recreational sports or driving a car, since otherwise there would be a risk of injuries from the airbag in an accident.
What limits the choice of glass somewhat, especially with higher prism effects, are the color fringes (chromatic aberration) that always occur with prism glasses, which are added to the already higher color fringes of the highly refractive material. Your optician will advise you on finding the best compromise for you between thin glasses and top image quality. A full anti-reflective coating should definitely be added. This is certainly not a tailor-made job, but makes a decisive contribution to optimizing the image quality.
Prism glasses online - disadvantages
There are no advantages to buying prism glasses online - on the contrary, there are only disadvantages.
You shouldn't buy your prism glasses online
Most online retailers therefore state on their websites that they do not manufacture glasses with prismatic lenses and do not carry out corresponding prescriptions - even if they come from an ophthalmologist.
Both the correction of heterophorias / cornered ametropia and, in particular, all types of manifest squint, require close-knit advice and support from your optician and ophthalmologist, often intensive cooperation. It must also be possible to check the exact adjustment at any time. No online retailer can guarantee that.
Save up to € 100 on your new glasses now!
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