Showing posts with label Progressive Lens. Show all posts
Showing posts with label Progressive Lens. Show all posts

Importance of Interpupillary Distance in Spectacles

Importance of Interpupillary Distance

Globally approximately 145 million people of the total population and approximately 40% of India’s population is having an uncorrected refractive error (means need a pair of glass/spectacle).

Refractive error is one of the most important ocular conditions that are affecting all age groups and a priority under the VISION2020 initiative. Most refractive errors can be easily corrected at the primary care level with the help of spectacles. Uncorrected refractive error is the second leading cause of vision impairment in developing countries (including India).

Interpupillary distance (IPD) is the distance between the centers of your two eyes (center of corneal reflex) which is measured in millimeter. IPD can be measured monocularly and binocularly. Monocular IPD is the distance between the center of each eye and the center bridge of the nose. Monocular IPD may be slightly different for each eye due to anatomical variation and is dependent on your eye placement. An adult IPD usually remains more than kids IPD. IPD measurement is required for all types of spectacles including buying spectacles for distance or near. The near IPD is usually smaller than distance IPD due to convergence of the eyes together to fuse an object at near.

Different ways to measure IPD:

•    Ruler and torchlight (old method)
•    Pupillometer
•    Essilor Visioffice (USA)
•    Eye-Ruler 2 (USA) 
•    Zeiss i-Terminal2

IPD Measurement with Ruler and torchlight

Examiner should be exactly in front of the patient maintaining the same eye level height and holding a ruler at arm's length distance. Examiner closes his right eye and the patient must be looking at the examiner’s left eye with his right eye and put the ruler at the patient’s nose bridge area (for better stability) while aligning ‘0’ at patient’s center of the right eye. And without moving off a ruler examiner should open his right eye (after closing his left eye) and the patient must be looking at the examiner’s right eye with his left eye. Now measure the distance between both eyes center with ruler – this is your distance IPD. Average distance IPD may fall in the range of 56-66mm. The drawback of this method is parallax may skew measurement, examiner PD and patient’s PD must be very similar and the patient’s pupil must have the same size.
Another way to measure IPD is the distance between the right eye’s outer limbus to left eye’s inner limbus. With this method, IPD measurement may be inaccurate.

IPD measurement with Pupillometer

IPD measurement with pupillometer is more convenient. A Pupillometer has forehead rest, nose bridge, dial to change the distance to focus, display for monocular and binocular PD. Putting dial at infinite while using pupillometer over patient nose and aligning forehead rest. Now patient must be looking at small light inside the pupillometer and the examiner should align the measuring slide (a line or bar) until vertical lines should coincide with the corneal reflex. And read the reading in the display for monocular and binocular IPD. 
This device is faster than using manual measurement and takes all necessary measurements for personalized progressive lens and provides numerous solutions to select the lenses and frames. This device assesses the different measurement centers of eye rotation, the distance between pupils, the dominance of eye, reading distance, pantoscopic tilt, the fitting height of the lens, and frame parameters ie. wrap angle, A, B, and DBL.

Zeiss i-Terminal2

Zeiss i-Terminal2 is faster than using manual measurement and captures and calculates the patient’s required parameters with the click of a button. This device enables the measurement of highly ametropic patients through vergence control technology.  Measurement can be taken for any type of frame including large-size sunglasses and sports frames. It can measurement all height persons (children, taller or wheelchair person) ranging from ~120-208cms.

IPD is used to align the center of eyeglass lenses with the center of your eyes, which will give you a comfortable and relaxed vision. If the centers of eyeglass lenses are not aligned properly as per your eyes centration, then you may experience eyestrain, headaches, distorted vision, double vision, blurred vision, and/or an inability to wear your eyeglasses for a longer duration.

There are so many spectacle users (patients) who complain of eye strain. Many spectacle users are unsatisfactory that is the result of incorrect estimation of the IPD for distance and near. So it is an important While making a pair of glasses the dispensing professional need to measure Interpupillary distance (IPD) of a person.

IPD measurement is not the part of an eye examination but the IPD measurement is the responsibility of the dispensing professional and it is an important part of a spectacle prescription. Other spectacle dispensing services includes selection of an appropriate frame, lens type, lens thickness and coatings, adjustment of the spectacle frame, and proper alignment of the bifocal or progressive lens height.
Sometimes being perfect IPD alignment and proper spectacle lens fitting still patient complain of eye strain or unsatisfied. What could be the next to find for eye strain or unsatisfactory? So it is very useful scientifically to check the front and back curvature of the lens, coating of the lens, design (aspheric or conventional) of the lens of old glasses as sometimes spectacle users may not very satisfy with new glasses because of this. Sometimes new spectacle lens may have different front and back curvature, different coating of the lens, and design (aspheric or conventional) of the lens.

There are some more factors including frame size (bigger or smaller size over face), pantoscopic tilt (the amount of tilt on the front of the frame), wrap angle (the amount the frame is curved towards your face) and frame alignment over the face which can lead eye strain or unsatisfactory.

There are some other measurements that your optician will take when fitting you for progressive or multifocal lenses. There are two important key measurements - the IPD and the segment height must be checked while fitting a multifocal lens. These two measurements will be providing a comfortable vision with lesser adaptation time.

Each spectacle lens manufacturer is having its version of customized progressives. Near-pupillary distance measurement is also important in progressive addition lenses as these lenses are having customization instead of being standard. There are few other measurements including the vertex distance (the distance between the frame and the cornea), the pantascopic tilt, and the wrap angle to be checked while making a selection of a frame. The latest progressive lenses are more user friendly, giving a wider area of the field of vision and are generally easier to adapt to, especially for those with higher power prescriptions.

In my practice, if any patient complains of eye strain, double vision (shadow feeling while watching TV, or reading text at near), headache after wearing new glasses then my first check would be refraction (repeat power) check-up, IPD measurement, vertex distance, pantoscopic tilt, etc. Most patients are satisfied with repeat refraction and IPD measurement and their comfort changes after reassuring these two checks.

Progressive Lenses vs Bifocal Lenses

 Progressive Lenses vs Bifocal Lenses

  Approximately at the age of 40 (+/-2 years) your eyes gradually lose the ability to see things for near. It is a normal part of aging. The term is called “presbyopia”, comes from a Greek word which means “old eye.” You will probably find that you hold reading materials farther away or stretch hands in order to see them clearly.
When you are young, the lens is soft and flexible, easily changing shape due to this you can focus both distance and near objects clearly. After 40 years of age, the lens becomes more rigid and cannot change shape easily. This makes you to read at near with efforts, thread a needle, or do other close-up tasks. There is no way to stop or reverse the normal aging process that loses the ability to see things at near. However, this can be corrected with eyeglasses, contact lenses or surgery but surgical choice may not be very good for everyone. If you do not correct presbyopia, you may experience eye stress, eyestrain, focus-defocus, headaches and sometimes you feel that your hands shorten.
Wearing eyeglasses is an easy way to correct presbyopia. Seeing clear vision with eyeglasses offers the opportunity to select from different types of lens options, frame designs and even lens coatings for various purposes.
There are different types of eyeglass lens designs: single vision lens – corrects only one distance vision, bifocal lens – corrects distance and near vision (does not has intermediate distance to see monitor), and multifocal or progressive lens – designed to correct all distances vision (the upper portion is focused for distance vision, middle portion focused for monitor, while the bottom portion is used for reading).
Bifocals have two zones to correct vision for reading on the bottom half of the lens and another for seeing at a distance on the top. Some special lenses may also have segments at the top for those who need to look upward at objects that are in the intermediate or near range these lenses are called double-D bifocals (but not used frequently).
Progressive or multifocal lenses function generally the same way as bifocals due to a smooth transition between distance and near focal areas instead visible dividing lines in bifocal.
If you don't want to use glasses for distance, you can use reading glasses only which can be purchased from any optical store. You will probably must change your eyeglass prescription from time to time (approx every 1-2 years) between the ages of 40 and 60 because your eye's natural lens will still lose flexibility and consequently focusing ability.

Progressive Lens Designs

 Progressive Lens Designs

 Population age than 40 years having confusion that which type of glasses they should use! As they find discomfort while seeing ingredient/price tag on product or teaching their child(ren).
Glasses with distance and near vision correction are called bifocal. This comes with 'D shape' or 'Circle shape' or 'straight line' bifocal. This type of lens can give confusion in vision while walking, climbing up & down on stairs, when vision falls at the junction of distance and near segment. It is difficult to see monitor with this type of glasses. And person may experience back and neck pain.
Progressive lens provides clear vision in vertically (distance to near) without image jump but creates distortion in peripheral vision (side vision). But this distortion may depend on the design of progressive lens. Progressive lens comes in different field of vision ie. Narrow (hard design), medium and wider (soft design) field of vision progressive lens.
Wider field of vision progressive lenses are the best choice to use where user may have very less distortion in side vision, better comfort while adaptation, lesser adaptation time.
Progressive lenses are available with top brands like Essilor, Zeiss etc.

History of Progressive Lens Development

History of Progressive Lens

Bi-focal eyeglasses are having two corrective lenses for distance and near. The lenses in the lower half are for near vision and lenses in the upper part are for far vision correction. Benjamin Franklin (the American statesman) was the inventor of bifocals. Bifocal eyeglasses are usually prescribed for presbyopia or approximately 40 years of age.

The story of the invention of the bifocal lens dates back to 1780s when Sir Benjamin Franklin was found deterioration in his vision and he found it difficult to switch far and near glasses. Around 1784, Benjamin Franklin cut two lenses in half and glued them together with a line extending across the entire width of the lens — one with a distance vision correction and one with a correction for near vision. It is now also called an Executive bifocal or straight-line bifocal. In this type of bifocal junction, the line was cosmetically unattractive and annoying.

In the 19th century the Franklin bifocal lens design was further refined. And the first fused bifocal was invented by John L. Borsch in the year 1899 but redesigned in 1908 and called the new ‘kryptok bifocal’.

In 1910, O’Conner produced a back surface grounded bifocal to improve chromatic aberrations which was found infused kryptok bifocal and provide a larger reading area. It is called as Ultex Bifocal.

In 1926, Univis Company introduced flat-top bifocal, which is called ‘D’ bifocal. In this bifocal unusable area at the top of the earlier (kryptok) bifocal was removed to decrease the prismatic image jump in vision.

In the 1950s, presbyopes were still using bifocals that divide the field of vision into two parts and were not looking for cosmetically attractive. A young French Optics and Mechanics engineer Bernard Maitenaz transformed the world of presbyopes by creating a lens that allows them to see at any distance without interruption.

 The story of the invention of the progressive lens starts when Bernard Maitenaz was studying at the École Supérieure d'Optique about the eye in his Optics and Mechanics engineering and thought that the eyes were magnificent organ which can see at all distances and have the capacity to see as 3D objects.

As the person’s eye ages and does not adjust/focus enough anymore, the only solution is to use a bifocal lens in front of it. He was in his twenties and full of enthusiasm so he thought to be a better solution. Being as an Optics and Mechanics engineer, he started as his project to calculated progressive surfaces of a lens, and prepared machines to produce it.

His objective was to achieve what he had imagined that is a lens with power in the lens varies progressively and allows compensating for this adjustment that disappears. This lens was a more physiological manner than with bifocal lenses without breaking the image.

He took 8 years to develop the first progressive lens called Varilux, which was launched in 1959. In 1972, his team introduced a modified progressive lens call Varilux 2, which was a fully aspheric lens. This lens was adopted world widely by presbyopes due to higher visual comfort.

Varilux (progressive lens) technologies are the innovation in the field of optics constantly involve addressing challenges in optical design, materials and the coating of lenses. Since 7 generations Varilux lens is continually improving and approaching a more and more natural vision. And now the Varilux S series is available with the latest technology and scientific progress.

Varilux is the lens available from the Essilor brand which is a leader in the optical market. Approximately 95% of Varilux wearer is very much satisfied around the world. Essilor brings different Varilux series:

Varilux Freedom 3.0
Varilux Comfort 3.0
Varilux Physio 3.0
Varilux E Series
Varilux X Series
Varilux S Series