Segmented Multifocals PDF

OPHT 2612 Segmented multifocals Chapter 19 27 March 2024 Introduction: “Multi” meaning more than one ― Distance, Intermediate or Near “Focal” = focus point Distance rx not equal to near rx not equal to intermediate rx “Multi” lenses is mainly used when px requires more than one rx Presbyopia Accommo...

OPHT 2612 Segmented multifocals Chapter 19 27 March 2024 Introduction: “Multi” meaning more than one ― Distance, Intermediate or Near “Focal” = focus point Distance rx not equal to near rx not equal to intermediate rx “Multi” lenses is mainly used when px requires more than one rx Presbyopia Accommodation insufficiency ― Accommodation support lens Type of MULTIFOCAL lenses: 3 main types: BIFOCAL = 2 distances (has visible line) TRIFOCAL = 3 distances (two visible lines) PROGRESSIVE ADDITION LENSES (PAL) = 3 main distances (no visible lines) Near addition: Required for patients with presbyopia What is presbyopia? ― Younger px can see distant and near with one prescription ― Gradually unable to focus on near objects with aging ― Near reading addition is required ― This is over and above the distance rx ― Additional plus segment at the bottom of a lens helps to focus at near points Near addition: If no distance correction is required, the only factor to be considered is the plus lens power to see clearly at near: The amount of plus power can be given in a single vision lens having the same power over the whole lens e.g. +2.50 D It can also be given in a segmented multifocal with the lens power in the main portion being PLANO and the +2.50D power in the lower portion of the lens Near addition: If the wearer does have a correction for distance, the extra power for near must be “added on” to the power found in the distance prescription The near addition is the same as a small plus lens placed in the lower portion of the lens It is often referred to as the near segment or seg A bifocal addition is just that = an addition to the distance power Near addition: Example: Distance power = -1.00 D Add = +2.00 D The total power at near is: Near portion = dx + add = -1.00DS + (+2.00DS) = +1.00 DS WHAT IS THE RX OF THE SINGLE VISION READER FOR THIS PX?? WHAT IS THE RX IN THE READING PORTION OF THE BIFOCAL LENS?? = The +1.00DS power is the power that would be used in a pair of single vision lenses intended to be used only for reading Near addition: Near power: The net power resulting from the combination of the distance power and the add An example how the near addition is written in Rx form: OD +2.00 DS OS +3.00 DS Add +2.00 DS ― UNDERSTAND: Both lenses have a near segment of +2.00D that adds much more power to that part of the lens ― Since an addition is made to the distance power, meaning the measured near power through the near portion of the right lens is +4.00 D sphere, and the measured near power through the near portion is +5.00 D sphere Near addition: Near power in a TORIC lens: Example: OD: +1.50 / -0.25 X 145 OS: +2.50 / -0.75 X 90 Add: +2.00 What is the near rx for OD and OS? Near addition: Near power in a TORIC lens: Example: OD: +1.50 / -0.25 X 145 OS: +2.50 / -0.75 X 90 Add: +2.00 Take the distance sphere + the reading add NB: Leave the cyl as is RE: +1.50 + 2.00 = +3.50 ‫ ؞‬+3.50/-0.25x145 LE: +2.50 + 2.00 = +4.50 ‫ ؞‬+4.50/-0.75x90 Near addition: Additional example: The distance power of a lens is -3.00DS. The near add power is +3.00DS. What is the power in the near portion (near power)? -3.00 + (+3.00) = plano ‫ ؞‬Near rx = plano If this px was a 60 year old pensioner, with financial problems (constraints), what is the best option with respect to spectacle options? Construction of segmented multifocals: 1. Fused Available only in glass The segment has a higher refractive index than in the main part of the lens No ledge or change of curvature on the front Curvature can not be felt Construction of segmented multifocals: 2. One – Piece Have a ledge that can be felt Made from one lens material All plastic lenses are made as one-piece multifocals Can be identified by feeling the segment border ― If either a ledge or a change in curvature is felt, the lens is not fused, it’s a one-piece design Construction of segmented multifocals: 3. Cement lenses Are made by gluing a small segment to the distance lens Usually in a form of small, round segments Types of Bifocals: Round segment Flat-top segment Curved top segment Panoptic segment Ribbon segment ― B-seg ― R-seg SELF REVIEW pg. 434 - 435 Types of Bifocals: 1. Flat Top Advantages Disadvantages 1. The segment is wide at the top, giving the wearer a wide reading area. = This makes the flat-top very comfortable to read with 1. The edges of the segment can intrude a little into the wearer’s peripheral vision Benefits: The flat-top is considered to be the best all-round bifocal It provides a good reading field without intruding too much into the distance field Segment sizes range from 22mm up to 45mm ― Most flattops used now are 28mm or greater Types of Bifocals: 2. Round segment Advantages Disadvantages 1. Round-segments are the least visible of the regular bifocal types 2. It can be rotated and still not look tilted 3. The segment does not intrude to much into the distance field 4. Their round shapes allows them to be placed in unusual positions e.g. the upper temporal corner of a golfer’s right lens 1. Because of its round shape, the wearer must drop his/her eyes further down into the segment to obtain a useful width 2. The lens has a smaller reading area than other bifocals 3. The optical center of the segment is at the GC of the segment = This results in the viewer encountering an extra amount of base down prism as the line of sight enters the segment, this causes “image jump” Also known as: 1. Kryptok 2. Round fused Types of Bifocals: 3. Ribbon Segment Advantages Disadvantages 1. Has a wide field of view. 2. The wearer can look through the distance portion under the segment, assuming the lens shape is deep enough (B segment) 1. The segment is only 9mm deep, limiting the wearer’s vertical eye movement Round segments with the top and bottom cut off There are two types B & R. ― B-only 9mm deep ― R-14mm deep Types of Bifocals: 4. Curved top and Panoptik Advantages Disadvantages 1. The lens is very similar to the flat top with the same advantages 2. It is argued that the eyes are more comfortable moving from side to side in an arc rather than straight across 1. The edges of the segment can intrude a little into the wearer’s peripheral vision Curved top and panoptik are very similar: Both have a curved top, but in the panoptic, the corners are rounded as well Trifocals Lenses: Some lenses have an intermediate area between distance and near portions This area is used when the area through the normal distance and reading areas are not clear enough The solution is then to use segmented multifocal lenses known as trifocals The trifocal power of the intermediate portion is normally 50% of the near add Example: Near add +2.50 ‫ ؞‬Intermediate = Near add / 2 = +2.50 / 2 = +1.25 D Trifocals Lenses: When do we use the trifocal lens? If the add power increases to above +1.50DS there will be intermediate areas of vision that are not clear through either the distance portion or the near portion of a normal bifocal A trifocal intermediate will give clear vision at this blurred in between distance Trifocals Lenses: Intermediate Power: The intermediate add value is added to the distance power to find the expected total intermediate power as measured in the lens meter. Example: Distance Rx Near add = +1.00D = +2.00D 1. What is the near rx? +1.00 + 2.00 = +3.00DS 2. What is the intermediate rx? First need to know the intermediate add Intermediate add = +2.00/2 = +1.00D ‫ ؞‬Intermediate rx: +1.00 + 1.00 = +2.00DS Trifocals Lenses: Example: Distance Rx = -7.00D Near add = +3.00D 1. What is the near rx? 2. What is the intermediate rx? Trifocals Lenses: Example: Distance Rx = -7.00D Near add = +3.00D 1. What is the near rx? -7.00 + (+3.00) = -4.00DS 2. What is the intermediate rx? First need to know the intermediate add Intermediate add = +3.00 /2 = +1.50DS ‫ ؞‬Intermediate rx: -7.00 + (+1.50) = -5.50DS Trifocals Lenses: TYPES Flat-top trifocal Franklin (Executive) trifocal E/D trifocal Round segment (ᴥ) Trifocal Types: 1. Flat-top It has a wide and distinct intermediate and near field The Flat-Top vary in width from 22mm to 35mm and in depth from 6mm to 14mm Less attractive Trifocal Types: 2. Executive (Franklin) It produces a maximum width on intermediate and reading It is a useful trifocal for occupations requiring a wide near and intermediate field The two dividing lines are unsightly, can collect dirt and grease, and may chip along the edges Trifocal Types: 3. E/D trifocal Provides a very useful intermediate field over most of the bottom half of the lens Requires minimal horizontal and vertical head movements Excellent segmented lens for working at a desk Not a very attractive lens as it attracts dirt and grease The executive line may restrict mobility (not easy to walk around in) Occupational Multifocals Any multifocal selected by the dispenser for a specific viewing purpose/situation may be classified as an occupational multifocal It’s specifically designed with certain work circumstances THREE TYPES: 1. Double D (occupational flat top) 2. Quadrafocal 3. Rede-rite (minus add upcurve) Occupational Multifocals Types: 1. Double D segment If you require intermediate or near viewing while looking upward ― Occupation? Upper segment comes in a variety of possibilities ― Upper segment is identical in power as lower segment = both segments contain near rx ― Upper segment with a power that is ½ D less than the lower segment = one seg is for intermediate and the other for near It requires awkward head movement Occupational Multifocals Types: 2. Quadrafocal This is a double-segment lens with a flat-top trifocal on the bottom and an upside-down flat top segment on the top Appropriate for those needing a trifocal and double segment Only available in glass lens Occupational Multifocals Types: 3. Minus add ‘Rede-rite’ bifocal It has a large round segment at the top, most of which is cut off after edging The top part of the lens is for distance viewing and the rest of the bottom part of the lens is for near work In essence = a bifocal with a large near portion and small distance viewing area Occupation? Near portion center vs distant portion center: What position are your eyes in when reading? ― Reading position = eyes are down and converged The near portion of the lens is placed infero-nasally because of convergence and the lower position of the eyes As a result near portion centre is not the same as distance portion centre Near centration point is usually 2mm in and 8mm down from the distance OC NVP = near visual point DVP = distance visual point Major reference point: The MRP of a lens is positioned in the same vertical plane as the pupil and a few millimeters below it The MRP and the distance optical centre is usually the same point if no prism is prescribed When prescribed prism is present in the distance portion, the optical center is no longer in the same location as the MRP Bifocal Terminology Seg width - size of the seg horizontally (measured across the widest section of the segment area) Seg Depth – is the longest vertical dimension of the seg Seg Drop - is the vertical distance between the major reference point (MRP) of the lens and the top of the seg Seg Height - is dependent on the frame for which the lenses has been edged and is measured vertically from the lowest point on the lens to the level of the top of the seg Bifocal Terminology Inset or outset - amount the MRP is moved laterally from the geometric center of the lens (also referred to as distance decentration) Seg Inset – the amount the center of the near segment is moved inward from the MRP. ― The horizontal distance between the MRP and the segment centre is the segment inset Total inset / Total seg inset – is the inset plus the seg inset ― The total distance from the geometric centre to the seg is the total inset The sum of the inset and the seg inset = total inset Ordering of reading spectacles: When a spectacle lens rx is written with an add power, it is normally written before a decision is made on what type of lenses to be used If an individual wants reading glasses only, the order form will not be written with an add, but will be written for single vision lenses Example: A Rx is written as follows: OD: +0.25 / -0.50 x 180 OS: +0.25 / -0.50 x 180 Add : +1.50 The wearer decides they do not want anything but single vision reading glasses. What power would be ordered? Ordering of reading spectacles: Example: OD: +0.25 / -0.50 x 180 OS: +0.25 / -0.50 x 180 Add : +1.50 Power of reading glasses? SOLUTION: Distance rx + ADD For both eyes: +0.25/-0.50 x 180 + (+1.50DS) = +1.75/-0.50 x 180 Ordering of glasses with an intermediate and near rx: Some individuals work in circumstances in which they need to see at intermediate and near-viewing distances only Example: the wearer has half-eye frames and needs to see at intermediate distances. A decision is made to place a bifocal lens in the half-eye frame Ordering of glasses with an intermediate and near rx: Example 1: OD: +0.25 / -0.25 x 170 OS: +0.25 / -0.25 x 010 Add: +2.50 What power lens should be ordered? Ordering of glasses with an intermediate and near rx: “A normal intermediate power is 50% of the near addition” ‫ ؞‬Intermediate add: +2.50/2 = +1.25DS The top of the new half-eye bifocal must be the wearer’s distance RX plus the intermediate add OD: +0.25 / -0.25 x 170 + (1.25DS) OS: +0.25 / -0.25 x 10 + (1.25DS) = +1.50 / -0.25 x 170 = +1.50 / -0.25 x10 These are the powers of the upper portion of the lenses. Ordering of glasses with an intermediate and near rx: The near portion of the half-eye bifocal will have the near prescription: Distance power + near addition = near power OD:+0.25 / -0.25 x 170 + (+2.50DS) = +2.75 / -0.25 x 170 OS: +0.25 / -0.25 x 010 + (+2.50DS) = +2.75 / -0.25 x 010 Ordering of glasses with an intermediate and near rx: The intermediate will be at the top of the bifocal and the near at the bottom. How would we write this RX for the lab?? Remember the intermediate acts as the “distance rx” section. And we know that: Near rx = distance rx + reading addition ‫ ؞‬Reading addition = near rx – distance rx Ordering of glasses with an intermediate and near rx: Information so far: Intermediate (the new distance rx in this example) OD: +1.50 / -0.25 x 170 OS:+1.50 / -0.25 x10 Near RX OD: +2.75 / -0.25 x 170 OS:+2.75 / -0.25 x 010 So if the intermediate is the upper portion, we write the upper rx and an add. What is the add?? Ordering of glasses with an intermediate and near rx: Reading addition = near rx – distance rx OD: reading add = +2.75/-0.50 x 170 – (+1.50/-0.50 x 170) = +1.25DS OS: reading add = +2.75/-0.50 x 10 – (+1.50/-0.50 x 10) = +1.25DS ‫ ؞‬Final ordering Rx: OD: +1.50 / -0.25 x 170 OS: +1.50 / -0.25 x 010 Add: +1.25 Ordering of glasses with an intermediate and near rx: Example 2: Distance rx OU: +2.00DS/-3.00 x 20 Add = +2.00DS Px wants BF with intermediate and near zones What is the Rx to order?? Ordering of glasses with an intermediate and near rx: 1. Intermediate add = (1/2) of add = +2.00/2 = +1.00DS 2. New distance rx for top portion of BF: = +2.00/-3.00 x 20 + (+1.00DS) = +3.00/-3.00 x 20 3. Reading add for ordering: = +2.00/-3.00 x 20 + (+2.00) Near rx = +4.00/-3.00 x 20 Reading add = +4.00/-3.00 x 20 – (+3.00/3.00 x 20) = +1.00DS Ordering of glasses with an intermediate and near rx: What to write on the order form? OU: ADD: +3.00/-3.00 x 20 +1.00DS Image jump: The bifocal section is a smaller lens on a larger lens When the segment is round, the OC will be exactly in the middle of the seg ― Not all are round, some are shaped with the upper section cut off Some are constructed so that the OC is exactly on the upper line The farther the eyes are looking from the OC, the greater the prismatic effect The value of the prismatic effect in the segment is dependent on the location of the segment optical center The sudden displacement of the image as the bifocal line is crossed is known as image jump Image jump: The bifocal section is a smaller lens on a larger lens When the segment is round, the OC will be exactly in the middle of the seg ― Not all are round, some are shaped with the upper section cut off Some are constructed so that the OC is exactly on the upper line Image jump: Prentice’s rule: The amount of image jump for a given style of bifocal is independent of the power in the distance portion It can be calculated using Prentices Rule Image jump: Example 1: How much image jump does a bifocal with a 22mm round segment have if the add is +2.00 D? SOLUTION: Since the segment is round with the seg OC in the middle, the upper bifocal border is 11mm above that center. △ = cF = (1.1)(2.00) = 2.20△ Therefore a 22mm round seg of +2.00 D add power has an image jump of 2.20△BD Image jump: Image jump = 2.20△BD Why base down? Image jump: Example 2: How much jump does a flat-top segment have with the following segment dimensions: Seg width = 25mm Seg depth = 17.5mm Add Power = +1.50 D SOLUTION: A flat-top style seg is essentially a small round lens with the top cut off Therefore to find the distance from the seg line to the seg optical center, subtract one half of the seg width from the seg depth. Distance (c) = seg depth – (seg width/2) Image jump: ‫؞‬c = 17.5mm – 12.5mm = 5mm = 0.5cm ∆ = cf = (0.5) (1.50) = 0.75 ∆ Therefore the image jump is 0.75∆BD Image jump: Example 3: How much image jump does a bifocal with a 24mm round segment have if the add is +3.00 D? (2) Example 4: How much jump does a flat-top segment have with the following segment dimensions: (3) Seg width = 26mm Seg depth = 18mm Add Power = +1.50 D Image jump: Example 3: How much image jump does a bifocal with a 24mm round segment have if the add is +3.00 D? (2) SOLUTION: Round BF, therefore △ = cF = (1.2)(3.00) = 3.60△BD c = 24mm/2 = 12mm = 1.2cm Image jump: Example 4: How much jump does a flat-top segment have with the following segment dimensions: (3) Seg width = 26mm Seg depth = 18mm Add Power = +1.50 D c = 18 – (26/2) = 5mm = 0.5mm ∆ = cf = (0.5) (1.50) = 0.75∆BD https://www.youtube.com/watch?v=cjnxSk_Cw10 END!