Lecture 7 - Complex Lenses Main CW 2024 PDF

OPT505 Clinical Skills and Refractive Management: Complex Lenses Claire Wright FBDO One Time Code NS-HU-FJ Core Competencies 4.1.1 Ability to advise on, order and to dispense the most suitable form of optical correction taking into account durability, comfort, cosmet...

OPT505 Clinical Skills and Refractive Management: Complex Lenses Claire Wright FBDO One Time Code NS-HU-FJ Core Competencies 4.1.1 Ability to advise on, order and to dispense the most suitable form of optical correction taking into account durability, comfort, cosmetic appearance, age and lifestyle 4.1.2 Ability to adjust a spectacle frame or mount to optimise physical and optical performance 4.2.1 Ability to measure and verify optical appliances, taking into account relevant standards One time code: NS-HU-FJ Learning Outcomes Recap lens design Understand dispensing options for: High Minus High Plus Lenses used for Near Vision Define Decentration Vertex Distance One time code: NS-HU-FJ Lens form and Optical Performance  Aberrations Material Lens form Transverse Chromatic Aberration Spherical aberration (TCA) Coma Oblique astigmatism Curvature Distortion Spectacle Lens Design Ideal lens produces point image of a point object on the far point sphere Point Focal Percival Form Minimum Tangential Error Form High Minus Using spherical surfaces, no real optical challenge until Rx exceeds -22.00D Lenses have thick edges Reduce lens aperture (smaller frame) Increasing refractive index is first choice High Refractive Index Is an option if we get the most suitable frame choice E.g. -10.00D lens power, an increase in lens size from 40mm to 50mm will result in edge thickness to increase by 40% and weight to double in a normal refractive index -10.00D in a 1.9 glass lens could have edge thickness of under 4mm compared to over 10mm in CR39 Aspheric Lenses Lenses which are not spherical or cylindrical Eliminates oblique astigmatism Less distortion Flatter Thinner and lighter Less spectacle magnification Aspheric Lenses Free-form surfacing Allows for more complex designs Powers digitally mapped across the lens Better control of aberrations Variability in design parameters Thinner, flatter lens forms and good optics Cylinders worked on both surfaces Add shared between front and back surfaces (bi-concave or bi- convex, bi-aspherics) Still limited to about +8.00DS Lenticular Designs Reduced aperture lenses Edge thickness has been reduced or flattened (Think of a fried egg shape, inverted for minus) Lenticular Lenses Optician article: Essential course in dispensing 8/8/14 High Plus Using spherical surfaces optical challenge when Rx exceeds +7.00D Lenses have thick centre substance Reduced field of view Increasing refractive index doesn’t help much  The angle subtended by the effective diameter of the lens at Ring scotoma the point conjugate with the centre of rotation is called the REAL FIELD OF VIEW y Θ’ R θ R’ s’ s Where s’ = 25mm, tanθ = y(40-F) and tanθ’ = y40 1000 1000  Calculate the real and apparent field of view when a +5.00D hypermetrope wears a lens of diameter 65mm at a distance of 25mm from the centre of rotation of the eye. In this example y = 32.5mm F = +5.00D  Calculate the real and apparent field of view when a +5.00D hypermetrope wears a lens of diameter 65mm at a distance of 25mm from the centre of rotation of the eye. In this example y = 32.5mm F = +5.00D Real field: tanθ = 32.5(40-5) = 1.1375 hence θ = 48.68o so 2θ = 97.36o 1000 Apparent field: tanθ’ = 32.5x40 = 1.3 hence θ’ = 52.43o so 2 θ’ = 104.86o 1000 Difference in total = 7.5o Higher - order aspherics Obtained by deforming a conicoid Known as polynomial surface Surface power reduces from the centre of the lens to the edge Main advantage is the absence of ring scotoma and jack in the box effect Smaller field of view so have to move head more Aphakia ( Px is Aphakic) As eyes develop cataract, eventually the crystalline lens may be surgically removed. Mostly a new plastic replacement lens is implanted into the eye, making the eye Pseudo-phakic. The power of the implant is calculated to make the eye as ‘emmetropic’ as possible. With no accommodation, so an add is required, based purely on working distance. Aphakia Before this process was done (>25 years) the eye may have been left without any replacement lens –Aphakic This would then leave a previously emmetropic eye needing a +9.00D to +14.00D correction just for distance. Depending on patients age, congenital cataract extraction may be left aphakic until eye is deemed fully developed. Aphakia may occur due to trauma  Aphakia Sph Cyl Axis Base ADD RE +10.00DS +3.00 LE +11.50 -1.50 180 +3.00 Aphakia Before Aspheric surfaces became available, one method of reducing the weight and thickness of a high positive lens is to produce a lenticular Complex Rx and Frame Choice We always suggest a smaller frame to our complex Rx patients why? A small frame will help with: Weight/thickness TCA Oblique Astigmatism But the main reason is……… Complex Rx and Frame Choice The main reason is to keep Decentration to a minimum By minimising decentration you will always achieve the best cosmetically appealing pair of spectacles What is decentration? This is the amount of we have to move the lens to ensure the optical centre sits in front of the patient's pupil.  Decentration A Eyesize or Lens Size Patients Pd x HCL Optical centre of lens Distance Between Centres (DBC) 1. Work out Total Decentration by: TD = A+ DBL – Patient Pd e.g. TD= 52 + 18 - 62 = 8mm so 4mm in each eye 2. So the Optical Centre of the lens needs to be moved from the Geometric Centre of the frame in by 4mm in each eye to align the Optical Centre of the lens with the Patients Pd.  Decentration A Eyesize or Lens Size Patients Pd x HCL Optical centre of lens & Px’s Pd coincide Distance Between Centres (DBC) 1. Work out Total Decentration by: TD = A+ DBL – Patient Pd e.g. TD= 52 + 18 - 62 = 8mm so 4mm in each eye 2. So the Optical Centre of the lens needs to be moved from the Geometric Centre of the frame in by 4mm in each eye to align the Optical Centre of the lens with the Patients Pd.  Vertex Distance Distance from the visual point of a lens to the corneal apex British Standards (3521) ‘recommends that the vertex distance be included in the prescription whenever the power exceeds…’ +/-5.00DS British Standards (3521) ‘recommends that the vertex distance be included in the prescription whenever the power exceeds….’  Vertex Distance –Back Vertex distance BVD Can be measured with a ruler or using a special calliper British Standards (3521) ‘recommends that the vertex distance be included in the Ideally you want the frame you dispense to sit in the same position prescription whenever the power exceeds….’ or BVD as the trial frame. This is where the Rx was found, if frame BVD does not match the trial frame BVD then the Rx may need to be compensated Vertex Distance –Back Vertex distance BVD British Standards (3521) ‘recommends that the vertex distance be included in the prescription whenever the power exceeds….’  Effectivity Calculation To prevent the Px having the incorrect Rx at the new position we have to calculate the new Rx taking into account the new vertex distance 𝐹𝐹 𝐹𝐹𝑂𝑂𝑂𝑂𝑂𝑂 𝑁𝑁𝐹𝐹 𝐹𝐹𝑂𝑂𝑂𝑂𝑂𝑂 𝑒𝑒𝑒𝑒 = 𝑁𝑁𝑁𝑁𝑁𝑁 = 1 ± 𝑑𝑑𝑑𝑑𝑂𝑂𝑂𝑂𝑂𝑂 1 ± 𝑑𝑑𝑑𝑑𝑂𝑂𝑂𝑂𝑂𝑂 - If Vertex decreases + If Vertex Increases d Is the change in Vertex  Effectivity Calculation +10.00DS tested at 10mm, frame dispensed at 15mm what's the new Rx? FFnFew = +11.00 Fnew = +10.00 1 + (0.005 x 10) = +9.50DS We know this to be correct as if the Rx wasn’t amended the power at the new BVD would be too strong Lens Form and Near Vision Near Vision Effectivity Error (NVEE) Near Vision Effectivity Error arises because lenses of the same power that manufactured in different forms are not interchangeable for near vision. In distance vision the vergence of light leaving the back surface of a spectacle lens is known as the Back Vertex Power (BVP) Lens form makes no difference so are interchangeable (for distance) In near vision light originates from a finite point therefore the vergence leaving the back surface is dependent on BVP and form and thickness of lens Lenses for near with the same BVP but different forms means resultant Rx will differ with different lens forms. NVEE Consider the following example: A +10.00D lens made with a -3.00D base curve, in glass, n = 1.5 with an axial thickness of 9mm is used for near vision with the near object placed at -33.3cm from the lens. The front curve of the lens will be +12.06D. Paraxial ray-tracing will show that the vergence leaving the back surface of the lens is +6.58D NVEE The difference between the vergence of light actually leaving the lens when the light originates from a near object (L2') and the anticipated vergence obtained by adding the incident vergence and the BVP of the lens (L1 + BVP) is known as the near vision effectivity error (NVEE). Form Vergence leaving lens Difference to trial lens Curved (meniscus) +6.58D -0.29D Flat (plano-convex) +6.69D -0.18D Flat (equi-convex) +6.87D 0 NVEE In general a curved strong positive lens will impress less vergence change for a near object than a bi-convex form and even less than a plano-convex form This will generally only affect single vision spectacles as multifocal segments which are on the front surface impress a greater vergence than the add suggests, negating and sometimes superseding the NVEE Generally high plus powers at near. Break! Prescription Analysis Sph Cyl Axis Add RE -10.00 -1.00 90 - LE -9.00 -0.50 90 - What to consider? Is the Rx to British Standards What's Missing? Type of Rx Options for lenses Options for frames Measurements Required Prescription Analysis Sph Cyl Axis Add RE -10.00 -1.00 90 - LE -9.00 -0.50 90 - High myopia No BVD Thick edges, heavy lenses, minification Limited frame and lens choices 1.74 aspheric plastic to make it as light as possible MAR – reflectance will be higher TCA – discuss to see if an issue – small frame with low BVD will minimise Plastic, small frame, regular shaped with similar PD to box centre distance to minimise decentration – rectangle and large frames will look bad Mono Pd’s, heights, BVD Prescription Analysis Sph Cyl Axis Base ADD RE +13.00DS +3.00 LE +14.50 -3.50 180 +3.00 What to consider? Is the Rx to British Standards What's Missing? Type of Rx Options for lenses Options for frames Measurements Required Prescription Analysis Sph Cyl Axis Base ADD RE +13.00DS +3.00 LE +14.50 -3.50 180 +3.00 Aphakic Rx No BVD Centre thickness Ring Scotma 1.74 Aspherics or Blended Lentics MAR Plastic, small frame, regular shaped with similar PD to box centre distance to minimise decentration Mono Pd’s, heights, BVD Recommended Reading Jalie, M. (2008) Ophthalmic Lenses & Dispensing, 3rd Ed, Chapter 4, 8, 9, 13. Jalie, M. (2001) The Principles of Ophthalmic Lenses, Chapter 11, 18, 21 Gilbert, P. (2013)Ophthalmic Lens Availability ABDO Tunnacliffe, A. (2007) Essentials Of Dispensing, 3.1,3.4,4.6,4.7, 4.8 ABDO Griffiths, A. (2003) Practical Dispensing, Chapter 17. ABDO Brooks, C (2013) System for Ophthalmic Dispensing. Butterworth and Heinemann Tunnacliffe, A. (2007) Essentials Of Dispensing, OPALs page 57-62. ABDO Griffiths, A. (2003) Practical Dispensing, Chapter 15. ABDO Freeman, S (2014) OMB considerations in Dispensing. Dispensing Optics. ABDO