VC160 Lens 2 - Final Exam Review Notes

Learning Objectives

• Identify and record information necessary to duplicate lenses covered in class.
• Apply formulas that relate to the function of spectacle lenses.
• Measure and apply appropriate formulas to determine the proper base curve for a given lens Rx.
• Analyze various lens treatments, enhancements, and materials.
• Identify various single vision, multifocal, and specialty lens types.
• Distinguish and describe various lens tints and their uses.
• Describe different lens aberrations and proper methods of limiting their unwanted effects.

Module 1: Lens Formulas

Review Lens 1 Concepts: Properties of Light

• Light can be seen as a particle (photon), wave, or ray.
• Light is a part of the electromagnetic spectrum.
• Visible light occupies a portion of the electromagnetic spectrum, ranging from 500 nm to 600 nm.
• White light can be broken down into its color components, with violet having the shortest wavelength and red having the longest.

Review Lens 1: Properties of Lenses

• Refractive index is the measure of the change in the velocity of light as it travels through a medium.
• Refractive index is represented as “n”.
• The equation to determine refractive index is: $n = \frac{186,000}{velocity \ of \ light \ in \ a \ medium}$
  • Example: The speed of light through water is 139,849 mps. What is the refractive index of water?
    $n = \frac{186,000}{139,849}$
    $n = 1.33$
• Water acts as a lens, bending or refracting light.
• The density of the water determines how much the light will be bent.
• Optical lenses are made from materials that perform the same job as water in refracting light.

Review Lens 1: Prism

• Prism is the basis for all ophthalmic lens designs.
• A prism is described as any transparent wedge-shaped material.
• The thinnest part of a prism is the apex, and the thickest part is the base.
• Prism is represented by the Greek letter Delta ($\triangle$).
• Prism power is measured in Prism Diopters.
• Prisms can appear to move an object in three-dimensional space.
• When light strikes a lens, it is absorbed, reflected, and refracted.
• The image is always displaced toward the apex of the prism.
• The ray leaving the prism is the “emergent ray,” which deviates toward the base.
• A one diopter prism will refract a ray of light one centimeter at a distance of one meter.

Lens 2 Nominal Power Formula

• Lenses are essentially prisms.
• Plus lenses are prisms base to base, converging light into a real image.
• Minus lenses are prisms apex to apex, diverging light into a virtual image.
• The patient’s prescription corrects their refractive error by moving the image onto the retina and addressing the area of least confusion for astigmatic refractive error (axis).

Nominal Power Formula

• Opticians use the Nominal Power Formula to determine the dioptric power required:
  $F<em>1 + F</em>2 = F_N$
  Where:

  • $F_1$ = Front of the Lens or Base Curve
  • $F_2$ = Back of the Lens or Ocular Curve (or curves if cylinder is required)
  • $F_N$ = The patient’s prescription – derived from the refraction

• The first step is to determine the BASE CURVE or $F_1$ of this formula.

  • The base curve is the curve around which the other curves are based.
  • This curve is derived by taking the spherical equivalent (SE) of the lens and adding +6.00D.
  • If the lens is a minus lens, the SE is divided by 2, THEN +6.00D is added.
  • This curve is a sphere and represents the front of the lens.

• The second part is $F_2$ which is the OCULAR side of the lens (facing the eyes). This curve is calculated by subtracting the difference between the Prescription and the Front Base curve.

• Nominal Power Formula Example: What is the nominal power of a lens with a base curve of +10.00 and the ocular curve is -7.00?

  • Answer: +3.00 sphere.
    • $F_1 = +10.00$
    • $F_2 = -7.00$
    • $F_n = +3.00$

Lens 2: Transposition

• Flat transposition is converting a prescription from plus cylinder to minus cylinder and vice versa.
  • Add the cylinder power to the sphere.
  • Change the sign of the cylinder power to the opposite sign.
  • Change the axis by 90 degrees (If greater than 90, then subtract 90. If less than 90, then add 90).
• Example: Transpose into minus cylinder the following prescription Rx: +6.00 +2.00 x 090
  • Step 1: Add the Cylinder to sphere (+6.00 +2.00 = +8.00)
  • Step 2: Change the sign of the cylinder power but keep the power the same (+2.00 = -2.00)
  • Step 3: Change the axis (090 + 90 = 180)
  • Answer: +8.00 -2.00 x180

Lens 2: Decentration

• Light travels through the optical center of the lens unchanged.
• Opticians need to use the measurements from the frame the A, B, and DBL.
• Additionally, the patient’s monocular pupillary distance, near measurements, and fitting height measurements to ensure that the optical center of the lens is positioned in front of the eye relative to its position to the geometric center of the frame.

Lens 2: Horizontal Decentration

• Decentration for distance is done as follows:
  • The patient’s monocular PD is taken ideally with a reflective pupillometer but can be taken with a digital pupillometer and (if in a pinch) the PD stick mm ruler
  • Distance PD is measured from the center of the pupil to the center of the pupil; however, for patients with dark irises, you can use outer limbus to inner limbus
  • The A and DBL measurements are taken from the frame, added together and divided by 2
  • The difference between the Frame PD and the Patients PD is calculated.

Lens 2: Vertical Decentration

• Vertical Decentration is when we measure the pt’s fitting height and then subtract it from ½ the B (also known the datum line).
• This lets us determine if the OC goes above or below the datum line.

Lens 2: Minimum Blank Size

• Once you have the horizontal decentration, we can then use this in addition to the Effective Diameter (ED) plus 2mm for excess to determine the Minimum Blank Size to be pulled for that order.
• Lens Blanks are semi-finished lenses with a base curve cut on the front of the lens and have yet to have their secondary curves ground onto the back.
• These curves can be done with a lens generator in two axis, or digitally using a type of CNC machine where multiple curves are cut into the back of the lens (this adjusts the curves to consider vertex, pantoscopic tilt, and wrap measurements of the frame).
• MBS Formula is as follows: $2(decentration) + ED +2mm$
  • Example: What is the minimum blank size for a frame with an ED of 50 and the decentration is 6?
    • $2(6)+50+2$
    • $12+50+2$
    • MBS = 64

Module 2: Lens Aberrations

Lens 2: Lens Aberrations

• Aberrations are lens anomalies inherent to the lens due to:
  • Density of the material
  • The way the light rays are hitting the lens
  • Size of the lens
  • Strength of the lens
• Chromatic Aberration – the dispersing of white light into its color components of the visible spectrum
  • The amount of chromatic aberration a lens substrate exhibits is given a numerical value (ABBE).
  • Lenses with Higher Abbe exhibit less chromatic aberration compared to lenses with lower abbe values.
  • The human eye has an ABBE value of 45, therefore the eye will not detect chromatism for lenses made from materials that have an Abbe greater or equal to 45.

Lens 2: Lens Aberrations - Examples of Materials with their Abbe Values

• CR-39 – 58
• Crown Glass – 58
• Trivex – 43
• Mid index 1.54 – 47
• Hi index 1.67 – 32
• Polycarbonate - 30
• Ideally, lens makers want to produce lenses with high Abbe values that are close or less than 45 for best optical clarity

Lens 2: Lens Aberrations

• Spherical Aberration – Broad peripheral rays of light focus at different points on the lens than the paraxial rays. The relative size of the pupil blocks out most of this.
• Coma – Broad rays of light pass through a lens obliquely, creates a comet-type image known as comatic flare.
• Radial Astigmatic Error (marginal astigmatism) – result of narrow parallel light rays pass obliquely through a lens, creates a focus in two areas resulting in unwanted astigmatic distortion. This is the greatest cause in the reduction of image quality (eliminated by aspheric/atoric lenses).
• Curvature of Field – Inherent curvature of the image plane and the residual of the lens curvature. Can be reduced by using lenses with higher refractive indices and flatter base curves.
• Distortion:
  • Pincushion – strong plus lenses produce more power on the corners of a square than the sides, giving the impression of a pin cushion as the corners come forward and the center is pushed back.
  • Barrel – strong minus lenses minify the corners of a square, giving the appearance that the center and middle are bulging forward like a barrel.
• Vertex Distance – the distance from the front of the eye to the back of the lens.
  • A plus lens moved closer to the eye LOSES plus power.
  • A minus lens moved closer to the eye GAINS minus power.
    • If the lens is moved AWAY from the eye the OPPOSITE happens!
• Martins Rule of Tilt:
  • For every 2° of pantoscopic tilt, the OC of the lens is moved 1mm UP.
  • It is, therefore, advisable that dispensers choose a frame where the wearer’s eyes are about 5mm within the datum line (1/2 B).

Module 3: Bifocals and Image Jump

Lens 2: Bifocals and Image Jump

• Types of multi-focal lens designs:
  • Bifocal
  • Trifocal
  • Occupational – Bifocal or trifocal with an additional segment at the top of the lens.
  • Progressive
• What is a bifocal?
  • A lens used to correct two or more refractive errors.
  • It is used to correct presbyopia (the loss of near vision).
  • Lower portion of the lens is the segment and is commonly used for the reading correction.
  • The segment contains the ADD power, additional plus power needed for reading.
  • Measured from the lower limbus or lower lid to the deepest part of the frame.
  • The total dioptric power of the segment is the sum of the sphere power for the distance and the add power.
    • Example: +2.00DS ADD +1.50 Total reading for this lens is +3.50
• Trifocal:
  • A distance lens with three separate viewing areas, the ribbon above the segment contains the intermediate, and the segment contains the near.
  • Measured with the top of the segment at the bottom of the pupil to the deepest part of the frame.
  • The intermediate power of a trifocal is ½ the ADD power.
    • If a trifocal near power is +2.50, then the intermediate will be +1.25
• Progressive Lens:
  • A multifocal lens that contains multiple viewing distances that gradually increases in plus power as the wearer looks from distance to near.
  • Measured with the fitting cross over the pupil to the deepest part of the frame.
  • Benefits:
    • There is no residual image jump associated with this type of lens.
    • No restrictive focal lengths
    • No lines
• Image Jump:
  • The inherent prismatic effect created by having two optical centers in a bifocal lens.
  • Since plus power is present in the segment portion of the lens, base down prism is always induced.
  • When the eye moves downward from the primary position (distance) and looks through the segment, the image appears to suddenly “jump” upward toward the apex.
  • The amount of jump is measured in prism diopters and is calculated using Prentice’s Rule.
    • Example: How much image jump is present in a pair of FT28 bifocals (10mm reading level)? Rx: -1.75DS ADD: +2.00 OU P = dD/10 Where:
      • P = Prism power
      • d = distance (10mm)
      • D = Reading Power (+2.00)
        Prism is 2.00D
• SLAB OFF (bi-centric grinding):
  • If a prismatic difference of more than 1.50D or more exists between the right and left eye in the vertical meridian, then slab off is used to neutralize this.
  • A slab off lens is recognizable by the horizontal line that dissects the entire lens, often at the top of the near segment.
  • Applied base up to the MOST MINUS lens.
    • BUMM (base up, most minus)

Module 4: Prism and Prentices Rule

Lens 2: Prentices Rule

• Prism has many applications for lenses.
  • Prism can correct Binocular Vision Dysfunction (BVD) where the eyes are not working together.
    • Prism is helpful for conditions such as strabismus, rectus muscle issues, diplopia, and symptoms from Traumatic Brain Injury.
• Two methods of prescribed prism are Adverse Prism and Therapeutic Prism
  • Adverse Prism – strengthens weak rectus muscles, and the APEX is positioned over the weak rectus muscle, forcing the eye to turn in the direction of the image
  • Therapeutic Prism – prescribed to relieve or resolve the issue rather than solve the root cause, and the BASE of the lens is positioned over the weak rectus muscle, causing the image to be displaced in the SAME direction as the eye.
• Prism is calculated using the formula “Prentice’s Rule”. This formula is as follows:
  $\triangle = \frac{dD}{10}$
  Where:
• $\triangle$ = Prism Diopters
• d = distance in mm
• D = Diopter of the lens
• Horizontal Prism: A common symptom of unwanted horizontal prism in excess is patients perceives flat horizontal objects appearing slanted
• Vertical Prism: Common symptoms of unwanted vertical prism are complaints that the floor appears to bow in or is higher towards the walls than the center of the room.
  • Unwanted base up prism makes the floor appear to bend downward toward the walls (Walking on a hill) (Standing in a bowl)
• Amounting Prism vs. Neutralizing Prism:
  • Amounting Prism (compounding) when the total prism equals the sum of the right and left lens.
    • Vertical – Base Up and Base Down
    • Horizontal – Base In and Base In, OR Base Out and Base Out
  • Neutralizing Prism (cancelling) opposite of compounding prism, the total prism equals the difference between the right and left lens
    • Vertical: Base up and Base up, or Base Down and Base Down
    • Horizontal: Base In and Base Out or Base Out and Base In
• Decentering for Prism
  • Prescribed prism can also be achieved by decentration during the layout process.
  • Prentice’s Rule can be used to determine the correct amount of decentration to achieve the prescribed prism. The following formula can be used: $d=\frac{\triangle(10)}{D}$
    • d= distance needed to move the lens in mm
    • D = Diopter of the lens in the 180 or 90
    • $\triangle$ = Prism

Module 5: Prism and Oblique Cylinders

Lens 2: Oblique Cylinders

• When considering spherocylindrical lenses or lenses with sphere and cylinder power, only a percentage of the total dioptric power is present (in effect) in each meridian.


• Consequently, each meridian has a different focal length


• To determine the dioptric power in each meridian, one of two methods can be used: Using a chart, or using a formula


• Using the chart:

1. To calculate the total dioptric power, find the difference between the prescription's axis and the given meridian
2. Locate the difference on the MDP chart and multiply the prescribed cylinder
3. Algebraically add the amount to the sphere power

Lens 2: Lens Enhancements

• Types of Lens enhancements include:
  • Tints
  • Polarized Filters
  • Anti-reflective coatings
  • Anti-scratch/Hard coatings
  • Oliphobic/Hydrophobic coatings
  • Photochromatic Lenses
• Photochromic Lens:
  • A lens that changes from transparent to dark when exposed to ultraviolet light and/or a change in temperature
    • Not designed to replace sunglasses
    • Intended as a comfort lens for people who need self-adjusting tint
    • Photophobic patients (light sensitivity)
    • Plastic or glass
    • Colored tints
• Tint with UV Filters
  • Gray – even light transmission, comfortable for an extended length of time, true color
  • G-15 – same features/benefits as gray
  • Brown – improves depth perception, increased contrast, filters blue light
  • Amber – excellent driving, skiing, better depth perception, increased contrast, blue light filter
  • Rose – reduces indirect glare
• UV Filters
  • UVA – 320 -380nm Longest
  • UVB – 280-320nm Longer
  • UVC - 200-280nm Long
  • UV Filters will block 100% UVA and UVB
    • UVC is absorbed by the ozone layer
• Blue Light
  • Blue Light 400-475nm
    • Can be just as challenging to ocular tissue as UVR and can contribute to AMD
    • IOLs only filter UVR not Blue Light, therefore filtering blue light can benefit post-op cataract patients
• Mirror Coatings
  • Lens treatment applied to the front of the lens
    • Can be Dielectric Coatings and Reflex
    • Dielectric is most popular due to their unlimited color collage
    • Reflex mirrors are single cosmetic coating in single colors such as gold, silver, and blue
    • Reflex coating are not recommended on sunglasses
• Polarized Lenses
  • Thin sheets of polyvinyl acetate (PVA) in one meridian
    • Channels are created and filled with iodine crystals, which cause them to align in one direction
    • Crystals block light in that direction like a window blind
  • Two methods of incorporating a polarized filter on a lens are:
    • Lamination – placing/sandwiching a polarized filter between two lenses
    • Molding – liquid monomer molded onto the lens
    • Colors avail: Gray, Brown, G-15, Amber
• Anti-Reflective Coatings
  • Used to eliminate specular reflection on the surface of a lens
    • Spin cast
    • Stacked (crucible in a vacuum chamber)
    • The lenses are grouped by substrate
    • Electron beam is used to apply a thin film to the surfaces of the lens
  • Light waves reinforce each other from their source we call this “in-phase wave motion or constructive interference”
  • Anti-Reflection coatings break this up by canceling the relationship known as “out of phase” or “destructive interference”
    • The secondary reflection created by the thickness and different refractive indices of the top layer applied to the substrate
    • A subtle residual reflection is call the “reflex color” is present on most AR coatings
  • Multiple layers can add hydrophobic, oliophobic, and dust resistance to the lens, making it easier to see when the lens needs to be cleaned
    • Can enhance properties of dense lenses such as Polycarbonate and Hi-Index lenses