Explanations for Review Question Answers

Physical Characteristics

1. The lens clock is only accurate when used on a lens surface of the same index of refraction as the lens clock is calibrated for. If the measured lens has a higher index its actual power is greater than the clock reading. If the index of the lens being measured is lower it has an actual power lower than the lens clock reading.
   The relevant formula is: ((nT - 1)/PT) = ((nC - 1)/PC)
2. Decentering the lens in means moving the optical center of the lens toward the bridge. Since a minus lens is thickest at the edge this will create a lens that is thickest on the temporal edge.
3. The lens clock works by measuring the sagittal depth of the surface being measured. The lens clock can be used to estimate or measure surface power so it can "measure" approximate power and cylinder power. It is typically calibrated for an index of 1.53.
4. When a lens is tilted, unwanted astigmatism is created along the line of sight. This is due to oblique astigmatism since the observer is no longer looking along the optic axis but along an oblique axis.
5. For most lens materials, the Abbe value (a measure of dispersion or chromatic aberration) is lower for higher index materials. The lower the Abbe value, the higher the dispersion.
6. The center thickness of a plus lens will increase if the edge thickness is iincreased, it's power is increased, or its base curve is made steeper. Increasing the index will decrease the center thickness because it will create flatter lens surfaces, all other factors being equal.

Optical Characteristics

1. Front Vertex Power is the refractive power of the lens at the front pole of the lens (the point where the optic axis intersects with the front surface). It can be obtained by hand neutralization (if neutralizing lens is held against front surface), lensometry (if front surface of lens is placed against the lens stop), or by calculation (requires knowledge of both surface powers, thickness, and index). It cannot be determined with a lens clock as it cannot provide either thickness or index.
2. Corrected curve is a term used to describe spectacle lenses designed to minimize or eliminate oblique astigmatism and curvature of field.
3. When pantoscopic tilt is added to a spectacle lens it introduces a change in sphere power (increases sphere power) and introduces cylinder power (sign the same as sphere power) and axis 180 degrees. For a plus lens this is an increase in plus sphere power and an introduction of plus cylinder power axis 180 degrees. Increasing pantoscopic tilt will lower the bifocal segment.
4. By increasing the vertex distance of their distance minus Rx they are effectively reducing its power and reducing the accommodative demand. If this makes it easier to see at near then they are becoming presbyopic.
5. Spectacle Magnification (SM) depends on base curve, CT, index, back vertex power, and vertex distance. The relationships are:
   
   • Increase Base Curve: increase SM
   • Increase CT: increase SM
   • Increase Index: decrease SM
   • Increase BVP: increase SM
   • Increase vertex distance: increase SM
6. An AR coating must satisfy two conditions for interference to reduce the reflection: 1) Amplitude (determined by index of coating) and 2) Path (determined by thickness of coating)
7. The optical thickness for an AR coating is 1/4 wavelength. In this case 600nm/4 = 150nm.

Ophthalmic Prism

1. Use the definition of prism diopter: 1 Prism Diopter = 1cm of displacement at 1 meter. In this case, displacement = 5 prism diopters x 0.5 meters = 2.5cm.
2. All references to prisms are from the base of the prism.
3. Prism Power = 10cm / 0.8m = 12.5 prism diopters
4. The induced prism is 0.8 x 3.00 = 2.4 prism diopters base up OD. This is the distance from the optical center in cm times the difference between the lens powers. The base up in front of the OD will create a right hyperphoria which is the same as a left hypophoria. Rember to visualize what happens as an eye is uncovered (it will move toward the apex of the prism). Since the problem is too much BU OD this can be neutralized with BD prism OD. If you use slab off prism it is applied to the most minus (least plus) lens.
5. The right hyperphoria in downgaze is caused by base up prism in front of the right eye. This means the only Rx that will work is the one that has more plus power in the right lens relative to the left lens. Since choice D has less minus in the right lens and both are minus powered, it has relatively more base up prism on the right side.
6. The solution is to reduce the prism induced in downgaze. This can be done with a single vision reading Rx since the patient looks through the optical centers when reading. Lens decentration will help as it reduces the amount of prism (d in Prentice's rule is less). Slab off prism will reduce the base down prism in the more minus lens and will also work.
7. The amount of induced prism is determined using Prentice's rule: prism = distance from optical center x lens power. The distance from optical center is the reading level 10mm or 1cm. The power is the difference in power in the vertical meridian: 4D. So the induced prism is 4 prism diopters base up OD. When the right eye is uncovered, the right eye will look down to regain fixation so it is a right hyperphoria.
8. When bifocal segments are of equal power and height, no additional vertical prism is induced in downgaze. When the segment are of unequal type (dissimilar) the optical center positions are different and vertical prism is induced. Unequal segment heights create prism as the optical centers are at different heights. And compensated segments are bifocal segments that are of similar type, but different seg OC positions to create prism in downgaze.
9. To avoid prism, lenses must be decentered when the frame PD is different than the patient's PD. When the patient's PD is larger than the frame PD the lenses will need to be decentered out. This is true for both plus and minus lenses.
10. Use Prentice's rule to solve this. At a point 5mm to the right of the OC of the right lens (tells us that d is 0.5) the displacement of an object 1.5m away is 4.5cm. Use the definition of prism diopter: 4.5cm / 1.5m = 3 prism diopters. Now apply Prentice's rule: 3 prism diopters / 0.5cm = 6.00D. The image is always displaced toward the apex so the base is to the right. That means this must be a minus meridian.
11. When the patient's lines of sight converge to read at near they will be looking nasal to the centers of the minus lenses. This induces BI prism in front of each eye. In order to maintain single clear vision they will have to diverge their lines of sight which requires negative fusional vergence.
12. The patient's lines of sight will not go through the OC of their lenses. Since their PD is smaller than the frame PD and the location of the OCs, the lenses will induce BO prism. The amount is Prism = dP (where d = 0.5 and P = 2.00DS. The lenses will induce 1.0 prism diopter of BO prism.
13. The frame PD is larger than the patient's PD so with a plus Rx they will get BO prism from each lens. The BO prism will require positive fusional vergence.

Multifocal Lenses

1. Near power and Near Rx are the same thing. The Add is the difference between the Near and Distance powers.
2. Image jump (sometimes called differential displacement) is the prismatic effect of a bifocal segment at the top of the segment. Use Prentice's rule where d is the position of the segment OC relative to the top and P is the add power. The positions of the common segment OCs are:
   
   • Executive: at segment top (d = 0)
   • Round: center of segment (d = 1/2 segment diameter)
   • Flat Top: 5mm down ( d = 0.5)

3. Reading only Rx: add the add power to distance sphere and keep the same cylinder correction.
4. Remember that when a hyperope switches from spectacles to contact lenses the demand for accommodation decreases. The opposite is true for a myope.
5. On a bifocal Rx without prism the segment OCs are inset so that the lines of sight at near are through the bifocal OCs. This means insetting the segments by the difference in distance and near PD. To create prism at near only we can change this inset to create the prism. Since this patient requires BI prism we need to increase the inset. The additional inset is determined with Prentice's rule where d is additional inset. In this case, we want 1 prism diopter of prism in each lens where the segment power is 2.00D. 1 prism diopter = d (2.00). Solve for d = 1 / 2.00 = 0.5cm = 5mm.
6. To create BO prism at near only we need to have less segent inset (or even outset). Use Prentice's rule: 0.50 prism diopter = d x 2.50D. Solve for d = 0.50 / 2.50 = 0.2cm or 2mm. Normally (no prism) the segment would each be inset 1/2 the difference between the distance and near PD (2mm in this case). Total inset is 0 because 2mm inset is offset by 2mm outset.
7. PAL lenses have several reference points. The major reference point (MRP) is the same as the prism reference point (PRP).
8. The total inset of the segment is (Frame PD - Near PD)/2. The Distance OC should be in front of line of sight in primary gaze and the Segment OC should be in front of the line of sight at near.

Anisometropia / Aniseikonia

1. The goal in prescribing a correction for aniseikonia is to reduce the difference in the two retinal image sizes. A review of the spectacle magnification formula will reveal changes that will help such as reducing vertex distance, decreasing center thickness (reducing lens size), and increasing index of refraction. Contact lenses will often help reduce aniseikonic symptoms because the reduction in vertex distance means less maginification.
2. Using Prentice's rule we can determine the vertical imbalance. The difference in power is p (5.00D) and the reading level distance is d (1.0cm). There is more minus in the right lens so slab off prism will be applied to that lens. Since slab off removes base down prism we can think of it as adding base up prism to the right lens.
3. Two design considerations in eikonic lenses design are base curve and center thickness. Increasing (steepening) the base curve will increase spectacle magnification. Increasing center thickness will also increase spectacle magnification. In this case we want to increase the magnification of the right lens and decrease the magnification of the left lens. We can flatten the more plus lens and increase the center thickness of the less plus lens.
4. The shape factor has 3 parameters: base curve, center thickness, and index of refraction. Back vertex power is part of the power factor.
5. Using equal base curves and center thicknesses is one approach to dealing with small amounts of anisometropia. Contact lenses is also often an option. Adding prism will not affect the aniseikonia.
6. The problem is the left lens is creating a larger retinal image than the right lens. We can reduce the magnification of the left lens (flatter base curve and thinner center thickness) or increase the magnification of the right lens (steeper base curve and thicker center thickness).

Absorptive Lenses

1. Total (ultimate) transmission is the product of the individual transmissions of each lens. It will always be less than the lowest transmission.
2. The transmission of light through a lens is 100% minus the reflection at the front surface minus the absorption by the lens minus the reflection at the back surface. In this case the lens is clear so their is no absorption by the lens.
3. AR coatings decrease the reflections and therefore increase the transmission through a lens.
4. Lenses get their color from the wavelenth of light that has maximum transmission through the lens. The lens appears blue because 450nm is blue light.
5. UV radiation is shorter than violet light. This means we need a warm colored tint to absorb it. In this case yellow is the warmest color listed and will absorb most UV radiation.

Considerations for High Powered Lenses

1. The corneal refraction is found using effectivity. For a plus lens the corneal refraction (CL Rx) will always be greater than the spectacle correction. There is only one choice here that is higher (+12.84D).
2. The advantages of aspheric lens design include: thinner and flatter lenses. These will also be lighter in weight. There are some improvements in peripheral optics, but not in central optics.
3. Higher index materials result in thinner and lighter lenses.They also result in flatter base curves. These are the only advantages to higher index materials. In general, they have more chromatic aberration and are more expensive.
4. The ring scotoma created by high plus powered spectacle lenses is a prismatic effect. The angular size of the ring scotoma can be calculated using Prentice's rule.
5. High plus powered spectacle lenses have several potential problems. It is more difficult to control aberrations with lens powers above +7.50D. The high amounts of induced BO prism require high positive fusional demands at near. The use of aspheric lenses is common for high plus spectacle prescriptions. The distortion created by high plus lenses is pincushion distortion not barrel distortion.