Instruments, ANSI, Lenses

Instruments

Ophthalmoscope
- BIO has a larger FOV and depth of focus, but a smaller mag
  - Shows inverted, real image
- Direct O has a smaller FOV and depth of focus, but is more magnified
  - Shows upright, virtual image
- What light exits the patient’s eye during ophthalmoscopy?
  - For a myope, their lens and cornea has more plus, so converging light exits the eye
  - For a hyperope, their lens and cornea has more minus, so diverging light exits the eye
Lensometer
- Measures back vertex power and prism of the lens
- x = f²Fv
  - x: distance target is moved
  - f: focal length of standard lens (m)
  - Fv: back vertex power of test lens
    - When x is negative, Fv is negative
    - If target has to be moved back, x is negative, so the lens is a minus lens and the patient is a myope
- To check for prism, look at where the cross hairs are in relation to the bullseye
  - Direction of crosshairs in relation to bullseye is direction of base
- To check for add, turn the glasses backwards in the lensometer
  - Measure distance sphere power and near sphere power
  - Distance between the two is the add power
Hand neutralization
- Minus lens: with motion
- Plus lens: against motion
Radiuscope
- Measures radius of curvature of rigid gas permeable lenses
- Clear image will be seen at surface of CL and at radius of curvature
- The distance between the 2 clear image locations is the radius of curvature of the RGP
Keratometer
- Measures radius of curvature of the center of the cornea
  - Looks at cornea as convex mirror and measures the size of the reflected image
- DK indicates dioptric power found using keratometry readings
  - Assumes the n_cornea= 1.3375
  - To convert between corneal power (DK) and corneal radius of curvature:
    - F = 337.5 / r
Lens Clock
- Measures sag of a lens
- Needs to be calibrated for the right n value
  - If not, use: F_L = (n_L-1 / n_LC-1) F_LC
Slit Lamp
- Consists of a Keplerian telescope with an inverting prism
- Galilean telescope is used for extra mag
Fundus Lens
- Creates a real, inverted image of the retina that is used as the object for the slit lamp
- Higher lens powers create less magnification with a larger field of view
  - Using fundus lens creates reverse telescope: M = -(F_eye)/(F_obj)
    - Using this equation, we can find the power of any fundus lens, knowing that the power of the eye is about 60 D
      - For a 60 D lens, M = -(60)/60 = -1X
        Image is minified and inverted
      - For a 78 D lens, M = -(60)/78 = -0.77X
        Higher lens power has less mag
    - The slit lamp magnifies and makes up for the minification of the fundus lens

ANSI

Sphere
- +6.50 D to -6.50 D
  - tolerance of ±0.13 D
- over ± 6.50 D
  - tolerance of 2% sphere power
Cylinder
- up to -2.00 D
  - tolerance of ±0.13 D
- -2.00 to -4.50
  - tolerance of ±0.15 D
- over -4.50 D
  - tolerance of 4% cyl power
Axis
- Cyl under -0.25 D
  - ± 14 degrees
- -0.25 to -0.50
  - ± 7 degrees
- -0.50 to -0.75
  - ± 5 degrees
- -0.75 to -1.50
  - ± 3 degrees
- Cyl over -1.50 D
  - ± 2 degrees
Thickness tolerance
- ± 0.3 mm
Base curve
- ± 0.75 D
Z87.1 Safety Standards
- High mass impact
  - 500 g pointed projectile dropped from 50 inches
- High velocity impact
  - 0.25 in steel ball fired at 150 ft per second

Lenses

Base curves
- Always on the front surface for single vision lenses
  - Sphere lens: BC is the front sphere curve
  - Plus cylinder
    - BC is the flatter front toric curve
    - Cross curve: steeper front toric curve
    - Sphere curve: back curve
  - Minus cylinder (cylinder/toricity is on the back surface of the lens)
    - BC is the front sphere curve
    - Toric BC: flatter back toric curve
    - Cross curve: steeper back toric curve
- BC is always on the back surface for CLs
Lens thickness
- t_e, t_c←→ s₁, s₂ ←→ r₁, r₂ ←→ F₁, F₂
  - Lens thickness can be related to sag
    - For a plus lens, t_c = t_e + s₁ + s₂
    - For a minus lens, t_c = t_e - s₁ - s₂
  - Sag can be related to radius of curvature
    - s = h²/ 2r
      - Remember that sag can be (+) or (-) depending on shape of lens and whether you are measuring front or back surface
        Sag is measured arc to chord (from the C shape out)
        If pointing to the left, sag is (-)
        If pointing to the right, sag is (+)
        For a convex shape, front surface is (+) and back surface is (-)
        For a concave shape, front surface is (-) and back surface is (+)
  - Power can be related to radius of curvature
    - F = n₂-n₁ / r
      - Remember that the n value can be (+) or (-), depending on whether you are measuring front or back surface
        For front surface, n2 is the n of the lens and n1 is the n of air: n2-1
        For back surface, n2 is the n of air and n1 is the n of the lens: 1-n1
Boxing system
- GC (geometric center): vertical halfway point of lens
- A (eye size/lens size): horizontal length of box
- B: vertical length of box
- DBL (bridge size): horizontal distance between lenses
- GCD/frame PD: distance between the geometric centers of each lens
- ED (effective diameter): longest diameter of the lens
- MRP (major reference point): point on the lens where line of sight/visual axis passes
  - Corresponds to optic center if there is no prism
- Decentration per lens (D): frame PD-wearers PD / 2
  - Wearer’s PD: distance between pupils
  - Frame PD: GCD of frame
- Minimum blank size: ED + 2(decentration) + 2 mm
Multifocals
- Distance between top of segment and optic center (where eye is)
  - Flat top (28 mm or less): 5 mm
  - Flat top (35 mm): 4.5 mm
  - Flat top (larger than 35 mm): 0 mm (OC at the seg line)
  - Franklin/Executive: 0 mm (OC at the seg line)
  - Round (Kryptok): distance between top of seg and OC is r (radius of the seg)
  - Curve-top, Panoptic, Ribbon-B: 4.5 mm
  - Ribbon-R: 7 mm
- Progressives
  - Hard design: short corridor, high add power
  - Soft design: long corridor, low add power
- Trifocals have an intermediate add that is half the add power of the near add
- Seg terminology
  - Seg width: horizontal seg diameter
  - Seg depth: vertical seg diameter (goes to the bottom of the seg and not the bottom of the lens)
  - Seg height: top of seg to lowest point of lens
  - Seg drop: vertical distance between MRP (where eyes are) and top of seg
- Inset
  - Inset: distance from GC (center of lens) to MRP (where eyes are)
    - Used for single vision and distance vision
    - Inset: (frame PD-distance PD) / 2
  - Seg Inset: distance from MRP (where eyes are) to center of seg
    - Used for seg and near vision
    - Seg Inset: (distance PD-near PD) / 2
  - Total Inset: distance from GC (center of lens) to center of seg
    - Total Inset: Inset + Seg Inset
    - Total Inset: (frame PD-near PD) / 2
Adjustments
- Adjusting seg height
  - Seg is too high:
    - Increase pantoscopic tilt
    - Decrease vertex distance
    - Spread nose pads
    - Raise nose pad arms
    - Stretch bridge
  - Seg is too low:
    - Decrease pantoscopic tolt
    - Increase vertex distance
    - Narrow nose pads
    - Lower nose pad arms
    - Shrink bridge
- Glasses falling down the nose
  - Pull in temples
  - Bend down temple tips
  - Pull in nose pads to tighten fit
- One lens is closer to the face than the other
  - Straighten temples
- Glasses touch cheek
  - Reduce pantoscopic tilt
  - Narrow bridge or pads to raise frame and/or increase vertex distance
- Glasses too close to face
  - Decrease face form
  - Narrow the pads or shrink the bridge
Lens Materials
- Ophthalmic Crown Glass
  - n: 1.523
  - Abbe: 58.9
- CR-39
  - n: 1.498
  - Abbe: 58
- Polycarbonate
  - n: 1.586
  - Abbe: 30
- Trivex
  - n: 1.53
  - Abbe: 44