📖 Optical Quick Reference

Fast lookup for optical terms, formulas, and calculations

⚡ Formulas 🔄 Conversions 📊 Ranges 📚 A-Z Terms

⚡ Common Formulas

Prentice's Rule

Δ = (P × d) / 10

P = lens power (D), d = decentration (mm), Δ = prism diopters

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Spherical Equivalent

SE = Sphere + (Cylinder / 2)

Average refractive power across all meridians

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Vertex Distance

F₂ = F₁ / (1 - d × F₁)

F = power (D), d = distance change (meters)

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Transposition

1. New sphere = old sphere + old cylinder

2. New cylinder = opposite sign, same value

3. New axis = old axis ± 90°

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K Reading to Diopters

D = 337.5 / mm

Convert corneal radius to power. Standard keratometry formula.

Prism Resolution

R = √(H² + V²)

Combine horizontal and vertical prism components

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Effective Diameter

ED = DBC + 2 × Longest Radius

Calculate minimum blank size for lens edging

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Sag Formula

Sag = r - √(r² - y²)

r = radius of curvature, y = semi-diameter

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🔄 Quick Conversions

Diopters ↔ Millimeters

D = 337.5 / mm

7.50 mm	= 45.00 D
7.80 mm	= 43.27 D
8.00 mm	= 42.19 D
8.50 mm	= 39.71 D

Distance → Near PD

Near PD ≈ Distance PD - 3mm

64 mm (distance)	→ 61 mm (near)
62 mm (distance)	→ 59 mm (near)
60 mm (distance)	→ 57 mm (near)

Abbe Value Reference

CR-39	58 (excellent)
Trivex	43 (good)
Polycarbonate	30 (poor)
High-index 1.67	32 (fair)

Refractive Index

CR-39	1.498
Standard plastic	1.50
Mid-index	1.54-1.59
High-index	1.60-1.67
Ultra-high	1.70-1.74

Prism Notation

Base Up (BU)	90°
Base Down (BD)	270°
Base In (BI)	Nasal
Base Out (BO)	Temporal

UV Protection Wavelengths

UVA	315-380 nm
UVB	280-315 nm
UVC	100-280 nm
Blue light	400-500 nm

📊 Normal Ranges & Values

Pupillary Distance (PD)

• Adult average: 62-64 mm
• Adult range: 54-74 mm
• Children: 50-60 mm
• Near PD: Distance PD - 3mm

Corneal Curvature

• Average: 43.00-44.00 D
• Flat cornea: <42.00 D
• Steep cornea: >46.00 D
• Normal astigmatism: 0.50-1.00 D

Contact Lens Specifications

• Soft lens diameter: 14.0-14.5 mm
• RGP diameter: 9.0-10.0 mm
• Soft lens movement: 0.5-1.0 mm
• RGP movement: 1-2 mm

Lens Thickness

• Spectacle center: 2.0-2.5 mm
• Soft CL: 0.05-0.20 mm
• RGP CL: 0.12-0.20 mm
• Min edge: 0.5-1.0 mm (spectacle)

Tear Film

• Thickness: 3-5 microns
• TBUT (normal): 10-15 seconds
• TBUT <7 sec: Dry eye likely
• 3 layers: lipid, aqueous, mucin

Accommodation

• Age 10: ~15 D amplitude
• Age 40: ~5 D amplitude
• Age 50: ~2 D amplitude
• Formula: 15 - (age/4)

Visual Acuity

• Normal: 20/20 (6/6)
• Legal blindness: 20/200 or worse
• Low vision: <20/70
• 20/20 = 1.0 decimal = 0.0 logMAR

Add Power by Age

• Age 40-45: +1.00 to +1.50
• Age 45-50: +1.50 to +2.00
• Age 50-55: +2.00 to +2.50
• Age 55+: +2.50 to +3.00

Oxygen Requirements (Dk/t)

• Daily wear minimum: 24
• Extended wear: 87+
• Overnight wear: 125+
• Corneal need: ~21 units

📚 A-Z Quick Definitions

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

Abbe Value	Measure of chromatic dispersion in lens material. Higher value means less color fringing. CR-39: 58 (excellent), Polycarbonate: 30 (poor).
Aberration	Optical imperfection causing distortion. Types include spherical, chromatic, coma, and astigmatism. Reduced by aspheric designs.
Accommodation	Eye's ability to change focus from far to near by changing crystalline lens shape. Lost with age, causing presbyopia around 40+.
Add Power	Plus power in bifocal/progressive reading zone for near vision. Typical range: +1.00 to +3.00 D, increasing with age.
Against-the-Rule (ATR)	Astigmatism with steepest meridian at 180° ± 30° (horizontal). More common in older patients. Opposite of with-the-rule.
Alignment Fit	RGP lens base curve 0.00-0.50D flatter than flat K. Shows thin, even fluorescein band. Optimal for most patients.
Aniseikonia	Unequal retinal image sizes between eyes. Can cause eyestrain, depth perception issues. Often from high anisometropia or unilateral aphakia.
Anisometropia	Significant refractive difference between two eyes. Generally considered >1.00 D difference. Can cause amblyopia in children.
Anti-Reflective (AR)	Coating that reduces surface reflections on lenses. Improves light transmission by ~8-10%. Essential for high-index materials.
Apex	Thickest point of a prism lens. Base is opposite the apex. Prism notation indicates base direction.
Aphakia	Absence of crystalline lens, usually after cataract surgery. Requires high plus correction (~+12.00 to +14.00 D) or IOL implant.
Aspheric	Non-spherical lens surface with gradually changing curvature. Flatter, thinner profile. Reduces spherical aberration and distortion.
Astigmatism	Refractive error with different powers in two meridians. Requires cylinder correction. Can be regular (correctable) or irregular.
Axis	Direction of cylinder correction, measured 1-180°. Indicates meridian with no cylinder power (perpendicular to cylinder power meridian).

B

Back Vertex Power (BVP)	Lens power measured from back surface. Standard for spectacle prescriptions. Critical for powers >±5.00 D due to vertex distance.
Base Curve	In spectacles: front surface curve. In contact lenses: back surface that sits on cornea. Measured in mm radius or diopters.
Base Direction	Orientation of prism: Base Up (BU/90°), Base Down (BD/270°), Base In (BI/nasal), Base Out (BO/temporal).
Bifocal	Lens with two distinct powers: distance (top) and near (bottom). Common types: D-28, Round 22, Executive, Franklin split.
Binocular PD	Total distance between pupils center to center. Adult average: 62-64mm (range 54-74mm). Used for spectacle lens centering.
Bitoric CL	RGP contact lens with toric curves on both front and back surfaces. Used for high corneal astigmatism with significant refractive astigmatism.
Blank	Uncut lens disc before edging. Must be large enough to accommodate frame shape, decentration, and bevel. See Effective Diameter.
Blue Light	High-energy visible (HEV) light, 400-500nm wavelength. Some evidence of retinal damage. Blue-blocking coatings available on lenses.
Boxing System	Frame measurement using smallest enclosing rectangle. A (horizontal width), B (vertical height), DBL (distance between lenses).
Burton Lamp	Portable UV light source for viewing fluorescein patterns in RGP contact lens fitting. Shows tear layer distribution.

C

Center Thickness (CT)	Lens thickness at optical center. Plus lenses: thick center, thin edge. Minus lenses: thin center, thick edge.
Chromatic Aberration	Color fringing from different wavelengths focusing at different points. Related to Abbe value. Higher Abbe = less chromatic aberration.
Compound Prescription	Prescription containing both sphere and cylinder powers. Corrects for spherical refractive error plus astigmatism.
Concave	Curved inward (minus/diverging lens). Thinner center, thicker edge. Used for myopia correction. Negative focal length.
Convex	Curved outward (plus/converging lens). Thicker center, thinner edge. Used for hyperopia and presbyopia. Positive focal length.
Cornea	Clear front surface of eye with 5 layers. Provides approximately 40 D of power (two-thirds of eye's total refractive power).
Corneal Edema	Corneal swelling from insufficient oxygen supply. Common with low Dk/t contact lenses or overwear. Causes blurred vision and halos.
CR-39	Standard plastic lens material (Columbia Resin #39). Refractive index 1.498, Abbe value 58. Good optical quality, lightweight, affordable.
Cross-Cylinder	Lens with equal plus and minus powers at 90° apart. Used in refraction to refine cylinder axis and power.
Cylinder Power	Astigmatism correction amount in diopters. Can be written in plus or minus notation. Power at 90° to indicated axis.

D

Daily Disposable	Single-use contact lenses worn once and discarded. Healthiest modality option as no cleaning needed. Most convenient, lowest infection risk.
DBL	Distance Between Lenses in boxing system. Bridge size measurement. Frame PD = A dimension + DBL.
Decentration	Horizontal or vertical displacement of lens optical center from pupil. Creates induced prism per Prentice's Rule (Δ = Pd/10).
Diopter (D)	Unit of lens refractive power. Reciprocal of focal length in meters (D = 1/f). Fundamental measurement in optics.
Dk	Oxygen permeability of contact lens material measured in barrer units. Higher = more oxygen transmission. RGP lenses: 100+ Dk.
Dk/t	Oxygen transmissibility including lens thickness (t). Minimum 24 for daily wear, 87 for extended wear, 125 for overnight.
Dominant Eye	Eye preferred for sighting tasks. Determined by sighting tests. Important for monovision contact lens fitting and refractive surgery.
Dry Eye	Insufficient tear production or poor tear quality. Symptoms: burning, redness, fluctuating vision. TBUT <7 seconds indicates dry eye.

E

Edging	Process of cutting lens blank to fit frame shape. Creates bevel for frame retention. Requires accurate frame tracer data.
Effective Diameter (ED)	Minimum lens blank diameter needed. Formula: ED = DBC + 2 × longest radius. Essential for determining blank size.
Emmetropia	Normal vision with no refractive error. Distant objects focus perfectly on retina without accommodation. 20/20 vision typically.
Esophoria	Latent tendency for eyes to turn inward. Detected with cover test. May cause eyestrain. Opposite of exophoria.
Exophoria	Latent tendency for eyes to turn outward. Common in myopes. May require base-in prism if symptomatic. Opposite of esophoria.

F

Flat Fit	RGP lens base curve significantly flatter than corneal curvature. Excessive movement, edge standoff. Poor tear exchange and comfort.
Fluorescein	Orange dye for evaluating RGP lens fit. Viewed under blue/UV light. Shows tear layer thickness between lens and cornea.
Focal Length	Distance from lens to focal point where parallel light converges. Related to power by f = 1/D (meters).
Focal Point	Point where parallel light rays converge after passing through lens. Plus lens: real focal point. Minus lens: virtual focal point.
Fovea	Central retinal area with highest visual acuity. Contains only cone photoreceptors. Approximately 1.5mm diameter, 0° fixation point.
Frame PD	Distance between frame lens geometric centers. Calculated as A + DBL in boxing system. Must match patient PD or require decentration.

G

Geometric Center	Center of smallest rectangle enclosing lens shape. May not coincide with optical center in decentered lenses.
Giant Papillary Conjunctivitis	Contact lens complication with large bumps under upper eyelid. Caused by protein deposits, mechanical irritation. Requires treatment break.

H

High-Index	Lens material with refractive index >1.60. Thinner, lighter lenses. Lower Abbe values (more chromatic aberration). Requires AR coating.
Hyperopia	Farsightedness where distant objects may be clear but near vision blurred. Light focuses behind retina. Corrected with plus lenses.

I

Image Jump	Sudden displacement when eyes drop into bifocal segment. Formula: IJ = h × (Add/100), where h = segment height below OC.
Impact Resistance	Lens ability to withstand impact. Polycarbonate and Trivex offer highest protection. ANSI Z87.1 standard for safety glasses.
Induced Prism	Prism created by decentration from optical center. Calculated using Prentice's Rule: Δ = (P × d) / 10.
Inset	Horizontal decentration of bifocal segment toward nose. Typically 2.5mm per eye for near PD convergence requirements.

J

Jump (Image)

See Image Jump. Visual displacement entering bifocal segment due to prismatic effect of add power.

K

K-Reading (Keratometry)	Corneal curvature measurement in two meridians. Expressed in mm radius or diopters (D = 337.5/mm). Essential for contact lens fitting.
Keratoconus	Progressive corneal thinning and steepening into cone shape. Causes irregular astigmatism. Often requires RGP lenses or corneal crosslinking.

L

Lensometer	Instrument for measuring lens power, axis, prism, and optical center location. Essential for verifying spectacle prescriptions.
Lens Clock	Manual tool measuring lens surface curvature. Three-pin design. Reads in diopters based on standard refractive index (usually 1.53).
Limbus	Border between cornea and sclera. Approximately 12mm diameter. Important landmark for soft contact lens fit assessment.

M

Macula	Central retinal area containing fovea. Approximately 5.5mm diameter. Responsible for detailed central vision. Degeneration causes vision loss.
Meridian	Any plane passing through optical axis of lens. In astigmatism, principal meridians are 90° apart with different powers.
Minimum Blank Size (MBS)	Smallest lens blank diameter needed for frame. Calculated from effective diameter plus decentration. Critical for ordering lenses.
Monocular PD	Distance from center of nose bridge to each pupil separately. Right PD + Left PD = Binocular PD. Essential for accurate centering.
Monovision	Contact lens strategy where one eye corrected for distance, other for near. Alternative to bifocals. Dominant eye usually for distance.
Multifocal	Lens with multiple focal zones for different distances. Includes bifocals, trifocals, and progressives. Available in spectacles and contact lenses.
Myopia	Nearsightedness where close objects clear but distance blurred. Light focuses in front of retina. Corrected with minus (concave) lenses.

N

Near PD	Pupillary distance for near vision. Approximately 3mm less than distance PD due to convergence. Used for bifocal/reading glasses.
Neutralization	Finding lens power by movement assessment with trial lenses or lensometer. Movement indicates residual power; no movement = neutralized.

O

Oblique Astigmatism	Astigmatism with principal meridians not near horizontal/vertical. Axis between 30-60° or 120-150°. Less common than WTR/ATR.
Optical Center (OC)	Point in lens where light passes without deviation (no prism). Should align with pupil unless prescribed prism present.
Orthokeratology	Overnight rigid contact lens wear to temporarily reshape cornea. Provides daytime clear vision without correction. Myopia reduction technique.

P

Pantoscopic Tilt	Forward tilt of spectacle frame bottom. Typical 8-12°. Aligns lens perpendicular to line of sight. Affects effective power slightly.
Peripheral Vision	Vision outside central 30° of fixation. Mediated by rod photoreceptors. Important for mobility, detecting motion. Poor in high minus lenses.
Phoria	Latent eye misalignment when fusion disrupted. Measured with cover test. Exophoria (out), esophoria (in), hyperphoria (up/down).
Photochromic	Lenses that darken in UV light, clear indoors. Transitions is popular brand. Useful for light sensitivity, eliminates need for separate sunglasses.
Plano	Zero power lens (0.00 D). No refractive correction. Used in non-prescription sunglasses or when only one eye needs correction.
Polycarbonate	High-impact lens material. Refractive index 1.586, Abbe 30 (poor optics). Excellent for children, sports, safety glasses. UV blocking.
Prentice's Rule	Calculates induced prism from decentration: Δ = (P × d) / 10. P = lens power (D), d = decentration (mm), Δ = prism diopters.
Presbyopia	Age-related loss of accommodation. Difficulty focusing at near. Typically begins age 40-45. Requires reading add or progressive lenses.
Prism	Wedge-shaped lens that deviates light without focusing. Measured in prism diopters (Δ). Used to correct eye alignment problems.
Prism Diopter (Δ)	Unit of prism power. 1Δ deviates light 1cm at 1 meter distance. Can be horizontal (BI/BO) or vertical (BU/BD).
Progressive Addition Lens	No-line multifocal with gradual power change from distance to near. Requires precise fitting measurements. Adaptation period for some patients.
Pupillary Distance (PD)	Distance between pupils. Adult average 62-64mm (range 54-74mm). Critical measurement for proper lens centering and optical performance.

Q

Quarter-Diopter

0.25 D increment. Standard step for prescription changes. Smallest change typically noticeable to patients in moderate powers.

R

Radius of Curvature	Measurement of surface curve steepness. Expressed in millimeters. Related to power: D = (n-1)/r for single surface.
Refractive Index	Measure of light bending in material. Higher index = thinner lenses. CR-39: 1.498, Poly: 1.586, High-index: 1.60-1.74.
Residual Astigmatism	Astigmatism remaining with spherical contact lens on eye. Due to lenticular (internal) astigmatism. Requires toric contact lens.
Retinoscopy	Objective refraction technique using light reflex from retina. Projects light into eye, observes movement. Starting point for subjective refraction.
RGP Lens	Rigid Gas Permeable contact lens. Excellent optics, high oxygen permeability (Dk 100+). Smaller than soft, requires adaptation period.

S

Sag	Height of lens curve from edge to center. Formula: Sag = r - √(r² - y²). Critical for contact lens fitting and frame clearance.
Segment Height	Vertical distance from bottom of lens to top of bifocal segment. Typical 14-19mm for D-28, 18-22mm for progressives.
Snellen Chart	Standard visual acuity test with letters decreasing in size. 20/20 line standard for normal vision at 20 feet distance.
Spherical Equivalent	Average refractive power: SE = Sphere + (Cylinder/2). Useful for simplified prescriptions and contact lens initial fitting powers.
Sphere Power	Main lens power correcting basic refractive error. Plus (+) for hyperopia, minus (-) for myopia. Same power in all meridians.
Steep Fit	RGP lens base curve too steep for cornea. Central pooling on fluorescein, minimal movement. Can cause corneal edema and discomfort.

T

TBUT	Tear Break-Up Time. Measures tear film stability. Normal: 10-15 seconds. Below 7 seconds indicates dry eye syndrome.
Toric Lens	Lens with different powers in two perpendicular meridians. Corrects astigmatism. Available in spectacles and contact lenses with stabilization.
Transposition	Converting prescription between plus and minus cylinder notation. New sphere = old sphere + cylinder. New axis = old axis ± 90°.
Trifocal	Lens with three focal zones: distance (top), intermediate (middle), near (bottom). Less common now due to progressive lens popularity.
Trivex	Impact-resistant lens material. Refractive index 1.53, Abbe 43 (better than poly). Lighter than poly, excellent for rimless frames.

U

UV Protection

Lens coating/material blocking ultraviolet light (280-380nm). Essential for eye health. Polycarbonate has built-in UV blocking to 400nm.

V

Vergence	Measure of light ray convergence or divergence. Positive (converging), negative (diverging). Fundamental concept in lens power calculations.
Vertex Distance	Distance from back lens surface to cornea. Standard 12-14mm. Affects effective power, especially in high prescriptions (>±4.00 D).
Visual Acuity	Clarity of vision, typically measured with Snellen chart. 20/20 is normal. Numerator = test distance, denominator = letter size.

W

Water Content	Percentage of water in soft contact lens. Higher = more comfortable but less oxygen transmission. Range: 38-75%.
Wrap Angle	Curvature of frame front around face. High wrap provides better peripheral coverage, UV protection. Requires base curve adjustment.
With-the-Rule (WTR)	Astigmatism with steepest meridian at 90° ± 30° (vertical). More common in younger patients. Opposite of against-the-rule.

X

X-Axis

Horizontal meridian in lens notation (180°). One of two principal meridians in astigmatic prescriptions.

Y

Y-Axis

Vertical meridian in lens notation (90°). One of two principal meridians in astigmatic prescriptions.

Z

Z87.1

ANSI standard for impact-resistant safety eyewear. Requires specific testing for basic and high-impact protection. Mark stamped on approved frames.

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