Ophthalmic Dispensing Theory

OPTOMETRY · SEMESTER 1 Ophthalmic Lenses Lecture 1 Ophthalmic Dispensing Theory START READING NOTES Contents of This Topic Ophthalmic Lenses Lecture 1 To understand the way light behaves for lenses we need to look at the nature of light itself. Note: The greater the amplitude the greater the intensity of the light. The wavelength of light that are visible to the human eye vary in length from 380 to 760nm. Reflection Angle of reflection; Angle at which the ray is reflected. 2. Speed of light and refractive index The medium of less resistance is called rarer and the medium of more resistance is called denser medium. Refractive index =speed of light in vacuum/speed of light in a new substance. 3. Refraction. Snell‘s Law. Given, n =1 (refractive index of air) 4. Dispersion (white light split into its constituent colors). OPHTHALMIC LENSES Ophthalmic Lenses Lecture 1 An understanding of lens optics begins with basic study of the action of a single ray of light and how it is affected when passing into or through a transparent optical surface. INTRODUCTION… To understand the way light behaves for lenses we need to look at the nature of light itself. When light travel it behaves in two ways, Like a wave generated by dropping a rock into a pond. As a particle or photon. THEORY OF LIGHT For our purposes, we can best understand light as a wave. Deflining light wave Note: The greater the amplitude the greater the intensity of the light. As wavelength changes, so does the perceived color of the light. The wavelength of light that are visible to the human eye vary in length from 380 to 760nm. Color of high or largest wavelength is more seen than with the short wavelength. Red (620-760), orange (590-620), yellow (560-590), green (490-560), blue (450-490) and violet (380-450). These are only a small part of the electromagnetic spectrum. The visible spectrum Reflection Is the bouncing back of light rays when interrupted on its path at an angle. Angle of incidence(i) ; The angle at which the light strikes the surface. Measured from a line perpendicular to the reflecting surface at the point of reflection known as the normal to the surface. Properties of light Angle of reflection; Angle at which the ray is reflected. ie i = r 2. Speed of light and refractive index Light is able to travel faster through some material than through others. Travels more faster in a vacuum , because there is nothing to resist than other medias. Speed of light in a vacuum is 3×10*8m/s . The medium of less resistance is called rarer and the medium of more resistance is called denser medium. The amount of resistance to the speed of light that slows it down is called Refractive Index. Note; The more the material slows the passage of light , the higher is its refractive index. Refractive index =speed of light in vacuum/speed of light in a new substance. The refractive index is always greater than one , the speed in the denominator is always slower and smaller number. 3. Refraction. Happens when light strikes a new transparent medium straight , the light slows down , but continues on the same direction. When light strikes a new substance , the change in speed in the new media causes the light to change direction. Ophthalmic Lenses Lecture 1 Note ; when light travel from a rarer media to denser media the direction of the refraction is towards the normal to the surface and vice versa. Snell‘s Law. If the refractive index of both media through which light is travelling are known, the angle of refraction for a given angle of incidence is predictable. Mathematically, nsini=n’sinr Given, n =1 (refractive index of air) n’= 1.523 (refractive index of glass) I = 30degrees r= ? d= ? Example, 4. Dispersion (white light split into its constituent colors). 5. Diffraction 6. Interference 7. polarization. 8. Dual nature (both wave and particles) 9. Absorption and transmission. Other properties of light. Introduction to ophthalmic lenses. NEXT TOPIC →VIEW MODULE NOTESVIEW SEMESTER NOTESALL OPTOMETRY NOTES Need These Notes as PDF? Request a formatted copy for offline study, printing or revision. GET PDF NOTES ON WHATSAPP

OPTOMETRY · SEMESTER 1 Types Of Ophthalmic Lenses Lecture 3 Ophthalmic Dispensing Theory START READING NOTES Contents of This Topic There are two main types of ophthalmic lenses these are] Types Of Ophthalmic Lenses Lecture 3 Optical properties of converging lens] Object beyond 2f – real, inverted , diminished image( btn f and 2f) Correction of hypermetropia Has negative (-) power Always forms a virtual , erect , and diminished image , regardless of object position. Correction of myopia TYPES OF OPHTHALMIC LENSES AND MATERIALS. There are two main types of ophthalmic lenses these are] Convex lens ( converging lens). Concave lens (diverging lens). OPHTHALMIC LENSES Types Of Ophthalmic Lenses Lecture 3 Is thicker at the centre and thinner at the edges. It converges (brings together) parallel rays of light to a point called principal focus. Convex lens Optical properties of converging lens] Has positive (+) power Focal length is positive Converges light rays ‘light is diplaced towards the base when pass through the prism. Object beyond 2f – real, inverted , diminished image( btn f and 2f) Object at 2f – real, inverted , same size. (at 2f) Object btn f and 2f – real, inverted , magnified (beyond 2f) Object at f – image at infinity Object btn f and optical center – virtual , erect , magnified. Image formation on convex lens. Correction of hypermetropia Near vision correction in presbyopia. Used in magnifiers, reading glasses, and optical instruments. Uses of convex lenses Types Of Ophthalmic Lenses Lecture 3 Is the thinner at the center and thicker at the edges. It diverges (spreads out) parallel rays of light, making them appear to come from virtual focus. Concave lens Has negative (-) power Focal length is negative Diverges light rays Optical properties Always forms a virtual , erect , and diminished image , regardless of object position. Image formation on concave lens Correction of myopia Uses of concave lens Key differences OPHTHALMIC LENS MATERIALS ← PREVIOUS TOPICNEXT TOPIC →VIEW MODULE NOTESVIEW SEMESTER NOTESALL OPTOMETRY NOTES Need These Notes as PDF? Request a formatted copy for offline study, printing or revision. GET PDF NOTES ON WHATSAPP

OPTOMETRY · SEMESTER 1 Lens Materials and Their Properties Ophthalmic Dispensing Theory START READING NOTES Contents of This Topic _ LENS MATERIAL & What is an Ophthalmic Lens ? LENS MATERIAL NATURAL NATURAL MEDIA , ° Clear natural crystals comparisons between glass & Glass lenses GLASS LENS MATERIAL DVIRES CROWN |} CROWN GLASS Wy) ¢ R.A- 1.523 ADVANTAGES * Low in costs DISADVANTAGES FLINT GLASS dense flint ADVANTAGE DISADVANTAGE BARIUM CROWN GLASS TYRES Refractive |Abbe Specific HIGH INDEX GLASS PLASTIC LENS MATERIAL CR – 39 * CR stands for Columbia Resin & it is 39% ¢ For years CR- 39 was used without anti ) * CR-39 plastic lenses do not fog up as i, * Refractive index : 1.498 ", ¢ The material is highly impact resistant and characteristics Disadvantages POLYCARBONATE History i] ¢ RL: 1.586 ¢ Very soft material — low scratch resistance ZIESS DURALATE LENSES ° High index plastics Refractive | Abbe value | Specific POLY METHA Properties of PMMA Trivex i, * Refractive index : 1.532 POLARIZED LENS Tinted lens A tinted lens could be available in:- Glass CR-39 Polycarbonat =) ; rT OPTICAL PROPERTIES Abbe number ¢ Abbe number of 60 is considered to have ¢ The amount of prism created together with REPEECTANCE , * The reflectance of the lens surface is , * The higher the refractive index, the greater Refractive Index i, * n= Velocity of light in air ABSORPTION ¢ Absorption of an ophthalmic lens generally Specific gravity Impact resistance Drop ball test for impact SCRATCH RESISTANCE ¢ Lenses may be dipped, or a thin layer of ELECTRICAL PROPERTIES CHEMICAL PROPERTIES THERMAL PROPERTIES * Therefore, a hypothetical ideal lens ¢ Unfortunately, there is no lens material that _ LENS MATERIAL & IT’S . \ PROPERTIES What is an Ophthalmic Lens ? , * It is the portion of the transparent medium bounded by 2 refracting surfaces at least | of which is curved. LENS MATERIAL , * Ophthalmic lens materials means all materials used during manufacturing, i.e. all materials entered into the composition of the basic ophthalmic lens. ji # * There are three types of lens material. } + (1) NATURAL MEDIA * (2) GLASS > * (3)PLASTIC NATURAL MEDIA GLASS PLASTIC NATURAL MEDIA ) * Quartz or rock made out of pure silica was exclusively used property made it ideal for spectacle lens s * Not used in optical instruments since it is doubly refracting material , ° Clear natural crystals of quartz are very rare to find ? « So their use is almost stopped comparisons between glass & plastic Glass lenses Plastic lenses f° Heavy * Light weight Low impact resistant ¢ High impact resistant Scratch resistant * Highly Scratch resistant Cheaper * Costlier 3 piece can’t be use * 3 piece can be use Not proffered for children * good proffered for children Glass lenses Less transmission Aberration is low Higher tendency to fog More affected It not necessary required coating Plastic lenses 2% greater transmission Aberration is high Lower tendency to fog Less affected must necessary required coating GLASS LENS MATERIAL ', * Amorphous compound ¢ Super cooled liquid J * Becomes softer & loses viscosity when heated © * No specific melting point DVIRES ° CROWN GLASS BARIUM CROWN GLASS FLINT GLASS HIGH INDEX GLASS ° 10 CROWN |} GLASS BARIUM HIGH CROWN INDEX GLASS GLASS CROWN GLASS ) * 70 % Silica , 14- 16 % sodium oxide , 11- 13 % calcium oxide and small percentage of potassium , borax , antimony , arsenic ( * Used for single vision. ji * * Distant portion of bifocal, trifocal. ¢ Most widely used in ophthalmic industry. Wy) ¢ R.A- 1.523 ¢ Abbe value – 59 § * Specific gravity — 2.54 * Transparency – 91.6% C ¢ U.V. absorption — 280 nm ADVANTAGES i Highly scratch resistant ¢ Resistant to solvents & temperature fluctuation ‘ ¢ Tinted by vaccum coating © * Good optical qualities ¢ High range curves blanks & addition available , * Available in photochromic sunglass option |, * Low in costs * Produce least amount of chromatic aberration * Blanks available in all sizes DISADVANTAGES ij, * Low impact resistance ¢ Heavier material § * Chips can easily form while edging & handling © ¢ Not appropriate for children & sport wear ¢ U.V. absorption not 100 % (upto 280nm) FLINT GLASS if, * Ingredients:- 45-65% lead oxide 25-45% silica 10% soda + potassium oxide dense flint Refractive |Abbe Specific index value gravity Dense flint | 1.649 33.8 3.90 | Extra 1.69 30.9 4.23 | ADVANTAGE i, * Used in kryptok bifocal * High prescription DISADVANTAGE i, * High dispersion ¢ High specific gravity § * Transparency less than crown glass BARIUM CROWN GLASS , * 25—40 % barium oxide * 30 % Silica Lime , zinc , aluminum , boron , zirconium TYRES LIGHT BARIUM CROWN GLASS DENSE BARIUM CROWN GLASS 23 Refractive |Abbe Specific index value gravity E573) 57.4 S22 1.616 55.1 3.36 24 ADVANTAGE , * High RI with out an appreciable increase in chromatic dispersion DISADVANTAGE i, * Chromatic dispersion more than crown glass HIGH INDEX GLASS ¢ Any lens having Refractive Index (RI) higher than 1.523 in glass is called high index lens * Available in 1.6, 1.7,1.8,& 19) * Contents- high % of Titanium oxide * Transmission- less than 92% * Useful for reducing the thickness for high powered lenses 2 PLASTIC LENS MATERIAL ¢ Organic substances based on molecular frame work of carbon with H, N, O | 4 °* Superior to glass in many aspects except their softness = * They are comparatively more attractive * Can be easily tinted for cosmetic appeal and sun wear PLASTIC LENS MATERIAL ¢Itisa polymeric material of large molecular wt. which can be shaped by flow formed by combining various organic ingredients with inorganic materials such as carbon, hydrogen, ,nitrogen, chlorine & Sulphur. PLASTIC LENS MATERIAL TYPES CR – 39 | * It is

OPTOMETRY · SEMESTER 1 Components Of Ophthalmic Lenses Used In Visual Correction Ophthalmic Dispensing Theory START READING NOTES Contents of This Topic LEARNING OBJECTIVES BIFOCAL LENS DESIGN Description of bifocal lens design Types of Bifocal Lenses Flat-Top (D-shaped) Segments Round Segments Curve-Top and Panoptik Segments Who Can Benefit from Bifocal Lenses? Multitaskers Instructions for new bifocal wearers 2. How to Look Through the Lenses 3. Safety and Movement 4. Adjusting Habits EFFECTIVE POWER Components Of Ophthalmic Lenses Used In Visual Correction Key Factors Affecting Effective Power LENS POWER AND THICKNESS Lens Thickness OPHTHALMIC PRISMS Parts of ophthalmic prisms How it Works Uses of ophthalmic prisms Therapeutic Uses: LENS TRANSPOSITION How to Transpose a Prescription (Simple Transposition) Example The Toric Transposition Formula Step-by-Step Rules COMPONENTS OF OPHTHALMIC LENSES USED IN VISUAL CORRECTION LEARNING OBJECTIVES At the end of this session student should be able to: Describe Bifocal lens design Explain effective power Describe lens power and thickness Describe ophthalmic prisms Explain lens transposition BIFOCAL LENS DESIGN What Are Bifocal Lenses? Bifocal lenses incorporate two different optical powers within the same lens to correct vision for both distance and near tasks. This dual functionality eliminates the need for switching between multiple pairs of glasses, making them a practical and convenient choice for many people. Description of bifocal lens design Bifocal lenses feature a distance prescription in the upper portion of the lens and a near prescription in the lower segment. This design allows wearers to focus clearly on objects far away or close up without removing or changing their glasses. How Bifocal Lenses Work The lens is divided by a visible segment line separating two areas of different lens powers. Above the line, the lens corrects distance vision. Below the line, the lens curvature changes for near-vision tasks such as reading. The optical power transition helps your eyes shift focus smoothly between these two zones. Types of Bifocal Lenses The basic styles include Round segments Flat-top segments Curved-top segments Panoptik Segments Executive-style segments or Franklin Flat-Top (D-shaped) Segments Also called 'half eyes,' these lenses have a D-shaped reading segment at the bottom of the lens. They provide a broad reading area with a clearly defined boundary, popular among those who want clear separation of vision zones and do not mind a visible segment line. Round Segments Round bifocal lenses feature a circular reading segment that tends to be less conspicuous but generally offers a smaller near-vision area. These are favored by individuals who prioritize cosmetics and a less noticeable lens design. Executive Segments Executive bifocals have a full-width horizontal segment at the bottom of the lens that provides a large near viewing area, ideal for extensive reading or detailed work. These lenses tend to be thicker and can be heavier, but they maximize your reading zone Fig. round segment Curve-Top and Panoptik Segments Curve-top segments look similar to fl at tops, except that the upper line is arched, rather than fl at. There is a dis tinct point on either corner. The top of panoptik seg ments are curved as well, but the corners are rounded Who Can Benefit from Bifocal Lenses? Bifocal lenses are primarily intended for those experiencing presbyopia or who require simultaneous correction for distance and near vision. They suit people whose lifestyles demand frequent focusing between different visual ranges. Presbyopia Presbyopia is a normal, age-related change in the eye's lens elasticity that makes close-up vision more difficult. It usually begins in the early to mid-40s and affects nearly everyone, leading to a need for reading magnification. This natural change is not a disease and does not mean your eyesight is failing. Multitaskers Individuals who switch often between near and far visual tasks, such as drivers checking navigation apps or professionals alternating between screen work and face-to-face interaction, benefit from the convenience of bifocals. Whether you are a teacher, office worker, or someone who values efficiency in daily activities, bifocals can reduce the frustration of constantly removing and replacing glasses. Occupational Use Bifocals serve well in occupations requiring integrated viewing distances, including teachers, office workers, technicians, and craftspeople. They allow for quick visual transitions without changing eyewear, improving efficiency and comfort throughout your workday. Instructions for new bifocal wearers 1. Initial Adjustment and Daily Wear Wear them consistently: Wear your new glasses all day to help your brain adapt faster. Do not keep switching back to your old pair. Start at home: Practice wearing them at home, especially while doing simple tasks like watching TV, before driving with them. Give it time: It takes about 1-2 weeks to fully adjust. 2. How to Look Through the Lenses Understand the zones: The top portion is for distance, and the bottom is for reading. Look above or below, not through the line: Avoid looking directly through the dividing line. Move your eyes up and down to choose the correct zone. Turn your head, not just your eyes: When looking around, turn your head toward the object rather than looking through the sides of the lenses. Lower your eyes, not your head: When reading, look down through the bottom section without lowering your head too much. 3. Safety and Movement Be careful on stairs and curbs: When walking down stairs, keep your chin tucked down and look through the top part of the lens to see your feet clearly, otherwise, the ground may look blurry or distorted. Take it slow: Avoid running or walking quickly until you are comfortable with your new depth perception. 4. Adjusting Habits Use proper lighting: Good light reduces eye strain. Position reading materials lower: Hold books or phones slightly lower than you are used to so you are naturally looking through the bottom segment. Keep them clean: Smudges make adjustment harder. 5. When to Seek Help If you are still having trouble after 2–3 weeks, or if you experience intense, persistent headaches, visit your eye care professional to ensure the frames are properly fitted or if a prescription adjustment is needed EFFECTIVE POWER The effective power

OPTOMETRY · SEMESTER 1 Frame Selection Ophthalmic Dispensing Theory START READING NOTES Contents of This Topic WHAT IS FRAME SELECTION FACTORS TO CONSIDER DURING FRAME SELECTION Factors to Consider Before Using the Wearer’s Old Frames Frame Selection COSMETIC CONSIDERATIONS FRAME SHAPE AND FACE SHAPE Facial Types 5. Triangular—the lower part of the face is wider than the upper part The oval face is considered normal and can wear almost any frame, so only the general rules apply. FRAME COLOR Key factors in frame color Hair Color Eye Color Personal Style & Goal FITTING CONSIDERATIONS The Bridge The Significant Nasal Angles for Fitting Frontal angle Splay angle The Crest Angle FRAME SELECTION WHAT IS FRAME SELECTION Frame selection is the process of choosing eyeglass frames that best suit your facial features, prescription needs, lifestyle, and personal style, often involving matching frame shapes to face shapes (e.g., angular frames for round faces) for balance, ensuring proper fit and comfort, and considering color and material for aesthetics and function. FACTORS TO CONSIDER DURING FRAME SELECTION USING THE WEARER’S OLD FRAME Sometimes a person wants to use their old frames instead of selecting something new. This may or may not be appropriate. There are a number of valid reasons for wanting to use the old frame and not purchase a new one. These include cost, comfort of the old frames, and sometimes the inability of the wearer to look in the mirror with any other frame and still have what they see look right to them. Factors to Consider Before Using the Wearer’s Old Frames Stress on the frame. Putting new lenses in an old frame may involve putting additional stress on the frame, Older frames may not withstand that stress very well. It is hard to predict how long an old frame will last. Will it last the life of the new lens prescription? If the frame breaks, it is not a simple task to find another frame into which those new lenses will fit. Frame Selection Availability of spare parts. If the old frame needs repairing in the future, will there be parts available? A used frame may already be discontinued. Emergency backup of old frame. Usually people keep their old glasses as a backup spare pair in case they lose or break their new pair. Using the old frames eliminates the emergency backup Frame Selection Are the old frames out of style or nearly out of style? If they are nearly out of style, what will these older frames look like by the time the wearer is ready for the next prescription change? COSMETIC CONSIDERATIONS Cosmetic frame selection involves balancing face shape, skin tone, and features like eyebrows, focusing on contrast (angular frames for round faces, softer frames for square faces) and complementary colors (warm tones for warm skin, cool tones for cool skin) to enhance features, while ensuring proper fit and size for overall harmony. FRAME SHAPE AND FACE SHAPE Since frames are exceedingly obvious on the face, their shape tends to emphasize or deemphasize characteristics of the face. A good frame selection can be simplified by considering first which facial lines are complimentary to the person. Because a hairstyle can also alter the apparent shape of the face, frames are generally chosen to compliment the face as it appears with the hairstyle being worn at the time of frame selection Facial Types Generally, there are seven basic facial shapes: 1. Oval—considered to be the ideal type 2. Oblong—thinner and longer than usual, with the sides of the head being more parallel to one another than in the oval type 3. Round—more circular than the oval 4. Square—again, the sides of the face are more parallel than in the oval, with the face being wider and shorter than usual 5. Triangular—the lower part of the face is wider than the upper part 6. Inverted triangular—the upper part of the face, the temple area, is wider than the lower jaw area 7. Diamond—the central section of the face is wider, with the upper and lower extremities of the face narrowing down considerably Frame Selection To simplify the face shapes to help choose frame width and depth, the seven shapes can be condensed to the following fi ve shapes. The oval face is considered normal and can wear almost any frame, so only the general rules apply. An oval face is longer than it is wide, with balanced, soft proportions, a gently curved jawline, and cheekbones that are the widest part of the face, resembling an upside-down egg, and is often considered the most versatile face shape for hairstyles and makeup Frame Selection The oblong face is simply referred to as long. Both the round and the square face fall into the category of the wide face. The erect or base-down triangular face is a category that does not lend itself to condensation. For fitting purposes, the diamond face is included in the inverted or base-up triangular classifi ca tion, since these shapes are all fi t in basically the same manner. Using this simplifi ed system, a face may deviate from the normal in four essential ways: it may be either too long, too wide, or too triangular, with the base of the triangle up or down. FRAME COLOR When selecting frame colors, consider your skin's undertone (warm: gold, brown; cool: silver, blue) for harmony, your hair and eye color for contrast or accentuation (e.g., orange for blue eyes, dark frames for light hair), and your personal style (classic vs. bold) to find shades that enhance features, make a statement, or blend in, aiming for a complementary look. Key factors in frame color Skin Tone Warm Undertones (golden, peach): Earthy tones like gold, brown, tortoise, warm greens, and orange. Cool Undertones (pink, blue): Silver, black, blue, purple, cool grays, and jewel tones. Neutral Undertones: Versatile; can wear both warm and cool colors. Hair Color Dark Hair: Bold colors (black, deep blues, rich browns) provide strong contrast; clear or pastel frames

OPTOMETRY · SEMESTER 1 Lens Forms Ophthalmic Dispensing Theory START READING NOTES Contents of This Topic Lens Forms with common forms including Convex (Converging): Both surfaces curve outwards, causing light rays to converge (come together). Types of Lens Forms (Examples) LENS FORMS Lens Forms Lens form refers to the specific curvature of a lens's front and back surfaces, which, along with its material, dictates its optical power and how it bends light.. with common forms including Convex (curves out), Concave (curves in), Plano (flat), and Meniscus (mixed curves) Convex (Converging): Both surfaces curve outwards, causing light rays to converge (come together). Concave (Diverging): Both surfaces curve inwards, causing light rays to spread out (diverge). Plano: One surface is flat (plano), and the other is curved (e.g., plano-convex). Meniscus: Both surfaces are curved, but one is more curved than the other, creating a lens that can converge or diverge light. Types of Lens Forms (Examples) Biconvex/Equiconvex: Both surfaces are convex (outward). Biconcave/Equiconcave: Both surfaces are concave (inward). Plano-convex: One flat, one outward-curving. Plano-concave: One flat, one inward-curving ← PREVIOUS TOPICNEXT TOPIC →VIEW MODULE NOTESVIEW SEMESTER NOTESALL OPTOMETRY NOTES Need These Notes as PDF? Request a formatted copy for offline study, printing or revision. GET PDF NOTES ON WHATSAPP

OPTOMETRY · SEMESTER 1 Lensometer: Measuring Spectacle Lenses Ophthalmic Dispensing Theory START READING NOTES Contents of This Topic FIACLE- AUST Lensometry is lensometer. PARTS OF LENSOMETER 10. Power drum Focus the eyepiece [accommodation and refractive error] In Focus User’s eye becomes part of optical system SPHERICAL LENS Focus the eyepiece Rotate the power wheel until target comes into sharp focus Segment side should be placed against lens stop By centering the target in the crosshair or center circle of the reticule of the lensometer. For Rigid:- For Soft:- Lens is immersed in a small chamber filled with saline Steepness of contact lens in relation to lens stop Target displaced from optical center of instrument (though in sharp focus) E.g. Mires displaced temporally on a scale of 2 Eyepiece not focused Position of lens vertex. Clinical Optics Treatment FIACLE- AUST KCMC- School of optometry LENSOMETRY Lensometry is lensometer. Use to check Spherical lens power Cylindrical lens power Axis Reading add Prism power and direction Optical center of lens Contact lens power Use to mark Actual optical center Required optical center when prism is present Axis direction PARTS OF LENSOMETER Eyepiece Chrome knurled sleeve Prism compensating device knob Lens holder handle Marking device control Gimbal Ink pad Spectacle table lever Spectacle table 10. Power drum 11. Locking lever 12.Prism axis scale 13.Prism compensating device 14.Prism diopter power scale 15. On – off switch 16. Lens stop 17. Cylindrical axis wheel 18. Filter lever 19. Lamp access cover Focus the eyepiece [accommodation and refractive error] Focus the mires Power drum sets on zero If testing a pair of glasses,always check the right lens first SET UP In Focus Out of Focus FOCUSING THE EYEPIECE User’s eye becomes part of optical system Any uncorrected refractive error in eye will affect actual measurement of lens power Also, user can accommodate to bring target into focus Operator uses Lensometer for long periods – fatigue may affect ocular condition Importance of focusing eyepiece -_x000b_ SPHERICAL LENS CYLINDRICAL LENS VIEW OF TARGET Focus the eyepiece Power drum sets on zero Place lens on lens stop with back(concave) surface against lens stop Centre the lens in the lens stop Procedure for spherical lens Rotate the power wheel until target comes into sharp focus Rotate power drum towards + or – till mires are focused Read power of lens from power wheel & record Focus the eyepiece Power drum sets on zero Center the lens First clear the mires in one meridian & record the reading in the same axis Eg:- -2.00×10º Then, clear the mires for other meridian Eg:- -4.25x 100º Then transpose in sphero-cyl form -2.00\-2.25 x 100º PROCEDURE FOR CYLINDERICAL LENS Segment side should be placed against lens stop For back surface segment (E.g. Ultex) – Add = BVP of near portion – BVP of distance portion For front surface segment (E.g. Fused & Executive) – Add = FVP of near portion – FVP of distance portion Procedure for add power By centering the target in the crosshair or center circle of the reticule of the lensometer. MARKING OPTICAL CENTER For Rigid:- Change the lens stop Focus the eyepiece Clean,rinse & dry the lens Center the lens Back(concave)surface against lens stop Obtain clear mire image Record BVP PROCEDURE FOR CONTACT LENS For Soft:- Direct measurement Wet cell method Focus the eyepiece Blot dry with lint free tissue Air dry for 10 sec Place lens with convex side against the lens stop Center the lens FVP is measured Within four mins of removing from bottle – power tends to increase. Direct measurement Lens is immersed in a small chamber filled with saline Entire system is placed on the lens stop of the lensometer Measures BVP or FVP. Multiply by conversion factor – compensation for difference in refractive indices. Not very accurate – varies with refractive index of lens. WET CELL METHOD Steepness of contact lens in relation to lens stop Improper placement of lens on stop. Spherical aberration in contact lens. Sources of error Target displaced from optical center of instrument (though in sharp focus) Amount of displacement = power of prism Measured by graticule (1 unit = 1 Prism D) Direction of displacement = direction of base of prism (telescopic system inverts image) Procedure for prism power & direction E.g. Mires displaced temporally on a scale of 2 Reading is 2 Prism base out (1 unit = 1 Prism D ) Eyepiece not focused Power drum not at zero Improper centration of reticule or target Inspite of no zero error high powers give too plus or too minus readings – lens stop is in incorrect position. Eg:- +20.00+21.00, -20.00  -18.00 Inspite of no zero error high powers read too high or too low – standard lens likely to be incorrect Eg:- +20.00+21.00 , -20.00 -21.00 Sources of error in lensometry Position of lens vertex. Misalignment of marking system . Mental arithmetic errors by the operator. Filter Drum backlash. Clinical Optics Bennett’s- ophthalmic prescription work CLAO Volume-I Optometric instrumentation References…. Treatment Preserving or restoring vision and improving appearance may involve one or more of the following: glasses to aid in focusing and straighten the eye(s) patching to force infants and young children to use and straighten the weaker eye eye drops or ointments as a substitute for patching or glasses or to make glasses more effective surgery to tighten, relax, or reposition eye muscles medication injected into an overactive eye muscle to allow the opposite muscle to straighten the eye vision training (also called eye exercises) ← PREVIOUS TOPICNEXT TOPIC →VIEW MODULE NOTESVIEW SEMESTER NOTESALL OPTOMETRY NOTES Need These Notes as PDF? Request a formatted copy for offline study, printing or revision. GET PDF NOTES ON WHATSAPP

OPTOMETRY · SEMESTER 1 Uses Of Lenses To Correct Ametropia Ophthalmic Dispensing Theory START READING NOTES Contents of This Topic AMETROPIA Types of refractive errors How lenses correct Ametropia Uses Of Lenses To Correct Ametropia USES OF LENSES TO CORRECT AMETROPIA AMETROPIA a refractive condition where light rays are not focused properly onto the retina, resulting in blurred or distorted vision. Types of refractive errors Myopia Hyperopia/hypermetropia Astigmatism Presbyopia How lenses correct Ametropia Lenses correct ametropia by adjusting how light enters the eye, ensuring that the image focuses precisely on the retina, which produces a clear image. The specific type of lens used depends on the nature of the refractive error. Uses Of Lenses To Correct Ametropia Myopia (Nearsightedness): In myopia, the eye is often too long or the cornea is too curved, causing light to focus in front of the retina. Correction: A concave (diverging or minus) lens is used. This lens is thinner in the center and thicker at the edges. It spreads light rays out slightly before they enter the eye, pushing the focal point backward to land directly on the retina Uses Of Lenses To Correct Ametropia Hyperopia (Farsightedness): In hyperopia, the eye is typically too short or the optical power is too weak, causing light to focus at a point behind the retina. Correction: A convex (converging or plus) lens is used. This lens is thicker in the center and thinner at the edges. It bends light rays inward, adding converging power to the eye's optical system, bringing the focus forward onto the retina Uses Of Lenses To Correct Ametropia Astigmatism: This condition results from an irregularly shaped cornea or lens, which causes light to focus on multiple points instead of a single one, leading to blurred or distorted vision at all distances. Correction: Cylindrical or toric lenses are used. These lenses have different powers along different meridians to compensate for the irregular shape and bring the multiple focal points into a single clear focus on the retina Uses Of Lenses To Correct Ametropia Presbyopia (Age-related farsightedness): This is the natural, age-related loss of the eye's ability to focus on near objects. Correction: Multifocal, bifocal, or progressive lenses are used. These lenses have different power zones to provide clear vision at various distances, such as for far and near vision ← PREVIOUS TOPICNEXT TOPIC →VIEW MODULE NOTESVIEW SEMESTER NOTESALL OPTOMETRY NOTES Need These Notes as PDF? Request a formatted copy for offline study, printing or revision. GET PDF NOTES ON WHATSAPP

OPTOMETRY · SEMESTER 1 Additional Study Notes: Prescription Transposition and Induced Prism Ophthalmic Dispensing Theory Additional Study Notes — newly authored explanations and examples. These sections supplement the supplied course material. START READING NOTES Contents of This Topic Learning objectives Sphere, cylinder and axis Transposition: original worked example Prentice’s rule: magnitude and units Applying the calculation in dispensing Learning objectives Transpose a spherocylindrical prescription, identify principal meridian powers and calculate the magnitude of induced prism using consistent units. Sphere, cylinder and axis A prescription S / C × axis specifies a spherical component S and a cylindrical component C. In the cylinder-axis meridian, the cylinder contributes no additional power, so the principal power is S. At 90° to the axis, the principal power is S+C. The axis describes orientation, not an extra power. Transposition: original worked example To transpose: add cylinder to sphere; reverse the sign of the cylinder; rotate the axis by 90°, expressing the result within 1–180°. Example: −2.00 / −1.50 × 180 becomes −3.50 / +1.50 × 90. The two forms describe the same optical correction. The original principal powers are −2.00 D at 180° and −3.50 D at 90°; the transposed prescription gives the same pair. Spherical equivalent is S+C/2. For this example it is −2.75 D. Spherical equivalent does not preserve the two principal powers and is not automatically a suitable substitute for the full correction. Prentice’s rule: magnitude and units For a thin lens, the magnitude of prism at a point away from its optical centre is P = cF, using c in centimetres and the relevant meridian power F in dioptres. Express the magnitude in prism dioptres (Δ); determine base direction separately from lens sign and point of gaze. Original example: a point 4 mm from the optical centre of a +5.00 D spherical lens has c = 0.4 cm and prism magnitude 0.4 × 5 = 2Δ. Using 4 instead of 0.4 would produce a tenfold error. A plus lens behaves as prisms with bases toward its optical centre; a minus lens behaves as prisms with bases away from it. For a spherocylinder, use power in the meridian of displacement rather than always using the sphere. For a principal-meridian displacement, obtain that power directly from the optical cross. Applying the calculation in dispensing Record monocular and binocular measurements with their units and intended viewing condition. Separate an observed measurement from an assumption. A calculation can estimate the effect of decentration, but the final appliance still needs measurement and wearer assessment. Study References Supporting source: System for Ophthalmic Dispensing, supplied in the module Supporting source: Geometrical and Visual Optics, supplied in the course External references checked 13 September 2026. Worked numerical examples and teaching activities are original. ← PREVIOUS TOPICVIEW MODULE NOTESVIEW SEMESTER NOTESALL OPTOMETRY NOTES Need These Notes as PDF? Request a formatted copy for offline study, printing or revision. GET PDF NOTES ON WHATSAPP

OPTOMETRY COURSE Ophthalmic Dispensing Theory — Optometry Notes Semester 1 · 9 topics. Source notes and Additional Study Notes are identified by their titles. Ophthalmic Lenses Lecture 1 Types Of Ophthalmic Lenses Lecture 3 Lens Materials and Their Properties Components Of Ophthalmic Lenses Used In Visual Correction Frame Selection Lens Forms Lensometer: Measuring Spectacle Lenses Uses Of Lenses To Correct Ametropia Additional Study Notes: Prescription Transposition and Induced Prism SEMESTER NOTESALL OPTOMETRY NOTES Need These Notes as PDF? Request a formatted copy for offline study, printing or revision. GET PDF NOTES ON WHATSAPP