4. Optical Prism (1).pdf

OPHD 2604 Optical Prism Part 1 Chapter 16 6 March 2024 Prisms: A prism causes light to be bent towards the BASE of the prism = the image is then displaced towards the APEX of the prism Prisms: Convex lenses: PLUS Two prism...

OPHD 2604 Optical Prism Part 1 Chapter 16 6 March 2024 Prisms: A prism causes light to be bent towards the BASE of the prism = the image is then displaced towards the APEX of the prism Prisms: Convex lenses: PLUS Two prisms with their bases together Concave lenses: MINUS Two prisms with their apices together Prisms: The prism used to neutralise a phoria/tropia is the opposite direction to the prism used to correct the alignment problem Example: a patient has a exo(phoria/tropia) BO is used to neutralise the deviation (used to get the amount of prism needed) BI prism will be used when prescribing a correction NB: remember – the prism displaces the image toward the APEX of the prism, therefore the apex must be placed in the direction that the eye is Splitting Prisms: Prism is generally prescribed to assist the binocular system If one prism is placed in front of one eye, it will effect the other eye Thus: total prism required for the BV system can be prescribed to one eye only, or split between the eyes ― Prism can be split evenly or unequally between the eyes ― Evenly split due to eye dominance, cosmesis, equalise lens thickness Splitting Prisms: HORIZONTAL: Generally horizontal prism split evenly BUT can be unequal Both will be either BI or BO Example: 5BO = R: 2.5 BO L: 2.5 BO R: 1BO L: 4BO L: 5BO Splitting Prisms: VERTICAL: Also split equal or unequal BUT OPPOSITES: BU in one eye gives BD in the other Example: 6BD OS = R: 3BU L: 3BD R: 2BU L: 4BD R: 6BU Splitting Prisms: Example: 16 HYPERPHORIA RE 16 HYPERPHORIA LE 8 EXOPHORIA R: 8BD L: 8BU R: 8BU L: 8BD R: 4BI L: 4BI Splitting Prisms: Example: Rx OD: -4.25/-0.50x80 OS: 0.00/-2.50x105 RE: 2.5BI 4.5BU LE: 2.5BI 4.5BD ADD: +2.50 RE: 5BI LE: 9BD Splitting Prisms: RE LE BO BO BI Is the same as BI BU BD BD BU Differential Prismatic Effect: Differential Prismatic Effect: The difference between the RE AND LE prismatic effects TOLERANCE IN THE VERTICAL MERIDIAN = 1 Δ D EXAMPLE RE:+5.00DS LE:+2.00DS Both look through a point 1cm above the OC of the lens. What is the prismatic effect produced in each eye? Prentice’s Rule!  Differential Prismatic Effect: EXAMPLE Centimeters! RE:+5.00DS LE:+2.00DS Both look through a point 1cm above the OC of the lens RE P = 1 x 5 = 5 Δ BD. (positive lens, viewing through a point above OC ‫ ؞‬BD) LE P= 1 x 2 = 2 Δ BD. (positive lens, viewing through a point above OC ‫ ؞‬BD)  Differential Prismatic Effect: EXAMPLE (continued): TO WORKOUT DIFFERENTIAL PE, ALWAYS WORK ONE EYE AT A TIME RE = 5 Δ BD LE = 2 Δ BD How many prism diopters is the right eye away from the left eye? 5 Δ BD TO 2 Δ BD = 3BD How many prism diopters is the left eye away from the right eye? 2 Δ BD TO 5 Δ BD = 3BU Differential prismatic effect: RE 3BD OR LE 3BU Differential Prismatic Effect: EXAMPLE 2: What is the differential prismatic effect if: RE: +4.00 DS LE: +7.00DS Px looking through a point 10mm above OC Differential Prismatic Effect: EXAMPLE 2: Right P=cxF=1x4 = 4 ΔBD Left P=cxF =1x7 = 7Δ BD 4 7 ‫ ؞‬DIFFERENTIAL PRISMATIC EFFECT: RE 3BU OR LE 3BD Differential Prismatic Effect: Eliminating differential prismatic effect: 1. Take away all prismatic effect in both eyes Thus each eye has zero PE ‫ ؞‬Differential PE = 0 2. Make the two eyes the same PE By neutralising the differential PE in one eye only Differential Prismatic Effect: Eliminating differential prismatic effect: 1. Take away all prismatic effect in both eyes 2. Make the two eyes the same PE Of the two methods, which is the most appropriate and why? Let’s look at the resultant prismatic effects that will be experienced by the patient When we eliminate all prismatic effect, each eye has zero prismatic effect When we eliminate the differential prismatic effect of the RE → both have 7 Δ BD When we eliminate the differential prismatic effect of the LE→ both have 4 Δ BD In this example: Since in general the tolerance in the vertical meridian is low (1 Δ), we rather eliminate all the prismatic effect Bi-Centric Lenses: We decided to eliminate all prismatic effect by introducing prisms in front of each eye How do we introduce prisms/prismatic effect?? 1. Decentre a lens 2. Literally place a prism into the spectacle lens Bi-Centric Lenses: Bi-centric lenses are formed when one places a prism into the spectacle lens Bi-centric means: ― Bi (two), Centric (centres) ― Meaning: having TWO OPTICAL CENTRES We know that at the optical center of a lens = NO prismatic effect Thus a bi-centric lens has TWO points at which there is no prismatic effect Bi-Centric Lenses: Bi-Centric Lenses: EXAMPLE: Before placing any prism in front of the eye, the patient was looking at a point 1cm above the optical centre When placing the 4 Δ BU, you eliminate all prismatic effect: ― Remember, the 4 Δ BU is placed to eliminate the 4 Δ BD (caused due to the eye looking through another point) ―At the point where the eye is now looking: it has 4 Δ BD and 4 Δ BU at the very same point ―This means the total prismatic effect at that point is ZERO ― Name given to a point where there is no prismatic effect: OPTICAL CENTRE Now, considering the lens already has its own optical centre, and now you have placed this prism and made ANOTHER POINT WITH NO PRISMATIC EFFECT = you have actually created a lens with two centres ‫ ؞‬BI-CENTRIC LENS Compounding and Resolving of Prisms: Compounding and Resolving of Prisms: Compounding – when two prisms are combined in power and base orientation to form one prism that is the equivalent of both COMPOUNDING = PUT TOGETHER Vertical component Horizontal component Resultant is in an oblique meridian Resolving – taking a prism whose base orientation is oblique and expressing it as two prisms orientated perpendicular to one another RESOLVING = BREAKUP Taking the oblique resultant and determining vertical and horizontal components Two Methods: 1. Mathematical 2. Graphical 1. MATHEMATICALLY Compounding Prisms: Compounding – when two prisms are combined in power and base orientation to form one prism that is the equivalent of both Steps involved: 1. Determine the prism amount (Prentice’s Rule) 2. Determine the base direction 3. Calculate the resultant prism amount R² = H² + V² 4. Determine the meridian of the resultant prism SOH CAH TOA 5. Put it all together in a single answer Compounding Prisms: +2.00 + (-4.00) = -2.00 RECAP: Optical Cross Example: +2.00/-4.00x180 +2.00  +2.00 + (-4.00) Compounding Prisms: = -2.00 Example 1: +2.00 Px looking through a point 2mm inwards RE: +2.00/-4.00 x 180 What is the PE? STEP 1 & 2: Right (horizontal) Eye c = 0.20cm F = +2.00DS = 0.2 x 2 = 0.40 ΔBO  +2.00 + (-4.00) Compounding Prisms: = -2.00 Example 1: +2.00 Px looking through a point 2mm down RE: +2.00/-4.00 x 180 What is the PE? STEP 1 & 2: Right (vertical) Eye c = 0.20cm F = -2.00DS = 0.2 x 2 = 0.40 ΔBD Compounding Prisms: Example 2: Px looking through a point 2mm in and 2mm down RE: +2.00/-4.00 x 180 What is the PE? Compounding Prisms: Step 1 & 2: Already done (see previous slides – example 1) Step 3: Resultant Prism amount R² = H² + V² R² = (0.4)² + (0.4)² R = (0.4)² + (0.4)² R = 0.565685… R = 0.57ΔD Compounding Prisms: Step 1 & 2: Prism amount and base direction (0.4BO and 0.4BD) Step 3: Resultant Prism amount (R = 0.57ΔD) Step 4: Prism angle BU x? TOA! BO 0.4 BI tan (x) = opposite/adjacent x tan (x) = 0.4/0.4 0.4 tan (x) = 1 ‫ ؞‬x = tan¯¹ (1) BD x = 45° Compounding Prisms: Step 1 & 2: Prism amount and base direction (0.4BO and 0.4BD) Step 3: Resultant Prism amount (R = 0.57ΔD) Step 4: Prism angle (x = 45°) BU Step 5: Put it all together BO 0.4 BI The resultant prismatic effect is: x 0.4 0.57ΔD BO and BD at 45° BD Compounding Prisms: On a test or exam Compounding Prisms: Example 3: What is the total prismatic effect if the px has in RE 4ΔBO and 3ΔBU? 1. Determine the prism amount (Prentice’s Rule) 2. Determine the base direction 3. Calculate the resultant prism amount R² = H² + V² 4. Determine the meridian of the resultant prism SOH CAH TOA 5. Put it all together in a single answer  Compounding Prisms: Example 3: What is the total prismatic effect if the px has in RE 4ΔBO and 3ΔBU? tan(α) = opp/adj Compounding Prisms: Example 4: Px has a 2BU and 4BI deviation RE. What is the resultant prism? 1. Determine the prism amount (Prentice’s Rule) 2. Determine the base direction 3. Calculate the resultant prism amount R² = H² + V² 4. Determine the meridian of the resultant prism SOH CAH TOA 5. Put it all together in a single answer Compounding Prisms: Example 4: Px has a 2BU and 4BI deviation RE. What is the resultant prism? Compounding Prisms: Example 5: Patient is looking through a point 3mm down and 3mm in from the optic centre. What is the resultant prismatic effect? RE: +2.00DS/ -4.00DC x 180 (10) Use the mathematical method and show ALL WORKING. Resolving Prisms: Resolving - taking a prism whose base orientation is oblique and expressing it as two prisms orientated perpendicular to one another Steps involved: 1. Plot the information you have on a Cartesian plane 2. Determine the different components SOH CAH TOA Resolving Prisms: Example 1: RE: +5.00/-3.00 X 90 RESULTANT PE = 5∆ BU AND BO @ 143,13 What are the vertical and horizontal prismatic effects for this eye?  Example 1: Resolving Prisms: RE: +5.00/-3.00 X 90 RESULTANT PE = 5∆ BU AND BO @ 143,13 OD BU 5Δ D x x = Vertical Meridian (component) = ?? 143.13° CAH BO BI y y = Horizontal Meridian (component) = ?? SOH BD  Resolving Prisms: Example 1: RE: +5.00/-3.00 X 90 RESULTANT PE = 5∆ BU AND BO @ 143,13 BU BO  Resolving Prisms: How can we complicate this even more? By asking to determine the decenteration needed = Determine where the eye is looking in relation to the vertical and horizontal meridian Steps involved: 1. Resolve the PE into V and H 2. Use power cross and get Fh and Fv 3. Use P = cF on H and on V to get c Resolving Prisms: Example 1: RE: +5.00/-3.00 x 90 RESULTANT PE = 5∆ BU AND BO @ 143,13 Exactly where is the eye looking in relation to the vertical and horizontal meridians?  Resolving Prisms: NB: Decentration direction? “c” refers to the position of the OC, thus centration has direction as well Remember to note the direction Draw the lens in each meridian, place the eye on either side of the OC and check what base is created Hint: ― Positive lens: decentration direction is the SAME as the base direction ― Negative lens: decentration direction is the OPPOSITE of the base direction ― UNDERSTAND WHY! Resolving Prisms: 1. Base up 2. Base in 3. Base out 4. Base up  Resolving Prisms: +5.00 Horizontal meridian = 4Δ BO; Vertical meridian = 3Δ BU Rx: +5.00/-3.00 x 90 +5.00 + (-3.00) = +2.00 Pv = cF Ph = cF 3 = c(+5.00) 4 = c(+2.00) c = 2 cm c = 0.6 cm c = 20mm out c = 6mm up Resolving Prisms: Example 2: What is the decentration along the vertical and horizontal meridian to produce a resultant prismatic effect of 8.50Δ BI and BD along 15? OS: -3.75/+5.00 x 90 Resolving Prisms: Resolving Prisms: Example 3: The patient used his spectacles while playing volleyball and it fell. The spectacles are misaligned. The resultant prismatic effect is 4.3ΔD BD and BO @ 165 What are the vertical and horizontal prismatic effect components? LE: +2.75DS/ -4.50DC x 180 (10) What is the decentration along the vertical and horizontal meridians? Use the MATHEMATICAL method and show ALL WORKING. END!