front 1 Shorter the radius creates ↑ diameter Steeper the curve creates ↑ saggittal depth Stronger the power because of the PLUS lacrimal lens created Tighter the fit distorts BEFORE the blink A "steeper than K" fit | back 1 Key Concepts |
front 2 Refractive Power of Cornea = 1.376 Refractive Power of Keratometer = 1.3375 Vertex Distance = Diopter/1-(mm-Diopter) Reference Point: 7.50 mm = 45.00 31.0D flatter cornea 61.0D steeper cornea Rule Of Thumb: When the power ↑ by .50D the curve ↓ by .10mm Find mm: 337.5/Diopter ....................... Find Diopter: 337.5/mm To find new power mm X .3775 | back 2 Key Concepts To extend the range of a Keratometer Flat Cornea down to 31D use a -1.00 trial lens Steeper Cornea up to 61D use a +1.25 trial lens |
front 3 Longer the radius creates ↓ diameter Shorter the curve created ↓ saggittal depth Weaker the power because of the MINUS lacrimal lens created Looser the fit distorts AFTER the blink A "flatter than K" fit | back 3 Key Concepts |
front 4 CARSA CA (corneal stig) + RA (risdual astig) = SA (spectale astig) Rigid Fit Rule if CA is more than +3D order a back toric surface if RA is more than +.50D order a front toric surface if BOTH CA is more than +3D and RA is more than .50D order BITOIC rigid lens | back 4 Key Concepts |
front 5 Soft Contact 1/3 Rule Transpose if neccessary (All CL RX are in negative CYL) if astig is ≤ .50D use SPH EQ, (compensate for VerDist if needed) if astig is ≥ 1.50D use toric lens, (compensate for VerDist if needed) if astig is between .50D and 1.50D divide SPH/CYL if ≥ 3 use SPH EQ, (compensate for VerDist if needed) if ≤ 3 use toric lens, (compensate for VerDist if needed) | back 5 spherical equivalent = SPH EQ ( sphere + 1/2 cyl) vertex distance = VerDist |
front 6 WTR - With the Rule = 30° of x90 ATR - Against the Rule = 30° of x180 Oblique - within x30 - x60 or x120 - x150 | back 6 Key Concepts |
front 7 Types of Bifocal Contacts Lenses Annular aka Aspheric Segmented aka Translating Concentric | back 7 Key Concepts |
front 8 Tear Index of Refraction = approx. 1.377 TBUT Test - Tear Break-up Time, tests the QUALITY of tears Schirmer test - tests the QUANITY of tears with a strip of paper in 5min with a good tear collection of 15mm. | back 8 Key Concepts |
front 9 Total Diamer of a Bicurve Lens Base Curve + 2X peripheral curve Example BC = 8.5 Peripheral curve = 1.2 total Diameter = 10.9 | back 9 Key Concepts |
front 10 ANSI = American National Standard Institute RGP............................ SOFT DIAMETER ........................................±.05mm......................±.25mm BASE CURVE....................................±.05mm.......................±.20mm CENTER THICKNESS.....................±.02mm .....................±.03mm 0.00-5.0D .............................................±.12D...........................±.25D 5.12D-10D............................................±.18D...........................±.25D | back 10 Key Concepts |
front 11 FIT EVAUATIONS Proper Fit Keratometry- mires not distorted before or after a blink Blinking Indicator - dips briefly and stabilizes right back Steep Fit - Too Tight Keratometry - mires distorted before blink, clear temporarily, then distort again Blinking Indicator - barely budges across the blink cycle Flat Fit - Too Loose Keratometry - mires intially clear, distort after blink, and clear again Blinking Indicator - jumps after blink, takes time to resettle altering vision | back 11 Key Concepts |
front 12 VERTEX DISTANCE - Calculate if RX is ± 4.0D Diopter / 1 - (mm - Diopter) Rule of Thumb 10-10-1 or 10-5- .50 The effective power of a 10D lense is moved a distance of 10mm will change by approximately 1D. Or the effective power of a 10D lense is moved a distance of 5mm will change approximately .50D. Remember: A minus lens gains power moved closer to the eye. Remember: A plus lense looses power moved closer to the eye. | back 12 Key Concepts |