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Single Vision corrects for one distance Multifocal Corrects with two or more prescriptions (or 'powers') Bifocal lens having two focus strengths Trifocal pertaining to having three focus strengths Calculating Prismatic Effects obtained at the center of an optical lens by producing a difference in edge thickness, either by grinding the back surface at an angle to the front surface during lens surfacing or, if the lens has sufficient power, de-centering the optical center of the lens Compounding Prism BI & BI BO & BO BU & BD Cancelling Prism BI & BO BD & BD BU & BU Prentice's Rule P=hcm x D P=amount of prism created h= number of cm OC moved cm= centimeters (convert to mm, divide by 10) D= power in meridian with error Vertical Imbalance The difference in vertical prism power at the reading level in a pair of lenses. OC Height's by lens SV = 1/2 of B Progressive = Fitting cross FT = 5mm above segment Vertex Compensation A plus lens when moved farther from the eye gains in plus power Distance is 0 with CL because they are on the eye Vertex compensation Formula De= Dl/(1 + d xDl) De= diopter effective value of error created D/= lens power (given power) d= amount in METERS lens moved from refracted position to position lens is worn Keep sign for what it is (+/-) - to d means moves away Horizontal Centration PD Vertical Centration OC Horizontal & Vertical Centration Formula Given PD/OC - Human PD/OC= mm moved Frame PD is found by adding A+DBL Power in Oblique Meridians Dt= (sina)²x Dc +Ds Dt= Total Power we need (sina)²= degree we have degree we need Dc= cylinder power Ds= sphere power Lensmeter is an ophthalmic instrument. It is mainly used by optometrists and opticians to verify the correct prescription in a pair of eyeglasses, to properly orient and mark uncut lenses, and to confirm the correct mounting of lenses in spectacle frames. (also known as a focimeter or vertometer) Lens Measurement Devices lens clock, calipers, millimeter ruler, circumference gauge Lens Clock is used to measure the surface powers base curve of a lens Lens Calipers Measures thickness of lenses in mm Millimeter ruler measures interpupillary distance, or seg heights, or OC heights Circumference gauge For quick and accurate measurements of lenses or patterns. Corneal reflex Pupilometer for precisely measuring and digitally recording pupillary distances. Used for measuring of monocular and binocular Hand Tools Screwdrivers, needle-nose pliers, tweezers, files scratchless screwdriver for working on drill mount or facetted lenses pickup screwdriver jaws with spring tension to hold screws while you put them in place, available with either type blade Big Brass screwdriver unscrew most screw, with a large brass handle plastic top screwdriver most popular and most dangerous Pad Popper used to remove snap-in nosepads without changing the alignment of the adjustable guard arms. the prongs are inserted behind the nosepad and in front of the nosepad box. a simple twist removes the pad Rimless mounting tool the hook is started at the hinge side of the nylon line that pulls around the lens. use caution, easy to chip lens eyewire sizing pliers used to check the size of the lens without damaging the barrels of the frame lens axis pliers allow you to turn the lens to adjust the axis while it is mounted in the frame Flat/round Nose Pliers used to change the curvature of pad arm loops. positioned so the loop is bent around the round tip Nylon Jaw flat/round nose pliers or Half-padded pliers they are used to protect the finish of metal frames during adjustments. nylon side should be positioned against the visible part of the frame