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There's More to Refraction Than

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There's More to Refraction Than There’s More To Refraction Than “Better One or Two” Light travels in waves like ripples on a pond. The wave front consists of rays perpendicular to its surface. When light passes through an interface (glass, water, etc), the light rays are bent. This is called refraction. Refraction is bending light rays and Points Regarding Vergence altering their vergence! * The curvature of the wave front depends Examples: on its radius. The further from its origin, the A wave 1 meter away has a vergence less the curvature. Curvature determines of 1D. Vergence at 0.5 meter is 2D. vergence. Vergence at 2 meters is 0.5D. * Vergence is measured in Diopters. * Curvature (vergence) varies inversely with the distance… shorter the distance, + Diopters = convergence greater the vergence. - Diopters = divergence Think about it …….. IF a patient is The ability of a farsighted (hyperopic), substance to slow the light rays are and bend rays of projected behind the light is described as eye. You need to use its refractive index. plus lenses to The refractive index of any medium will converge the rays be greater than 1.0. The greater the medium forward onto the retina. slows light, the higher its refractive index. eyecaretyler.com mrziegler.com 1 Divergence Light rays Natural light strike a emits rays that are surface at an divergent and have angle. negative vergence. When the ray passes through the interface it has a new angle (angle of refraction). gcsescience.com faxswitch.com Refractive Errors Convergence “Ametropia” Refractive errors are discussed in terms of “far points” Far Point: that point in space which is conjugate with the fovea when accommodation is relaxed. IF you locate the far point, you will It is called positive vergence when a lens know the power of the lens. EX: IF rays landing on the fovea come from converges light rays. infinity ( ∞ ), the patient is emmetropic (no error). IF they fall elsewhere, they are ametropic ! Where Else Could They Fall? Emmetropia Hyperopia: focuses behind Absence of refractive error. the eye Parallel rays of light (from infinity) are Myopia: focuses in the focused on the fovea. vitreous area No lenses are needed so we consider the Astigmatism: patient plano. two focal points drugs.com lincolnvisioncenter.com 2 Spherical Ametropias Ametropias can occur because of: Refractive surfaces of the eye curves a. variations in the axial length equally in all meridians then the eye has a of the eye spherical refraction. The light is b. shifts in the lens location brought to a focus point by the cornea and the lens. The cornea’s curvature is c. errors in the curvature of ideally matched to the the cornea or lens length of the normal eye. d. any of the above There is no astigmatism that needs to be Lenses are needed to correct these. corrected for. shorelinevision.com Hyperopia Myopia “Farsighted” “Nearsighted” Deficient refractive power – Excessive refractive not enough positive or power where parallel plus power. The cornea is rays are refracted too not curved enough or much and are focused in the eye is too long. front of the retina. The Rays are being focused behind the eye at a point cornea is too curved or the eye is too long. beyond infinity. Plus The far point is located somewhere between lenses will converge the infinity and the eye. Minus lenses diverge rays on the retina. the rays backward onto the retina. shorelinevision.com Aspherical Ametropias In the average person, the cornea does not Light is focused in (2) have the same refracting surfaces in all meridians differently. meridians. When this Flatter 180° refracts the happens, the patient least whereas the steeper 90° refracts the has astigmatism. This most! type of corneal Each of these meridians surface is called applies a different toric (shaped like a vergence so there are two football) principal focal points. researchcenters.info 3 “Sins When Performing Refraction vs.. Refractometry Refractometry” Refraction: The sum of the subjective When I was learning refractometry, I was told the number one sin and objective steps that was overminusing a patient. leads to a decision by the I actually believe the number one doctor of a prescription. sin is underplussing a patient, followed closely by overminusing - and in both cases- they occur for Refractometry: Measurement of the this main reason……..listening too refractive error… what much to what the patient “wants” the technicians do and not giving them what they “need”. Underplus Common complaints: Hyperopic eyes are what I call martyr eyes. * I used to love to read but now They need glasses – but their brain doesn’t want the help. So, I am so tired at the end of the the brain works and works to keep day it’s no fun things in focus. They get tired * I used to wear glasses when I and some people even complain of was younger but I outgrew it headaches or upset stomach. * I am 33 years old and need They are not hurting themselves, bifocals to read they are “bugging” themselves. Overminus Myopic eyes are “piggy eyes”. Be careful of listening too They want all the anatomical much to what they “want” . power present and more. They They may “need” +3.50 to don’t need more… they just want correct their hyperopia, but it. their brain might tell IF you give it to them, and they them they only need +1.00. My personal are under 35 y.o. or so, they usually best: gentleman came to our office (5) times don’t have an issue (except for the in a year with the same complaints. 34 y.o. fact you have given too much power). After 35 or so, (5) different techs. He “wanted” to be a they begin to have issues with reading, and will tell you -3.00sp… he “needed” to be a +4.50sp they take their glasses off to read !!!! 4 What is the Answer ?? So, in this case, you are listening too much The best way to know if and doing what the patient “wants” you are overminusing or – and not doing what they need. underplussing someone is the When you are refining, ask retinoscope. The retinoscope will give you them : “ better one, better two the anatomical answer to what the patient’s or about the same”… if you don’t give them an out, they will continue true refraction is. Then you do a minimal to pick more minus because it subjective “one or two”. That way the patient makes it look darker – and to the gets what I see (what they need) and not what brain – darker is better. they want (overminus or under plus) ! Pearls And…..more pearls “If it ain’t broke, don’t fix it” • Don’t give them what they want, give them what they need. IF a patient states they see well, no complaints, and their vision is good (20/30 or better), don’t make • Learn to retinoscope! big changes to their Rx unless they “need” it – my gentleman actually saw 20/25 • Listen to the patients complaint – and then with his wrong Rx – he was just miserable. find the answer for the complaint. For example: original: -1.00+0.75 x 180 ( 20/25-2) • For every 0.25D you give a patient – they should new: -1.75 + 1.25x 21 (20/25+2) When I asked the tech why the big change – the improve one line. answer was ???????????????????? • Patients will tolerate bigger changes to their sphere than they will to their cylinder and axis. Make sure it’s a good change! Refractor Parts of the Phoropter “Phoropter” Three groups of discs… * spheres * cylinders * accessories The lenses are in 0.25D increments ranging from -19.0 to +17.0 and cylinders to 6.0D 5 Vertex Distance is 12mm when the phoropter is in the correct position SubjectiveTesting Patients experience a great deal of anxiety Large dial controls over this testing procedure especially when sphere the examiner goes too quickly or gives the Cylinder controls for patient too many choices…. power and axis “ Better one or two…three or four…. five or six…” Occluder, pinhole Be careful of badgering the patient or appearing to be bored Pupillary distance with the “routine”… and – give Level the patient an out !!!! Order of Refinement “Better one or two – or about the same” !! A. Sphere until they get to 20/40 B. Cylinder axis IF you don’t give them an out, you will get C. Cylinder Power chased around the dial because you are Always go for the most plus (or less minus) asking them for an answer. The goal is to that you can give. Put the chart up on the get them where it is about the same. screen to show 20/200 through 20/20 and Patients get frustrated and will begin to have them read the smallest line they can see. guess at the answer they think you want ! EX: The patient reads 20/60 . Example This is (6) lines away from 20/20. Give them another 0.25 sphere. (6) x 0.25D = 1.50 + whatever cylinder They now read 20/40 fighting. Using the Jackson cross part of After performing the auto refractor or your phoropter, adjust the cylinder axis. retinoscopy - put the prescription in the When the patient states that they are about phoropter. This is your starting point . power the same, adjust the cylinder – In our head we will not want to increase or again 0.25 at a time. And again, looking for decrease that 1.50 unless they can read improvement in the vision measured by another line better.
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