INTRODUCTION TO LENSOMETRY

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1 INTRODUCTION TO LENSOMETRY Susan Brummett, COMT Little Rock, Arkansas January 2007

2 Terminology 1 Lensmeter instrument designed to measure the prescription of an optical lens. Also defined as an instrument to determine the spherical power, cylinder power with axis, and optical center of a lens 2.

3 Mires measurement images viewed by technician. Reticle eyepiece or ocular adjusted for examiner s focusing point. Instrument consists of an ocular for viewing the mires, a flat stage or table for supporting the spectacle frame, a power dial, and an axis wheel 3.

4 Sphere lens with optical power being the same in all meridians (expressed in diopters). Cylinder lens that has different refractive/optical power in each meridian. It is used to correct astigmatism. Axis meridian of cylinder with the minimum power perpendicular to maximum power meridian. Expressed in units of degrees.

5 Steps to Neutralizing Spectacles 2 Adjust the eyepiece Adjustment is critical for obtaining accurate measurement Turn power drum to plano Turn ocular to most PLUS position (counterclockwise) Slowly turn ocular toward minus until the numbers and circles appear sharp and focused. STOP!!!!

6 Check calibration of lensmeter Rotate the diopter dial until all mires come into focus without a lens on the frame table. If the diopter dial reads 0 once all mires are in focus, the lensmeter is properly calibrated.

7 Position spectacles Place glasses on platform so that both lenses are resting on stage with the temples facing away from you (for distance RX) This prevents movement of the lenses and inaccurate reading of the axis

8 Locate optical center of lens Beginning with the right lens (as with most ophthalmic procedures) center the lens on the platform and lock in place with the lens holder. Upon viewing the lens through the ocular, the mires should be centered. If not, possibly have ground-in prism present 3.

9 Neutralizing the lens Mires in lensmeters vary from each manufacturer. The most common format is three thin lines close together positioned at 90 0 from three thicker lines spaced further apart 1. Example of mires in lensmeter: Thick lines Thin lines

10 Neutralize Sphere, Cylinder, and Axis by plus cylinder method Turn the diopter/power drum to high minus power, then slowly rotate the drum toward you until the first set of lines come into focus. If all lines (thin & thick lines) come into focus, the lens is a sphere lens. If either the thin or thick lines come into focus first, the lens is a spherocylindrical lens. If the thick lines come into focus first, rotate the axis wheel so the thin lines are in sharp focus. Record the number on the power drum where the thin lines are in focus as the sphere power. After determining the sphere power, the lines may appear broken but in sharp focus. Adjust the axis wheel until thin lines are complete. Example of focused broken thin lines After rotating axis wheel note all thin lines are complete This gives you the axis degree of the cylinder. Continue rotating the power drum toward you until the 3 thick lines are in focus. Record this power and axis.

11 Recording prescription - Distance Sphere first power drum reading with thin lines focused. Cylinder second power drum reading with thick lines focused. Recorded as difference between first reading and this one. Axis degree reading with thin lines focused. This measurement notates the POWER of the cylinder meridian.

12 Neutralize Add/Bifocal Power Adjust glasses on platform by turning glasses so that temples are toward you and bottom of glasses are positioned for viewing to determine add power. Rule of thumb: For determining add power, turn glasses around to read from front vertex.

13 Bifocal power is determined by rotating the power drum until the thin lines are in focus again. The difference between this number and sphere reading power is the amount of add to the lenses 2.

14 PRISM!! Now, a prism bends light rays toward its base. Meaning.. the image is shifted in that direction. SO if a patient has an eye deviation, prism is used to correct this deviation so that both eyes see the image in the same place. Example: right eye is misaligned temporally; prism placed in what direction to bring the image to the center?

15 Neutralizing prism in the lensometer: Position the glasses on the platform with optical center in the middle as if reading sphere power. When viewing the mires, lines may be offcenter. The rings inside the lensometer are measured at 1 prism diopter. The number of rings from the center of the reticle that the lines are over is the amount of prism in the glasses. Example: Ground in prism measures 2 base up

16 Prism direction: This is determined by the direction of displacement of the intersection of the thin and thick lines. If the position of the lines is toward the nosepiece, it is base-in prism. If the position of the lines is toward the top of the lens, it is base-up prism

17 EXAMPLES: 1 st power drum reading: nd power drum reading: Axis wheel reading: 090 Answer:

18 1 st power drum reading: nd power drum reading: Axis wheel reading: rd power drum reading: Answer:

19 1 st power drum reading: nd power drum reading: Axis wheel reading: rd power drum reading: 0.00 Mires are centered over 2 nd ring towards the nosepiece. Answer:

20 BONUS:::::::: 1 st power drum reading: nd power drum reading: Axis wheel reading: 090 No bifocal Mires centered over 3 rd ring temporally and up 1 ring. ANSWER:

21 References 1. Stein HA et al: The Ophthalmic Assistant: A Guide for Ophthalmic Medical Personnel; 7 th ed. Mosby, Inc Cassin, B: Fundamentals for Ophthalmic Technical Personnel, 1995, WB Saunders. 3. DuBois, L: Clinical Skills for the Ophthalmic Examination; Basic Procedures, 2006, Slack Inc.

PART 3: LENS FORM AND ANALYSIS PRACTICE TEST

PART 3: LENS FORM AND ANALYSIS PRACTICE TEST 1. 2. To determine the power of a thin lens in air, it is necessary to consider: a. front curve and index of refraction b. back curve and index of refraction