LENSOMETER---7.pptx

this gives an elaboration on the principle, different types of lensometers, their uses and places where these are useful.

1. LENSOMETER LAVANYA KALIKIVAYI, M.Opt., FLVPEI., FIACLE., (PhD in process) Associate Professor, Ahalia School of Optometry & Research Centre

2. Lensometry Definition “Lenso” = lens “metry” = measurement of

3. Manual Lensometer :- Manual lensometer gives the accurate power of a lens and were used in optical industries. A manual lensometer is portable and can be carried anywhere But a person needs to have a better idea to measure the power of a lens. (specially in case of a toric lens) Automated Lensometer :- It is a fully automatic well programmed device mostly used in clinics. It is easy and faster and can print prescription. It is less accurate when compared to manual lensometer .

4. Lensometer Purpose Neutralizing a pair of glasses To determine the prescription Verifying a pair of glasses To confirm the accuracy of fabricated glasses Duplicating a pair of glasses To determine the prescription, and the lab duplicates the exact prescription

5. Lensometer It is used to measure the back vertex power or front vertex power of the lens

6. Lensometer To find the back vertex power, place the concave side of lens against lens stop

7. Lensometer To find the front vertex power, place the convex side of lens against lens stop

8. Lensometer In the case that the lens is a sphero -cylindrical prescription, the lensometer is used to determine the cylinder axis It is used to locate the optical center of the lens The lensometer is used to measure the amount of prism in the lens

9. Lensometer Systems Observation system (Keplerian telescope with two plus lenses, an inverted target image) Objective lens Eyepiece (ocular lens) Reticle (concentric circles and cross hairs focused by eyepiece)

10. Lensometer Schematic

11. Observation System The Keplerian telescope consists of an objective lens, an eyepiece, and a reticle The two plus lenses are positioned so that their two focal points coincide with each other The unknown lens whose power is to be measured or neutralized is positioned at the lens stop (the location of the secondary focal plane of the standard lens)

12. Lensometer Operation With the instrument set at zero, an illuminated target (light source) is positioned at the focal length of a plus lens (standard lens) usually a +20.00D lens Diverging rays of light from the illuminated target are bent by the standard lens and parallel light emerges from focusing system into the observation system, which is viewed through the telescope by the observer

13. Lensometer Operation When the lens of unknown power is introduced, the image of the illuminated target is thrown out of focus

14. Lensometer Operation The target is movable By moving the target closer to or farther from the standard lens, the refractive power of the unknown lens can be neutralized Closer to standard lens for plus lens neutralization Farther from standard lens for minus lens neutralization

15. Lensometer Operation The physical distance forward or backward that the target moves indicates the power of unknown lens for the meridian being measured

16. eyepiece Lens holder Lens stop Axis wheel Power wheel Lens stage Risley prism Lensometer Anatomy

17. Adjustment of Eye piece:- It is very important to focus the eye piece so that the observer’s eye is relaxed and to avoid errors . In adjusting the eye piece the blurred black protractor is focused . At first the eye piece is rotated completely Anti-clock wise, the protractor views blurred . Then slowly rotate the eye piece clock wise and stop at once where it is sharp and clear . Now the eye piece is set and focused . Blurred protractor Focused protractor

18. Placing the spectacles lens on the device with its front surface facing towards the eye piece. Placing the spectacle lenses on the device. There are 2 knobs on the lensometer :- Lens stop (holds the lens/frame in place ) Frame leveling knob (makes sure that the frame is leveled and helps in accurate axis measurement )

19. Target It is green in colour and appears when the device Is switch on. It shows the position of the optical center of the lens. There is a ring of round dots at the centre of the target. This represents the power orientation of the lens as it rotates with the lens rotation These round dots become small lines oriented in one direction in case of a toric lens

20. • Move the lens such that the target is exactly at the centre. •Rotate the power drum until target is clear and sharp (all the dots at the center should be separate & sharp ) •Stop rotating the power drum when the target is sharp for the first time. •Always count the upper mark on the power scale for accuracy of the spherical power of the lens. Analyzing the Spherical lenses

21. Analyzing the Cylindrical lens The process of Neutralizing a toric lens when compared to spherical lens is completely a different concept. However , with the help of few tips we will be able to measure the lens power very fast and accurately. The key and the most important thing to remember is the central orientation of the dots.

22. Process of toric lens neutralization :- • Step 1: Move the lens so that the target is at the center of the protractor. •Step 2: Rotate the power drum so that the central lines (oriented in one direction) are sharp. •Step 3: Rotate the axis wheel so that one meridian of the target is parallel to the orientation of the central lines. Note that only one meridian of the target will be sharp •Step 4: Rotate the power drum so that the opposite meridian in the target is sharp. Note that the central lines will change in direction and will be oriented in the opposite direction.

23. Step 1:- • Move the lens so that the target is at the centre of the protractor. •This gives the optical centre location.

24. Step 2:- •Rotate the power drum so that the central lines (oriented in one direction) are sharp

25. Step 3 :- Rotate the axis wheel so that one meridian of the target is parallel to the orientation of the central lines. Note that only one meridian of the target will be sharp. As the 180° meridian is sharp in this picture, we will write the power and the axis as 180° Ex : ---DC X 180s°

26. Rotate the power drum so that the opposite meridian in the target is sharp. Note that the central lines will change in direction and will be oriented in the opposite direction. As the 90° meridian is sharp in this picture , we will write the power and the axis as 90° Ex : ----DC X 90 ° Writing the power • As the Lensometer gives the readings of a toric lens power in 2 cylinder format, we need to transpose the power and write in sphero – cylindrical format • The power which we got in the example shall be written as: -----DS/----DC X 180 °

27. Important points to remember :- Always focus the eyepiece before using the instrument Concentrate on the central circular orientation of dots (especially in case of toric lenses) One meridian of the target should always be parallel to the orientation of central small lines (in toric lens) Each dot at the center will not be round, instead they will be small lines oriented in one particular meridian Except the protractor, everything will be blur when a lens is placed The target moves with the movement of the lens Instrument should not be used more on battery

28. Automated Lensometer

29. Automated Lensometer offers a fast and easy way to accurately measure all lenses including: Single Vision, Bi-focal, Progressive, and Prism . Automatic measurements can be taken quickly by simply positioning the lens on the lens table. Progressive lenses are recognized and measured automatically, with the rate of addition and diopter change graphically displayed . Features:- UV transmittance measurement Pupillary distance measurement Measurement of soft and hard contact lenses Space saving vertical orientation Full color tilt-screen for easy viewing while standing or seated Easily accessible menu to customize your settings Data output options including a built-in printer and USB ports Can be configured to interface with EMR systems

30. LENS GAUGE OR LENS CLOCK A  lens clock  is a mechanical dial indicator that is used to measure  dioptric power of a lens . A lens clock measures the curvature of a surface, but gives the result as an optical power in  diopters , assuming the lens is made of a material with a particular refractive index .   It is called as Geneva Lens Gauge, Lens measure or Lens Clock

31. GENEVA LENS CLOCK TO MEASURE THE BASE CURVE OF THE SPECTACLE LENS

32. Appearance The lens clock has 3 legs            The 2 outside legs do not move         The center one moves in and out        The difference in height position of the center leg and the 2 outside leg is sag for the arc of a circle Doesn’t show the actual sag measurement but shows the dioptric value for the surface power.

33. Using The Lens Clock            Place the clock on a flat surface, so that all 3 pins are equal, your clock should measure zero– If not, your lens clock is defective         The lens clock must be held perpendicular to the surface of the lens        Tilting the clock by 10° from the perpendicular, can create as much as 2 diopters of error in your reading.

34. Base Curve Determination         Defined as the beginning curve upon which the net power is based.          The lens clock can be used to measure this.          Modern lenses have spherical front surfaces (F1)– The base curve will be the lens clock reading  of the front surface of the lens.          Back surface is called (F2 ).            When measuring the F1 of the lens, you will need to read the black scale.          When measuring the F2 of the lens, you will need to read the red scale.            If there are more than 1 curve on the front surface, the lens is either warped or is a plus-cylinder lens form.          The base curve is the least curved of the 2 readings.

35. Index          The lens clock is designed for materials where n = 1.53 (crown glass ).          Measuring a lens where n > 1.53 – The lens clock will read too LOW.          Measuring a lens where n < 1.53 – The lens clock will read too HIGH.

36. Nominal Power of a Lens Since the lens clock directly measures the surface values of a lens, we can use it to approximate the power of lenses. – Only works for materials with index of refractions close to 1.53.          F1 measures +6.00D          F2 measures -4.00D          Ft = +2.00D (Power)    

37. Power Determination The lens clock can be used to measure sphere and cylinder power. 1. Hold the lens clock so that the center leg is at the center of the lens and perpendicular to the lens surface. 2. Rotate the lens clock around the center leg. 3. If the needle on the lens clock remains unchanged, the surface is spherical. 4. If the needle shows a change in value, the surface is toric with 2 separate curves. 5. Read the maximum and minimum values. (The orientation of the three legs where the maximum and minimum readings are will correspond to the major meridians of lens power)  

38.     Modern lenses are of Minus cylinder form . So while measuring the power of the lens you might find cylinder in F2 and the F1 will always be SPHERICAL   1.       When rotating the lens clock on the front surface of a lens, all meridians read +4.00D. On the back surface, the clock reads -6.25D Then the power will be (+ 4.00) + (-6.25) = -2.25D 2.       When rotating the lens clock on the front surface of a lens, all meridians read +6.50D. On the back surface, the clock reads -7.50D when the 3 legs are along the 180° meridian, and -6.00D when the 3 legs are along the 90°meridian.  Here you can calculate the nominal power of the lens if the lens made in minus cylinder form as given below.       + 6.50 +6.50 -7.50 -6.00 = + 0.50 -1.00 F2 +0.50/-1.50cylx90