Lensometry.

As Seen in clinical setup in nepal.

1. Lensometry BY Anuja dhakal Phase II,NEH

2. What is a lensometer ? A   lensometer  or  lensmeter  is a standard ophthalmic device sometimes referred to as  vertometer or focimeter . The techniques and procedures for operating a lensometer are termed as lensometry or focimetry . The most common use of lensometer is to verify the accurate configuration/prescription in a pair of eye lenses by optometrists and opticians. 

3. “ A focimeter is used to measure the vertex power of a lens.” 

4. Who invented the lensometer ? Antoine Claudet designed the photographometer in 1848.  This device was used for measuring the intensity of photogenic rays. In the following year, he designed the focimeter that was used for finding the correct focus in photographic portraiture. In 1922 the first projection lensometer was patened .

5. What is the lensometer used for? The lensometer helps orient appropriately and mark uncut lenses, verifying the power of single vision, bifocal and trifocal lenses, and checking the right mounting of eye lenses in spectacle frames.  The device is used by the ophthalmologists or optometrists to evaluate certain parameters specified on the patient’s prescription, such as cylinder, axis, sphere, add, and in some cases, prism.  This instrument is often used to verify the accuracy of  progressive lenses .

6. It is also used to determine and mark the center of a lens and several other physical measurements that are essential for the proper functioning of the lens.  At times, the lensometer is used for a prior examination of a patient’s spectacle power to match with the previous prescription that the patient was given.  Measures ranges up to +20D to -20D . Contact lens power measurement.

7. Detection of lens type Possible to detect weather the lens is spherical, Astigmatic,or a Prism. Ways to detect lens type: central peripheral thickness magnification / minification movements

8. Hand neutralization The power of an unknown lens can be determined by neutralising it with another lens of known power.  Neutralisation is based on the fact that if you look at an object through a convex or concave lens and move the lens from side to side (right and left or up and down), the image that you see through the lens will also move.  If the lens has no power - or has been neutralised by placing a lens of equal power but opposite sign against the unknown lens - then there is no image movement.

9. With movement

10. Against movement

11. 1)Spherical Lenses Spherical lenses causes no distortion of the object seen, however when lens is moved from ‘side to side’ and ‘up and down’ it appear to move . Use lenses of opposite movements : Against Movement: Use Concave Lenses. With Movement: Use Convex Lenses.

12. 2)Astigmatic lenses. For a toric lens ,rotational motion is used to find the axis if observe “against motion” use plus cylinder axis. If observe “with motion” use minus cylinder axis.

13. 3)prism A lens with a  ground prism  will displace the cross image as follows : Base up: displaces image downwards Base down: displaces image upwards Base in: displaces image outwards Base out: displaces image inwards

14. Errors in Hand Neutralization Error can be induced in Hand held neutralizing technique such as: It is somewhat inaccurate for curved lenses of more than about 2 diopter power. Error upto 0.50D may be incurred with powerful lenses.

15. Types of lensometer : 1) Manual lensometer 2)Automated lensometer

16. lensometer working principle A lens having a focal length  (f)  is used to image a target (usually a crossed set of lines). The user then places the spectacle lens (under test) at the lens’s rear focal point ( f ). The light rays emerging from the spectacle lens then pass into an eyepiece having an internal reticle . The target is axially shifted simultaneously by the user until it is in focus with the reticle . Then, a condition occurs when the light rays emerging from the test spectacle lens becomes collimated.

17. The formula relates the target position d and the power of the spectacle lens Φ: 

18. DIAGRAMMATIC REPRESENTATION

19. 1)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) 

20. Working principle of manual lensometer . The working of a manual lensometer is grounded on the Badal principle . According to the principle, when we place the eye at the focal point of a positive lens, we can detect an object’s virtual image, between the anterior focal point and the lens subtending the same visual angle.

21. Badal optics The Badal principle is Knapp’s law applied to lensometers .  

22. General description of manual lensometer 1  .Adjustable eyepiece  2  . Reticle   3  . Objective lens  4  . Keplerian telescope  5  . Lens holder  6  .Unknown lens  7  .Standard lens  8  . Illuminated target  9  . Light source  10  . Collimator  11  . Angle adjustment lever  12  . Power drum (+20 and -20 Diopters )  13  .Prism scale knob

23. Parts of a Manual Lensometer 1 . Eyepiece The eyepiece used for obtaining reading accuracy is mounted over a screw-type focusing mechanism. A rubber guard is also placed n the eyepiece to avoid the scratching of the user’s eyewear.  2 . Prism Compensating Device Knob The Prism Compensating Device Knob helps in reading prism amounts that are greater than five prism diopters .  3.Chrome Knurled Sleeve ( Lensometer Reticle Adjustment Knob) The Chrome Knurled Sleeve helps in rotating the lensometer Reticle to align the prism base.  4 . Lens Holder handle The lens holder handle is present to hold a lens in place alongside the aperture.  5 . Spectacle Table Lever The spectacle table lever helps in raising or lowering the level of the spectacle table according to the user.

24. 6 . Spectacle Table The spectacle table provides a resting place for the spectacle frame when the lens power is being neutralized. 7 . Marking device control (Lens Marker) Lens markers are pins controlled by the handle and used to mark the lens at the optical center or the prism reference point. 8 . Power Drum The power drum is a handwheel that has numbered scale readings ranging from +20.00 to -20.00 Diopter . The scale of the reading interval is in 0.12 diopters steps, and for higher powers, the interval is of 0.25 diopters steps. 9 . Prism Axis scale The prism axis scale helps in the orientation of the prism axis. 10 . Prism Compensating Device Prism compensating device helps to verify a large amount of prism. 11.Prism Diopter Power Scale The prism diopter power scale displays the prism amount.

25. 12 . Locking Lever The locking lever helps in elevating or depressing the position of the instrument according to the user’s height or posture. 13.Cylinder Axis Wheel The cylinder axis wheel helps in orienting and neutralizing the cylindrical axis. 14 . Filter Lever The filter lever can incorporate or remove the green filter. 15 . Lamp Access Cover The lamp access cover is used for changing/altering the manual lensometer bulb.

26. Methods to operate manual lensometer  Focus the eyepiece of the lensometer for the examiner’s eye . With the power wheel set on zero, turn the eyepiece as far counter-clockwise as possible. Then slowly turn it clockwise until the reticule first comes into sharp focus. Insert the spectacles . If testing a pair of glasses, always check the right lens first .

27. Place the pair of glasses in the lensometer with the ocular surface away from you The lens is held in place by the lens holder and is held level on the lens table Center the lens by moving it so that the image of the lensometer target is aligned in the center of the eyepiece reticle

28. Mires formed in manual lensometer 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

29. Determination of spherical power if all the lines or dots of the target are in focus at a given setting of the power wheel, the lens is spherical, Marking the optical centre 1. Check that the centre of the lens coincides with the centre of the target. 2 . When this is so, the lens is correctly positioned and the optical centre could be marked . 3. While there is no centre dot, the whole set of dots can be ‘framed’ within the lines of the graticule to locate the centre . 4. Repeat the same procedure for the other lens .

30. Determination of spherocylinder Step 1 (finding the sph : power) : Rotate the power wheel until one set of lines (stretched dots) becomes clear. Start with the higher positive power (or lower negative power). The axis drum will need to be rotated to ensure that the lines are unbroken. Note the power on the power wheel. Step 2 (finding the cyl : power) : Rotate the power wheel until the second set of lines (stretched dots)becomes clear. The second power reading minus the first reading will give the power of the cyl (and its correct sign ). Step 3 (finding the axis) : Note the direction of the lines (stretched dots) at the second reading.This is the axis. The rotatable line in the graticuleis used to line up with the stretched dots to determine the axis.

31. Determination of bifocal  For measuring the distance correction of a bifocal lens, we want to measure the lens at the optical center. • The optical center of a conventional bifocal lens is just above the center of the horizontal line of the bifocal lens and read the near correction from the center of the lower segment . • For determining add power, turn glasses around to read from front vertex.

32. To determine the ADD power on a bifocal : 1) Flip the bifocal over so that the lens stop of the lensometer touches the front surface of the lens. NOTE: Remember the Base Curve and ADD segment are both located on the FRONT surface of the lens. 2) Adjust the lens so it touches the distance portion of the lens. usually start with the right lens. 3) Record the power reading using the lensometer as you would in a single vision lens. The power reading is known as the Front Vertex Power for the DISTANCE/carrier Rx. 4) Adjust the lens so that the lens stop is now touching the ADD/NEAR SEGMENT portion of the bifocal lens. 5) Record the power of this segment using the same procedure as before for the Distance Rx portion of the lens. This power is known as the TOTAL NEAR Power.

33. 6) To determine the ADD power, use the following equation: TOTAL NEAR Power = Distance Rx + ADD power, thus ADD power = Total Near Power - Distance Rx NOTE: If the Distance Rx contains a cylinder, just take the difference between the spherical portion of the Rx from the Total Near Power. 7) Repeat for the other lens. For example, if the Distance Rx was determined to be +2.00-1.00 x 180, and the Total Near Power was +4.00 from lensometry , then the ADD power would be: ADD power = Total Near Power - Distance Rx = +4.00 - (+2.00) = +2.00 D.

34. Determination of trifocal As with the bifocal , read the distance correction just above the segment line, and read the near correction from the center of the lower segment.

35. Determination of progressive lens Locate the centre to make the reading accurate, otherwise is similar to the simple lensometery . All new pair contain sticker on it, which describe geometry of lens . The add power is read through the lower/nasal area of the lens Conventional progressive addition is measured from front vertex, while free form progressive from back vertex. However , automated is more suitable for this purpose.

36. Determination of prism Prism moves (deflects) the lensometer target away from the center of the reticle . • The target is deflected in the direction of the prism base. • When viewing the mires, lines may be off-center . • 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.

37. Important points to be remembered . 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

38.  specifications of veatch Aries Lensometer   Power range -20 D to +20 D Axis range 0° to 180° Prism range 0.25D to 7.00D directly,to 14.00D with auxiliary prisms included Lens diameter 13 to 84 mm Light source High luminosity LED Dimensions Height 370 mm Length 500 mm Width 170 mm

39. Limitations of manual lensometer Although it is good for single vision lenses, but may misguide in addition determination of bifocal/trifocal lenses. Power varies with 0.25D difference don’t calculate 0.12D difference. Difficult in case of progressive lenses. Can cause damage to the contact lenses.

40. 2)Automated lensometer

41. 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.

42. Principle: The degree to which a beam of light is deflected as it passes through the lens depends on the focal and prismatic power of the lens and the distance from its optical centre Working: A square pattern of four parallel beam of light is passed through the lens to be tested. It is quick and easy, minimum expertise required to handle . Accuracy is variable with machine to machine. .

43. Features of automated lensometer  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

44. Advantages over manual lensometer : It can used for accuracy in all conditions: Proper centration / decentration ,Axis Marking. Measuring power of near addition lens (bifocal, trifocal, progressive) made easier. Some models also measure Abbe no. The lens also be measured with 0.01D or 0.12D difference. IPD measurement of lens mounted spectacle . Less time consuming in clinical steup .

45. References CLINICAL OPTICS –American Academy of Opthalmology -section 3. Theory and Practice of Optics & Refraction, A. K. Khurana . Grosvenor PRIMARY CARE OPTOMETRY. Internet sources https://lambdageeks.com/a-detailed-overview-on-lensometer-working-uses-parts/ http ://www.medilexicon.com/medicaldictionary.php?t=53289 http ://www.allaboutvision.com/eyeglasses/eyeglass-prescription.htm https ://en.wikipedia.org/wiki/Lensmeter

46. Thank you thank you!