Lensometer and Prism. [Name of the Writer] [Name of the Institution]

Transcription

1 Running Head: SCIENCES PHYSICS 1 Lensometer and Prism [Name of the Writer] [Name of the Institution]

2 SCIENCES PHYSICS 2 Abstract The invention of lensometer has played a vital role by effectively assisting opticians and optometrists in prescribing accurate values of lenses to the patients. The device is also known as focimeter, but normally referred as Lensometer, which is actually a trademark name by an optical equipment manufacturer. The development in this instrument is highly contributed by different people and corporations who have worked for invention of devices which can measure various parameters of a lens; however a breakthrough in the technology was achieved in the year 1921 when US based manufactures patented their design of Lensometer. These instruments have been effectively helpful in verifying the prescribed prismatic correction. However today there exists digital and more advanced version of these instruments called Lensometers. They are fully automatic and easy to operate. In this paper much details are covered for manual lensometers and there use in finding prismatic corrections.

3 SCIENCES PHYSICS 3 Lensometer and Prism Hypothesis Statement Invention of lensometers made it possible for opticians and optometrists to effectively find accurate prescription for pair of glasses and contacts for their patients and minimize inaccuracies caused by conventional methodologies. It is a highly effective instrument for prismatic corrections. Introduction The lensmeter mostly often referred as Lensometer is an optical instrument which is used for measuring the optical parameters of an eyeglass lens or contacts. It is also called a focimeter. Lensometer is actually trademark of an American optical instrument manufacturer Bausch and Lomb. Ophthalmologists use it frequently for finding out the correct and accurate value of lens to be used in frames required for the patient. Opticians use it for checking that either the newly pair of glasses or contacts are correctly made or not. In technical terms a lensmeter is used for measuring the cylinder axis, the refractive power, Prismatic power and optical center of prescription lenses. The history of lensmeter is deeply rooted into the end of 19 th century and with time the technology has highly improved, toady there exist state of the art automatic lensmeter with capability of displaying results as digital values, and they are computer-driven and well more accurate then ever. In the current paper we will be more focused on the manual lensmeter, the history of the technology and its effectiveness for prescribing best option to the patient suffering with optical impairments specially the use of this technology for prismatic correction related issues (Kerr, 2010). Body A brief History Ernst Abbe was a physicist, as an undergraduate student his majors were mathematics and physics. In 1866, before he met Carl Zeiss, a German optical instrument manufacturer, Abbe became very much involved in solving optical problems. In the year 1867, he formed a partnership with Zeiss. The purpose of the partnership was: Abbe worked and designed an instrument with the capability to resolve the focal power of lenses. In 1912, another man Troppman launched an instrument called focimeter ; which was an instrument through which when light is passed through a lens in order to directly convert these values to those dioptric values, which is a unit for measuring optical powers. The development made until that time were limited in scene that the source of illumination, the device used for measuring purposes and the microscope by which the operator observed the lens and deduce results, were still immovable. In the year 1915, Busch, an American optical instrument manufacturer came up with a classic and efficient design, which proved as a long lasting solution. The design of the instrument was actually deduced from the astronomical telescope and a test target. In the device, the external light was used a light source, by using mirroring techniques. Future models supported built-in lamps as internal light source; however until the

4 SCIENCES PHYSICS 4 year 1920s the level of illumination achieved was not so good. In 1921 The American Optical Company patented their lensmeter/focimeter under the brand name Lensometer. In the coming years, there have numerous developments in electronics and optics fields and it resulted in more advanced and accurate equipments. Today manual Lensometers are used but they are somehow replaced by state of the art modern automatic Lensmeters. However in the current research we will be focusing on the different details of manual Lensometer and its use in the prescribing prismatic corrections (college-optometrists.org, 2014). Lensometer parts and functions Different lensometers have different specifications and performance parameters; due to which there may be dissimilarities between two different models. However in all the lensometers following components are common (amconlabs.com, n.d) Eyepiece The eyepiece is fitted with a spiral focusing unit, normally the focusing range of the unit +/-5D so that it may cater various refractive properly. It is represented in Fig 2. Eyepiece Reticle and Marker The field of view of eyepiece is called reticle it is shown in Fig. 3. The Reticle is divided with two scales. These are measuring scale and dividing disc of prism degree. Integer degree of prism is directly read from the measuring scale while the decimal one is anticipated from the degree on the measuring scale. Dividing disc has marking with an interval of 5º. Marker reticle is shown in Fig. 4. These are three long horizontal green lines. Reading Division Board In the eyepiece s field of vision, the lower part of eyepiece reticle has reading window. Reading eyepiece reticle has range of +/-25D. There is 0.125D marking interval between 0 and +/-5D. It is shown in Fig. 5. Prism Compensation Device While measuring a lens diopter value above 5Δ, then the prism compensation device is needed. Prism compensation device have 2 lines of graduation. The upper for measuring 0º to 180º, upper line is used at 5º intervals. For the range from 0 to 15Δ, lower lines prism are used in the in each direction at 1Δ intervals. It is represented in Fig. 6. Lens Pressing Unit These are three plastic pressing feet with springs; they are used to press the surface of lens without damaging it the part is shown in the Fig 7.

5 SCIENCES PHYSICS 5 Lens Pushing Unit This unit is used make the position of the lens to be fixed so measurement can be taken. The part is shown in the Fig 8 Printing Unit The part of the instrument has three same point making pens connected in line. Middle pen is used to set the center of the lens. To distinguish the angle of axis astigmatism and base angle of the prism lens; the connected line of printing points drawn by these three pens are used. It is represented in Fig 9. Objective Lens Bearing Seat This nylon part is used to provide a safe and secure supports to the surface of lens with any kind of shape without damaging it. It also has three lock screws. It is represented in Fig 10. Diopter Measuring Hand wheels These are located on either side of the lensometer. In order to adjust the focus until the observer can clearly see the diopter scale, these screws are rotated gently and smoothly. It is also equipped with mounting screws. It is represented in Fig 11. Astigmatism Axis Measuring Hand wheel In order to measure and fix the axis of the cylindrical lens and the base angle of the prism lens, astigmatism axis Measuring hand wheel is used. It is represented in Fig 12. Illuminating Bulb Chamber It is the main light source of the lensometer; it is represented in Fig 13. Instrument s Inclination Regulating Handle This handle is used to adjust the inclination angle of the instrument so that the operator can easily and comfortably use the equipment by adjusting it in accordance with his/her own posture. It is represented in Fig 14. Prismatic Correction Using Lensometer Strabismus' is an optical impairment in which the patient s eyes fail to directly look at the same object with both eyes simultaneously (.nlm.nih.gov,2014). The reason behind this impairment is extraocular muscles of the patient s eyes are not balanced with each other. This phenomenon is prevalent with growing age. Patients suffering with such diseases are prescribed prismatic correction in order to get one singly merged image rather than double-vision.

6 SCIENCES PHYSICS 6 Because when eye muscles are unable to move together in synchronization then it results in an unclear vision. When inspecting finished eyewear or contacts in which prism is prescribed; It is of immense important to reconfirm that the degree and position of prism has been made-up precisely, which also includes an inspection of the direction as well as degree of prescribed prism too, before handing over the wearable to patient. It is also important to confirm that the lenses may not incorporate any undesired prism or it may not have horizontal or vertical prism disparity or may be even a mixture of the both. In case of an unwanted prism, patient may suffer with various visual problems and discomfort, including double vision, nausea and headaches. In case, when the measurement of PD is switched off then it may lead to a horizontal prism which is not required. If the optical center placements are inaccurate then it may result into vertical prism. However a slight amount of horizontal or vertical prism is tolerable, but greater values are required to be rectified. PD is greatly important for finding prism.pd is the interpupillary distance i.e. that is the distance between the pupils of the patient s both eyes. The PD measurement actually measures the horizontal position of the lens optical center. Unless and until the prism correction is prescribed to a person, the horizontal position of the optical center of each lens should be correlating to monocular pupillary measurement corresponding to each eye. The measure of monocular pupillary distance is from the pupil to the center of the face, and it is regarded as per eye measurement. Another type of measurement is the binocular interpupillary distance. This is the actually the measurement taken between the centers of the both the pupils of a patient s eye. But it is not used mostly due to the reason: the patient face is hardly have a perfect symmetry, therefore the monocular pupillary distance is used Verifying prism Lensmeter calculate the focal length of a lens and give measurements in diopters after converting the focal length into D value. These reading are directly interpretable, and are read on power wheel of the lensmeter manual lensmeter has various functions which includes finding out the cylinder component and axis of a lens, making and indicating position of the optical center, and also about the prism reference point (PRP). It is the point at which the prescribed amount of prism becomes equal to the prism calculated by inspection. It possible for operator of such lensometers to detect areas of a lens which are distorted or have some kind of deviation in it. The operator can also find direction of prism from it. An instrument setup generally used for verifying prism is discussed below stepwise (Carlton, 2012) The Reticle The image of circles and numbers having an orientation lines and a protractor scale is reticle. It is used to measure the optical center (OC) of lens. Also it used to measure and point out direction of prism base. Most of manufacturer of these instruments in U.S.A has marking like that the 1/2 of a diopter of prism is represented by first prism ring; the dashes with in this ring represent one-third prism diopter. It is recommended to refer to specific manufacture s manual for knowing the exact values of the circles in diopters. Focus the eyepiece. The eyepiece of lensmeter has series of lenses. In order to neutralize the lens it is order to focus the eyepiece by turning the power drum zero power, after that turning the eyepiece in to its extreme plus value. It is done without

7 SCIENCES PHYSICS 7 putting the lenses into the instrument for verification. Now examine the reticle through the eyepiece. When observing through the reticle, it is required to adjust the eyepiece by turning it in minus values. It is done until the circles and numbers becomes sharp, clear and focused, at the time when the values seems to me most clear, then stop. This complete procedure is called focusing. Position the frame. In this step the frame is properly placed on the display table. During this procedure it is confirmed that eyewires are placed evenly. In case of even slight displacement in the eyewires, there can be a significant error in results Clamp the lens in place. The device on the lensmeter which is used for holding down the lens firmly in place is the lens holder. Using this device, while obtaining the power of a lens is recommended, so that to make it sure that values are calculated accurately. It is required to be cautious while clamping a lens into place so to avoid any kind of damages to lens under inspection It is also necessary to check the lens holder. It is done so in order to confirm for any kind of damage to the lens holder itself, as this can also result in scratches or damages to the lens. The clamps which are holding lens are with rubber pads, it is expected from the operator to check them for any wear and tear as in case these pad dry out and may scratch the lens surface. Verifying ground-in prism Firstly it is required to examine the patient s or customer s order, so that to confirm prescribed prism amount and its direction of the lens pair. After doing that the lens optical center (OC) is located. This is done by finding the central point, which is actually the point at which the crosshairs coincides. When the optical centers have been located then the lens are locked into place with the help of the lens holder. Now through the eyepiece the optical center of the strongest lens is viewed. After that it is observed that how the OC is corresponding with the reticle markings. The reticle rings have markings such that to find out the prism diopter value Prism direction is calculated using its base. There are four quadrants in reticle, which are used in measurement of the prism direction. It is necessary to know that in which quadrant out of the four, the OC appears. In order to find out the prism direction the correlation of the bridge to the optical center is checked. If the OC is ground, and it is on the line of 180 degree then in case of Oculus Dexter (OD) lens, the prism direction would be base in. This reading will show results having base in. This is so due to face nasal area is the extreme inward position for the right lens. It would have base out (BO), while using OD lens, in case if the OC would ground in sequential position. As it would be the most outward position of the right lens. Now in the end the ink-marking tool is used to mark the lenses and then to measure the PRP.This PRP value should correspond with the PD measurement which is mentioned on the job order. If required then sometimes the prism ground is also found out in the

8 SCIENCES PHYSICS 8 left lens. This is done by repeating the above steps. It is necessary that settings of the frames remain unaltered. The prism compensating device (PCD) is used to examine lenses which have demands a high prism correction.pcd is placed near the eyepiece in the lensmeter, and capable of moving horizontally to find out horizontal prism. Also PCD can be locked in vertical position to examine the lenses which have vertical prism. Before checking lenses with requirement of prism correction, firstly it is required to move the device into proper location, and afterward lock it, while in 180 degree position this is the case while lenses having horizontal prism are examined. The device is locked into 90 degree position while examining lenses with vertical prism. The lens is positioned properly. Now the silver knob of the PCD is rotated till the OC of the lens under inspection is first centered in the view of reticle. Then the inside prism scale is used to indicate the degree of percentage to which prism is present in the lens. In this case the direction of prism is by device position and bridge position. Lens is marked with marking device and process is repeated for other lens accordingly Verifying induced prism Induced prisms have capability so that they are movable to various positions on reticle. Such prism are verified in a way such that firstly the optical center is aligned in the centre of the reticle, one that is done then the 2 nd step is using Prentice s Rule. Depending upon the power of the lens it found that what the value of induced prism in the lens is. After when the amount of induced prism is calculated then the 2 nd step is to find out the induced prism direction Current State of the Technology Due to technological advancements there have been enormous improvements in the capabilities and accuracy of the lensometers. In today s world of automation and advanced computer technology, we also have fully automated and sophisticated computerized lensmeters. The modern lensmeter are very easy and straightforward to operate. They are equipped with a clear and large LCD screen. The modern instruments are incorporated with advanced processing technologies, due to which they are capable of processing data not only automatically but at a faster rate. It also eliminates the inaccuracies and errors usually caused as a result of human error. These instruments are capable of providing positive results for all type of hard and soft lenses quickly because the modern lensmeter have capability to automatically distinguish between different types of lens. Some of the modern technologies are also incorporating built-in memory in the instrument so that the patient data can be stored and manipulated accordingly with the help of just few click and touches (birminghamoptical.co.uk, 2014). Conclusion A lensometer which is also known as a focimeter is an optical instrument which is most commonly used by opticians and eye specialists for prescribing lenses to be used in eye glasses or as contacts for handling issues related to vision impairments. A lensometer is used for measuring prism, cylindrical power, spherical power, axis, prism and also the different optical centre positions of a lens. There history of lensometer is quite old, and today there are automatic and more precise version of these instruments. Most of the lensometers have different specifications and therefore there are slight differences in the configuration and different parts in

9 SCIENCES PHYSICS 9 lensometers from different manufacturers. However some of the parts are common to all the lensometers, which have been discussed already in the paper. The use of lensometer is quite effective in prescribing prismatic corrections. Prisms are prescribed to patients who have such kind of vision impairments such that they are unable to focus on a single object from both eyes simultaneously. This problem occurs mostly in the old ages, and it is due to some issues in eyes muscles function due to which patient can not focus on the objects in a harmony. In such issues prism are prescribed to the patient but the proper and correct prism values are immensely important to be confirmed before handing over the pair of glasses or contacts to the patients, as an incorrect value may result into discomfort and uneasiness to the patient. An important parameter for finding out prism value is the interpupillary distance that is regarded as PD. It is the distance between the pupils of the eyes. There are two type of prism and methods for verifying them are discussed in detail. One is the ground prism and the other one is the induced prism. The prism values in each case may be either horizontal or vertical and a methodology for both the cases is discussed accordingly. In today s world there is still use of manual lensometers, however there are state of the art digital lensmeter which are making it even more easy, convenient and accurate for the operator to check and verify different parameters of the lenses.

10 SCIENCES PHYSICS 10 References Amcon Labs (n.d). Official Website: Amcon Lensometer user s manual EQ-6001 Data retrieved from Birmingham Optical. (2014). Official Website: Nidek LM-1800 Auto Lensmeter. Data retrieved from lensmeter/ on April 12, 2014 Carlton, J.(2012). Understanding Prism Part 2: Verifying prescribed and unwonted prism.. Data retrieved on April 12, 2014 Kerr, A.D.(2010). The Focimeter: Measuring Eyeglass Lenses. Data retrieved from on April 12, 2014 The College of Optometrists (2014). Official Website: Focimeters.Data retrieved from cfm on April 12, 2014 U.S National Library of Medicines (2014). Official Website: Strabismus Data retrieved from on April 12, 2014