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PURPOSE To measure the back vertex refractive power, the cylinder axis, the optical center, and the prismatic power of prescription lenses．
EQUIPMENT
·Lensometer．
·Loose lenses.
BASIC COMPONENTS OF A LENSOMETER
Because lensometers differ, the examiner should review the location of the components and the specific instructions provided by the manufacturer for the lensometer he is using．The following components are common to all lensometers and are important to the examiner in reading a lens prescription：
EXTERNAL PARTS
1．Adjustable eyepiece for focusing the instrument for the examiner’s eye．
2．Lens holder and lens table to sport the lens that is being measured．
3．Power wheel for reading the refractive power of the lens．
4．Axis wheel in 1° increments from 1°to 180°to read the axis of cylindrical lenses．
5．Inkwell and pens for dotting the optical center of the lens．
INTERNAL PARTS
1．Reticle for focusing and for determining prism power．The reticle is focused by the eyepiece of the instrument．
2．A target consisting of two sets of lines perpendicular to one another for reading the power of the lens．These lines are focused by the power wheel．
SET-UP
·With the power wheel set on zero, the examiner focuses the eyepiece by turning the eyepiece as far counterclockwise as possible，and then slowly turning it clockwise until the reticle first comes into sharp focus．
·If testing a pair of glasses, always check the right lens first．
·Place the lens or 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 (see Figure 3-2)．
·Center the lens by moving it so that the image of the lensometer target is aligned in the center of the eyepiece reticle．Note：If the lens has been made with prism ground into it, it may be impossible to center the target in the reticle．See the section in this chapter for methods to deal with prism．
STEP-BY-STEP PROCEDURE
SINGLE-VISION LENSES
1．Determine which part of the target is used for determining the spherical power of the lens and which part is used for determining the cylindrical power of the lens．This varies depending on the manufacturer of the lensometer, as shown in Figure 3-3．
2．Rotate the power wheel until the target comes into sharp focus．If the spherical and cylindrical lines of the target come into focus at the same time, the lens is spherical．Read the power of the lens from the power wheel and record．
3．If the spherical and cylindrical lines do not come into focus at the same time, the lens has a cylindrical component．To read the power of a spherocylindrical lens，stat with enough plus power to blur the lensometer target, then rotate the power wheel toward less plus until the spherical line comes into focus. At the same time, orient the axis wheel of the lensometer so that the spherical line is perfectly continuous．Read the power from the power wheel and record it as the spherical portion of the prescription．
4．Focus the cylindrical line by rotating the power wheel toward more minus power．The difference between the power when the spherical portion of the target is in focus and the cylindrical portion of the target is in focus is the amount of minus cylinder power in the lens．The axis of the cylinder is read directly from the lensometer’s protractor．
5．Before moving the glasses，dot the optical center(OC)of the lens with the lensometer’s marking device．
6．Repeat steps 2 to 5 for the left lens．
7．When both lenses have been measured and dotted, measure the distance between the optical centers of the lenses and compare it to the patient’s interpillary distance．If the patient’s PD and the PD of the spectacles are not the same, or if there is a vertical discrepancy in the heights of the optical centers of the two lenses, calculate the amount of induced prism using Prentice’s rule (see step 4 under Prism)．
8．The power of rigid and soft contact lenses can also be read in the lensometer．The procedures are described in Chapter 6．
MULTIFOCAL LENSES
1．Read and record the power of the distance portion of each of the two lenses(the carrier)as described in steps 1 to 7 above．
2．Turn the glasses around so that the ocular surface faces you．
3．Recheck one meridian in the carrier and compare the power in this meridian to the power in the same meridian through the near portion(the segment)of the lens(ie, compare the spherical power to the spherical power or the cylindrical power to the cylindrical power)．The difference between these powers is the add．It is often necessary to reset the axis orientation during this step．
4．Although the adds in the right and left lenses are usually the same, the add should be determined separately for each lens．
5．Progressive addition lenses are read in the same manner as other multifocal lenses, but the examiner must use the guidelines of the manufacturer for locating the near portion of the lens．
PRISM
1．Locate the center of the target in the center of the eyepiece reticle．
2．Dot this location on the lens．This is the optical center of the lens.
3．Subsequently dot the location of the patient’s line of sight on the lens．
4．When the optical center of the lens and the location of the patient’s line of sight do not coincide，compute the induced prism using Prentice’s rule：
△ = DC/10
where D is the power of the lens in diopters and C is the linear distance between the patient’s line of sight and the optical center of the lens in mm．Vertical and horizontal prism are calculated separately．
5．If the lens has been made with prism ground into it, it may be impossible to center the target in the reticle．In this case, dot the lens at the location of the patient’s line of sight and position the lens so the dot is in the center of the reticle．Read the amount of prism using the prism scale in the lensometer．
Note：With high amounts of prism, it is difficult or impossible to locate the center of the target．In this case, hand-held prisms are added to center the target and to determine the amount of prism in the lens．
6．Record the amount and direction of prism in the glasses．
Note：First calculate the prism induced by each lens separately．The prism induced by the spectacles is the net discrepancy between the prism induced by each of the two lenses．
RECORDING
·Record the Rx for each lens separately in standard Rx form．
·Record the amount of induced prism in the glasses，if applicable.
EXAMPLES
·OD -2.75 sph
0S -2.25 = -1.00×10
·OD +2.75 = -.50×90，Add +2.00
OS +3.25 = -1.25×110．Add +2.00
·0D -2.00 = -1.00×180，Add +.50
OS +2.75 = -1.50×160, Add +2.00
·0D +2.00 = -1.00×165 = 2△BI．Add +1.00
0S +2.00 =- 1.00×10 = 2△BI，Add +1.00 |
