Question 20 of 48

(demo test)

1) focus the eyepiece so that the reticle is sharp.  First, turn eyepiece into the plus area (counter clockwise) until the reticle becomes blurry.  The plus area is marked by a plus sign or in black (as apposed to the minus red numbers).  Then turn the dial is slowly clockwise until the reticle comes FIRST comes into focus.  If the observer has no glasses prescription or if the observer is wearing their glasses, this should be at 0.  This method prevents the observer from accommodating which falsely adds minus power to the prescription.  If the observer wears contact lenses or glasses, they should be worn while reading the glasses prescription or the observer's prescription should be put into the eyepiece.  

2) The glasses are now placed into the lensometer with the temples pointed away from the observer.  The right lens is read first.  The glasses are placed onto the lens stage and held into place against the lens holder by the lens clamp.  It is very important to adjust the lever of the lens stage to control the vertical position of the glasses.  Do NOT hold the glasses into the lensometer by hand.  This can induce tilt and result in an inaccurate reading.  Pulling back on the spring-loaded lens clamp holder will unlock the lens clamp.  The lens clamp with then go toward the glasses and hold them in place against the lens holder.  

3) Determine if the lens is spherical or has cylinder.  Look through the lensometer and adjust the power wheel focus until the single or the triple line mires are seen.  Center mires in the reticle by adjusting the position of the glasses in the lensometer until the mires are perfectly in the middle of the reticle.  If you are unable to center the mires in the middle of the reticle, this indicates that there is prism in the glasses (this is covered below).  Once the glasses are centered, if both sets of mires are in focus at the same time, this indicates that the lens is a spherical lens.  If one set of mires comes into focus while the other is out of focus, the glasses have cylinder.  

4) Determine the prescription if the glasses have cylinder.  Get one of the lines of mires in focus.  Rotate the axis wheel until there are no breaks in the mire lines.  Record the number on the power wheel.  Then focus the power wheel until the second set of mires is in focus and record the number on the power wheel AND the number on the axis wheel.  The spherical correction of the glasses prescription will be the first number recorded.  The difference between the second and first number respectively will be the amount of cylinder in the prescription.  For example, suppose the first set of mires was in focus at -2.00 sph and the second set of mires was in focus at -3.00 axis 178.  The power of the glasses will be -2.00 sph.  The cylinder will be -3.00 - (-2.00) = -1.00.  The axis will be the axis of the second number recorded called 178.  So, the overall prescription will be -2.00 -1.00 x 178.  

5) Transpose the prescription to the type of cylinder that is preferred.  Most doctors will want the glasses prescription in either positive or negative cylinder.  Converting the prescription from positive to negative cylinder notation or vice versa is called transposition.  It is done by: 1) adding the sphere to the cylinder to get the new sphere, 2) changing the cylinder sign (either from positive to negative or negative to positive) and 3) changing the axis power 90 degrees.  For example, let's say you have a prescription of -2.00 -1.00 x 178.  To get the new spherical power, you would add -2.00 to -1.00 to get -3.00.  The cylinder would change from -1.00 to +1.00.  The axis would change by 90 degrees to become 88.  Remember that the axis is always between 0 and 180.  So, if you add 90 degrees to the axis and it is over 180, you need to subtract 90 degrees.  For example, if the axis is 178, the new axis would be 88 and NOT 268.  The final overall transposed prescription would be +2.00 -3.00 x 88.  

6) Figure out the add on a straight top bifocal.  Get one of the sets of mires in focus and record the power on the power wheel.  Technically you are now supposed to turn the glasses 180 degrees so the temples are facing the observers.  Practically this is not relevant unless the patient has a high add power.  Now, move the lens stage up (which will move the glasses up) until you are in the middle of the bifocal.  Readjust the power wheel until the SAME set of mires is in focus.  The power of the add will be the difference between the second number and the first number.  It will always be positive.  For example, if the distance reading was -4.00 sph and the bifocal add reading was -2.00, the bifocal would be -2.00 - (-4.00) = +2.00.  If the glasses have a trifocal, you should technically read this as well.  However, it is almost always just half of the bifocal add.  For instance, if the bifocal add was +2.00, the trifocal would be +1.00.  

7) Figure out the add on a progressive bifocal.  A progressive bifocal is also known as a "no-line bifocal".  It is called progressive because the power of the bifocal progressives gets stronger as the patient looks toward the bottom of the glasses.  So, when you are reading the add on a progressive bifocal, it is best to move the lens stage up so that you are reading as close to the bottom of the lens as possible.  Progressive lenses also generally have a marking on the lens that tells the power of the lens.  This is the most reliable way to determine a progressive add power.  

8) Measuring Prism

Prism is measured as: base in, base out, base up, or base down.  Base in means that the mires are deviated toward the nose and base out means the mires are deviated away from the nose.  Note that you have to know which lens is being measured.  If the mires are deviated to the right on the right lens, it is base in prism.  If the mires are deviated to the right in a left lens, the prism is base out.  The numbered circles in the lensometer represent the amount of prism.  Make sure you adjust the glasses so that the middle mire of one set of mires crosses the middle mire of the other set of mires on the horizontal line of the lensometer.  

a) Find the optical center.  This is done by putting the cross-hair target in the middle of the concentric circles.  This can be marked with the ink marking system on the lensometer.

b) Measure horizontal prism.  First, measure the patient's pupillary distance (PD).  Take the patients glasses and make an ink dot on the glasses so that the PD is marked out and symmetric on the glasses.  Try to make the ink marks centered on the glasses from a horizontal and vertical standpoint.  Line the ink marks up in the center of the lensometer.  The amount of prism is the deviation from the center and measured by the mires (circles).  The first circle is generally 1/2 prism diopter.  The total prism is the addition of the prism in the right eye and left eye.  Note: you don't need place an ink mark if you are certain that the optical center of the lenses were made properly by separating the optical centers by the patient's PD.  If this is not the case, a decentered optical center can induce prism.

c) Measure vertical prism.  First measure the lens with the highest power in the vertical meridian.  Center the reticle in the lensometer with this lens.  Without moving the position of the lens table, move the glasses over to the other lens.  The amount of deviation as measured by the mires (circles) is the total amount of vertical deviation.