Module 17 Section 3

Home

Catalog

Contact us

Go to Post-test

Module 17:

Clinical Optics

Part 1

Section 3:

This point can be found and marked using a manual American type lensometer by centering the target in the crosshair or center circle of the reticule of the lensometer. This does not apply to the progressive lens; more on that latter. If you are using an auto-lensometer you will need to read the instructions for your particular lensometer.

Marking the optical center is simple with a spherical lens because the single and and triple line targets are in focus at the same time. Make sure the frames are sitting flat on the lens stage. Move the glasses horizontally on the stage, and move the stage vertically, until the intersection of the single and triple line targets is in the crosshair or center circle of the reticule.

Mark the optical center by using the marking device of the lensometer. If the marking device is not in working order, you can use a Sharpie® (Sanford) or similar fine tipped marker. Make the mark just large enough for you to see it. The marks can be removed easily with alcohol.

If the lens is cylindrical, it is more difficult to find the optical center. You will need to align the lens so that the center of the reticule lines up with each target individually, as they will not be in focus at the same time. This will involve shifting focus between the single line target and the triple line target as you intersect them with the center circle.

What if you are unable to intersect the targets with the center circle, no matter how much manipulation you do with the lens position? This is an indication that there is prism in the lens. This is why it is important to try to locate the optical center whenever performing lensometry, even if you don’t intend to mark the optical center.

Finding the optical center of a progressive lens

Because of the way progressive lenses are made, you will not be able to use a lensometer to find the optical center.

There are usually two to four small engraved markings on a progressive lens. In order to see them you will need to hold the lens close (you may need magnification if you cannot otherwise focus at close range). Viewing the lens at an angle and using a light source as a background may help.

The optical center of the progressive lens is found by drawing a straight line from one of the engraved circles to the other. The optical center is marked in the middle of the line. You can make an accurate estimate of the OC without actually drawing a line once you have located the engraved circles. Make sure you are not mistaking a logo for a circle.

Sometimes you will find the nasal circle almost up against the edge of the frame.

Be aware that the optician is instructed by the manufacturer to fit the frame so that the pupil is lined up with a fitting cross that is 3 to 4 mm above the optical center. The finished lens comes to the optician with the fitting cross marked on it so that the proper fit can be confirmed. The markings are then removed by the optician before dispensing the lens. You can approximate the position by marking a dot about 3mm above the optical center.

Optical Center Decentration

If the optical centers of spectacle lenses are not aligned with the optical axis (pupils) of the wearer’s eyes, an unwanted prismatic effect may result. Let us assume we have already found and marked the optical centers of the spectacle lenses. In order to evaluate possible decentration, we need to know the patient’s PD. 

If the patient’s PD does not match the OCD (distance between optical centers), there are four possible situations that produce either a base-in or base-out prismatic effect.

Here’s how it works:

We know from basic optics that a plus lens can be considered to be two prisms aligned base to base:

A minus lens can be considered to be two prisms apex to apex:

By definition, the optical center of a lens is the thickest part of a plus lens, or the thinnest part of a minus lens. Either mentally, or on paper, we draw out the effect of decentration. 

1) In this case we have a plus lens and a PD smaller than the OCD, producing a base-out prismatic effect.

2) A combination of plus lenses and a PD greater than the OCD produces a base-in prismatic effect.

3) A combination of minus lenses and a PD greater than the OCD produces a base out effect.

4) A combination of minus lenses and a PD less than than the OCD produces base-in prism.

How much of a prismatic effect is produced for any of the above combinations?

That depends on the lens power and the difference between the PD and OCD for the particular lens. The Prentice Rule states that the effective prism is equal to the lens power in diopters, multiplied by the distance the viewing point (pupillary center) is from the OCD, as measured in centimeters.

Let’s look at an example:

The glasses Rx is measured as:

OD -4.00 SPH OS -5.00 SPH

The optical center of each lens is marked.  Use a mm ruler to measure the distance between optical center marks (OCD). Take a PD measurement on the patient.

Let us assume the OCD was measured as 68mm and the PD was measured as 64mm. The PD is less than the OCD and will be placed inside the OCD on our diagram. The diagram indicates a base-in prismatic effect.

Figure the magnitude of the effect by taking the difference between the OCD and the PD (68-64=4mm) and dividing by 2 to get the monocular disparity (4/2=2mm). This number (2mm) is converted to centimeters (2/10=.2cm) and is multiplied by the diopter lens power for each lens— 

OD 4D x .2cm = .8 prism diopters

OS 5D x .2cm = 1.0 prism diopters

Important note:  Remember that the formula uses centimeters in the calculation and that your measurement will be in millimeters and will need to be converted.  Remember that the the OCD/PD discrepancy must be divided by 2 when doing monocular calculations.  These are the two factors that cause most of the calculation errors when using this formula.

We can conclude from the formula that optical center decentration does not have much effect in the lower lens powers. Conversely, as the lens power increases, the optical center position becomes more and more important. Frame adjustment becomes a critical issue for the aphakic patient and the high myope wearing glasses.

This prism and decentration stuff isn’t so difficult, is it? Well, I have to make mention of one more factor…

The example we worked through was for spherical lenses. If the lens has a significant cylindrical component, it will need to be accounted for. This is one of those situations where it is easier to work and think in minus cylinder.

The point to remember is that the power of a cylinder is 90 degrees to the axis. This means that a lens with a minus cylinder correction with axis 90 degrees will have a thicker edge at 180 degrees, increasing the horizontal prismatic effect of a minus lens, and decreasing the horizontal prismatic effect of a plus lens.

Let’s modify our previous example:

The glasses Rx is measured as:

OD -4.00-2.00x90

OS -5.00-1.00x90

We will assume the same PD and OCD of the previous example, meaning the multiple will still be .2cm. The minus cylinder axis for each lens is 90 degrees, which translates into an effective horizontal power of 6 diopters (-4 - 2 = -6) for the right lens and 6 diopters for the left lens (-5 - 1 = -6). The increased edge thickness at 180 degrees will increase the horizontal prismatic effect of decentration.

OD 6D x .2mm = 1.2 diopters of induced prism in the horizontal meridian

OS same as above

If the the lens has a plus spherical component, the minus cylinder at 90 degrees will reduce the horizontal prismatic effect— 

+5.00-2.00x90 ► +5.00-2.00= 3D

A minus cylinder at axis 180 will have no effect on this horizontal calculation because the lens edge thickness is at 90.

If the lens has a cylindrical component at axis 45 degrees, the horizontal effect will be half of the cylinder power—

+5.00-2.00x45 ► +5.00-1.00= 4D

The horizontal effect of other cylinder axis readings can be approximated with a reasonable degree of accuracy in the final result.  For example:

+5.00-200x80

Since 80 is very close to 90 degrees (88 percent to be exact), you can approximate the power of the cylinder at 180 to be -1.75 (88 percent of -2.00 is actually -1.76).  Thus the total lens power in the 180 degree meridian would be +3.25 D (+5.00-1.75= +3.25 D).

The power cross can be used to graphically estimate or calculate the lens power in the 90 and 180 degree meridians.  For a discussion of the power cross, see Module 19, Section 1.

Conclusions

So, does this mean that I am going to have to go through these calculations every time the OCD does not exactly match the PD on a patient's glasses?

Practically speaking, no.  There are some generalizations that can be made for most situations.  However, for the COT and COMT tests you will need to be familiar with the various applications of the Prentice Rule and the calculations.

Generalizations:

Look what happens when a +2.00 D lens is decentered 2mm:

Prentice Rule:   2 D x .2 cm = .4 prism diopter

Less than half a diopter of prismatic effect, which practically speaking, does not have much effect.  The American National Standard Institute (ANSI) states that the error should not exceed 2/3 prism diopter in the horizontal plain to be acceptable.  This means that a glasses prescription of 2D OU could have a PD/OCD discrepancy of 3mm and still be within tolerance (2 x .3 = .6, which is less than .75).

Here is a quick calculation that works reasonably well for glasses corrections with cylinder powers one diopter or less:

1) Figure the spherical equivalent of each lens.

2) Average the spherical equivalents of both lenses.

3) Multiply this number by the PD/OCD discrepancy.

4) If the result is above7.5, send the lenses back to be re-made with a note that the OCD is out of tolerance.

5) If the result is below 7.5, assume that the lenses are within tolerance.

Example:

Rx:   OD +3.00+1.00x35     OCD = 68mm  OCD, PD difference = 3mm

         OS +2.00+0.50x20        PD = 65mm

Spherical equivalent:  OD +3.50

                                      OS +2.25

Average lens power:  ~ 3 D

Calculation:  3 x 3 = 9

9 is greater than 7.5, so the lenses are not within tolerance and should be

remade.

As with all generalizations, simplifications, and quick calculations, they break down in some situations.  If the cylinder amount is above one diopter in at least one of the lenses, you will need to break down the calculation as discussed previously, taking into account the axis of the cylinder.

The exact amount of induced prism can be calculated quickly using the calculator included with the Clinical Optics Calculator.