CONTINUING EDUCATION, 1 CE Credit – $9.99, 1 Hour, General Knowledge, Level 1, Release date: October 2007, Expiration date: October 31, 2012

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Continuing Education, 1 Credit Hour, $14.99

1 CE Credit – $9.99
1 Hour, General Knowledge, Level 1
Release date: October2007
Expiration date: 


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   In Pursuit of Precision


Alvaro Cordova, ABO, NCLE

Alvaro Cordova is a licensed ophthalmic dispenser in NJ and has an Associates in Applied Science degree in Ophthalmics from Raritan Valley Community College in North Branch. He is a contributing author for OptiCourier Magazine. Alvaro is a member of OAA, OANJ, ATO and CLSA and maintains a blog at

Learning Objectives 

This course is intended to give rudimentary knowledge of:

1. Basic understanding of how precision in measuring affects optics.
2. Take facial and frame measurements such as:

  • Ocular Center, Segment and Progressive Heights

  • Pupillary Distance measurements

  • Understand when and how to take a vertex distance measurement

3. Layout single vision, basic bifocal, trifocal, and progressive lenses for edging.


Historically, lens companies would advertise the precision of their optics versus their competitors. The theory went that if the optics were measured to a certain threshold, the visual aid would perform better and more predictably. This idea isn’t far from the truth. In many ways, poor measurements and optics can introduce optical aberrations that would otherwise remain unnoticed. 

It is safe to say that the pursuit for ever precise measurements and optics remains at the center of the optical industry to this day, and well into the future. It is not a coincidence that many lens companies today are pursuing wave-front technologies that rely heavily on facial and ocular measurements. The future, it seems, will be driven by precision just as it has in the past. This module will focus on properly measuring the patient for heights, vertex distance, and pupillary distances, as well as frame dimensions in order to properly layout lenses for edging. 

Frame Measurements: The Boxing System 

In the desire for precision, the optician is given the concept of the boxing system. The boxing system consists of a rectangle that encloses the lens. Each horizontal and vertical line touches the outermost edge of the lens. The horizontal and vertical lines remain parallel to each other respectively. The horizontal lines are perpendicular to the vertical lines. As these lines enclose the lens, it becomes apparent that the center of the rectangle is also the geometric center of the lens. By placing diagonal lines, one can easily see where the geometric center of the lens is (refer to Figure 1.)


Frame Measurements: The Boxing System

Figure 1.


The A dimension is the widest horizontal measurement and the B dimension is the longest vertical measurement. It is measured with a ruler in millimeters. Remember that the A dimension and B dimension include the bevel of the lens. When one measures just the opening of the frame, add about 1 mm because the bevel of the lens isn’t immediately visible. Half of the B dimension is also known as the horizontal midline or the datum line.The datum line was primarily used in the datum system. The datum system is no longer in use.The DBL is measured from the end of the lens to the beginning of the other. It is not necessarily equal to the width of the bridge at the horizontal midline. As the name suggests, it is the distance between the lenses. In Figure 1, one can see that the DBL is slightly higher than the horizontal midline.

The benefit of using the boxing system is consistency. So when one specifies a height, it is assumed to be from the bottom of the rectangle. This system ensures that all persons involved in the processing and fabrication refer to the same representation. The distance between each geometric lens center is called the Frame PD or the “Distance Between Centers.” This measurement is important because it is needed when decentering a lens for edging. It happens that the “Distance Between Centers” is equal to the A dimension plus the DBL. 

DBC = Frame PD = A + DBL 

Once all the frame dimensions are collected, we need to find out what the patient’s PD, or pupillary distance, is. This measurement can be taken with a ruler or a pupillometer. 

Facial Measurements: Taking Pupillary Measurements 

If one is going to use a ruler it is preferable to use a penlight so that the corneal reflex becomes apparent. Have the patient sit down in front of you at 16 inches (40 cm) away and at eye level. Have the patient’s head inclination facing you so that it doesn’t look like the patient is looking up or down to see you. Your left is the patient’s right, so start with your left. Shut your right eye and have the penlight directly underneath your open left eye and have the patient look at the light or your eye. 

Your head or the patient’s head should not move at all during the measurement. Position the ruler so that the 0 matches with the reflection. Make sure you hold the ruler across the patient’s nose with the top of the ruler tilted away from you. The numbers should be on the top. Don’t press too hard against any features and try to avoid any sawing action while you position the zero of the PD ruler underneath the pupillary reflection. Once it is “zeroed,” open your right eye and close your left and move the penlight underneath the right eye. Have the patient look at the light or your eye, but be consistent. Don’t have them look at your eye and then the light when you switch eyes or vice versa. Once the patient is looking at the object of choice (penlight or eye), measure where the corneal reflection is. 

Keep in mind that neither head should have moved during the entire process. I highly recommend practicing this on a co-worker several times before taking a measurement on a patient. You must make sure the penlight isn’t crooked when taking any measurements and make sure your head didn’t move. This measurement will give you a binocular distance PD which is a single number.

To take a monocular PD, follow the rules above and obtain the binocular PD, but simply note the measurement at the center of the nose to obtain the right monocular PD. Subtract the reading you took from the binocular PD to obtain the left eye monocular PD. This process is simpler with a pupillometer. The light is provided inside the device and the corneal reflex is easily visible. The pupillometer also rests naturally on the nose like a pair of spectacles would. The measurement itself is taken by lining up the vertical line with the corneal reflex. One must make sure that the pupillometer is set to optical infinity and that the nose and forehead rests are touching in order to obtain an accurate distance PD. Monocular PDs generally look like; Right Eye/Left Eye.

The near PD, taken with a ruler, is similar in method to taking the distance PD except that the patient will be looking at an object at their working distance, which will usually be 16 inches (40 cm) away. One places the penlight at one’s nose. Like before, the ruler is placed on the patient’s nose and the zero of the ruler is underneath the corneal reflex of the patient’s right eye. 

Without changing the eye, one looks to see what the other corneal reflex reads at. With the pupillometer, it is virtually the same as the distance PD except that the setting is at the working distance. Monocular PDs should be taken especially when the patient will be wearing an aspheric lens, progressives, has an asymmetric face or is strabismic. If the patient has strabismus, one must occlude each eye before taking a measurement.  

Facial Measurements: Taking Heights 

Heights are required for any vertical measurement. Before any height is taken, it is extremely important to adjust the frame to the patient. A typical bifocal measurement is found at the lower lash line. A trifocal height is usually measured at the lower pupillary margin or in the middle of the lower iris. Typically, an OC height will be taken at the pupil or a few millimeters underneath it to compensate for the pantoscopic tilt. Progressive heights are taken at the center of the pupil or as indicated by the manufacturer. The patient must be at eye level for any accurate measurement and at 16 inches (40 cm) away directly in front of you. 

There are several ways to take a height. One method includes marking up the demo lenses with a marker while the patient looks at one’s eye or finger directly ahead of the patient. It is important that the patient’s head faces you in such a way that the eyes are neither looking up or down, also known as primary position. One makes a mark on the demo lens depending on the height one wants and measures according to the boxing system.

My personal preference is to place the ruler up to the frame on the patient’s head. The zero of the ruler should be placed at the bottom B dimension or the zero placed on the point of interest and measured to the bottom B dimension. This requires a little visualization on your part in order to box in the lens and determine where the bottom B dimension lies. 

An OC height is becoming more important with the prevalence of aspheric lenses. If the patient does not look through aspheric lenses properly, they may experience discomfort or blurry vision even though the PDs are correct. Aspheric lenses may be less forgiving, but they offer considerable advantages that spherical lenses do not. It is also important to note that when dealing with children the height may be specified at a different location. Contact their eye care provider to determine where the multi-focal lens height is to be measured.

It is important to simulate the desired distance. If taking a progressive or OC height, one wants the patient to look into the distance. Since the patient is directly in front, have the patient look at the eye directly ahead of the eye being measured. For segmented bifocals, one wants the patient to look at their working distance which will usually be between 14 (35 cm) and 16 inches (40 cm). Have the patient sit down as before, but have them look at your finger in front of your nose or situate one of your eyes between the patient's eyes and have them look at your eye or your finger beneath your eye. You don't need to switch eyes as in the near PD example.Take all measurements according to the boxing system. 

Facial Measurements: Vertex Distance 

When a prescription reaches a certain power, the distance away from the eyes while wearing spectacles should match the distance the eyes are away from the phoropter during the eye examination. A patient may start to experience problems with their spectacles at around 6 diopters of power. At around 12 diopters, 1mm of difference from the distance the refraction was taken can cause an eighth of a diopter of error from the patient’s point of view. It is important to make a phone call to find out the vertex distance at the time of the examination and either duplicate that distance or compensate the optics at the distance the spectacles will be dispensed at. In any case, one will be required to use a distometer. 


The distometer measures the vertex distance and even compensates for the average eyelid thickness. Have the patient close their eyes with the spectacles on in a relaxed fashion and place the flat end on the patient’s eyelid directly behind the lens. The elongated button is pushed and the distometer arm will come out of its resting position. Once it lightly touches the back of the lens, take the reading. From here, you can either adjust the frame or make a decision to fit the glasses at that distance. If you decide to fit the glasses at a different distance then the examination, make sure your distance is feasible and that the prescription is compensated correctly at that distance. It is important to understand that the further away a prescription sits, the smaller the field of view. 

Single Vision Lens Layout 

Each single vision lens to be used should be already marked up before being decentered. The major reference point (MRP) will generally look like three dots or a cross, the MRP being found at the cross or middle dot. The cross and dots are used to horizontally align the lens in the blocker. Many confuse the optical center with the major reference point. The optical center is the point which light passes undeviated. The major reference point is where, upon inspection with a lensmeter, the full prescription will be found. In many cases the, OC and the MRP will be equal. If a prescription calls for prism, then the major reference point will differ from the OC. The MRP is what is being decentered.

Once a lens is marked up correctly, it is time to figure out how much decentration is required.  Some patternless edgers do not require decentration. A monocular or binocular PD can be directly keyed in. Usually if cutting a rimless frame or if there is an auto-beveling feature, it is best to decenter so that the mechanical center and geometric center of the lens coincide and do not interfere with the chuck. 

Most blockers will use a grid, the origin of which coincides with the geometric center of the boxing system. So when one decenters a lens, it is in reference to the boxing center. Some blockers may have a slider that will allow for the proper positioning of bifocals and allow for half millimeter estimates. To properly decenter a lens horizontally, two equations need to be introduced depending on the information given: 

Horizontal Decentration per Lens = ((Frame PD) / 2) – (Monocular PD)

Horizontal Decentration per Lens = ((Frame PD) – (Binocular PD)) / 2 

If the answer is positive, one decenters “IN,” if negative, one decenters “OUT.” If your blocker requires that the lens face ocular side towards you, then you decenter a right lens “IN” by moving the lens to the left and moved to the right if you need to decenter “OUT.” It is opposite for a left lens. If the ocular side is away from you, than a right lens is decentered “IN” by moving it to the right and is moved left for “OUT.” The movement is opposite for a left lens. 

Decentration up or down is determined by half the B dimension minus the height.If the answer is positive, then one decenters up, negative down. 

Vertical Decentration per Lens = Ocular Center Height - (B / 2) 

Segmented Bifocal Lens Layout 

Segmented bifocals include bifocals (FT-28, FT-35, FT-45 etc…), trifocals, executives and quadrafocals. For bifocal lenses, one decenters the segment based on the near PD and any inset from the distance PD would have been surfaced into the lens. The one exception to this would be with executive or “Franklin” bifocals. Executive bifocals are decentered using the distance PD, but must be checked to have the proper inset. Ideally, bifocals should look, more or less, symmetrical unless the patient has significant facial asymmetry. The top most segment is where the height will be. This includes lenses such as Executive D trifocals (ED Trifocal) and regular trifocals. The Executive D trifocal is like any ordinary Franklin style bifocal with one major exception. The bottom half of the lens will contain a D-shaped segment. The intermediate section is located in the bottom half, with the obvious exception of the D-shaped segment. The near PD will bisect the full powered segment. (Refer to Figure 2).

Segmented Bifocal Lens Layout
Figure 2 

Vertical Decentration per Lens = Segment Height - (B / 2) 

It will probably be easier to explain with an example. 

Consider a frame with a B dimension measurement of 40 mm. The height taken was at 18.Using the equation for vertical decentration we obtain -2 (-2 = 18 – (40 / 2)).That means the height will be decentered 2 BELOW or DOWN the horizontal midline. From this, we can gather that a negative number will equal “BELOW” and a positive number will be “UP” according to the boxing system. 

Progressive Addition Lens Layout 

Progressive lenses are very similar to the decentering of single vision lenses with an OC height. The only difference is that there will be a progressive fitting cross. The cross is what will be decentered. Progressives provide several markings to properly align the lens. 

This lens in particular should be properly decentered with great care. They are both expensive and extremely unforgiving if not precise enough. Patients may complain of “swimming” or maintain unnatural head postures to see clearly out of them.

It is important to know what the different markings are.

Progressive Additional Lens Layout
1.Distance Reference Point (DRP)

2.Fitting Cross
3.Hidden Marking (Add Power Underneath)
4.Prism Reference Point (PRP)
5.Near Reference Point (NRP)
6.Hidden Marking (Manufacturers Logo Underneath) 

The hidden markings can be difficult to see but contain useful information such as the add power or manufacturer logo and sometimes the material. 

Putting it All Together: Three Examples 

Lets put all this information together. Try to determine the answer for yourself before checking the answer.  

Single Vision Example
Q1. What is the decentration required to properly layout this lens? 

A=50   B=35   DBL=18   OC Height=18   OU Patient's Near PD=59
                                                Patient's Distance PD=31/32
FPD = 50 + 18 = 68 

Horizontal Decentration per Lens = ((Frame PD) / 2) – (Monocular PD)
(Right Eye)Horizontal Decentration per Lens = ((68 / 2) – 31) = 3 = 3 IN
(Left Eye)Horizontal Decentration per Lens = ((68 / 2) – 32) = 2 = 2 IN 

Vertical Decentration per Lens = Ocular Center Height - (B / 2)
Vertical Decentration per Lens = 18 – 17.5 = 0.5 = 0.5 UP 

3IN 0.5 UPOD
2IN 0.5 UPOS

Segmented Bifocals Example  
Q2. What is the decentration required to properly layout this lens?

A=30   B=22   DBL=14   Seg Height=10   Patient's Near PD=45  
Patient's Distance PD=47.5
FPD = 30 + 14 = 44

Horizontal Decentration per Lens = ((Frame PD) – (Binocular PD)) / 2
(Both Eyes) Horizontal Decentration per Lens = ((44 - 45 ) / 2) = -0.5 = 0.5 OUT

Vertical Decentration per Lens = Segment Height - (B / 2)
Vertical Decentration per Lens = 10 – 11 = -1 = 1 DOWN 

0.5 OUT 1 DOWN OU 

Progressive Lenses Example  
Q3. What is the decentration required to properly layout this lens?

A=48   B=32   DBL=19Seg   Height=22/23   Patient’s Near PD=31/32
                                                                        Patient's Distance PD=33.5/34.5
FPD = 48 + 19 = 67

Horizontal Decentration per Lens = ((Frame PD) / 2) – (Monocular PD)
(Right Eye) Horizontal Decentration per Lens = ((67 / 2) – 33.5) = 0

(Left Eye) Horizontal Decentration per Lens = ((67 / 2) – 34.5) = -1 = 1 OUT

Vertical Decentration per Lens = Segment Height - (B / 2)
(Right Eye) Vertical Decentration per Lens = 22 – (32 / 2) = 6 = 6 UP
(Left Eye) Vertical Decentration per Lens = 23 – (32 / 2) = 7 = 7 UP

6 UP OD  


We all want our work to pass inspection. If special care isn’t taken, the optical device will not work properly and the job will have to be reordered wasting time and money. What other products or devices are as tailored to an individual as a pair of spectacles? Very few, I would suppose. The optician’s craft is precision. 

Depending on what state one resides, there will be a set of standards one abides by.For most people in the United States, that standard will be ANSI. Amazingly these standards are “recommendations” when regarding “dress wear” and not used as a regulatory standard in which to judge one’s work. In other states, a regulatory body regulates according to its own standards of which all licensed opticians must abide by. Regardless of which standard is followed, opticians should strive for greater degrees of accuracy. It’s what this industry is about.

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