Measuring the correct pupillary distance and pupil heights for your customers is quintessential to proper optical dispensing. Get it right and your customers will love their new glasses, get it wrong and they will promptly be back to return their glasses.
In this lesson, you will learn why are these measurements so important, as well as how they are used during the lens edging & mounting process.
Why Are Pupillary Distance and Pupil Height Measurements So Important?
To gain a deeper understanding let’s first review a few important points from previous lessons:
- Lenses create a focal point (from lesson: Lens Power)
- For clear vision, the focal point has to be exactly on the the retina (from lesson: Basic Eye Optics)
Here’s an example:
In this example, we have a myopic eye. A minus lens diverges light slightly, then the cornea (a plus lens) converges the light to a focal point that is on the retina.
Side note: Compared to ray diagrams in The Optics of Vision course, you might think that I’ve drawn an absurdly large lens in this diagram. In actuality, this is much closer to the real-life proportions of the lenses we work with.
Now let’s focus in on the area of interest from the diagram above; the focal point forming on the retina.
In reality, we don’t just want the focal point to be on the retina, we want it to be exactly on the macula.
What is the Macula?
The macula is a small red area on the retina that is able to produce very sharp vision. Click here for more information.
What is the Difference Between the Retina and the Macula?
The retina is the entire layer that lines the inside back portion of the eyes. Most of the retina cannot produce sharp vision.
The macula makes up a very small portion of the retina, but it is the only portion able to produce sharp vision.
This image shows the macula. It appears as a small red spot inside the eyes. It is on that tiny little red spot that light must come to a focus for us to see clearly. The rest of the retina senses light, but does not produce sharp vision.
The macula produces sharp vision, the rest of the retina does not.
Light must focus on the macula for vision to be clear.
Here is how I will represent the macula in a cross-section diagram.
How Do We Get The Focal Point Onto The Macula?
In order to properly position the focal point on the macula, we have to know:
- Where a lens’ focal point will be.
- Where the macula is.
Where Is The Focal Point?
A lens’s focal point is always in line with its optical center.
Here is a diagram that illustrates a plus lens creating a focal point. Notice that lens’s optical center and its focal point both lie on the same straight line (green).
Where Is The Macula?
The macula is always in line with the center of the pupil.
Here is a cross section of an eye. Notice that the center of the pupil and the macula both lie on the same straight line (green).
Combine It All Together
We now know that that focal point is in line with the lens’ optical center, and the macula is in line with the center of the pupil. Combining these two pieces of information tells us that aligning the lens’ optical center with the center of the pupil will place the focal point on the macula.
This is a diagram showing the optical center of the lens being lined up with the center of the pupil. As a result, the focal point falls exactly on the macula.
This is a diagram showing what happens with the optical center of the lens is not lined up with the center of the pupil. As a result, the focal point does not form on the macula.
For vision to be clear, the lens must focus light exactly on the macula.
For a lens to focus light exactly on the macula, the optical center of the lens must be aligned with the center of the pupil.
Note to all the seasoned dispensers reading this If you’re screaming ‘INDUCED PRISMATIC EFFECT‘ at the screen right now; cool it. This is a beginners course.
Now, let’s shift gears and talk about glasses.
How To Align the Optical Center of a Lens With The Center of the Pupil
Glasses frames do one thing; they hold the lenses in front of the eyes. But before the lenses are placed in the frames, they are just large discs called semi-finished lenses.
Semi-finished lenses have prescription in them, and coatings on them, but they are not yet cut to the shape of the glasses frame they will eventually be mounted into.
|A semi-finished lens.||A drawer full of semi-finished lenses in different prescriptions.|
Semi-finished lenses typically have a diameter of 70mm. This is intentionally bigger than most glasses frames so that they can be cut down to fit any size/shape of frame.
So if we’re starting with a lens that is way bigger than the glasses frame, here’s the problem: Which part of the lens are we going to put into the frame?
Just imagine that the semi-finished lens blank is a sheet of rolled out cookie dough and the shape of frame is the cookie cutter. We have to cut out a piece of the lens and put it in the frame. But which part of the semi-finished lens (cookie dough) are you going to cut out and use for the glasses?
Let’s remember what the goal is.
The goal is to align the optical center of the lens with the center of the pupil.
This means we’re going to need to know 2 things:
- Where the optical center of the lens is
- Where the center of the pupil is when the glasses are worn
Where is the Optical Center of a Semi-Finished Lens?
For now, you can assume that the optical center is in the exact middle of the semi-finished lens. This is not always the case, but for the purpose of this lesson we can assume that it is.
Where is the center of the pupil when the glasses are worn?
Where the center of the pupil falls when glasses are worn will be different for every person and for every frame. Here is an example of me wearing a pair glasses:
Look at where the center of my pupils are on both the left and right sides. That is where the optical center of the lenses should be placed.
In order to place the optical center of the lenses exactly where my pupils are, we need two measurements:
- The pupillary distance
- The pupil height
The Pupillary Distance
The pupillary distance (PD) is the distance between the center of the two pupils, measured in millimeters (mm). It is a very useful measurement to take because for any given (adult) person, the PD never changes. This means that you only have to measure a person’s PD once, and it can used with any frame.
The image below shows that my pupillary distance is 60mm.
The pupils height, often just called the height, is the distance from the center of the pupil to the frame’s bottom rim, measured in millimeters (mm). This measurement is frame specific, meaning that for any given person, wearing a different frame will result in a different height measurements.
The image below shows that my pupil height with this frame is 23mm.
Why are Heights Frame Specific but the Pupillary Distance is Not?
You do not have to understand this, but if that’s what you’re wondering: I like where you’re head’s at. You should consider becoming an optician.
Once the pupillary distance and pupil heights have been measured, you have essentially established a set of coordinates. These coordinates will tell the edging lab exactly where and how to cut the semi-finished lenses in order for the optical center of the lenses to be perfectly aligned with the center of the pupils when the glasses are worn.
Measuring Pupillary Distance and Pupil Heights
There are many ways to measure both the pupillary distance and pupil heights. In order to properly explore all the relevant techniques, I will dedicate a separate lesson for this.
Let’s Do A Run Through
The customer has chosen a frame and wants to get his prescription put into them. We have a semi-finished lens blank with the customer’s prescription in it. What do we need to do in order to properly position the lens within the frame?
Remember: The goal is to position the lens’ optical center directly over top of the center of the customer’s pupil.
Step 1: Measure the customer’s PD (glasses off). This gives us the horizontal position for the lens.
Step 2: Measure the customer’s pupil height (glasses on). This gives us the vertical position for the lens.
We now have all the information to determine exactly where the center of pupils will be when the customer wears the glasses.
This tells the edging lab exactly what part of the semi-finished lens to cut out and put into the frame to give the customer the best vision possible.
Making glasses is not as simple as just putting a lens into a frame. Many things have to be taken into careful consideration, including where the optical center of lens is in relation to the customer’s eyes.
Here is a summary of the key take-homes:
- Every prescription lens has an optical center – the part of the lens that offers the best possible vision
- The goal when making glasses is to align the lens’ optical center with the center of the customer’s pupil
- To pinpoint where the center of the customer’s pupil will be when the glasses are worn, two measurements need to be taken – the pupillary distance and the pupil height
In the next lesson we will learn the techniques and instruments used to take these measurements.