Frames: Measurements & Markings

Frames may be measured using either one of two systems, the Datum System or the Boxing System.

Datum System of Measurements

When measuring an eye size using the datum system, an imaginary line is drawn through the middle of the eyewire both in the horizontal and vertical planes. The length of those lines corresponds to the horizontal and vertical measurements of the eyewire. Given a shape such as that illustrated above, it is easy to see how inconsistencies in measurement can result from using this system. This system is not used by lens manufacturing laboratories or ophthalmic frame manufacturers.

Box System of Measurements

The boxing system is more accurate and more widely used in the optical industry today. In this system an imaginary box is drawn around the area to be measured. The distance between opposite, parallel sides of the box determine the length of the distance being measured. These measurements are of the lens, not the frame. All measurements are made from tip-of-bevel to tip-of-bevel even if it’s nested deeply in the frame. This system yields accurate measurements even with unusual shapes.

Frame measurements may be determined using different methods. The below information discusses taking actual measurements of the frame using a PD ruler as well as determining the measurements based on what’s provided on the frame.

Actual measurements can be taken by placing your PD ruler on the frame and taking each measurement individually. 

• ‘A’ is the longest horizontal dimension of the eye wire.
• ‘B’ is the longest vertical dimension of the eye wire.
• ‘ED’ stands for effective diameter and is a measure of two times the longest radius of the lens shape (it is not always diagonal!)
• ‘DBL’ means distance between lenses, not bridge size. We measure where the lenses are the closest to one another even if they are nested deep inside the frame or not in line with the actual bridge, meaning your PD ruler must sit inside of each eye wire or rim.
• ‘FPD’ (‘GCD’) can be measured from the outer eye wire on the left to the inner eye wire on the right, on or as close to the same plane as DBL as possible.

Unless you remove the lens from the frame to measure, you will have to add a small amount to your actual measurement to account for how much is inserted in the frame. Adding 1 mm to your ‘A’ and ‘B’ will usually suffice. However, you will need to subtract 1 mm from the ‘DBL’ to compensate for the added amount in measurements.

‘ED’ must be the largest measurement, so it can never be smaller than ‘A’ or ‘B’. Most of the common ways to measure ‘ED’ in our industry are estimates. For example, opticians often measure the longest diagonal and add 2mm or simply add 4mm to the ‘A’ measurement. Estimates are okay, until they aren’t. The more non-symmetrical the lens shape is (e.g. cat-eyes, aviators, or notched/hooked sunglasses), the bigger the ‘ED’ will be as compared to the ‘A’. The most accurate way to measure the ‘ED’ is to first mark the geometric center of the lens, where half the ‘A’ and half the ‘B’ meet. Now, from that point, measure to the furthest tip of the bevel and multiply that length by two. You can remember the formula: ED = 2 x the longest radius of the lens shape. This is the most important measurement for the lab to determine proper cut-out and thickness of the lens.

In theory, these measurements should correspond to the markings on the frame itself. But your measurements might be more accurate, so beware of using frame markings alone.

Frame markings can also be reviewed to assess frame measurements. The eye size corresponds to ‘A’. The bridge corresponds to ‘DBL’. Most frames are not marked with ‘B’ or ‘ED’ measurements, and you will need to take these by hand, so it is recommended to verify all of the measurements manually. 

Additionally, if a frame size and bridge are separated by a small box, it indicates that the manufacturer also uses the standard box system. It looks like this:  48□18, with the 48 representing the ‘A’ measurement and the 18 representing the ‘DBL’ measurement. Any other marking such as a dash (-) or slash (/) means that the manufacturer uses their own system.

A competent optician is capable of taking accurate measurements, and this skill plays an especially important role as it relates to frame dimensions.

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