When choosing new eyewear, consumers often encounter a series of numbers stamped on the frame. These measurements represent a highly standardized system used across the optical industry to ensure lenses fit correctly. This system provides precise dimensions for the space reserved for the lens, which is necessary for accurate manufacturing. These numbers refer to the lens size, not the overall physical size of the frame material, leading to misunderstandings about how height and width are calculated. Understanding this standardized method is key to translating the numbers on your frames into tangible dimensions.
Understanding the Frame Boxing System
The optical industry relies on a precise method called the Boxing System to define the dimensions of a spectacle frame’s lens opening. This system standardizes measurements by imagining a rectangle, or “box,” drawn around the space where the lens sits. The sides of this box are drawn tangent to the outermost horizontal and vertical edges of the lens shape.
This imaginary box creates the reference points for the three primary measurements typically found stamped on the frame arm or bridge. The ‘A’ Dimension, or eye size, is the horizontal distance between the vertical sides of this box, representing the widest part of the lens opening. The Distance Between Lenses (DBL) measures the shortest gap between the nasal edges of the two lens openings, often referred to as the bridge size. Both the ‘A’ dimension and the DBL are measurements of the internal space available for the lens, not the width of the surrounding frame material.
The Boxing System’s definition of these measurements as internal is fundamental to optical manufacturing consistency. Focusing on the lens space ensures that lenses are cut to the exact shape and size required to fit the frame’s groove. This method ensures that all standard measurements are independent of the varying thickness, shape, and style of the frame rim itself.
The ‘B’ Dimension: Measuring Lens Height
The vertical measurement, known as the ‘B’ dimension, measures the lens height. The ‘B’ dimension measures the maximum vertical distance between the horizontal sides of the imaginary box tangent to the lens shape. It does not include the physical material of the frame rim.
This exclusion is necessary because frame rims can vary dramatically in thickness, from a fine wire to a thick acetate border. If the ‘B’ measurement included the frame, the reported size would change based on the cosmetic design of the rim, making the measurement useless for standardized lens production. The standardized ‘B’ dimension ensures that the vertical size of the lens itself is accurately conveyed, regardless of the frame’s aesthetic bulk.
The ‘B’ measurement is taken parallel to the DBL line and represents the maximum lens height at its tallest point. Therefore, the measurement is a dimension of the lens space carved out by the manufacturer, not the overall vertical footprint of the entire eyewear structure. This focus on the internal space allows the ‘B’ dimension to serve as a reliable parameter for lens fitting.
Practical Importance of Lens Height
The ‘B’ dimension is a significant measurement that determines the suitability of a frame for certain lens designs, particularly for multi-focal prescriptions. The vertical height of the lens space is directly related to the ability to incorporate different power zones within a single lens. A sufficient ‘B’ dimension is required to accommodate the gradual change in power from distance vision at the top to reading vision at the bottom.
Lenses with specialized designs, such as progressive addition lenses (PALs), require a minimum vertical height to create a functional corridor of vision. This corridor must contain the distance, intermediate, and near viewing zones without compressing them excessively. Many modern PAL designs require a minimum ‘B’ dimension, often in the range of 28 to 30 millimeters, for proper segment placement.
When the ‘B’ dimension is too small, the near vision zone can be compromised or entirely cut off during the lens cutting process. This means a frame may look appealing but be visually unsuitable for the wearer’s prescription. The lens height measurement is a functional requirement that directly impacts the comfort and clarity of specialized vision correction.