Lens Measurements for Glasses – Pupillary Distance
Obviously choosing the correct lenses when you buy spectacles online is one of the most important elements to ensure that your new spectacles are comfortable and fit for use.
As well as deciding what type of lenses you would like for your new spectacles, such as varifocal glasses lenses, reading glasses lenses, bifocal glasses lenses or single vision glasses-it is helpful to also understand how the right size and shape of lenses for your new glasses are derived. Today spectacles lenses conform to a standard measurement system called the boxing system which is the official standard for the industry as defined by the optical manufacturers Association.
This spectacles lens measuring system basically has three main elements:
The first measurement for the glasses lens sets out a rectangle that encompasses the glasses lens. The X and Y measurements for the rectangle are tangents to the lens and define the effective height and width maximums for the glasses lens. The resulting measurements are known as the “B” dimension for the maximum height of the lens, and the “A” dimension for the width of the lens. The first size is often called the lens depth and the second the eye size. This system of boxing the lens also gives a simple geometric method for determining the center of the lens, which is ideally where the glasses wearer would center their visual field.
A comparable measurement of the glasses lens is used to define the minimum lens size that will fit in a particular spectacles frame. This measurement is usually referred to as the effective lens diameter. It is established by defining the minimum circle that the glasses lens will fit within and so as a corollary will provide in combination with other lens parameters a means to determine the minimum thickness at the edge of the glasses lens for any particular frame. Another important use for this measurement is when combined with the geometric center, it allows a calculation to specify the angular location for grinding the spectacles lens across the visual field-this is particularly critical when determining a lens map for varifocal glasses.
The final metric for the boxing system in prescription spectacles is called the “DBC” or is much more often referred to as the “PD” or interpupillary distance for the glasses frame. The simple definition of interpupillary distance is the measurement between the geometric center of the two glasses lenses. It is obviously a combination of the distance of each geometric center to its relative lens edge plus the distance between the two glasses lenses. This distance between the glasses lenses is called the bridge size for the spectacles frame and is important not only to ensure that the eye is centered on the geometric center of the glasses lens, but also used to make sure of a comfortable fit for the glasses as they rest on the wearers nose. This PD measurement between the lens geometric centers is sometimes called the binocular or “center to center” interpupillary distance. Another way to define the PD measurement is to split it into two components: taking the geometric center of each lens to the center of the bridge and then either providing each of the two measurements (now called monocular PD) or summing them together to provide a more accurate frame PD.
This method of determining the correct PD for the spectacles is very common and in most cases very accurate, there is and even more accurate method available using an instrument called a “Corneal reflection pupillometer”. This instrument can be used by an optician to use reflections of light from the cornea to determine the exact pupil center and then to the center of the bridge for the specific spectacles frame-hence giving an extremely accurate value for the monocular PD and so the glasses PD.
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