What Measurements are Used When Ordering Glasses?

Posted on Feb 7, 2024

Ordering glasses may seem like a straightforward process, but there are various measurements involved to ensure a proper fit and optimal visual correction. From the pupillary distance to the optical center, segment height, and prism reference point – each measurement plays a crucial role in delivering glasses that provide maximum comfort and visual acuity. In this blog, we will explore these essential measurements and their significance in the process of ordering glasses.


Pupillary Distance (PD)

Pupillary distance refers to the measurement between the centers of each pupil. It is a key measurement used to position the optical centers of the lenses correctly in line with your eyes. PD measurement ensures that your eyes are properly aligned with the focal point of the lenses, allowing for precise vision correction. The pupillary distance can vary from person to person, and an accurate measurement is essential to avoid visual discomfort.


Optical Center (OC)

The optical center is the point on the lens that provides the best visual acuity. It is determined based on the pupillary distance. The optical center ensures that your eyes are looking through the right spot on the lens to achieve the sharpest and most accurate vision. When ordering glasses, the lenses are positioned so that the optical center aligns with your pupils for optimal visual performance.


Segment Height

Segment height is a critical measurement used in the production of multifocal lenses, such as bifocals or progressive lenses. It refers to the vertical distance between the bottom of the lens and the top of the segment (or the line separating the distance and near vision correction areas). Accurate segment height measurement ensures that the bifocal or progressive lens segments are positioned correctly, allowing for a smooth transition between the different vision zones.


Prism Reference Point (PRP)

Prism correction is used to address certain eye conditions, such as double vision or eye misalignment. The prism reference point is a measurement used to position the prism correction accurately. It refers to the point on the lens where the prism is needed to correct the vision problem. The PRP ensures that the prism effect is applied at the correct location on the lens, aligning with the eyes and providing the needed correction.


Importance of Accurate Measurements Ordering Glasses

Accurate measurements are crucial when ordering glasses, as they directly impact the fit, comfort, and visual performance of your eyewear. Here’s why these measurements matter: 

Visual Clarity: Properly positioned optical centers and accurate pupillary distance ensure that your eyes are looking through the right spot on the lens, providing the clearest vision possible.

Comfort: Incorrect measurements can lead to eyestrain, headaches, or discomfort, as your eyes may need to strain to achieve proper focus. Accurate measurements ensure a comfortable fit.

Corrected Prescription: Correct segment height measurements are essential for multifocal lenses to ensure the appropriate correction for both near and distance vision.

Management of Eye Conditions: Accurate prism reference point measurements ensure that the prism correction is applied correctly to address specific eye conditions, allowing for improved vision and eye alignment.

Obtaining Accurate Measurements

To ensure accurate measurements, it is recommended to visit a qualified optician or eye care professional. They have the expertise and tools necessary to obtain precise measurements for your glasses. Some opticians may use manual measurement techniques or rely on advanced digital scanning or imaging technologies to capture the necessary data accurately. 

When ordering glasses online, many reputable vendors provide clear instructions on how to measure specific parameters, such as the pupillary distance, and offer guidance on obtaining accurate measurements at home. It’s crucial to follow their instructions diligently or consult with your eye care professional to ensure accurate results.


Our eye doctors at Eye Theory in Houston, TX excel in the prescription of contact lenses, glasses and various eye diseases.  Call our optometrist at 832.831.7386 or schedule an appointment online if you would like to learn more about the importance of measurements when ordering glasses for optimal visual performance.  Our eye doctor, Dr. Jonathan Tsao, provides the highest quality optometry services and eye exams in the Midtown, Downtown, Museum District, Montrose, East Downtown, and Southside Commons (Southside Place) vicinities of Houston, Texas as well as our newest location in the Stone Oaks neighborhood of San Antonio, Texas.

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