Prism glasses are specialized eyewear designed to correct binocular vision problems, primarily double vision, by bending light before it reaches the eye. These lenses contain a precise wedge shape that redirects the image onto the retina, allowing the brain to fuse two separate images into one clear picture. While the internet offers convenience and lower costs, purchasing prism glasses online is a cautious “yes,” though significant caveats regarding measurement accuracy and prescription complexity must be considered.
Decoding the Prism Prescription
A prescription for prism correction contains unique elements that differ from a standard spherical or cylindrical lens order. The first measurement is the prism value, expressed in prism diopters (PD), which quantifies the required light deviation. Prescriptions typically range from 0.5 to 10 PD per lens, indicating the degree of eye misalignment that needs to be compensated for.
The second component is the base direction, which specifies the orientation of the prism’s thickest edge. This direction dictates how the light is deflected and is denoted by four possibilities: Base In (BI), Base Out (BO), Base Up (BU), or Base Down (BD). Accurately transcribing both the numerical value and the exact base direction is essential for successfully ordering a prism lens remotely.
These prescribed prism details are crucial because they ensure the light is shifted precisely to counteract the eye deviation. Any error in the diopter value or base direction will result in the eyes still receiving misaligned images. This misalignment can lead to headaches, eye strain, or a worsening of the original double vision.
Critical Measurements for Remote Orders
For any prescription glasses, the Pupillary Distance (PD) is an important measurement, but for prism lenses, its accuracy becomes exponentially more critical. The PD measures the distance between the center of the pupils, ensuring the optical center of the lens is placed directly in front of the eye’s visual axis. When ordering online, it is essential to provide the monocular PD, which is the distance from the center of the nose bridge to the center of each pupil separately.
This dual measurement is necessary because even a small shift in the optical center of a lens with power can inadvertently introduce unwanted prism, a concept explained by Prentice’s Rule. If the lens is centered incorrectly, the power of the lens itself creates an unintended prismatic effect that compounds or negates the prescribed correction. For example, being off by just a few millimeters in a high-power prescription can induce several prism diopters of error, which can completely spoil the intended correction.
Another critical factor is the optical center (OC) height. This is the vertical measurement from the bottom of the frame to the center of the pupil. Since the prism correction needs to be centered precisely both horizontally (via PD) and vertically (via OC height), supplying this measurement is non-negotiable for success.
Patients must obtain these measurements either by requesting them from their eye doctor or by utilizing special online tools or having a friend carefully measure them with a millimeter ruler. Unlike a standard prescription where a slight PD error might only cause minor discomfort, an inaccurate PD or OC height with a prism prescription can render the glasses unusable.
When to Choose an In-Person Dispenser
While online purchasing is possible for simple prism prescriptions, certain complexities make an in-person dispenser the only safe and reliable option.
High Prism Values
High prism values, generally those exceeding four or five prism diopters per eye, are inherently difficult to manufacture and fit correctly in a remote setting. Lenses with strong prism corrections must have a significantly thicker edge. Managing this thickness to ensure it fits the chosen frame and remains cosmetically acceptable requires professional expertise.
Complex Combination Prescriptions
Complex combination prescriptions also require mandatory in-person fitting. This includes prescriptions that combine prism correction with multifocal designs, such as progressive addition lenses. Progressive lenses already require extremely precise fitting heights and horizontal centering to function correctly. Adding a prism correction introduces another layer of complexity that is nearly impossible to manage remotely.
Frame Selection and Fitting
Frame selection plays a significant role in the success of prism lenses. Frames with a high degree of wrap or those with unusual shapes can distort the intended prismatic effect. An optician can physically measure the frame’s specific pantoscopic tilt and face-form wrap, making necessary adjustments and ensuring the lens is ground to account for these variables. Choosing an in-person dispenser in these complex scenarios outweighs the potential cost savings of ordering online, as the risk of receiving an uncomfortable or unusable pair of glasses is significantly reduced.