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The prism reference point, or PRP, is a specific location on an eyeglass lens used for measurement and quality control. It is the exact point on the lens where an optician or lab technician measures the amount of prism to verify that it matches what the doctor prescribed. It is a critical checkpoint for ensuring the accuracy of your glasses.
read more about prism reference point prp ...Sometimes a doctor will prescribe prism to help correct eye alignment problems. This prescribed prism must be made very accurately. The prism reference point is the official, standardized spot on the lens to check this accuracy. An optician will place the lens in an instrument called a lensometer and measure the prism at the PRP to confirm it is correct.
The location of the PRP depends on the type of lens. In a simple single vision lens, the PRP is the same as the optical center. In a progressive lens, however, the PRP is a distinct, permanently marked spot. It is located a few millimeters below the "fitting cross," which is the mark that gets aligned with your pupil. This specific location is needed to get an accurate reading on a complex progressive lens.
The prism reference point is a central part of the quality control process in making glasses. Both the optical lab that makes the lenses and the optician who dispenses them use the PRP as their checkpoint. This step helps guarantee that the prism you need matches the prism in the finished lens. If the measurement at the PRP is wrong, the lens must be remade.
In progressive lenses, the PRP is also used to check for "prism thinning" or "equi-thinning." This is a technique where the lab grinds a specific amount of base-down prism into both lenses equally. This does not affect your vision because the prism is equal in both eyes. However, it physically thins the lens profile, making the glasses lighter and more attractive by removing unnecessary thickness from the top edge.
Yes, all prescription lenses have a point where prism can be verified. The term PRP is most formally used and is a physically marked spot on progressive lenses because of their complex design.
On progressive lenses, the PRP is part of a set of tiny, laser-etched markings that are made to be almost invisible. You cannot see them when you are wearing the glasses, but an optician knows how to find them to check your lenses.
Wearing glasses with an incorrect amount of prism can cause significant problems, including double vision, headaches, and a feeling of being off-balance. This is why verifying the prism at the PRP is so important.
The PRP is a technical mark for professionals, so you would not normally need to discuss it with your optician. It is one of the many behind-the-scenes details they manage to ensure your glasses are made perfectly. However, if you have a prism prescription and your new glasses are causing you discomfort, your optician will use the PRP as the first place to check for errors.
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