A trial contact lens is a temporary sample provided by an eye care professional, typically at no cost, to evaluate the suitability of a specific lens. These lenses are not blank placeholders; they always contain a specific, corrective prescription designed to match your visual needs. The purpose of these short-term samples is to ensure the chosen lens parameters, including the optical power, provide clear vision, comfortable wear, and proper eye health before a bulk supply is ordered. The trial phase is a mandatory step in the contact lens fitting process.
The Purpose of Prescribed Trial Lenses
The prescription in the trial lens is necessary because the lens serves as the final, real-time diagnostic tool for the eye care specialist. A primary purpose is confirming visual acuity, verifying that the chosen optical power provides crisp and stable sight. Unlike eyeglasses, contact lenses rest directly on the eye, requiring a slight power adjustment from a glasses prescription due to the difference in distance from the cornea.
A second evaluation area is the physical comfort of the lens, which is difficult to predict solely from eye measurements. Patients wear the trial lenses for a short period—often one to two weeks—to check for sustained irritation, dryness, or a feeling of a foreign body. This test ensures the lens material and design are compatible with the patient’s unique tear film and sensitivity.
The third focus is the fit and movement of the lens on the cornea, observed using a specialized microscope known as a slit lamp. The lens must center correctly and move slightly with each blink to allow for tear exchange and oxygen flow beneath it, maintaining corneal health. If the trial lens is too tight, it can restrict oxygen, while a loose lens can cause excessive movement and blurry vision.
Steps to Determine the Trial Prescription
Determining the appropriate trial prescription begins with a standard vision examination, or refraction, which measures the sphere power needed to correct nearsightedness or farsightedness. A contact lens prescription involves additional parameters unique to the physical dimensions of the eye, requiring the specialist to use a keratometer or corneal topographer to precisely map the curvature of the cornea.
This measurement is used to select the Base Curve (BC), which dictates the curvature of the back surface of the contact lens. The Base Curve, typically expressed in millimeters (e.g., 8.6 mm), must closely match the shape of the cornea to ensure a proper fit. A mismatch in this curvature can lead to discomfort or potential corneal damage.
Another specific measurement is the Diameter (DIA), which determines the overall width of the lens. The Diameter ensures the lens extends properly over the cornea and onto the surrounding white part of the eye (sclera) for stability. The final trial prescription is a combination of the corrective optical power, the Base Curve, and the Diameter, chosen before the lens is placed on the eye for evaluation.
Key Differences: Trial Lenses Versus Purchased Supply
Trial lenses are logistically distinct from the supply a patient ultimately purchases, though the lens specifications are intended to be identical. The most immediate difference is the packaging; trials are typically provided in individual blister packs, often unmarked, rather than in multi-pack retail boxes. This format allows the eye care office to keep a stock of various powers and parameters for immediate dispensing.
The quantity is limited, as trial lenses are provided for short-term testing, not for extended use. A patient usually receives just a few pairs to cover the evaluation period before their follow-up appointment. This allows the patient to confirm that the lens is comfortable and functional in their daily environment.
A final difference is that the prescription on the purchased box may be slightly adjusted based on the patient’s feedback during the trial period. If a patient reports that vision was nearly perfect but comfort was poor, the eye care professional may alter the Base Curve or switch to a different lens material on the final order. The trial lens, therefore, acts as a temporary prototype before the long-term supply is finalized and ordered.