Measuring Prism in Progressive Addition Spectacle Lenses

Background: Patients frequently go to eye care providers without knowing the prescription of their glasses. Progressive addition lenses can already be difficult to measure, especially when the prescription is unknown and lens markings are not visible. With the addition of ground-in prism, this problem is further compounded, and there is no standard methodology to minimize user error.

Purpose: This paper emphasizes the problems inherent in measuring such spectacle lenses, and describes a technique that best approximates prism power in this situation.

Methods: The distance viewing point of the lens is first marked, and effective vertical and horizontal prism powers are measured separately. Finally, the add power is estimated by measuring the increase in power, in the plus direction, from the top of the lens to the lowest portion of the progressive corridor. Staying within the progressive corridor is aided by avoiding the appearance of induced astigmatism when straying toward the edge of the corridor. 

Conclusions: This manuscript provides a standardized and repeatable method for measuring progressive add spectacles with prism. 

Keywords: Progressive addition lenses, prism in progressive addition lenses, progressives, prism, PAL, PALs, prism reference point, measuring prism, measuring spectacle prism.