Estimating the power of a minus spectacle while worn by the patient

This paper presents a new method of estimating the power of a minus spectacle lens without removing the spectacle from the patient's face. The examiner shines a small light toward the spectacle and measures the separation between the images reflected from the front and rear surfaces of the lens. The study presents a relationship between separation of reflected images and the power of minus lenses from 0 to -12 D. The article also contains a theoretical explanation of the phenomenon.