Aspheric Lens Irregularity And Strehl Ratio

All optical systems have a theoretical performance limit known as the diffraction limit. Strehl ratio is a specification used to compare the real performance of an optical system with its diffraction-limited performance. For aspheric lenses and other focusing optics, Strehl ratio is defined as the ratio of peak focal spot irradiance of the manufactured optic to the diffraction-limited peak irradiance (Figure 1).¹ Different aspheric lens manufacturing techniques impart unique surface errors that affect Strehl ratio differently. Understanding these effects is important to correctly specify aspheres, preventing unnecessary and costly overspecification while ensuring that desired specifications are met. This becomes increasingly crucial for fast aspheres with small f/#'s, and affects aspheres manufactured through conventional grinding and polishing more than those made through other methods. The industry standard threshold to classify a lens as “diffraction-limited” is a Strehl ratio greater than 0.8.